Cortical visual maps

In the visual system, cortical visual areas can be mapped using a variety of methods. Systematic electrophysiological techniques have revealed cortical areas with partial or complete organized maps of the visual field, with different emphasis on the central or peripheral representations (reviewed in Gattass et al., Reference Gattass, Nascimento-Silva, Soares, Lima, Jansen, Diogo, Farias, Botelho, Mariani, Azzi and Fiorani2005). Fig. 1 shows the location of the visual areas described so far using as template the brain of a macaque or capuchin monkey; because the organization in these species is similar, according to our studies, this figure represents a “generic” summary that applies to both species. In addition to V1, V2, and V3, the figure summarizes the visual topographies of the fourth visual area (V4), of two areas located in the occipito-temporal transition [cytoarchitectural area TEO of von Bonin and Bailey Reference von Bonin and Bailey1947, and the temporal ventral posterior area (TVP), which overlaps with cytoarchitectural area TF], of motion-sensitive areas within the superior temporal sulcus (MT, its adjacent dorsal zone—V4t or DZ, and the medial superior temporal area—MST) and of areas PO and POd, located in the anterior bank of the PO sulcus [These overlap with areas “V6” and “V6A” recognized by other groups (e.g., Galletti et al., Reference Galletti, Fattori, Battaglini, Shipp and Zeki1996, Reference Galletti, Fattori, Gamberini and Kutz1999a,Reference Galletti, Fattori, Kutz and Gamberinib)].

Anisotropies in the visual maps

Most cortical topographic maps show marked emphases on specific regions of the sensory surface. In the visual cortex, this is usually in the form of a magnified representation of the fovea, and gradual decrease in the cortical magnification factor (CMF) toward the representation of the retinal periphery (Daniel & Whitteridge, Reference Daniel and Whitteridge1961; Gattass & Gross, Reference Gattass and Gross1981; Gattass et al., Reference Gattass, Gross and Sandell1981; Gattass et al., Reference Gattass, Sousa and Rosa1987; Rosa et al., Reference Rosa, Sousa and Gattass1988b). Superimposed in these gradients is another type of irregularity, anisotropy. In brief, the CMF at a specific point of the map varies depending on the direction along which a measurement is performed; this typically results in the map being “stretched” in one direction, in comparison with the hypothetical situation in which the CMF is defined solely by the eccentricity of RFs recorded therein (Gattass et al., Reference Gattass, Sousa and Rosa1987; Rosa et al., Reference Rosa, Sousa and Gattass1988b). We have used anisotropy measurements to make distinctions between V1, V2, V3, and PO. In both V1 and V2, we have observed that, at any given eccentricity, the CMF is larger when measured between points located along an isopolar line (i.e., between cells with RFs at different eccentricities, but sharing a same polar angle), rather than along an isoeccentric line. We have also noticed that the anisotropy in visual representation is less pronounced in V1 than in V2, where the isopolar CMF is usually 50% higher than the isoeccentric one (Gattass et al., Reference Gattass, Sousa and Rosa1987; Rosa et al., Reference Rosa, Sousa and Gattass1988b). More recently, we have been studying the regularity of visual maps in other areas of the capuchin monkey. For example, in V3v, the anisotropy is even more pronounced than that in V2, with the CMF estimated along isopolar lines being at least twice that measured along isoeccentricity lines (Rosa et al., Reference Rosa, Piñon, Gattass and Sousa2000).

In areas PO and POd of the dorsomedial pathway (Gattass et al., Reference Gattass, Rosa, Sousa, Piñon, Fiorani and Neuenschwander1990; Neuenschwander et al., Reference Neuenschwander, Gattass, Sousa and Piñon1994; Nascimento-Silva et al., Reference Nascimento-Silva, Gattass, Fiorani and Sousa2003, Reference Nascimento-Silva, Piñon, Soares and Gattass2014), we found a different type of irregularity: it is always possible to define the representation of isopolar lines but not those of the isoeccentric lines. At present, we have no direct evidence regarding the functional significance of the observed isopolar order versus disorder in the isoeccentric domain in these two areas. One may speculate that centrifugal and centripetal organizations of directionality, such as those observed in higher-order posterior parietal areas (which are connected to PO), demand interactions between neurons that analyze the regions of space sharing a similar polar angle but at different eccentricities. It may be that the intermixing of RF eccentricities in adjacent columns of areas PO and POd allows these interactions to occur within local circuits. This arrangement could be considered an atypical visuotopic map (Fig. 2) due to its dissimilarities with neighboring V1, V2, and V3.

Fig. 2. Different types of visuotopic organization based on isoeccentric and isopolar domains. Examples of visuotopic organizations with a preponderance of isoeccentric (MT) or isopolar domain representation (PO and POd). Conventions as in Fig. 1.

Area V2 and its connectivity

In macaques, the major cortical projection target of area V1 is area V2 (Kuypers et al., Reference Kuypers, Szwarcbart, Mishkin and Rosvold1965; Cragg & Ainsworth, Reference Cragg and Ainsworth1969; Zeki, Reference Zeki1969, Reference Zeki1971, Reference Zeki1976; Zeki & Sandeman, Reference Zeki and Sandeman1976; Rockland & Pandya, Reference Rockland and Pandya1979, Reference Rockland and Pandya1981; Lund et al., Reference Lund, Hendrickson, Ogren and Tobin1981; Weller & Kaas, Reference Weller and Kaas1985; Van Essen et al., Reference Van Essen, Newsome, Maunsell and Bixby1986). V2 is located within area 18 of Brodmann and corresponds to area OB of von Bonin and Bailey (Reference von Bonin and Bailey1947). Our group was the first to systematically map the entire extent of projection targets of V2, by injecting anterograde tracers in this area (as opposed to using retrograde traces in other areas, which receive V2 projections). In the macaque, we found that all V2 sites project topographically back to V1 and forward to V3, V4, and MT (Gattass et al., Reference Gattass, Sousa, Mishkin and Ungerleider1997). There is also a topographically organized projection from V2 to V4t, but this projection is limited to the lower visual field representation. Thus, V2 appears to project to virtually all the visual cortex within the occipital lobe. In addition to these projections to occipital visual areas, V2 sites representing eccentricities of about 30 deg and greater project to three visual areas in dorsal extrastriate cortex, namely, the MST, the PO, and the ventral intraparietal (VIP) areas.

The second visual area, when stained for the mitochondrial enzyme cytochrome oxydase (CytOx), shows a pattern of alternating thick and thin CytOx-rich stripes, running perpendicular to the V1/V2 border, separated by CytOx-poor interstripes regions (Livingstone & Hubel, Reference Livingstone and Hubel1982). These three types of stripes differ in their neuronal properties and connections (DeYoe & Van Essen, Reference DeYoe and Van Essen1985; Zeki & Shipp, Reference Zeki and Shipp1989; Roe & Ts'o, Reference Roe and Ts'o1995; Levitt et al., Reference Levitt, Kiper and Movshon1994; Olavarria & Van Essen, Reference Olavarria and Van Essen1997). In capuchin monkey V2, histological sections tangential to the cortical surface stained for CytOx revealed a pattern similar to the one described in the Saimiri and Macaca (Livingstone & Hubel, Reference Livingstone and Hubel1982; Wong-Riley & Caroll, Reference Wong-Riley and Carroll1984; DeYoe & Van Essen, Reference DeYoe and Van Essen1985; Shipp & Zeki, Reference Shipp and Zeki1985; Gattass et al., Reference Gattass, Sousa and Rosa1987; Rosa et al., Reference Rosa, Gattass and Fiorani1988a; Zeki & Shipp, Reference Zeki and Shipp1989). Thus, to evaluate whether there is a segregation of the streams of visual information processing in CytOx-modules of V2, we injected fluorescent retrograde tracers into V4, MT, and PO and studied the distribution of the labeled cells in these modules (Nascimento-Silva et al., Reference Nascimento-Silva, Piñon, Soares and Gattass2014). The distribution of labeled cells provides evidence for three streams of visual processing with origins in different CytOx-modules in area V2. The data thus support the subdivision of the dorsal stream of visual information processing into a dorsomedial and a dorsolateral component (Nascimento-Silva et al., Reference Nascimento-Silva, Gattass, Fiorani and Sousa2003, Reference Nascimento-Silva, Piñon, Soares and Gattass2014), as originally proposed by Gattass et al. (Reference Gattass, Rosa, Sousa, Piñon, Fiorani and Neuenschwander1990). Within this scheme, PO would constitute the first processing stage of the dorsal-medial stream, coding for the full-field flow of visual cues during navigation.

Area V3 in the Old World macaque monkey

The existence of a representation of the upper visual quadrant adjacent to that of area V2, in the ventral prestriate cortex, was first suggested by Cragg and Ainsworth (Reference Cragg and Ainsworth1969) and Zeki (Reference Zeki1969) after anatomical tracing experiments in Old World (macaque) monkeys. This region was considered to be part of a “third visual area” (V3), which wrapped around V2 both dorsally and ventrally. However, there are currently two different views regarding the organization of V3 in macaques. Based on electrophysiological mapping studies, Gattass and colleagues (Gattass et al., Reference Gattass, Sousa and Gross1988) have argued that the entire region bordering V2 anteriorly is a single visual area which contains a representation out to 30–40 deg eccentricity in both the upper (V3v) and lower (V3d) visual fields. Gattass et al. (Reference Gattass, Sousa, Mishkin and Ungerleider1997) demonstrated that, although V1 may project asymmetrically to V3, V2 does not. Whereas the upper field representation of V2 projects to V3v, the lower field representation of V2 projects to V3d. The results from the feedforward projection studies in dorsal and ventral V2 serve to delineate the dorsal and ventral portions of V3. H³ amino acids injections in V2 (Fig. 3A) demonstrate that this area projects topographically back to V1 (data not shown) and forward to V3, V4, and MT. Peripheral injections in V2 project to both V3d, as well as to PO (Sousa et al., Reference Sousa, Piñon, Gattass and Rosa1991; Gattass et al., Reference Gattass, Sousa, Mishkin and Ungerleider1997). Note, for example, that injections number 9, 8, 7, 1, 2, 3, and 4 situated, respectively, along the ventral to dorsal extension of V2 (see Fig. 3A), project in an orderly fashion to the strip of cortex immediately anterior to it (i.e., V3v and V3d, respectively).

Fig. 3. Connectivity studies support the existence of a dorsal and a ventral V3, anterior to V2. Summary maps of the V2 feedforward (A) and the V4 feedback (B) projections in the extrastriate cortex. Two-dimensional reconstruction of the macaque monkey cortex, showing the location of the extrastriate visual areas delimited by myeloarchitectonic borders. Injection and projection sites are labeled with the same color to aid visualization. (A) Injection sites (1–11) are plotted with a thick black outline, while their corresponding projections sites are plotted with the same color, but thin outline. Blobs with dotted outline correspond to injection or projection sites in the peripheral representation. Adapted from Gattass et al. (Reference Gattass, Sousa, Mishkin and Ungerleider1997). (B) The corresponding injection and projections sites are labeled with the same number and color. To avoid clutter, all blobs are plotted with thin outlines. Adapted from Ungerleider et al. (Reference Ungerleider, Galkin, Desimone and Gattass2008). Please refer to Fig. 1 concerning the topographic organization of the various areas and other conventions.

Injections of anterograde and retrograde tracers in different topographical locations of V4 are also capable of revealing the borders of dorsal and ventral V3. Central injections in V4 (Fig. 3B) showed feedback projections to V3 both dorsally and ventrally, while far peripheral injections in V4 resulted in projections to V3v, V3d, and PO (Ungerleider et al., Reference Ungerleider, Galkin, Desimone and Gattass2008). Note that injections number 17, 16, 15, 1, 2, 3, 7, 10, 12, 13, 11 placed along V4 (see Fig. 3B) project to V3 following the same pattern as described for V2 above. Altogether, densely spaced tracer injections indicate that V2 and V4 send topographically organized projections to both V3v and V3d, suggesting that these two segments are part of a single and homogeneous area. Our projection results also show that the visual field representation within V3 may extend beyond 40 deg eccentricity, but it does not extend all the way out to 80 deg. Thus, there is a reduction in the extent of cortical visual field representation as one moves from V2 to V3, a finding that is consistent with our electrophysiological findings (Gattass et al., Reference Gattass, Gross and Sandell1981, Reference Gattass, Sousa and Gross1988, Reference Gattass, Nascimento-Silva, Soares, Lima, Jansen, Diogo, Farias, Botelho, Mariani, Azzi and Fiorani2005). Finally, the anatomical results show that V3d shares the representation of the vertical meridian (VM) with areas V4 and PO, which is again consistent with our electrophysiological findings (Gattass et al., Reference Gattass, Sousa, Covey, Chagas, Gattass and Gross1985, Reference Gattass, Sousa and Gross1988).

By contrast, several authors have argued that the cortex anterior to V2, dorsally and ventrally, contains different visual areas, based on differences in projections from V1, myeloarchitecture, and neural response properties (Burkhalter et al., Reference Burkhalter, Felleman, Newsome and Van Essen1986; Newsome et al., Reference Newsome, Maunsell and Van Essen1986; Van Essen et al., Reference Van Essen, Newsome, Maunsell and Bixby1986; Felleman & Van Essen, Reference Felleman and Van Essen1987). These investigators have termed the upper and lower visual field representations anterior to V2 as areas VP and V3, respectively. The name VP was coined on the basis of an anatomical study of interhemispheric connections in the New World (owl) monkey, which observed a conspicuous strip of callosal terminals in the ventral cortex anterior to V2 (Newsome & Allman Reference Newsome and Allman1980). A similar band of callosal terminations was observed in the ventral cortex of Old World monkeys (Van Essen et al., Reference Van Essen, Newsome and Bixby1982; Boussaoud et al., Reference Boussaoud, Desimone and Ungerleider1991), supporting a similar organization in both groups of simians.

In the Gattass et al. (Reference Gattass, Sousa and Gross1988) study of visual topography, we reported two types of organization for V3d in Macaca fascicularis. Fig. 4 shows the visual map of dorsal V3 in the two types of animals, and a pictorial diagram showing the hypothetical transformations (splits) in the representation of the visual field in the two V3 variants. In one type (Type I animal), the anterior border of V3 was simply the representation of the lower VM. In the other type (Type II animal), the lower VM representation was split into two segments, with a portion of the lower visual field representation lying in between. It is important to emphasize, however, that Type II animals were an exception to the rule. Type I animals, where V3 showed a continuous and smooth anterior border, were the most common variant observed.

Fig. 4. Visuotopic organization of area V3 varies across individuals. The Type I (left) and the Type II (right) variants of dorsal V3 organization described in the Macaca fascicularis. See text for details. Adapted from Gattass et al. (Reference Gattass, Sousa and Gross1988). Conventions as in Fig. 1.

Recently, Lyon and Connolly (Reference Lyon and Connolly2012) concluded that evidence obtained in several primate species supports the hypothesis that an elongated V3 area, forming a complete map of the visual field, occupies the region anterior to V2. One central prediction of Lyon & Connolly's scheme is that cells in the dorsal half of V3 always have RFs representing the lower half of the visual field. Specifically (see Fig. 5), sampling neurons along any sequence of sites starting near the V2d border, and moving rostrally, should reveal RFs that move from near the HM of the visual field toward the VM, in the lower half of the visual field.

Fig. 5. Visual cortical organization in gyrencephalic and lissencephalic monkeys diverged in a significant way along evolution. (A) Visuotopic organization proposed for the cortical visual areas anterior to V2 in the macaque monkey. Adapted from Lyon and Connolly (Reference Lyon and Connolly2012). (B) The corresponding organization proposed for small New World monkeys (marmoset). Adapted from Rosa et al. (Reference Rosa, Angelucci, Jeffs and Pettigrew2013). In the marmoset, VLP would correspond to V3v, and VLA to V4v. VLA is equivalent to part of DL. Conventions as in Fig. 1.

Our scheme of V3, with both a V3v and V3d, finds support also in functional magnetic resonance imaging (FMRI) (Arcaro et al., Reference Arcaro, Pinsk, Li and Kastner2011) and anatomical studies (Lyon & Kaas, Reference Lyon and Kaas2002) in the macaque. Arcaro et al., (Reference Arcaro, Pinsk, Li and Kastner2011), using fMRI in the awake macaque trained to maintain fixation (Fig. 6), described a large-scale visuotopic organization along lateral portions of the posterior parietal cortex (PPC) anterior to area V3A, and extending into the lateral intraparietal sulcus (LIP). By considering both the polar angle and the eccentricity phase estimates, they identified four visuotopically-organized areas representing the contralateral visual field within the PPC. In the dorsal portions of the prelunate gyrus, they identified an area, DP that predominantly contained a representation of the lower visual field. The portions of DP located within the PO sulcus represented the horizontal and lower VMs. Fig. 6A depicts the organization of the dorsal visual areas in the macaque, as mapped by Arcaro et al. (Reference Arcaro, Pinsk, Li and Kastner2011). Note the correspondence with the map originally proposed by Van Essen et al. (Reference Van Essen, Drury, Dickson, Harwell, Hanlon and Anderson2001), also in the macaque (Fig. 6B). Caution is needed when interpreting topographical studies carried out using fMRI methods. Topographical studies using electrophysiological methods in anesthetized monkeys benefit from the fact that eye movements in this kind of preparation can be pharmacologically abolished. In Arcaro et al. (Reference Arcaro, Pinsk, Li and Kastner2011), the authors went through the heroic task of training monkeys to fixate for several minutes in order to carry out their mapping in the awaken monkey. The downside is that their fixation window had to be enlarged to 4 deg by 4 deg diameter in order to accommodate for eye movement behavior. Variability in eye fixation, associated with the comparatively noisy nature of the fMRI signal gave rise in Arcaro et al. (Reference Arcaro, Pinsk, Li and Kastner2011) to maps with imprecise or ambiguous landmarks, such as the horizontal and VM representations, which are instrumental for areal delimitations. Overall, however, the fMRI results by Arcaro et al. (Reference Arcaro, Pinsk, Li and Kastner2011) are impressive and consistent with our electrophysiological recordings in V3d and V4d (Gattass et al., Reference Gattass, Sousa and Gross1988). Additionally, they found area DP to represent mainly the contralateral lower visual field (Andersen et al., Reference Andersen, Asanuma, Essick and Siegel1990). These results are also consistent with fMRI studies that have found representations of the lower visual field within the dorsal prelunate gyrus (Fize et al., Reference Fize, Vanduffel, Nelissen, Denys, Chef d'Hotel, Faugeras and Orban2003).

Fig. 6. Functional imaging studies in the macaque reveal a continuous dorsal V3 anterior to dorsal V2. (A) Functional magnetic resonance imaging of the dorsal posterior cortex in the alert macaque monkey reveals the layout of the cortical regions anterior to V2. Note the continuous arrangement of V3d. Stars indicate the foveal representation. (B) Same as in (A), but derived from a combination of electrophysiological, connectivity, and myeloarchitectonic methods. Adapted from Van Essen et al., (Reference Van Essen, Drury, Dickson, Harwell, Hanlon and Anderson2001). The border of the gray patches represents the points of lowest curvature on the cortex, with the midpoint across the dark gray region representing the fundus of the sulcal convexity. The green dashed lines in (B) indicate the border of the sulci. Regions delimited by the green dashed lines indicate the cortical areas, which are buried inside sulci, corresponding to the gray patches in Fig. 1B. Adapted from Arcaro et al., (Reference Arcaro, Pinsk, Li and Kastner2011).

Area V3 in the New World capuchin monkey

Anatomical connections of V1 and MT in the diurnal New World capuchin monkey suggested a lower field representation of V3 anterior to the lower field representation of V2, dorsally (Sousa et al., Reference Sousa, Piñon, Gattass and Rosa1991; Rosa et al., Reference Rosa, Soares, Fiorani and Gattass1993). In Fig. 7, we illustrate the results from retrograde tracer injections in V1. Note that the connectivity between V1 and V3 follows the same precise topographic pattern as the one observed between V1 and V2. Rosa et al. (Reference Rosa, Piñon, Gattass and Sousa2000) studied the visuotopic organization of the “third tier” visual cortex on the ventral surface of the occipital and posterior temporal lobes in the capuchin monkey. Unlike in the dorsal cortex anterior to V2, where there is evidence for multiple visual field representations, in both New World and Old World monkeys (for reviews, see Felleman & Van Essen, Reference Felleman and Van Essen1991; Kaas, Reference Kaas, Rockland, Kaas and Peters1997), the entire strip of cortex inserted between the ventral subdivisions of V2 and V4 formed a single, systematic representation of the upper contralateral quadrant. This region (for which we adopted the designation V3v) is also distinct from adjacent areas in terms of myeloarchitecture and neuronal RF size. With the exception of a slight invasion at the V2v/V3v border, no lower quadrant representation was observed in V3v or in the surrounding ventral cortex. Thus, two possibilities exist: either V3v is an area in itself that contains a representation limited to the upper half of the visual field (as suggested for Old World monkeys by Newsome et al., Reference Newsome, Maunsell and Van Essen1986), or V3v is part of a larger area that encompasses a representation of the lower quadrant in dorsal or dorsolateral extrastriate cortex. This lower quadrant representation could be continuous with the upper quadrant representation or be located in a segregated “island” of cortex, as suggested by Gattass et al. (Reference Gattass, Sousa and Gross1988) for the macaque (see Fig. 1).

Fig. 7. Extrastriate feedback projections to V1 indicate that ventral and dorsal V3 comprise a single homogeneous area, distinct from PO. (A) Flattened reconstruction of the posterior cerebral cortex similar to the illustration shown in Fig. 1B. (B) Flattened cortex exhibiting the injection sites (dotted outline blobs) located throughout area V1 (injections 1–7), along with their corresponding projection sites in the extrastriate cortex. Injection and projection sites are labeled with the same color to aid visualization. (C) Projection onto the contralateral visual hemifield of the location and extent of the seven injection sites. Thin lines in (A) and (B) indicate the lips of cortical sulci, while dotted lines indicate myeloarchitectonic borders of selected extrastriate visual areas. Refer to Fig. 1 for the corresponding topography of the various areas. Adapted from Sousa et al. (Reference Sousa, Piñon, Gattass and Rosa1991).

Rosa et al. (Reference Rosa, Piñon, Gattass and Sousa2000) have argued against the idea that V3v represents the entire extent of a visual area in New World monkeys. The sector of the visual field represented in V3v largely excludes the lower quadrant, with the exception of occasional RFs mapped for cells located near the V2 border. These RFs, which cover only a few degrees below the HM, were assigned either to V2 or V3v on the basis of their size. There are no other lower quadrant representations adjacent to V3v in the ventral cortex. Thus, previous references to “a highly compressed representation of the lower quadrant” (Weller & Kaas, Reference Weller and Kaas1985; Burkhalter et al., Reference Burkhalter, Felleman, Newsome and Van Essen1986) appear to refer solely to the V2/V3v border region. The degree of invasion of the lower quadrant by RF centers of cells at the V2/V3v border is, in the vast majority of cases, within the margin of error introduced by the technique used for estimating the position of the fovea (0.5–1 deg) and was never found to exceed 2 deg. Moreover, in areas with a second-order representation, there is usually a slight overlap between the sectors of the visual field represented on each side of the field discontinuity (e.g., between the parts of the visual field represented in dorsal and ventral V2; see Rosa et al., Reference Rosa, Sousa and Gattass1988b, Reference Rosa, Schmid and Pettigrew1994, Reference Rosa, Fritsches and Elston1997, Reference Rosa, Piñon, Gattass and Sousa2000). Thus, the existence of RFs that invade the lower visual field quadrant cannot be used as a valid argument in favor of a V3v as a “complete” area, representing both quadrants. In addition, the myeloarchitecture of the ventral and dorsolateral areas is similar (Rosa et al., Reference Rosa, Soares, Fiorani and Gattass1993, Reference Rosa, Piñon, Gattass and Sousa2000; Piñon et al., Reference Piñon, Gattass and Sousa1998). We therefore conclude that there is no compelling reason to view V3v in the capuchin monkey as an individual area restricted to the ventral cortex, or complete in terms of visual field representation. However, as pointed out by Rosa et al. (Reference Rosa, Soares, Fiorani and Gattass1993), it is still an open question as to how far V3d extends dorsally in the capuchin monkey.

Continuous versus split representation of V3 fovea

Two proposals have been made for the organization of the V3 fovea. Initial work in the macaque by Zeki (Reference Zeki1969) suggested that the dorsal and ventral portions of V3 would be segregated into two islands, with the gap coinciding with the foveal representation. The second proposal, based on studies in small New World Monkeys (Rosa et al., Reference Rosa, Angelucci, Jeffs and Pettigrew2013), is that the entire V3 (i.e., VLP), including the fovea, is a continuous cortical segment (see Fig. 5). Defining the borders of a cortical area is a challenging task, particularly at the foveal representation and in regions buried inside sulci. The various techniques typically used to delimit areal borders (myeloarchitecture, electrophysiology, and connectivity) rely on the specific methodology being employed. For example, the angle of histological sectioning may influence the precision of border detection. Border detection based on myeloarchitecture is best achieved when the plane of sectioning is orthogonal to the border. Coronal sections are adequate for analyzing parafoveal and peripheral regions, while horizontal sections are best for analyzing central field representations. Parasagittal cuts transect the foveal region tangentially, which makes border detection difficult.

RF mapping using electrophysiological techniques has also proven useful. What we usually seek with sequential electrode penetrations are reversals in RF mapping progressions, often associated with changes in RF size and property. Electrophysiology has a resolution of approximately 0.3–1 mm when investigating cortical regions with high cortical magnification, such as the foveal representation. Thus, border detection becomes masked in places where V3 width narrows down to 1.5 or less. To address this challenge, we usually combine RF progression reversals with myeloarchitectonic transitions in order to draw the border between visual areas. In each histological section, the myeloarchitectonic transition zone usually ranges from 0.5 to 1 mm. We usually place the border at the center of the transition zone.

For the reasons stated above, we feel that V3 foveal organization cannot be definitely resolved with the currently available data. Our results hint that V3 in the macaque is divided into dorsal and ventral portions separated by an intrusion of foveal V4 (Gattass et al., Reference Gattass, Sousa and Gross1988). This model for V3 organization is similar to that proposed by Zeki (Reference Zeki1969). In capuchins, we found cases where V3 seemed continuous but narrowed down to a small strip of cortex at the foveal region (one case in Sousa et al., Reference Sousa, Piñon, Gattass and Rosa1991). However, the most common arrangement was a V3 split into dorsal and ventral domains (Piñon et al., Reference Piñon, Gattass and Sousa1998).

VP or V3: In conclusion

To summarize the present situation, there is widespread agreement that, in Old World monkeys, V3v has a strip-like shape with a systematic representation of the upper quadrant. The same seems to be true for small New World monkeys (Newsome & Allman, Reference Newsome and Allman1980; Rosa & Tweedale, Reference Rosa and Tweedale2000). Anatomical connections of V1 in the capuchin monkey (Sousa et al., Reference Sousa, Piñon, Gattass and Rosa1991) revealed a lower field representation for area V3 anterior to the lower field representation of V2, dorsally. Nonetheless, Kaas (Reference Kaas, Rockland, Kaas and Peters1997) has argued that V3v (VP) itself might contain a complete representation of the visual field, distinct from that found in dorsolateral and dorsal areas. This is an important issue for understanding the organization of the cortex anterior to V2 and the homologies between species. In particular, the demonstration of a substantial lower quadrant representation in V3v would support the distinction between this region and others located dorsal to it. This distinction has been questioned both by studies that propose that V3v continues dorsally, to include part of the dorsolateral cortex interposed between V2 and MT (Rosa, Reference Rosa, Rockland, Kaas and Peters1997; Rosa et al., Reference Rosa, Fritsches and Elston1997), and by those proposing that it is part of a much larger area which, like the “original V3”, includes the dorsal cortex anterior to V2 (Zeki Reference Zeki1969, Reference Zeki1977; Gattass et al., Reference Gattass, Sousa, Covey, Chagas, Gattass and Gross1985, Reference Gattass, Sousa and Gross1988).

There is currently no compelling reason for ruling out a dorsal extension of V3v, although the exact extent of this area remains the subject of further studies. Thus, it is likely that V3v will prove to be part of a larger visual area in the capuchin monkey. Why, then, have previous investigators concluded that V3v is an area restricted to the ventral cortex?

Early studies in both the macaque and owl monkey illustrate a V3v (or “VP”) that is bordered dorsally by V4 (or DL) (e.g., Newsome & Allman Reference Newsome and Allman1980; Van Essen, Reference Van Essen, Peters and Jones1985). Neuroimaging studies in humans (e.g., Sereno et al., Reference Sereno, Dale, Reppas, Kwong, Belliveau, Brady, Rosen and Tootell1995; Tootell et al., Reference Tootell, Mendola, Hadjikhani, Ledden, Liu, Reppas, Sereno and Dale1997), preliminary electrophysiological observations in New World monkeys (Rosa et al., Reference Rosa, Sousa and Gattass1988b, Reference Rosa, Fritsches and Elston1997), and single-unit responses in macaques (Zeki Reference Zeki1983; Burkhalter & Van Essen, Reference Burkhalter and Van Essen1986) all converge to support the view that an area including V3v also encompasses a lower quadrant representation that extends at least to the dorsolateral cortex.

Recently, Lyon and Connolly (Reference Lyon and Connolly2012), in their review of V3, have concluded that evidence obtained in several primate species supports the hypothesis that an elongated V3 forming a complete map of the visual field occupies the region anterior to V2. One central prediction of Lyon & Connolly's scheme is that cells in the dorsal half of V3 will always have RFs representing the lower half of the visual field. However, Rosa et al. (Reference Rosa, Angelucci, Jeffs and Pettigrew2013) disagreed with the conclusions of Lyon and Connolly (Reference Lyon and Connolly2012) regarding the dorsal location and extent of V3. They argued that studies that mapped in detail the RFs (Allman & Kaas, Reference Allman and Kaas1975; Sereno et al., Reference Sereno, McDonald and Allman1994; Rosa & Schmid, Reference Rosa and Schmid1995) and anatomical connections (Rosa et al., Reference Rosa, Palmer, Gamberini, Burman, Yu, Reser, Bourne, Tweedale and Galletti2009; Jeffs et al., Reference Jeffs, Federer, Ichida and Angelucci2013) of the cortex immediately rostral to the dorsal half of V2, particularly in small New World monkeys, demonstrated the existence of an upper field representation in this region, which they called the dorsomedial area (DM). According to this scheme, a V3-like area, which is less extensive than that proposed by Lyon and Connolly (Reference Lyon and Connolly2012), occupies only the lateral and ventral aspects of the third tier cortex, and is referred to as the ventrolateral posterior area (VLP) (Rosa & Manger, Reference Rosa and Manger2005). Electrophysiological recordings in the cortex adjacent to dorsal V2 in the marmoset and owl monkeys have consistently revealed RFs that drift toward the upper VM, as shown in Fig. 5. Indeed, there is no evidence for even a partial lower quadrant representation between V2 and DM, either in this sequence or in many others illustrated in previous studies of marmoset and owl monkeys (Allman & Kaas, Reference Allman and Kaas1975; Krubitzer & Kaas, Reference Krubitzer and Kaas1993; Sereno et al., Reference Sereno, McDonald and Allman1994; Rosa & Schmid, Reference Rosa and Schmid1995; Rosa et al., Reference Rosa, Palmer, Gamberini, Burman, Yu, Reser, Bourne, Tweedale and Galletti2009). In summary, the exact location and extent of the lower field representation of V3 remains controversial, with the possibility that lissencephalic (e.g., owl and marmoset monkeys) and gyrencephalic (e.g., rhesus and capuchin) monkeys vary to some extent in the configuration of this brain region.

Area PO

Area PO was initially defined as a myeloarchitectonically distinct region, containing a complex visuotopic map with a de-emphasis of the central field representation (Covey et al., Reference Covey, Gattass and Gross1982; Gattass et al., Reference Gattass, Sousa, Covey, Chagas, Gattass and Gross1985; Neuenschwander et al., Reference Neuenschwander, Gattass, Sousa and Piñon1994). In our first assessment of area PO (Covey et al., Reference Covey, Gattass and Gross1982; Gattass et al., Reference Gattass, Sousa, Covey, Chagas, Gattass and Gross1985), we overestimate its size. The original description included two myeloarchitectonic areas that were later named areas PO and POd (Neuenschwander et al., Reference Neuenschwander, Gattass, Sousa and Piñon1994). The initial injections carried out by Colby et al. (Reference Colby, Gattass, Olson and Gross1988) were restricted to the ventral portion of the anterior bank of the PO cleft, in the myeloarchitectonically homogeneous region we currently call PO. Thus, PO was initially defined as comprising the entire anterior bank of the PO cleft (Gattass et al., Reference Gattass, Sousa, Covey, Chagas, Gattass and Gross1985). In a later review, we proposed that the anterior bank of the PO sulcus might be composed of more than one visual area (Gattass et al., Reference Gattass, Sousa, Covey, Chagas, Gattass and Gross1985). The study of the connections and myeloarchitecture of the anterior bank of the PO sulcus revealed that area PO is located ventrally in the anterior bank and that it receives projections from the peripheral representation of both V1 and V2 (Colby et al., Reference Colby, Gattass, Olson and Gross1988).

Covey et al. (Reference Covey, Gattass and Gross1982) proposed that area PO in the macaque, which borders V2 in the ventral portion of the PO cleft, is homologous to area M of the Aotus. In the Aotus, the medial visual area M is located within cytoarchitectonic area 19 and has a complete representation of the contralateral visual field, with little emphasis on the representation of central vision. Area M borders V2 anteriorly and shares with it part of the HM representation. A similar arrangement has also been described for the macaque (Covey et al., Reference Covey, Gattass and Gross1982), where area PO was included within cytoarchitectonic area OA of von Bonin and Bailey (Reference von Bonin and Bailey1947).

Borders of areas PO and POd

In order to precisely determine the boundaries of areas PO and POd, Neuenschwander et al. (Reference Neuenschwander, Gattass, Sousa and Piñon1994) studied the correspondence between electrophysiological data and myeloarchitectonic transitions in the parietal cortex of the capuchin monkey. In addition, connectional data in the capuchin also supported the subdivision of the rostral bank of the PO sulcus into dorsal (POd) and ventral (PO) areas (Colby et al., Reference Colby, Gattass, Olson and Gross1988; Sousa et al., Reference Sousa, Piñon, Gattass and Rosa1991; Rosa et al., Reference Rosa, Soares, Fiorani and Gattass1993). The myeloarchitectonic pattern of PO corresponds to the conjunct-striated pattern described by Sanides (Reference Sanides and Bourne1972). Several myeloarchitectonic features are apparent when viewing a histological section of the PO region stained with the Gallyas' method (see Fig. 8). The infragranular layers of PO are heavily myelinated, the inner and outer bands of Baillarger are confluent, and the layers above the outer band of Baillarger are pale and less myelinated than those of the surrounding cortex. Myeloarchitectonic transitions between PO and the laterally and medially located areas are easily determined in coronal sections, while transitions between PO and the ventral and dorsal areas are better determined in parasagittal sections. The border of PO with POd coincides with a decrease in myelination in the infragranular layers. POd is less myelinated than PO and presents a more differentiated and less dense outer band of Baillarger. Posteriorly, PO is bordered by V2, an area that has a homogeneous pattern of myelination in the infragranular layers (Rosa et al., Reference Rosa, Sousa and Gattass1988b) but not as dense as that of PO. In addition, the outer band of Baillarger in V2 is less conspicuous, and less dense than those of PO and POm. In some animals, the lateral portion of PO has an approximately 1.5 mm wide myeloarchitectonic transitional zone, which borders area V3d. This transition zone is less dense than the core of PO, but it is more myelinated than V3d. Based on topographic data, the representation of the visual field in this transition zone complements that of the core of PO and was therefore considered as an integral part of PO. In other animals, no myeloarchitectonic transitional zone was observed, and the V3d/PO border was therefore not clear in all sections. The dorsal border of PO with POd is usually determined in parasagittal sections. POd is less myelinated than PO and presents individualized inner and outer bands of Baillarger (Fig. 8). The inner band gradually joins the white matter. The outer band is thinner than that of PO but well differentiated.

Fig. 8. Myeloarchitecture delimits the borders of areas V3, PO and POd. See text for details. Gallyas' stained parasagittal section from the same animal described in Colby et al. (Reference Colby, Gattass, Olson and Gross1988).

We used the procedure described by Maunsell and Van Essen (Reference Maunsell and Van Essen1987) to assess which portions of the visual field are over-represented in areas PO and POd, as well as to determine their visuotopic orderliness. In brief, this procedure consists in back-transforming onto the visual field a grid defined by an array of interpolated coordinates corresponding to equally spaced points in the cortex. For the case of a precisely organized cortical map, a square grid on the cortex would translate into an orderly cobweb pattern in the visual field (Schwartz, Reference Schwartz1980; Van Essen et al., Reference Van Essen, Newsome and Maunsell1984; Maunsell & Van Essen, Reference Maunsell and Van Essen1987; Fiorani et al., Reference Fiorani, Gattass, Rosa and Sousa1989). This analysis showed interanimal variability regarding the degree of visuotopic orderliness in PO and POd. We observed a considerable organization along the isopolar dimension, and a more irregular organization along the isoeccentric dimension (Fig. 2). The back-transformed maps reported by Neunschwander et al. (Reference Neuenschwander, Gattass, Sousa and Piñon1994) suggest a greater representation of the visual field region ranging from 20–50 deg eccentricity. Area PO also resembles area M in having a limited central vision representation. We observed no RF centers with eccentricity values smaller than 15 deg. For this reason, our visuotopic map of area PO (Fig. 2) does not contain a representation of the central field. However, this result does not imply that the central field is not represented in PO in as much as some of the RFs do include the fovea.

The topographical peculiarities of areas PO and POd were evident in the back-transformed maps we generated using our electrophysiological data. Our assumption that PO is an area distinct from POd was based primarily on the topographic analysis performed for one animal we studied. We considered the multiplicities of central and peripheral field representations as evidence for the existence of two distinct visual areas, each containing a virtually complete representation of the visual hemifield. Neunschwander et al. (Reference Neuenschwander, Gattass, Sousa and Piñon1994) showed back-transformed grids (“webs”) obtained for PO and POd that superimposed on each other in the visual field. In one case, an unquestionable re-representation was observed in area POd, defined initially on myeloarchitectonic grounds. Likewise, the dorsal portion of PO contained a re-representation of a peripheral portion of the visual field. One could interpret these data as a result of irregularities due to the complex representation of the visual field in PO and, thus, equivalent to the local representations found, for example, in area V2 (Rosa et al., Reference Rosa, Sousa and Gattass1988b). We do not favor this hypothesis inasmuch as the representations shown in the back-transformed grids in each case were comparable to those of the back-transformed maps of all other cases we studied.

A case for PO as a unique cortical area

Connectivity studies by Van Essen et al., Reference Van Essen, Newsome, Maunsell and Bixby1986 were the first to suggest that PO constituted the peripheral representation of V3d. Particularly, they found that layer 4B of V1 projected to V3d and PO, but not to V3v, which hinted that V3d and PO might constitute a single area, distinct from V3v.

In addition to our myeloarchitonic data (see above), here we also review some of our connectivity studies, where we used densely spaced tracer injections in the macaque and capuchin monkeys, to make the case that area PO constitutes a single and distinct visual area. Particularly, we argue that PO is not a part of V3d, where it would represent the peripheral portion of the visual field, nor that PO is part of a larger area V6, as proposed by Galletti et al. (Reference Galletti, Fattori, Gamberini and Kutz1999a, Reference Galletti, Gamberini, Kutz, Fattori, Luppino and Matelli2001). In Fig. 3A, we reviewed data from tracer injections in V2 that served to delineate the extent of V3v and V3d in the macaque monkey. The same tracer injections, when carried out in the more peripheral portions of V2 (eccentricities of 30 deg and greater), are also capable of revealing the extent of PO and its border with V3d. Take, for example, injections number 5, 6, and 11 in Fig. 3A, all placed in the peripheral representation of V2. Note that, injection 5 gives rise to two projection sites in the medial-dorsal cortex anterior to V2d (i.e., two green blobs; one attributed to V3d and the other to PO). Additionally, injection 6, which was placed at a more peripheral representation in V2d, shows a projection site which is more lateral in PO compared to the projection of injection 5 (i.e., olive blob in between the two green blobs). If it were indeed the case that PO constitutes the peripheral representation of V3d, then only one projection site should have been observed for injection 5, and injection 6 should have been located more medially, obeying the orderly progression of isoeccentricity lines for V3d. On the contrary, the projection site of injection 6 labels both V3d and PO, along their border region, as it would be expected for projections representing the far periphery in these areas. Injection 11, which was placed at the peripheral representation of V2v labels the medial region of PO (i.e., yellow blob), also as expected from the topography we propose (Covey et al., Reference Covey, Gattass and Gross1982; Gattass et al., Reference Gattass, Sousa, Covey, Chagas, Gattass and Gross1985; Colby et al., Reference Colby, Gattass, Olson and Gross1988; Neuenschwander et al., Reference Neuenschwander, Gattass, Sousa and Piñon1994, see Fig. 1).

The above findings are corroborated by tracer injections in PO, also in the macaque. Fig. 9 shows feedforward projections from the lower and upper field representations of the far periphery of V1, V2, and V3 to PO (Colby et al., Reference Colby, Gattass, Olson and Gross1988). Specifically, injections 1, 3, and 5 project to neighboring V3d, emphasizing that PO does not constitute the peripheral representation of V3d, but rather a unique cortical area. The other nearby injections in PO (2 and 4) project to the far periphery upper field representation of V2, V3 and V4 (ventral cortex), also as predicted from our proposed topography for PO. Note the lack of organization based on isoeccentricity lines, despite the fact that the upper versus lower polarity of hemifield representations is still preserved (e.g., medial injections in PO project exclusively to ventral V2, V3, and V4; see red and blue blobs in Fig. 9). Notably, all the five injections shown in Fig. 9 occupy the entire extent of PO. The resulting projection sites label exclusively the peripheral visual field representation in the surrounding cortex, leaving little or no room in which the fovea could be represented. We believe that these results are incompatible with the proposal put forward by Galletti et al. (Reference Galletti, Fattori, Gamberini and Kutz1999a, Reference Galletti, Gamberini, Kutz, Fattori, Luppino and Matelli2001), where PO would be part of a larger area V6. Finally, note that PO is connected to other topographically organized areas in the parietal cortex, designated here as POm and MIP.

Fig. 9. Feedforward projections to PO support its status as a single cortical area, distinct from V3d. (A) Illustration of a flattened cortex equivalent to the one shown in Fig. 1A. The rectangle delimits the extrastriate cortical region depicted in (C). (B) Projections onto the contralateral visual field of the location and extent of the five PO injections. (C) Injection sites in PO (dashed outline circles, numbered 1–5) and their corresponding projection sites in striate and extrastriate cortex. Injection and projection sites are labeled with the same color to aid visualization. A, anterior; P, posterior; D, dorsal and V, ventral. Adapted from Colby et al. (Reference Colby, Gattass, Olson and Gross1988).

Altogether, our connectivity studies disagree with the initial finding presented by Van Essen et al., Reference Van Essen, Newsome, Maunsell and Bixby1986. First, we found that both ventral and dorsal V1 and V3 send projections to PO (Colby et al., Reference Colby, Gattass, Olson and Gross1988; see Fig. 9). These projections obey the upper versus lower polarity of visual field representation. The V1 projecting neurons were located both in layer 4B and in the supragranular layers. The V3 projecting neurons, both in its ventral and dorsal segments, were concentrated in the supragranular layers. Second, both V3d and V3v send feedback projections to V1 (Sousa et al., Reference Sousa, Piñon, Gattass and Rosa1991, see Fig. 7), suggesting that they are a homogeneous visual area. Despite the fact that we did not study the projections from V1 to V3, we assume that feedforward and feedback projections between early visual areas are generally symmetrical.

Adding to the diversity in nomenclature, Galletti et al. (Reference Galletti, Fattori, Battaglini, Shipp and Zeki1996, Reference Galletti, Fattori, Gamberini and Kutz1999a) have proposed the name of “V6” to visual area PO of the macaque. Based on visual topography, RF size and myeloarchitecture, we showed two distinct areas in the anterior bank of PO sulcus of the capuchin monkey, and named them PO and POd (Neuenschwander et al., Reference Neuenschwander, Gattass, Sousa and Piñon1994). POd has larger RFs and a distinct lighter myeloarchitecture. More recently, POd was renamed as “V6A” by Galletti et al. (Reference Galletti, Fattori, Kutz and Gamberini1999b, Reference Galletti, Gamberini, Kutz, Fattori, Luppino and Matelli2001).

Area V6 of Galletti et al. (Reference Galletti, Fattori, Gamberini and Kutz1999a, Reference Galletti, Gamberini, Kutz, Fattori, Luppino and Matelli2001) is roughly at the same cortical location as the one we reported for PO (Colby et al., Reference Colby, Gattass, Olson and Gross1988; Neuenschwander et al., Reference Neuenschwander, Gattass, Sousa and Piñon1994). However, V6 and PO exhibit very different topographic organizations: (1) area V6 is described as having a central field representation, with the fovea located laterally. We do not observe any fovea representation in PO; (2) V6 is organized in isoeccentric and isopolar lines, while PO shows no clear organization in the isoeccentric domain; (3) the representation of V6 upper visual field is more medial than that described for area PO.

Altogether, the organization of V6 lower visual field resembles that of area DM described in the marmoset (Rosa et al., Reference Rosa, Angelucci, Jeffs and Pettigrew2013), despite the fact that their reported organizations do not suggest they are homologous. Therefore, there is no evidence to support the notion that V6, as described by Galletti et al. (Reference Galletti, Fattori, Gamberini and Kutz1999a, Reference Galletti, Gamberini, Kutz, Fattori, Luppino and Matelli2001), corresponds to area PO that we have described in the macaque and capuchin monkeys (Colby et al., Reference Colby, Gattass, Olson and Gross1988; Neuenschwander et al., Reference Neuenschwander, Gattass, Sousa and Piñon1994).

Comparisons with small New World monkeys

Discussions regarding the nomenclature of cortical areas are especially useful in a review article such as this one. Grasping the homologies across species has been hindered by the variety of terms and acronyms often used to designate the same visual area. Area MT, for example, was first described as a fully defined visual area in the owl monkey (Allman & Kaas, Reference Allman and Kaas1971), and shortly after in the Galagos (Allman et al., Reference Allman, Kaas and Lane1973). Therefore, when we first described the visual topography of the striate recipient area located in the superior temporal sulcus of the macaque (Gattass & Gross, Reference Gattass and Gross1981), we chose to designate it area MT in an effort to standardize the nomenclature used in the literature. Zeki (Reference Zeki1980) also used the term MT when studying the corresponding visual area in the owl monkey. Surprisingly, however, he did not acknowledge that MT and the macaque movement area (V5) correspond to homologous regions in the two species.

Fig. 5 summarizes a comparative view for the organization of the posterior cortical visual areas in Old World (macaque) and New World (marmoset) monkeys. Despite the fact that the capuchin is a New World monkey, our results indicate that its cortical organization is much more closely related to the macaque than to the marmoset. The recent review by Lyon and Connolly (Reference Lyon and Connolly2012) suggests that in the macaque an elongated V3 is located anterior to V2, forming a complete map of the visual field. This view supports our original proposal based on electrophysiology (Gattass et al., Reference Gattass, Sousa and Gross1988; Piñon et al., Reference Piñon, Gattass and Sousa1998) and anatomy (Gattass et al., Reference Gattass, Sousa, Mishkin and Ungerleider1997; Ungerleider et al., Reference Ungerleider, Galkin, Desimone and Gattass2008). It is also confirmed by a recent study of the topographic organization of dorsal extrastriate cortex using fMRI and phase-encoded retinotopic mapping in monkeys (Arcaro et al., Reference Arcaro, Pinsk, Li and Kastner2011). These results, which are shown in Fig. 6, corroborate the notion that V3d is a continuous region spanning a large extent of V2's anterior border. This organization differs from that proposed for the marmoset (Rosa et al., Reference Rosa, Angelucci, Jeffs and Pettigrew2013) where the dorsal portion of VLP (V3) swings forward, thereby allowing DA and DM to border V2 medially. The study where we injected retrograde tracers in area V1 (Sousa et al., Reference Sousa, Piñon, Gattass and Rosa1991, see Fig. 7) clearly shows that the areal layout found in the capuchin monkey is similar to the one found in the macaque. Note in Fig. 7 that V1 receives orderly projections from a continuous segment of area V2. V1 also receives projections from another continuous strip of cortex located anterior to V2, namely area V3. We did not observe any swinging forward of V3 in the capuchin, contrary to the organization described for VLP (the homolog of area V3) in the marmoset (Rosa et al., Reference Rosa, Angelucci, Jeffs and Pettigrew2013). Additionally, there is evidence that V3d in the capuchin borders area PO, medially. Note that injection site #7 (located at V1 periphery, upper hemifield representation) in Fig. 7 labels areas V2v and V3v but also area PO, which is located dorsally. The complementary injection site #6 (located at V1 periphery, lower hemifield representation) projects to the peripheral representation of V2d but also to area PO. A DM-like organization would have predicted projection site 7 (yellow) to be located more medially compared to projection site 6 (green). Finally, the cluster of labeled cells is contiguous, with no evidence for an intervening visual area between V2d and PO.

In our view, V3 in the macaque and capuchin monkeys is homologous to VLP in the marmoset, while PO is homologous to area M and not area DM, as it has been previously proposed by Rosa et al. (Reference Rosa, Angelucci, Jeffs and Pettigrew2013). The following pieces of evidence support our claim: (1) both PO and area M emphasize the peripheral, as compared to the foveal representation, while DM has a clear representation of the central vision; (2) DM has a clear representation of the isoeccentricity lines, while PO does not; (3) PO and DM have distinct myeloarchitecture (Colby et al., Reference Colby, Gattass, Olson and Gross1988; Sousa et al., Reference Sousa, Piñon, Gattass and Rosa1991; Neuenschwander et al., Reference Neuenschwander, Gattass, Sousa and Piñon1994; Rosa et al., Reference Rosa, Angelucci, Jeffs and Pettigrew2013).

We are convinced that the topography of the areas anterior to V2 varies not only across primate species but also across individuals within the same species (Gattass et al., Reference Gattass, Sousa and Gross1988; Piñon et al., Reference Piñon, Gattass and Sousa1998). Dorsal V3, for example, is continuous and borders area PO medially in most animals. In some cases, however, area PIP can stand in-between V3d and PO (Colby et al., Reference Colby, Gattass, Olson and Gross1988). The reasons for this diversity are still unclear. We think that future work should also address the topographic organization of recently described areas located in the caudal region of the intraparietal sulcus, such as CIP-1 and CIP-2. Specifically, we think that systematic studies using multiple electrode arrays and automatic RF mapping methods will shed new light into the functional organization of areas PO, POd, as well as describe new areas located in the annectent gyrus and in the intraparietal cortex.