The influence of neurons projecting from the pretectal nuclear complex to the ipsilateral dorsal lateral geniculate nucleus (LGNd) was investigated in awake cats. Responses from relay cells in the A-laminae of the LGNd were extracellularly recorded and analyzed during saccadic eye movements and visual stimulation in association with reversible inactivation of the ipsilateral pretectum with the GABA agonist, muscimol. Pretectal inactivation (PTI) resulted in spontaneous nystagmic eye movements in the dark with slow phases directed away from the injected side. In the control situation, all Y-cells and about two thirds of X-cells were excited during saccades or saccade-like visual stimulation but one third of X-cells were inhibited. During PTI all recorded X-cells were inhibited, either during saccades or saccade-like visual stimulation. The PTI-associated inhibition was stronger than in inhibited X-cells in control experiments only during saccades but not during stimulation with a moving pattern while the eyes were stationary. In Y-cells a reduction in the response peak width at half-height was seen during PTI, again only during saccades but not during stimulation with a moving pattern. These results indicate that during saccades the pretecto-geniculate pathway has a stronger influence on X LGNd relay cells than on Y-cells. The findings are discussed in terms of saccadic suppression and postsaccadic facilitation.

We have examined the role of neurotrophins in promoting survival of mammalian rod bipolar cells (RBC) in culture. Retinas taken from 8- to 10-day-old Long-Evans rats were dissociated and cultured in media supplemented with either nerve growth factor (NGF), neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), or basic fibroblast growth factor (FGF-2). Survival was measured by the number of cells that were immunoreactive for α-, β-, γ-PKC, a bipolar cell-specific marker. Compared to untreated cultures, CNTF had no effect on RBC survival, while NGF and NT-3 increased survival only slightly. BDNF, however, increased survival by approximately 300%. Similar results were obtained with FGF-2. Both nerve growth factor (NGF) and an antibody (anti-REX) which interferes with binding to the 75-kD low-affinity neurotrophin receptor (p75NTR) eliminated BDNF-promoted survival, but had no effect on FGF-2-mediated survival. Interestingly, p75NTR was expressed by retinal glia (Müller cells), but not by the bipolar cells themselves, providing for the possibility that BDNF might induce Müller cells to produce a secondary factor, perhaps FGF-2, which directly rescues RBCs. In support of this hypothesis, an antibody that neutralizes FGF-2 attenuated the trophic effects of BDNF, and dramatically reduced survival in cultures with no added growth factors, indicating that there may be an endogenous source of FGF-2 that promotes survival of RBCs in culture. We suggest that BDNF increases production or release of FGF-2 by binding to p75NTR on Müller cells.

The impulse discharge of single ganglion cells was recorded extracellularly in superfused eyecup preparations of the tiger salamander (Ambystoma tigrinum). Contrast flashes (500 ms) were applied at the center of the receptive field while the retina was light adapted to a background field of 20 cd/m2. The incidence of cell types in a sample of 387 cells was: ON cells (4%), OFF cells (28%), and ON/OFF cells (68%). Quantitative contrast/response measurements were obtained for 83 cells. On the basis of C50, the contrast necessary to evoke a half-maximal response, ON/OFF cells fell into 3 groups: (1) Positive Dominant (26%), (2) Balanced (23%), and (3) Negative Dominant (51%). Positive Dominant cells tended to be relatively contrast insensitive. On the other hand, many Negative Dominant cells showed remarkably low C50 values and very steep contrast/response curves. Contrast gain to negative contrast averaged 8.5 impulses/s/% contrast, some four times greater than that evoked by positive contrast. In most ON/OFF cells, the latency of the first spike evoked by a negative contrast step was much shorter (40–100 ms) than that evoked by a positive contrast step of equal contrast. OFF cells typically showed higher C50 values, larger dynamic ranges, and longer latencies than those of Negative Dominant ON/OFF cells. Thus, different pathways or mechanism apparently mediate the off responses of OFF and ON/OFF cells. In sum, the light-adapted retina of the tiger salamander is strongly biased in favor of negative contrast, as shown by the remarkably high contrast sensitivity and faster response of Negative Dominant cells, the remarkably low incidence of ON cells, and the insensitivity of Positive Dominant cells. Some possible underlying influences of bipolar and amacrine cells are discussed.

We measured the variability in latency of the first spike seen in cells of the cat's lateral geniculate nucleus following the onset of a visual stimulus. We found that, in each of the 11 cells tested, this variability was significantly lower during burst than during tonic firing. We suggest that this difference confers an advantage in signal detection during burst compared to tonic firing. This complements other reported advantages of burst firing for signal detection seen in signal-to-noise ratios and in the ability to efficiently drive postsynaptic cells.

Previous studies of cat visual cortex have shown that the spatiotemporal (S-T) structure of simple cell receptive fields correlates with direction selectivity. However, great heterogeneity exists in the relationship and this has implications for models. Here we report a laminar basis for some of the heterogeneity. S-T structure and direction selectivity were measured in 101 cells using stationary counterphasing and drifting gratings, respectively. Two procedures were used to assess S-T structure and its relation to direction selectivity. In the first, the S-T orientations of receptive fields were quantified by fitting response temporal phase versus stimulus spatial phase data. In the second procedure, conventional linear predictions of direction selectivity were computed from the amplitudes and phases of responses to stationary gratings. Extracellular recording locations were reconstructed histologically. Among direction-selective cells, S-T orientation was greatest in layer 4B and it correlated well (r = 0.76) with direction selectivity. In layer 6, S-T orientation was uniformly low, overlapping little with layer 4B, and it was not correlated with directional tuning. Layer 4A was intermediate in S-T orientation and its relation (r = 0.46) to direction selectivity. The same laminar patterns were observed using conventional linear predictions. The patterns do not reflect laminar differences in direction selectivity since the layers were equivalent in directional tuning. We also evaluated a model of linear spatiotemporal summation followed by a static nonlinear amplification (exponent model) to account for direction selectivity. The values of the exponents were estimated from differences between linearly predicted and actual amplitude modulations to counterphasing gratings. Comparing these exponents with another exponent—that required to obtain perfect matches between linearly predicted and measured directional tuning—indicates that an exponent model largely accounts for direction selectivity in most cells in layer 4, particularly layer 4B, but not in layer 6. Dynamic nonlinearities seem essential for cells in layer 6. We suggest that these laminar differences may partly reflect the differential involvement of geniculocortical and intracortical mechanisms.

The metabotropic receptor mGluR6 is localized to the dendrites of On bipolar cells and mediates synaptic input from photoreceptors. The binding of glutamate to the receptor activates a phosphodiesterase (PDE), which then hydrolyzes cGMP. A nonselective cationic conductance, believed to be gated directly by cGMP, is turned off as a result of the fall in cGMP levels, and the cell hyperpolarizes. Here we present evidence for regulation of the conductance by an additional mechanism that it is independent of cGMP. Whole-cell recordings were obtained from On bipolar cells in slices of tiger salamander retina. Dialysis of cells with 1 μM KN-62 or 10 μM KN-93, two inhibitors of type II calmodulin-dependent protein kinase (CaMKII), depressed cGMP-dependent currents. This depression persisted when hydrolysis of cGMP was prevented with IBMX, a broad-spectrum PDE inhibitor, suggesting that CaMKII acts downstream from the PDE in the cascade. The depression of cGMP-dependent currents was probably not due to a direct interaction of the inhibitors with the channels as neither 1 μM KN-62 or 10 μM KN-93 was found to have any effect on cyclic nucleotide-gated channels when applied directly to excised patches of rod outer segments. We propose that phosphorylation by CaMKII may be an important mechanism for regulating the cGMP-dependent conductance of On bipolar cells.

Electrophysiological studies of the isolated retina involve perfusing the tissue with a physiological Ringer's. Organic pH buffers such as HEPES have become increasingly popular in recent years because for many purposes they offer a convenient and reliable alternative to the more traditional bicarbonate/CO2. In this paper, however, we report that important functional properties of rods, bipolar cells, and horizontal cells in the salamander, Ambystoma tigrinum, are sensitive to the choice of buffer and, in the case of horizontal cells, that sensitivity is acute. In bicarbonate/CO2 Ringer's, the dark potential of the horizontal cell was typically near −50 mV and saturating light caused it to hyperpolarize to about −75 mV. On switching to HEPES-buffered Ringer's at the same pH, horizontal cells depolarized in darkness to about −20 mV, close to the chloride equilibrium potential, and the kinetics of their light responses changed. The cone-driven components of light responses increased in size relative to rod-driven components. Saturating lights still hyperpolarized the cells to −75 mV, however. Horizontal cells, being coupled via gap junctions, form a syncytium and syncytial length constants, measured in bicarbonate/CO2 Ringer's, were generally in the range 150–225 μm. On switching to HEPES-buffered Ringer's, length constants increased substantially to 250–330 μm. All these changes were reversible. We discuss our findings within the context of the cell's ability to regulate its internal pH.

Choroidal blood flow (ChBF) in birds is regulated by a neural circuit whose components are the retina, the suprachiasmatic nucleus, the medial division of the Edinger-Westphal nucleus (EWM), the ciliary ganglion, and the choriod. We have previously shown that lesions of EWM appear to result in pathological alterations in the retina. To determine whether EWM lesions also lead to altered visual functions, we have examined the effects of EWM lesions on visual acuity in pigeons. Bilateral lesions of EWM were made electrolytically, and visual acuity for high-contrast, square-wave gratings was determined behaviorally about 1 year later and compared to that of a group of pigeons that had received sham lesions of EW about 1 year prior to acuity testing. Because lesions targeting EWM invariably resulted in damage to the adjoining lateral part of the Edinger-Westphal nucleus (EWL), which controls pupillary constriction and accommodation, two additional control groups were studied. In one such control group, bilateral lesions in the area pretectalis (AP), which innervates the pupillary control part of EWL and thereby controls pupillary constriction, were made and the effects on visual acuity determined about 1 year later. In the second such control group, the effects of acute accommodative and pupillary dysfunction on acuity were studied in pigeons made cycloplegic. The accuracy of all lesions was later confirmed histologically. The mean acuities of birds with AP lesions (9.1 ± 1.4 cycles/deg) and sham lesions (7.1 ± 1.5 cycles/deg) were not significantly different from normal, based on published normative data on pigeons. In contrast, pigeons with lesions that completely destroyed EW bilaterally showed visual acuity (2.7 ± 0.1 cycles/deg) that was well below the acuity of the sham and AP-lesion control groups. The acuity of the cycloplegic pigeons (4.8 ± 0.3 cycles/deg) and one pigeon with a nearly complete bilateral EWL but a unilateral EWM lesion (6.4 cycles/deg) indicated that only about half of the loss with a bilateral EW lesion could be attributed to accommodative dysfunction. Thus, bilateral destruction of EWM appears to have led to a loss in visual acuity. This conclusion suggests that disruption of adaptive neural regulation of ChBF may impair visual function. Destruction of EWM was, however, associated with damage to the somatic components of the oculomotor and trochlear nuclei. The possibility cannot be excluded that such damage also contributed to the acuity loss.

We examined cytochrome oxidase (CO) activity in striate cortex of four macaque monkeys after monocular enucleation at ages 1, 1, 5, and 12 weeks. These animal experiments were performed to guide our interpretation of CO patterns in occipital lobe specimens obtained from two children who died several years after monocular enucleation during infancy for tumor. In the macaques, the ocular dominance columns were labelled by injecting [3H]proline into the remaining eye. After enucleation at age 1 week, ocular dominance columns were eliminated in layer IVcβ, resulting in a uniform pattern of autoradiographic label and CO staining. However, columns could still be seen in wet, unstained sections and with the Liesegang silver stain. Autoradiographs through layers IVcα and IVa showed residual, shrunken columns belonging to the missing eye, indicating that enucleation has less drastic effects in these layers. In the two human cases, enucleation at age 1 week also resulted in uniform CO staining in layer IVc. In the macaque after enucleation at age 5 weeks, ocular dominance columns belonging to the missing eye were severely narrowed, but still occupied 20% of layer IVcβ. CO revealed wide, dark columns alternating with thin, pale columns in layer IVcβ. The CO pattern and the columns labelled by autoradiography matched perfectly. After enucleation at age 12 weeks, only mild shrinkage of ocular dominance columns occurred. Enucleation at ages 1, 5, and 12 weeks did not alter the pattern of thin-pale–thick-pale stripes in V2. The main findings from this study were that (1) CO histochemistry accurately labels the boundaries of columns in layer IVcβ of macaque striate cortex after early monocular enucleation, making it a suitable technique for defining the critical period for plasticity of ocular dominance columns in human striate cortex; (2) enucleation causes more severe shrinkage of ocular dominance columns than eyelid suture; (3) early monocular enucleation obliterates ocular dominance columns in layer IVcβ, but their pattern remains visible in wet sections and with the Liesegang stain; and (4) enucleation does not affect CO staining in V2.

Extrastriate cortical area MT is thought to process behaviorally important visual motion signals. Psychophysical studies suggest that visual motion signals may be analyzed by multiple mechanisms, a “first-order” one based on luminance, and a “second-order” one based upon higher level cues (e.g. contrast, flicker). Second-order motion is visible to human observers, but should be invisible to first-order motion sensors. To learn if area MT is involved in the analysis of second-order motion, we measured responses to first- and second-order gratings of single neurons in area MT (and in one experiment, in area V1) in anesthetized, paralyzed macaque monkeys. For each neuron, we measured directional and spatio-temporal tuning with conventional first-order gratings and with second-order gratings created by spatial modulation of the flicker rate of a random texture. A minority of MT and V1 neurons exhibited significant selectivity for direction or orientation of second-order gratings. In nearly all cells, response to second-order motion was weaker than response to first-order motion. MT cells with significant selectivity for second-order motion tended to be more responsive and more sensitive to luminance contrast, but were in other respects similar to the remaining MT neurons; they did not appear to represent a distinct subpopulation. For those cells selective for second-order motion, we found a correlation between the preferred directions of first- and second-order motion, and weak correlations in preferred spatial frequency. These cells preferred lower temporal frequencies for second-order motion than for first-order motion. A small proportion of MT cells seemed to remain selective and responsive for second-order motion. None of our small sample of V1 cells did. Cells in this small population, but not others, may perform “form-cue invariant” motion processing (Albright, 1992).

Photocurrents from isolated rods of adults and sub-adults of three species of amphibians, Rana pipiens, Ambystoma tigrinum, and Xenopus laevis, were measured with suction pipette electrodes. The intensity for a half-maximal response was 0.91 ± 0.48 photons μm−2 flash−1 (mean ± s.d., 10-ms flashes) for Rana, 0.92 ± 0.44 for Ambystoma, and 6.14 ± 1.33 for Xenopus. The mean number of photoisomerizations at half-saturation was 22 ± 12 for Rana, 50 ± 24 for Ambystoma, and 221 ± 48 for Xenopus. The photocurrent per photoisomerization is several times smaller in Xenopus rods than in the other two species. Spectral sensitivity was measured from 277–737 nm with light polarized both parallel and perpendicular to the planes of the membrane disks. Dichroism fell in the near UV and was absent in the region of absorption by tryptophan and tyrosine. Maximum sensitivity of Rana was at 503.9 ± 2.6 nm (n = 86), and of Ambystoma, 505.8 ± 1.8 nm (n = 24). Animals from these same batches that were sampled by HPLC had no 3-dehydroretinal (retinal2). Xenopus containing about 94% retinal2 and 6% retinal1 had λmax at 519.3 ± 2.7 nm (n = 11). Spectral position of the β-band, estimated by the method of Stavenga et al. (1993), appears to be at longer wavelengths in amphibian photoreceptors than in other vertebrates. Fits of log sensitivity to a normalized-frequency template that tracks the long-wavelength tail of the α-band (Lamb, 1995) show that the rod pigments of Rana and Ambystoma are slightly narrower than those found in the photoreceptors of fish and mammals.

First-order (Fourier) motion consists of stable spatiotemporal luminance variations. Second-order (non-Fourier) motion consists instead of spatiotemporal modulation of contrast, flicker, or spatial frequency. In spite of extensive psychophysical and computational analysis of the nature and relationship of these two types of motion, it remains unclear whether they are detected by the same mechanism or whether separate mechanisms are involved. Here we report the selective impairment of first-order motion, on a range of local and global motion tasks, in the contralateral visual hemifield of a patient with unilateral brain damage centered on putative visual areas V2 and V3 in the medial part of the occipital lobe. His perception of second-order motion was unimpaired. As his disorder is the obverse of that reported after damage in the vicinity of human visual area MT (V5), the results support models of motion processing in which first- and second-order motion are, at least in part, computed separately at the extrastriate cortical level.

Connections of the thalamo-hyperstriatal system of hatchling chicks were investigated using multiple injections of cholera toxin B subunit (CTb) in the wulst. In the diencephalon, cells with CTb-like immunoreactivity (CTb-LI) were seen bilaterally in n. dorsolateralis anterior thalami, pars lateralis dorsalis and ventralis, n. dorsolateralis anterior thalami, pars magnocellularis, and pars lateralis rostralis. Within this complex, more CTb-LI cells were observed in the ventral portions of the ipsilateral side, whereas more labeled cells were found in the dorsolateral portions of the contralateral side. Moreover, CTb-LI cells were seen bilaterally in n. superficialis magnocellularis. In the nonvisual thalamic structures, numerous CTb-LI cells were seen in n. dorsolateralis anterior thalami, pars medialis and n. dorsolateralis posterior thalami. In the ventral thalamus, intense CTb-LI fibers/terminals were present in the external half of the external laminae of n. geniculatus lateralis, pars ventralis. Moderate to minor concentrations of fibrous labeling were found in n. intercalatus thalami and n. ventrolateral thalami. Moreover, efferent projections of the wulst were evident in the most ventral half of the optic tectum and the pretectal areas. The latter included n. pretectalis medialis, n. spiriformis medialis, n. principalis precommissuralis, n. lentiformis mesencephali, pars magnocellularis, and n. superficialis synecephali. Also, CTb-LI fibers were seen in n. basal optic root. The present study provides strong evidence that neuronal connections of the thalamo-hyperstriatal system are well established by the time of hatching. Additionally, efferent projections from the wulst to the diencephalic, mesencephalic, and pretectal structures are evident.

The role of cortical area V4 in complex shape discriminations was studied by testing the effects of V4 lesions in macaques on the ability to visually discriminate between images of three-dimensional (3D) objects from different viewpoints. Stimuli were presented in pairs in the lower left or lower right visual field quadrants about 4 deg from the fovea, and the monkeys judged on each trial whether the two views were of the same or of different objects. Object similarity was varied to determine a threshold shape difference. V4 lesions caused profound, retinotopic, and apparently permanent disruptions of discrimination, regardless of whether the images represented single or multiple viewpoints. In V4 lesioned portions of the visual field, monkeys could discriminate objects only when they differed much more grossly in shape than was true in control locations. These effects of the lesion were virtually identical for discriminations that had been learned before lesions were placed and for those learned afterwards. As in previous studies, V4 lesions elevated contrast thresholds by approximately a factor of two, but control observations showed that this was not the basis of the disruption of shape discrimination. Manipulation of cues to shape showed that in control locations, monkeys maintained excellent shape discrimination despite a variety of stimulus alterations, whereas in V4 lesioned areas their performance was easily disrupted. This finding suggests that V4 may support visual shape discriminations by facilitating the use of multiple visual cues. However, the fact that single-viewpoint and multiple-viewpoint discriminations were similarly affected indicates that the disruption was not specific to 3D shape discrimination, but may apply to a variety of subtle discriminations.

ON direction-selective (DS) ganglion cells were identified by electrophysiological recordings in DAPI labeled, isolated rabbit retinas. Their responses to a flashing spot were sustained. Their responses to moving stimuli were strong in the preferred direction and weak in the null direction. Injection of the recorded cells with Lucifer yellow revealed that the cells had a distinct dendritic morphology, consistent with that described previously (Buhl & Peichl, 1986; Amthor et al., 1989; Famiglietti, 1992a). When neighboring cells were injected, an extensive dendritic co-fasciculation was observed. The pattern of fasciculation restricts the possible synaptic connections of the ON DS cell.

Ganglion cells with intraretinal axon collaterals have been described in monkey (Usai et al., 1991), cat (Dacey, 1985), and turtle (Gardiner & Dacey, 1988) retina. Using intracellular injection of horseradish peroxidase and Neurobiotin in in vitro whole-mount preparations of human retina, we filled over 1000 ganglion cells, 19 of which had intraretinal axon collaterals and wide-field, spiny dendritic trees stratifying in the inner half of the inner plexiform layer. The axons were smooth and thin (∼2 μm) and gave off thin (<1 μm), bouton-studded terminal collaterals that extended vertically to terminate in the outer half of the inner plexiform layer. Terminal collaterals were typically 3–300 μm in length, though sometimes as long as 700 μm, and were present in clusters, or as single branched or unbranched varicose processes with round or somewhat flattened lobular terminal boutons 1–2 μm in diameter. Some cells had a single axon whereas other cells had a primary axon that gave rise to 2–4 axon branches. Axons were located either in the optic fiber layer or just beneath it in the ganglion cell layer, or near the border of the ganglion cell layer and the inner plexiform layer. This study shows that in the human retina, intraretinal axon collaterals are associated with a morphologically distinct ganglion cell type. The synaptic connections and functional role of these cells are not yet known. Since distinct ganglion cell types with intraretinal axon collaterals have also been found in monkey, cat, and turtle, this cell type may be common to all vertebrate retinas.

Being utilized by over 40% of the amacrine cells, glycine is considered to be a major inhibitory neurotransmitter in the retinas of all vertebrate species examined. Localization of gephyrin, which is a 93-kD peripheral membrane glycine receptor-associated anchoring protein, has been used in several studies to identify the sites of glycinergic interactions in the retina and other regions of the central nervous system. Recent studies have shown that gephyrin colocalizes with GABAA receptors which, like those for glycine, are also inhibitory amino acid receptors usually associated with a chloride channel. In the present study, we have used two antibodies which recognize either gephyrin (mAb7a), or the α and β subunits of the glycine receptor (mAb4a) in order to determine to what extent gephyrin is associated with glycine receptors in the mammalian retina. Single-label studies showed extensive punctate staining throughout most of the inner plexiform layer with each antibody. Double labeling showed that nearly 90% of the glycine receptor sites were also immunoreactive for gephyrin. However, nearly 60% of the total punctae immunoreactive for gephyrin were not stained for glycine receptors. This distinction was most pronounced in the most proximal inner plexiform layer where only 24% of the gephyrin-immunoreactive sites were glycine receptor positive. This study suggests that although most glycine receptors in the rabbit retina colocalize with the anchoring protein gephyrin, a significant proportion of the gephyrin-labeled sites are not associated with glycine receptors. In light of studies showing gephyrin association with GABAA receptor subunits, the localization of gephyrin may be indicative of chloride-mediated inhibitory amino acid transmission in general and not solely that of glycinergic. Given several studies which show that bipolar cells express glycine receptors and respond to glycine but do not express gephyrin, the 10% of glycine receptors not colocalized with gephyrin shown in the present study may represent a subtype of glycine receptors found on bipolar cells which do not require gephyrin for the functional clustering of receptor subunits.

In developing marsupials, the path taken through the optic chiasm by ipsilaterally projecting retinal ganglion cells is complicated. Just prior to entry into the chiasm, ganglion cells destined for the ipsilateral optic tract separate from the remainder of axons by turning abruptly downwards to take a position in the ventral part of the optic nerve. In this report, it is shown that a discrete population of about 10–15 large glial cells transiently form a linear array across the prechiasmatic part of the optic nerve, precisely at this axon turning point. The distinct morphology of these cells and their novel location may reflect a specialized role in axon guidance.