The morphology and distribution of neurons labeled specifically by the lectin, Vicia villosa (VVA), were examined in striate cortex of adult macaque monkeys. Following incubation with VVA conjugated to histochemical markers, fine punctate reaction product appears to cover the surface of the soma and proximal dendrites of a population of cortical neurons. Although a small number of VVA-labeled cells are located in layers 2, 3A, 5, and 6, approximately 75% are located in a strip of cortex overlying layers 3B through 4Ca. Layers 1 and 4Cβ are virtually devoid of labeled cells. The morphology of labeled cells varies throughout the layers. In the supragranular layers, the labeled cells generally display a round or multipolar soma with a small number of radially disposed dendrites. In deeper layers, labeled cells are multipolar or horizontal, and their proximal dendrites are often more densely labeled. There is no clear correlation between the distribution of labeled cells and the pattern of cytochrome oxidase staining in supragranular layers.

Double labeling of single sections for VVA and for GABA (gamma-aminobutyric acid) immunoreactivity revealed that most VVA-labeled cells are also immunoreactive for GABA. The double-labeled cells comprise approximately 30% of all GABA immunoreactive cells. Soma size analysis of double-labeled cells shows that medium-to-large GABA cells in each layer are labeled by VVA. The soma size, laminar distribution, and morphology of the VVA-labeled GABA cells suggest that they include the large basket cells originally observed in Golgi preparations.

The photoreceptor layer of F. heteroclitus was examined by light and electron microscopy. We identified four cone visual pigments with maximum absorbance (λmax) in the UV (363 nm), short (400 nm), middle (463 nm), and long (563 nm) wavelength regions of the spectrum and a rod visual pigment that peaked in the middle wavelengths (503 nm). Electron-dense bodies, ellipsosomes and pseudoellipsosomes, were present in the distal ellipsoids of long/middle (L/M) and long/long (L/L) wavelength double cones and in single short wavelength (S) cones, respectively. The light absorption of ellipsosomes indicated the presence of reduced cytochrome-c with the highest optical densities found in the M members of L/M double cones. By contrast, S cones contained pseudo-ellipsosomes which had very low optical density. UV cones were present everywhere as part of square or row mosaics in the retina of F. heteroclitus. Cone packing was on average higher for locations in the upper half of the retina while the highest cone density was found in the centro-ventral retina. An analysis of potential quantum catches for each cone type as a function of retinal sector and underwater irradiance characteristics revealed higher overall quantum catches for cones in the upper retina when the light field was assumed homogeneous, and higher quantum catches for cones in the lower retina when downwelling, horizontal, and upwelling irradiances were considered separately. At dusk, quantum catch was highest for M cones and the contribution to the overall retinal quantum catch by UV and S cones was much greater than during daylight hours. We propose that UV and S cones may be used to detect targets of interest against the background irradiance sensed by double cones.

The retinas of anchovies have two unique photoreceptor types: “bifid” and “long” cones (Fineran & Nicol, 1976). The outer segments of these cells contain multiple layers of membranes (lamellae) oriented longitudinally (axially). This orientation is distinct from that in all other vertebrate rods and cones, where the lamellae are stacked transversely with their planes perpendicular to the incident light path. Although the common arrangement provides optimal absorption for normally incident light rays, it is also insensitive to the rays' direction of vibration (i.e. their polarization). In contrast, the two mutually perpendicular sets of axially oriented lamellae segregated into bifid and long cones could function as the principal analyzers for linearly polarized light, as previously hypothesized (Fineran & Nicol, 1976, 1978). Here, we report on a microspectrophotometric study that shows (1) the presence of two spectrally distinct visual pigments in the three photoreceptor types of the bay anchovy retina; these are typical vertebrate pigments in that they bleach, when exposed to light, and have absorption spectra like all other vitamin A1-based visual pigments; (2) that the rods and cones exhibit dichroic absorption of light in accordance with their lamellar orientation, and (3) that the two cone types of the retina contain a spectrally indistinguishable pigment with peak absorbance (λmax) around 540 nm, while the rods contain a rhodopsin-like pigment with λmax near 500 nm. Compared to other vertebrates, anchovies are remarkable for using a monochromatic cone system with unusual specializations supportive of a polarization detection system.

The hamster intergeniculate leaflet (IGL), part of the circadian rhythm regulatory system, has very extensive interconnections with subcortical visual nuclei. The present investigation describes IGL connections with the hamster diencephalon and telencephalon and compares them with ventral lateral geniculate nucleus (VLG) connections and retinal projections. Connections of the geniculate nuclei were evaluated using anterograde transport of iontophoretically injected Phaseolus vulgaris leucoagglutinin and by retrograde transport of cholera toxin β fragment. The cholera fragment was also injected intraocularly to trace retinal efferents. The IGL has ipsilateral and contralateral projections to the anterior and posterior hypothalamic nuclei, the ventral preoptic, lateral and dorsal hypothalamic areas, but not to the core ventromedial nucleus and very sparsely to the paraventricular nucleus. There are also IGL projections to the medial and lateral zona incerta, anteroventral, anterodorsal, reuniens, parataenial, paraventricular, centrolateral, central medial, and laterodorsal thalamic nuclei. IGL projections to the telencephalon are found in the horizontal limb of the diagonal band, olfactory tubercle, nucleus of the lateral olfactory tract, posterior bed nucleus of the stria terminalis, ventral pallidum, and in nuclei of the medial amygdala. The only substantial VLG projections are to bed nucleus of the stria terminalis, IGL, medial zona incerta, central medial and laterodorsal thalamic nuclei. Several of the IGL targets, the bed nucleus of the stria terminalis and zona incerta in particular, send projections back to the IGL and VLG. In addition, cells are present in the caudal cingulate cortex that project to both nuclei. Retinal projections are found in many of the regions receiving IGL innervation, including nuclei of the medial basal telencephalon, the posteromedial bed nucleus of the stria terminalis, and nuclei of the hypothalamus. A retinal projection is also visible in the lateral olfactory tract from which it extends rostrally, then medially along the base of the rhinal fissure. Fibers also extend caudally, in a superficial location, to perirhinal cortex. The results further demonstrate the widespread connections of the IGL and support the idea that the IGL modulates olfactory, photic, and circadian rhythm regulation of regulatory physiology and behavior.

To determine if changes in metabolic capacity revealed by cytochrome oxidase (CO) histochemistry are related to sustained changes in energy-utilizing neuronal activity, we assayed CO levels and recorded multiunit firing rates along nearly tangential penetrations of V1 in seven adult macaque monkeys before and after single, monocular injections of TTX. Within as little as 14 h, TTX blockade began to reduce CO staining in zones of layer 4C that received dominant input from the injected eye. Since simple monocular occlusion has only minor effects on cortical CO levels (Trusk et al., 1990), the changes in activity that were specifically associated with CO depletion were isolated by comparing spike rates during monocular TTX blockade and during monocular occlusion. Five second samples of multiunit spike rate were obtained after 2-min adaptation to each of four adapting fields: black, gray, white, and textured. Results were similar for these four conditions. In layer 4C, ocular dominance zones with input from the TTX eye had ongoing spike rates that were 48% of the rates in zones with input from a normal but occluded eye. In six animals, it was possible to record activity at a single site before, during, and after the onset of TTX blockade. Background activity at these interpuff sites decreased as much as 3-fold in less than 1 h but stabilized within 3–4 h to an average of 53% of pre-TTX rates. These data support the interpretation that energy utilization linked to sustained spike rates partially regulates CO levels under normal conditions, at least in layer 4. Furthermore, changes in neuronal activity induced by retinal TTX preceded the detectable reduction in CO activity in V1 suggesting that the adjustment of CO levels was in response to the altered activity.

Macaque monkeys are widely used as a model species for investigations of the biology of human vision. Previous measurements suggest that the cone-based spectral sensitivity of these two primates is greatly similar, but perhaps not identical. We measured the photopic spectral sensitivity of 42 male macaque monkeys from two species (Macaca mulatto, M. fascicularis) using an objective index, electroretinogram flicker photometry. The variations among individuals and between the two species were very small and there was no evidence for any significant cone pigment polymorphism in this sample. There are small but systematic differences in spectral sensitivity between macaque monkeys and equivalently tested human subjects–the monkeys were slightly more sensitive to short wavelengths (<520 nm) and slightly less sensitive to wavelengths longer than this value. The results obtained from the curve fitting of standard photopigment absorption spectra to the spectral-sensitivity functions suggest that the difference between human and macaque monkey spectral sensitivity principally reflects differences in the relative proportions of the long- and middle-wavelength cones in the retinas of the two species.

Two groups of retinal cone bipolar cells (CBCs) in rats were found to express voltage-gated Na+ channels. The axon terminals of the first group stratify in sublamina 2 of the inner plexiform layer (IPL) and partially overlap with the OFF-cholinergic band. This group was identified as type 3 CBCs. The axon terminals of the second group stratify in sublamina 3 of the IPL, slightly distal to the ON-cholinergic band. Cells of this second group resemble type 5 CBCs. In addition, we observed another group of ON-type CBCs with terminal stratification similar to that of the second group. However, this latter group did not show any Na+ current, instead exhibiting a large hyperpolarization-activated cyclic nucleotide-gated cation current, suggesting the existence of two subclasses of physiologically distinct type 5 CBCs. Both groups of Na+-expressing bipolar cells were capable of generating a rapid tetrodotoxin-sensitive action potential as revealed by current injection. Multiple spike-like potentials were also observed in some of these cells. Results of this study provide valuable insights into the function of voltage-gated Na+ channels of retinal bipolar cells in retinal processing.

Male Cebus monkeys are all dichromats, but about two thirds of the females are trichromats. M and P retinal ganglion cells were studied in the male Cebus monkey to investigate the relationship of their morphology to retinal eccentricity. Retinal ganglion cells were retrogradely labeled after optic nerve deposits of biocytin to reveal their entire dendritic tree. Cebus M and P ganglion cell morphology revealed by biocytin retrograde filling is similar to that described for macaque and human M and P ganglion cells obtained by in vitro intracellular injection of HRP and neurobiotin. We measured 264 and 441 M and P ganglion cells, respectively. M ganglion cells have larger dendritic field and cell body size than P ganglion cells at any comparable temporal or nasal eccentricity. Dendritic trees of both M and P ganglion cells are smaller in the nasal than in the temporal region at eccentricities greater than 5 mm and 2 mm for M and P ganglion cells, respectively. The depth of terminal dendrites allows identification of both inner and outer subclasses of M and P ganglion cells. The difference in dendritic tree size between inner and outer cells is small or absent. Comparison between Cebus and Macaca shows that M and P ganglion cells have similar sizes in the central retinal region. The results support the view that M and P pathways are similarly organized in diurnal dichromat and trichromat primates.

Ganglion cells in the albino rat retina were retrogradely labeled with the fluorescent dye, diamidino-yellow, from the superior colliculus. Preembedding and postembedding immunocytochemical techniques were employed in conjunction with computer-assisted image processing to visualize SP- and GABA-immunoreactivity. Examination of flatmount and sectioned retinas revealed that approximately 3% of the ganglion cells projecting to the contralateral superior colliculus exhibit SP-immunoreactivity. Moreover, these cells were found to comprise a subpopulation of the GABA-immunoreactive cells projecting to the rat tectum.

The retinae of lower vertebrates undergo a number of structural changes during light adaptation, including the photomechanical contraction of cone myoids and the dispersion of melanin granules within the epithelial pigment. Since the application of dopamine to dark-adapted retinae is known to produce morphological changes that are characteristic of light adaptation, dopamine is accepted as a causal mechanism for such retinomotor movements. However, we report here that in the teleost fish, Aequidens pulcher, the intraocular injection of 6-hydroxydopamine (6-OHDA), a substance known to destroy dopaminergic retinal cells, has no effect on the triggering of light-adaptive retinomotor movements of the cones and epithelial pigment and only slightly depresses the final level of light adaptation reached. Furthermore, the retina continues to show circadian retinomotor changes even after 48 h in continual darkness that are similar in both control and 6-OHDA injected fish. Biochemical assay and microscopic examination showed that 6-OHDA had destroyed dopaminergic retinal cells. We conclude, therefore, that although a dopaminergic mechanism is probably involved in the control of light-induced retinomotor movements, it cannot be the only control mechanism, nor can it be the cause of circadian retinomotor migrations. Interestingly, 6-OHDA injected eyes never reached full retinomotor dark adaptation, suggesting that dopamine has a role to play in the retina's response to darkness.

Mechanisms underlying the development of the primate area of high acuity (AHA) remain poorly understood. Finite-element models have identified retinal stretch and intraocular pressure (IOP) as possible mechanical forces that can form a pit (Springer & Hendrickson, 2004). A series of Macaca nemestrina monkey retinas between 68 days postconception (dpc) and adult were used to quantify growth and morphological changes. Retinal and pars plana length, optic disc diameter, disc-pit distance, and inner and outer retinal laminar thickness were measured over development to identify when and where IOP or stretch might operate. Horizontal optic disc diameter increased 500 μm between 115 dpc and 2 months after birth when it reached adult diameter. Disc growth mainly influences the immediate surrounding retina, presumably displacing retinal tissue centrifugally. Pars plana elongation also began at 115 dpc and continued steadily to 3–4 years postnatal, so its influence would be relatively constant over retinal development. Unexpectedly, horizontal retinal length showed nonlinear growth, divided into distinct phases. Retinal length increased rapidly until 115 dpc and then remained unchanged (quiescent phase) between 115–180 dpc. After birth, the retina grew rapidly for 3 months and then very slowly into adulthood. The onset of pit development overlapped the late fetal quiescent phase, suggesting that the major mechanical factor initiating pit formation is IOP, not retinal growth-induced stretch. Developmental changes in the thickness of retinal layers were different for inner and outer retina at many, but not all, of the ten eccentricities examined. Peripheral inner and outer retinal layers thinned appreciably with age, consistent with retinal stretch having a larger effect on the retinal periphery. Central inner retina around the area of high acuity (AHA) changed tri-phasically. It increased in thickness prenatally, thinned transiently after birth, and then resumed thickening. Transient postnatal inner retinal thinning around the pit coincided with the resumption of retinal growth and with cone packing providing evidence that a small amount of growth-induced central retinal stretch may account for cone packing as previously hypothesized (Springer, 1999). Central outer retina around the AHA progressively thickened over the fetal period. It reached asymptotic thickness at birth and continued to thicken into adulthood at some temporal, but not nasal, central eccentricities. These data indicate that peripheral outer and inner retina progressively thin with age because of eye growth-induced stretch, while central retina is minimally affected by stretch. Outer and inner retinal laminar thickness at the same locus can change in different directions, suggesting that they shear with respect to one another. This shearing induces the elongation of Henle axons, while their angle reflects the direction of shear.

The effects of monocular deprivation on cytochrome-oxidase (CO) expression were used to reveal ocular dominance columns in flatmounts of the striate cortex in macaque (Macaca fascicularis) and talapoin (Miopithecus talapoin) monkeys. This procedure allowed the first direct visualization of the complete pattern of ocular dominance bands in macaque monkeys, and less complete reconstructions in talapoin monkeys. In a second macaque monkey, the ocular dominance organization was revealed by injecting wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into one eye.

The organization of ocular dominance columns in the macaque monkeys conforms to previous descriptions, but the flat-mounted hemispheres provide accurate details on (1) the arrangement of columns, (2) the orientation of the representation of the optic disc, and (3) the breakdown of the bands in the cortex between the optic disc and monocular representations into a pattern of dots activated by the ipsilateral eye and larger surrounds related to the contralateral eye.

Talapoin monkeys, the smallest of Old World monkeys, have sharply segregated ocular dominance bands. The columns in talapoins are narrower than those in macaques, so that even with less striate cortex than macaques, talapoins have more ocular dominance hypercolumns.

On the basis of cortical connections and architectonics, inferior temporal (IT) cortex of squirrel monkeys consists of a caudal, prestriate-recipient region, ITC; a rostral region, ITR; and possibly an intermediate region along the border of ITC and ITR, ITI (Weller & Steele, 1992). ITC contains dorsal (ITCd) and ventral (ITCv) areas. The subcortical connections of these subdivisions of IT cortex were determined in the present study from the results of cortical injections of wheat-germ agglutinin conjugated to horseradish peroxidase, [3H]-amino acids and fast blue. ITC and ITR receive afferents from the locus coeruleus, dorsal raphe, nucleus annularis, central superior nucleus, pontine reticular formation, lateral hypothalamus, paracentral nucleus, and central medial nucleus; send efferents to the superior colliculus, reticular nucleus, and striatum; and have both afferent and efferent connections with the pretectum, pulvinar, claustrum, amygdala, and basal nucleus of Meynert. ITC and ITR have different patterns of connections with a number of subcortical structures, including the pulvinar and amygdala. Injections in ITC strongly label multiple nuclei of the inferior pulvinar and the medial division of the lateral pulvinar (PLM), and moderately label the medial pulvinar (PM), whereas injections in ITR strongly label PM and moderately label PLM. Injections in ITC label sparse projections to the lateral nucleus of the amygdala, in contrast to injections in ITR that label strong projections to the lateral and basal nuclei of the amygdala. Injections in “IT” produce a pattern of subcortical label that has some features of that observed from injections in ITC and that observed from injections in ITR. Although most of the connections of ITCd and ITCv appear similar, only injections involving ITCd label the middle nucleus of the inferior pulvinar (PIM).

Comparison of the subcortical connections of subdivisions of IT cortex in squirrel monkeys and what is presently known of the subcortical connections of subdivisions of IT cortex in macaque monkeys supports the previous suggestion that ITC of squirrel monkeys may be comparable to area TEO of macaques, IT, may be comparable to posterior area TE, and ITR may be comparable to anterior area TE (Weller & Steele, 1992).

Many studies on human retinal development have cited the third gestational month as a period when the posterior retina undergoes rapid differentiation and maturation, including a lining up of cone precursors. Ultrastructural data on the posterior retina during the third month are very limited, and totally lacking for the cone monolayer. We have examined two human fetal retinas between ten and 11 gestational weeks. Before the appearance of the cone monolayer, the outer neural retina consists of a homogeneous population of undifferentiated neuroblasts. Mitotic figures are still evident, even posteriorally. There is no outer plexiform layer (OPL). The interface of neural retina to retinal pigment epithelium (RPE) is largely featureless. By 11 weeks, the posterior retina has a thin OPL that separates the many rows of cells in the developing inner nuclear layer from the single tier of macular cone precursors. The RPE monolayer consists of cuboidal cells whose apical surface elaborates ridges of cytoplasm and branched processes that project into the subretinal space. The large, cuboidal cones are linked to each other and Müller cells at the outer limiting membrane. They show definitive signs of the structural polarity typical of vertebrate photoreceptors. Their apical cytoplasm contains many organelles common to the inner segment, while the basal cytoplasm has synaptic ribbons and vesicles, and receives invaginating contacts from processes in the OPL neuropil arising from differentiating second-order neurons. Lateral cone surfaces are mutually underlain by large subsurface cisterns.

We evaluated the dynamic aspects of the orientation tuning of the input to cat visual cortical neurons by analyzing the postsynaptic potentials (PSPs) evoked by flashing bars of light. The PSPs were recorded using in vivo whole-cell technique, and we analyzed the orientation tuning during subsequent temporal windows after stimulus onset and offset. Our results show that the amplitudes of the postsynaptic potential are reliably tuned to orientation and matching that of the spike responses only during certain temporal windows. During the first 100 ms after stimulus presentation, orientation tuning of the membrane potential underwent regular changes. Within particular intervals, orientation tuning of the input was much sharper than that estimated according to the whole response. In most cells, optimal orientation was usually stable over the whole period. In several cells which had a second hump of EPSPs in the response, this second hump was tuned to the same orientation as the first one, but always showed sharper tuning. Estimation of the integration time revealed sufficient delay between the appearance of EPSPs and spikes, to let inhibition influence spike generation. These results show that orientation selectivity of the input to cortical cells is a dynamic function, and also indicate the possibility of temporal coding in the visual system.

Results from earlier experiments indicate that different species of rodent vary both in the number of cone types found in their retinas and in the spectral sensitivities of the cone pigments. These features have now been examined in two types of hamster commonly used for research purposes: Syrian golden hamsters (Mesocricetus auratus) and Siberian dwarf hamsters (Phodopus sungorus). Electroretinogram (ERG) flicker photometry, behavioral discrimination tasks, and opsin antibody labeling were used to investigate hamster photoreceptors and their visual consequences. Results from the three approaches support the following conclusions: (1) The retinas of both species have an abundant population of rods containing a photopigment with peak sensitivity of about 498–500 nm; (2) Siberian dwarf hamsters have two classes of cone: one with maximum sensitivity in the ultraviolet (c. 360 nm), the other with peak sensitivity closely similar to that of its rod; and (3) Syrian golden hamsters have a class of cone with peak sensitivity at about 506 nm, but they lack a second cone type. Implications of these alternative arrangements are discussed.

The developmental time courses for vernier acuity and grating acuity were measured longitudinally in infant Macaca nemestrina monkeys. Behavioral measurements of vernier and grating acuity were made at regular intervals during development. Near birth, grating acuity is relatively more mature than vernier acuity. The proportional rate of vernier acuity development is faster than that for grating acuity. During the course of development, grating acuity improves approximately 15-fold whereas vernier acuity improves about 60-fold. Both visual functions approach adult levels at about the same age, around 40 weeks postnatally. Although grating acuity develops about four times faster in monkeys than in humans, vernier acuity development in monkeys and humans does not appear to reflect the same relationship. Adult levels of vernier acuity for the monkeys are about a factor of 2 poorer than are typically reported for humans. The differential development of vernier acuity and grating acuity does not necessarily reflect development at different levels of the visual system.

We evaluated the color vision of mercury-contaminated patients and investigated possible retinal origins of losses using electroretinography. Participants were retired workers from a fluorescent lamp industry diagnosed with mercury contamination (n = 43) and age-matched controls (n = 21). Color discrimination was assessed with the Cambridge Colour Test (CCT). Retinal function was evaluated by using the ISCEV protocol for full-field electroretinography (full-field ERG), as well as by means of multifocal electroretinography (mfERG). Color-vision losses assessed by the CCT consisted of higher color-discrimination thresholds along the protan, deutan, and tritan axes and significantly larger discrimination ellipses in mercury-exposed patients compared to controls. Full-field ERG amplitudes from patients were smaller than those of the controls for the scotopic response b-wave, maximum response, sum of oscillatory potentials (OPs), 30-Hz flicker response, and light-adapted cone response. OP amplitudes measured in patients were smaller than those of controls for O2 and O3. Multifocal ERGs recorded from ten randomly selected patients showed smaller N1–P1 amplitudes and longer latencies throughout the 25-deg central field. Full-field ERGs showed that scotopic, photopic, peripheral, and midperipheral retinal functions were affected, and the mfERGs indicated that central retinal function was also significantly depressed. To our knowledge, this is the first demonstration of retinal involvement in visual losses caused by mercury toxicity.

The major neurons and neural circuits that are involved in the transmission of color signals through the human retina to produce the color and spatially opponent P cell or midget ganglion cell responses are described. The older findings of single cone to midget bipolar connectivity is reviewed, and the single midget bipolar cell to midget ganglion cell connectivity as revealed by a recent serial section electron microscope study is described in detail. Our present knowledge concerning the discrimination of the blue-cone subtype from the other longer wavelength cones in the human at the outer plexiform layer is summarized, and our most recent findings concerning horizontal cell connectivity to the different spectral types of cones are discussed. Finally, a hypothetical pathway is proposed for color-opponent surrounds of midget ganglion cells using both horizontal cells at the outer plexiform layer and amacrine cell pathways at the inner plexiform layer.

Our previous light-microscopic study indicates that unilateral retinal impulse blockage with tetrodotoxin (TTX) causes a reversible decrease of cytochrome oxidase (CO) in alternating rows of metabolically active zones (puffs) in the adult macaque striate cortex (Wong-Riley & Carroll, 1984b). The goal of the present study was to determine if TTX blockade adversely affects all neurons or only a subpopulation of neurons within the puffs. Three major neuronal types were identified based on mitochondrial CO activities and morphological characteristics. Type A neurons were the most prevalent, consisting of small pyramidal and nonpyramidal neurons that received only symmetrical axosomatic synapses. They had little cytoplasm and relatively low levels of CO activity, and showed the least change with TTX treatment. Type B cells were medium-to-large pyramidal neurons that received exclusively symmetrical axosomatic synapses and were moderately reactive for CO. Impulse blockage caused a decrease in mitochondrial size and packing density, but somal size remained within the control range. Type C cells were medium-sized nonpyramidal neurons contacted by both asymmetrical and symmetrical axosomatic synapses. They contained abundant darkly reactive mitochondria and presumably are metabolically the most active. This cell type suffered the greatest decrease in somal size and packing density of mitochondria, particularly the darkly reactive ones. A rare fourth cell type, type D, was a small, darkly reactive nonpyramidal variety that gave rise to somatodendritic synapses. Their low occurrence prevented statistical analysis under normal and TTX-treated conditions. These data indicate that retinal impulse blockade is most detrimental to the metabolically most active neurons in the adult primate cortical puffs. The alterations are not permanent, because the effects of TTX are fully reversible (Carroll & Wong-Riley, 1987).