Effects of selective pressure block of Y-type optic nerve fibers on the receptive-field properties of neurons in area 18 of the visual cortex of the cat

Immediate Impact of Acute Elevation of Intraocular Pressure on Cortical Visual Motion Processing

Purpose

To physiologically examine the impairment of cortical sensitivity to visual motion during acute elevation of intraocular pressure (IOP).

Methods

Motion processing in the cat brain is well characterized, its X and Y cell visual pathways being functionally analogous to parvocellular and magnocellular pathways in primates. Using this model, we performed ocular anterior chamber perfusion to reversibly elevate IOP over a range from 30 to 90 mm Hg while monitoring cortical activity with intrinsic signal optical imaging. Drifting random-dot fields and gratings were used to characterize cortical population responses to motion direction and orientation in early visual areas 17 and 18.

Results

We found that acute IOP elevations at 50 mm Hg and above, which is often observed in acute glaucoma, suppressed cortical motion direction responses. This suppression was more profound in area 17 than in area 18, and more profound in central than peripheral visual field (eccentricities 0°–4° vs. 4°–8°) within area 17. In addition, orientation responses were more suppressed than motion direction responses for the same IOP modulation.

Conclusions

In contrast to human chronic glaucoma that may cause greater dysfunction in large-cell magnocellular than in small-cell parvocellular visual pathways, our direct measurement of cortical processing networks implies that the small X-cell pathway shows greater vulnerability to acute IOP elevation than the large Y-cell pathway in visual motion processing. The results demonstrate that fine discrimination mechanisms for motion in the central visual field are particularly impacted by acute IOP attacks, suggesting a neural basis for immediate visual deficits in the fine motion perception of acute glaucoma patients.

Glaucoma -state of the art and perspectives on treatment

Glaucoma is a chronic optic neuropathy characterized by progressive damage to the optic nerve, death of retinal ganglion cells and ultimately visual field loss. It is one of the leading causes of irreversible loss of vision worldwide. The most important trigger of glaucomatous damage is elevated eye pressure, and the current standard approach in glaucoma therapy is reduction of intraocular pressure (IOP). However, despite the use of effective medications or surgical treatment leading to lowering of IOP, progression of glaucomatous changes and loss of vision among patients with glaucoma is common. Therefore, it is critical to prevent vision loss through additional treatment. To implement such treatment(s), it is imperative to identify pathophysiological changes in glaucoma and develop therapeutic methods taking into account neuroprotection. Currently, there is no method of neuroprotection with long-term proven effectiveness in the treatment of glaucoma. Among the most promising molecules shown to protect the retina and optic nerve are neurotrophic factors. Thus, the current focus is on the development of safe and non-invasive methods for the long-term elevation of the intraocular level of neurotrophins through advanced gene therapy and topical eye treatment and on the search for selective agonists of neurotrophin receptors affording more efficient neuroprotection.

Zif268 mRNA Expression Patterns Reveal a Distinct Impact of Early Pattern Vision Deprivation on the Development of Primary Visual Cortical Areas in the Cat

Pattern vision deprivation (BD) can induce permanent deficits in global motion perception. The impact of timing and duration of BD on the maturation of the central and peripheral visual field representations in cat primary visual areas 17 and 18 remains unknown. We compared early BD, from eye opening for 2, 4, or 6 months, with late onset BD, after 2 months of normal vision, using the expression pattern of the visually driven activity reporter gene zif268 as readout. Decreasing zif268 mRNA levels between months 2 and 4 characterized the normal maturation of the (supra)granular layers of the central and peripheral visual field representations in areas 17 and 18. In general, all BD conditions had higher than normal zif268 levels. In area 17, early BD induced a delayed decrease, beginning later in peripheral than in central area 17. In contrast, the decrease occurred between months 2 and 4 throughout area 18. Lack of pattern vision stimulation during the first 4 months of life therefore has a different impact on the development of areas 17 and 18. A high zif268 expression level at a time when normal vision is restored seems to predict the capacity of a visual area to compensate for BD.

Binocular neurons in parastriate cortex: interocular 'matching' of receptive field properties, eye dominance and strength of silent suppression

Spike-responses of single binocular neurons were recorded from a distinct part of primary visual cortex, the parastriate cortex (cytoarchitectonic area 18) of anaesthetized and immobilized domestic cats. Functional identification of neurons was based on the ratios of phase-variant (F1) component to the mean firing rate (F0) of their spike-responses to optimized (orientation, direction, spatial and temporal frequencies and size) sine-wave-luminance-modulated drifting grating patches presented separately via each eye. In over 95% of neurons, the interocular differences in the phase-sensitivities (differences in F1/F0 spike-response ratios) were small (≤0.3) and in over 80% of neurons, the interocular differences in preferred orientations were ≤10°. The interocular correlations of the direction selectivity indices and optimal spatial frequencies, like those of the phase sensitivies and optimal orientations, were also strong (coefficients of correlation r ≥0.7005). By contrast, the interocular correlations of the optimal temporal frequencies, the diameters of summation areas of the excitatory responses and suppression indices were weak (coefficients of correlation r ≤0.4585). In cells with high eye dominance indices (HEDI cells), the mean magnitudes of suppressions evoked by stimulation of silent, extra-classical receptive fields via the non-dominant eyes, were significantly greater than those when the stimuli were presented via the dominant eyes. We argue that the well documented ‘eye-origin specific’ segregation of the lateral geniculate inputs underpinning distinct eye dominance columns in primary visual cortices of mammals with frontally positioned eyes (distinct eye dominance columns), combined with significant interocular differences in the strength of silent suppressive fields, putatively contribute to binocular stereoscopic vision.

Stimulus-entrained oscillatory activity propagates as waves from area 18 to 17 in cat visual cortex

Previous studies in cat visual cortex reported that area 18 can actively drive neurons in area 17 through cortico-cortical projections. However, the dynamics of such cortico-cortical interaction remains unclear. Here we used multielectrode arrays to examine the spatiotemporal pattern of neuronal activity in cat visual cortex across the 17/18 border. We found that full-field contrast reversal gratings evoked oscillatory wave activity propagating from area 18 to 17. The wave direction was independent of the grating orientation, and could not be accounted for by the spatial distribution of receptive field latencies, suggesting that the waves are largely mediated by intrinsic connections in the cortex. Different from the evoked waves, spontaneous waves propagated along both directions across the 17/18 border. Together, our results suggest that visual stimulation may enhance the flow of information from area 18 to 17.

Phase sensitivities, excitatory summation fields, and silent suppressive receptive fields of single neurons in the parastriate cortex of the cat

We have recorded single-neuron activity from cytoarchitectonic area 18 of anesthetized (0.4–0.7% isoflurane in 65% N2O-35% O2 gaseous mixture) domestic cats. Neurons were identified as simple or complex on the basis of the ratios between the phase-variant (F1) component and the mean firing rate (F0) of spike responses to optimized (orientation, direction, spatial and temporal frequencies, size) high-contrast, luminance-modulated, sine-wave drifting gratings (simple: F1/F0 spike-response ratios > 1; complex: F1/F0 spike-response ratios < 1). The predominance (∼80%) of simple cells among the neurons recorded from the principal thalamorecipient layers supports the idea that most simple cells in area 18 might constitute a putative early stage in the visual information processing. Apart from the “spike-generating” regions (the classical receptive fields, CRFs), the receptive fields of three-quarters of area 18 neurons contain silent, extraclassical suppressive regions (ECRFs). The spatial extent of summation areas of excitatory responses was negatively correlated with the strength of the ECRF-induced suppression of spike responses. Lowering the stimulus contrast resulted in an expansion of the summation areas of excitatory responses accompanied by a reduction in the strength of the ECRF-induced suppression. The spatial and temporal frequency and orientation tunings of the ECRFs were much broader than those of the CRFs. Hence, the ECRFs of area 18 neurons appear to be largely “inherited” from their dorsal thalamic inputs. In most area 18 cells, costimulation of CRFs and ECRFs resulted in significant increases in F1/F0 spike-response ratios, and thus there was a contextually modulated functional continuum between the simple and complex cells.

Influence of adapting speed on speed and contrast coding in the primary visual cortex of the cat

Adaptation is a ubiquitous property of the visual system. Adaptation often improves the ability to discriminate between stimuli and increases the operating range of the system, but is also associated with a reduced ability to veridically code stimulus attributes. Adaptation to luminance levels, contrast, orientation, direction and spatial frequency has been studied extensively, but knowledge about adaptation to image speed is less well understood. Here we examined how the speed tuning of neurons in cat primary visual cortex was altered after adaptation to speeds that were slow, optimal, or fast relative to each neuron's speed response function. We found that the preferred speed (defined as the speed eliciting the peak firing rate) of the neurons following adaptation was dependent on the speed at which they were adapted. At the population level cells showed decreases in preferred speed following adaptation to speeds at or above the non-adapted speed, but the preferred speed did not change following adaptation to speeds lower than the non-adapted peak. Almost all cells showed response gain control (reductions in absolute firing capacity) following speed adaptation. We also investigated the speed dependence of contrast adaptation and found that most cells showed contrast gain control (rightward shifts of their contrast response functions) and response gain control following adaptation at any speed. We conclude that contrast adaptation may produce the response gain control associated with speed adaptation, but shifts in preferred speed require an additional level of processing beyond contrast adaptation. A simple model is presented that is able to capture most of the findings.

Contrast gain control is drift-rate dependent: an informational analysis

Neurons in the visual cortex code relative changes in illumination (contrast) and adapt their sensitivities to the visual scene by centering the steepest regions of their sigmoidal contrast response functions (CRFs: spike rate as a function of contrast) on the prevailing contrast. The influence of this contrast gain control has not been reported at nonoptimal drift rates. We calculated the Fisher information contained in the CRFs of halothane-anesthetized cats. Fisher information gives a measure of the accuracy of contrast representations based on the ratio of the square of the steepness of the CRF and the spike-rate dependency of the spiking variance. Variance increases with spike rate, so Fisher information is maximal where the CRF is steep and spike rates are low. Here, we show that the contrast at which the maximal Fisher information (C(MFI)) occurs for each adapting drift rate is at a fixed level above the adapting contrast. For adapting contrasts of 0 to 0.32 the relationship between C(MFI) and adapting contrast is well described by a straight line with a slope close to 1. The intercept of this line on the C(MFI)-axis is drift-rate dependent, although the slope is not. At high drift rates relative to each cell's peak the C(MFI) offset is higher than that for low drift rates. The results show that the contrast coding strategy in visual cortex maximizes accuracy for contrasts above the prevailing contrast in the environment for all drift rates. We argue that tuning the system for accuracy at contrasts above the prevailing value is optimal for viewing natural scenes.

Preservation of functional architecture in visual cortex of cats with experimentally induced hydrocephalus

We investigated how neural function is preserved or matured in the visual cortex of cats, following the induction of hydrocephalus by kaolin injection. In vivo optical imaging of intrinsic signals in 11-17-week-old hydrocephalic cats revealed orientation maps showing the orderly arrangement of preferred orientations when stimulated by grating stimuli at a low spatial frequency, whereas stimulus-evoked intrinsic signals in response to gratings at a high spatial frequency were often too weak to construct orientation maps. Furthermore, in two of the three hydrocephalic cats, initially deteriorated orientation maps became almost regular maps in the second imaging experiments conducted 8 and 11 weeks, respectively, after the first imaging. This indicates that, despite large structural deformation of the hydrocephalic brain, orientation maps are elaborated sufficiently after the age of 5-6 months, by which time the orientation map formation is usually completed in normal cats. Single unit recording from the decompressed visual cortex revealed that many neurons showed normal orientation selectivity, whereas the binocularity of these neurons was found to be reduced. These results suggested that the deformed visual cortex of hydrocephalic cats exhibits a high plasticity, retaining its functional organization.

Relationship Between Contrast Adaptation and Orientation Tuning in V1 and V2 of Cat Visual Cortex

Previous studies investigating the response properties of neurons in the primary visual cortex of cats and primates have shown that prolonged exposure to optimally oriented, high-contrast gratings leads to a reduction in responsiveness to subsequently presented test stimuli. We recorded from 119 neurons in cat V1 and V2 and found that in a high proportion of cells contrast adaptation also occurs for gratings oriented orthogonal to a neuron's preferred orientation, even though this stimulus did not elicit significant increases in spiking activity. Approximately 20% of neurons adapted equally to all orientations tested and a further 46% showed at least some adaptation to orthogonally oriented gratings, whereas 20% of neurons did not adapt to orthogonal gratings. The magnitude of contrast adaptation was positively correlated with adapting contrast, but was not related to the spiking activity of the cells. Highly direction selective neurons produced stronger adaptation to orthogonally oriented gratings than other neurons. Orientation-related adaptation was correlated with the rate of change of orientation tuning in consecutive cells along electrode penetrations that traveled parallel to the cortical layers. Nonoriented adaptation was most common in areas where orientation preference changed rapidly, whereas orientation-selective adaptation was most common in areas where orientation preference changed slowly. A minority of neurons did not show contrast adaptation (14%). No major differences were found between units in different cortical layers, V1 and V2, or between complex and simple cells. The relevance of these findings to the current understanding of adaptation within the context of orientation column architecture is discussed.

Laminar differences in plasticity in area 17 following retinal lesions in kittens or adult cats

Circumscribed retinal lesions in adult cats result in a reorganization of circuitry in area 17 such that neurons in the lesion projection zone (LPZ) can now be activated, not from their original receptive fields (RFs) but from regions of normal retina adjacent to the lesion ('ectopic' RFs). We have studied this phenomenon further by making circumscribed monocular retinal lesions in 8-week-old kittens and recording responses to visual stimuli of neurons in the LPZ of area 17 when these cats reached adulthood. These responses have been compared with those in adult-lesioned cats either of relatively short postlesion survival (2-24 weeks) or long postlesion survival (3.5-4.5 years). In both kitten-lesioned and adult-lesioned animals most LPZ neurons recorded from the supragranular layers (II and III) not only exhibited new ectopic RFs when stimuli were presented via the lesioned eye but the RF properties (e.g. the sizes of excitatory RFs, orientation and direction selectivities, velocity preferences and upper cut-off velocities) were often indistinguishable from those seen when stimuli were presented via the nonlesioned eye. Similarly, in both kitten-lesioned and adult-lesioned animals, most LPZ neurons recorded from the granular and infragranular layers (IV, V, VI), like those recorded from the supragranular layers, were binocular. However, in adult-lesioned but not in kitten-lesioned animals, the responses and the upper cut-off velocities of LPZ cells recorded from the granular and infragranular layers to stimuli presented via ectopic RFs tended to be, respectively, substantially weaker and lower than those for stimuli presented via the nonlesioned eye. The age-related laminar differences in reorganizational plasticity of cat striate cortex correlate with the lamino-temporal pattern of distribution of N-methyl-d-aspartate glutamate receptors in striate cortex.

Global motion detection is impaired in cats deprived early of pattern vision

We investigated global motion detection in binocularly deprived cats (BD cats) and control cats (C cats). The cats were trained in the two-choice free running apparatus for a food reward. The positive stimulus was a moving random-dot pattern with all dots moving in one direction, the negative stimulus was the same random-dot pattern but stationary. The BD cats were severely impaired in detection of global motion stimulus as compared with the C cats. In contrast, their level of performance in a simple relative motion detection task (one square) did not differ from that in the C cats. However, in more complex relative motion detection task (two squares) the performance of the BD cats was impaired. The deficit in the detection of global motion in BD cats may be due to impairments of their Y-pathway.

Topographic reorganization in area 18 of adult cats following circumscribed monocular retinal lesions in adolescence

Circumscribed laser lesions were made in the nasal retinae of one eye in adolescent cats. Ten to sixteen months later, about 80 % of single neurones recorded in the lesion projection zone (LPZ) of contralateral area 18 (parastriate cortex, area V2) were binocular but when stimulated via the lesioned eye had ectopic discharge fields (displaced to normal retina in the vicinity of the lesion). Although the clear majority of binocular cells recorded from the LPZ responded with higher peak discharge rates to stimuli presented via the non-lesioned eye, the orientation and direction selectivities as well as preferred and upper cut-off velocities for stimuli presented through either eye were very similar. Furthermore, the sizes of the ectopic discharge fields of binocular cells recorded from the LPZ were not significantly different from those of their counterparts plotted via the non-lesioned eye. Thus, monocular retinal lesions performed in adolescent cats induce topographic reorganization in the LPZ of area 18. Although a similar reorganization occurs in area 17 (striate cortex, area V1) of cats in which monocular retinal lesions were made either in adulthood or adolescence, in view of the very different velocity response profiles of ectopic discharge fields in areas 17 and those in area 18, it appears that ectopic discharge fields in area 17 are largely independent of excitatory feedback input from area 18.

The receptive fields of cat retinal ganglion cells in physiological and pathological states: where we are after half a century of research

Studies on the receptive field properties of cat retinal ganglion cells over the past half-century are reviewed within the context of the role played by the receptive field in visual information processing. Emphasis is placed on the work conducted within the past 20 years, but a summary of key contributions from the 1950s to 1970s is provided. We have sought to review aspects of the ganglion cell receptive field that have not been featured prominently in previous review articles. Our review of the receptive field properties of X- and Y-cells focuses on quantitative studies and includes consideration of the function of the receptive field in visual signal processing. We discuss the non-classical as well as the classical receptive field. Attention is also given to the receptive field properties of the less well-studied cat ganglion cells-the W-cells-and the effect of pathology on cat ganglion cell properties. Although work from our laboratories is highlighted, we hope that we have given a reasonably balanced view of the current state of the field.

Plasticity in adult cat visual cortex (area 17) following circumscribed monocular lesions of all retinal layers

1. In eight adult cats intense, sharply circumscribed, monocular laser lesions were used to remove all cellular layers of the retina. The extents of the retinal lesions were subsequently confirmed with counts of alpha-ganglion cells in retinal whole mounts; in some cases these revealed radial segmental degeneration of ganglion cells distal to the lesion. 2. Two to 24 weeks later, area 17 (striate cortex; V1) was studied electrophysiologically in a standard anaesthetized, paralysed (artificially respired) preparation. Recording single- or multineurone activity revealed extensive topographical reorganization within the lesion projection zone (LPZ). 3. Thus, with stimulation of the lesioned eye, about 75 % of single neurones in the LPZ had 'ectopic' visual discharge fields which were displaced to normal retina in the immediate vicinity of the lesion. 4. The sizes of the ectopic discharge fields were not significantly different from the sizes of the normal discharge fields. Furthermore, binocular cells recorded from the LPZ, when stimulated via their ectopic receptive fields, exhibited orientation tuning and preferred stimulus velocities which were indistinguishable from those found when the cells were stimulated via the normal eye. 5. However, the responses to stimuli presented via ectopic discharge fields were generally weaker (lower peak discharge rates) than those to presentations via normal discharge fields, and were characterized by a lower-than-normal upper velocity limit. 6. Overall, the properties of the ectopic receptive fields indicate that cortical mechanisms rather than a retinal 'periphery' effect underlie the topographic reorganization of area 17 following monocular retinal lesions.

Structure and dynamics of receptive fields in the visual cortex of the cat (area 18) and the influence of GABAergic inhibition

Receptive fields (RFs) in the visual cortex are characterized by spatiotemporal profiles that have been described in detail for area 17 simple cells. In this study, we analyse spatial and temporal RF properties of simple and complex cells in layer II/III of area 18 of the anaesthetized adult cat, using the reverse correlation method with brief 50 ms presentations of flashing bright and dark bars. Stimuli were presented with preferred orientation as previously determined by moving bars. Simple cell RFs were characterized by spatially and temporally separable ON and OFF subfields, while in complex cells ON and OFF subfields were superimposed. To discriminate possible contributions of GABAergic inhibition to RF structure and response dynamics in area 18, we have used three-barrelled micropipettes for single cell recordings and microiontophoresis, and have documented ON and OFF responses before, during and after application of bicuculline methiodide for blockade of GABAA receptors. During blockade of GABAergic inhibition, the stimulus-induced and resting discharge frequency increased, and in about 50% of the cells both ON and OFF subfields changed significantly in space and/or time in a reversible manner. In space, blockade of inhibition widened RF subfields, whereas in time, it shortened the duration of the excitatory cell response in simple and complex cells. ON and OFF subfields separated in space and time (simple cells), or time (complex cells) became less isolated or even superimposed. The results indicate substantial local inhibitory processing contributing to spatiotemporal RF properties in layers II/III of area 18 of the cat.

Excitatory convergence of Y and non-Y channels onto single neurons in the anterior ectosylvian visual area of the cat

Numerous functional and hodological studies of the anterior ectosylvian visual area (AEV) of the cerebral cortex of the cat suggest that this area plays an important role in processing information about visual motion. In the present study, in cats with selective conduction block of Y fibres in one optic nerve, we have examined the extent of the excitatory convergence of Y (presumed 'motion channel') and non-Y information channels on single neurons in AEV, as well as the contribution of the Y channel to the receptive field properties of AEV neurons. While in normal cats all neurons recorded from AEV were binocular, i.e. could be photically activated via either eye, in cats with selective conduction block of Y fibres in one optic nerve, a significant proportion (about 15%) of AEV cells could be photically activated only via the normal eye. In comparison to those in normal cats, the peak discharge rates of AEV neurons in the Y-blocked cats were drastically reduced not only when photic stimuli were presented via the Y-blocked eye, but also when they were presented via the normal eye. Selective block of Y input also resulted in a significant shift in velocity preferences towards the lower velocities. However, the direction selectivity indices of AEV neurons were not affected by selective Y block. Thus: (i) the responses of AEV neurons to a high velocity of motion are dependent on the integrity of the Y input; (ii) the 'spontaneous' (i.e. not photically evoked) discharges of Y retinal ganglion cells exert a facilitatory influence on the responses of AEV cells to photic stimuli; (iii) although the responses of AEV neurons are dominated by the Y inputs, AEV neurons also receive significant non-Y excitatory inputs; and (iv) the strong direction selectivity revealed in most AEV neurons does not dependent on the integrity of Y input.

Excitatory convergence of Y and non-Y information channels on single neurons in the PMLS area, a motion area of the cat visual cortex

We analysed the receptive field properties of neurons in the posteromedial lateral suprasylvian (PMLS) visual cortical area of anaesthetized cats in which there was selective conduction block of the largest (Y-type) fibres in one optic nerve. As in normal cats, in cats with selective block of one optic nerve the great majority of PMLS cells could be activated by photic stimulation through either eye. However, the responses evoked by stimulation via the eye with the selectively pressure-blocked optic nerve ('Y-blocked eye') were significantly weaker than those of the same cells evoked by the stimulation via the normal eye. Accordingly, eye dominance histograms were shifted markedly in favour of the normal eye. Furthermore, there was a significant shift towards lower preferred velocities when PMLS cells were photically stimulated via the Y-blocked eye. Finally, when stimulated via the Y-blocked eye, PMLS cells responded poorly or not at all to high stimulus velocities (> or = 100 degrees/s). On the other hand, a number of receptive field properties, such as the spatial organization of receptive fields, the size of the discharge fields, orientation tuning and direction selectivity indices, were not significantly affected by the removal of the Y input. We conclude that virtually all neurons in the PMLS area of the cat receive excitatory input from both Y and non-Y information channels, although the Y channel provides the dominant input and makes the principal contribution to the detection of high-velocity motion.

Limits of parallel processing: excitatory convergence of different information channels on single neurons in striate and extrastriate visual cortices

1. It has been postulated that the distinct parallel retino-geniculo-cortical information channels characterizing visual pathways of virtually all mammals are selectively linked to parallel motion, colour and/or form information processing 'streams' distinguishable within the primary visual cortices, extrastriate cortical areas of occipital lobes and the temporal and parietal visual cortices. 2. Using selective pressure-blocking of the large-fibre channel (the so-called Y-channel) in the optic nerve of the cat, we have experimentally examined the 'selective excitatory parallel links' hypothesis. We conclude that the majority of neurons in the primary visual cortices (areas 17, 18) as well as in the two 'higher order' visual areas, area 21a and posteromedial lateral suprasylvian (PMLS) area, constituting, respectively, part of the 'form' and part of the 'motion' processing streams, receive their excitatory inputs from both Y- and non-Y-information channels. In areas 17, 18 and 21a (but not in PMLS area), there are, however, subpopulations of cells that apparently receive excitatory inputs from only one information channel. 3. Review of the relevant work on the macaque monkey suggests that the situation is similar in the primate: that is, there is a substantial degree of excitatory convergence of different retino-geniculo-cortical information channels on single neurons in the primary visual cortices and the extrastriate cortices constituting parts of the form/colour or the motion processing streams. 4. Despite this high degree of excitatory convergence of different information channels, the large-fibre channels (the Y-channel in the cat and the magnocellular or Y-like channel in macaque), are in both carnivores and primates the principal contributors to the motion processing cortical streams.

Prenatal monocular enucleation induces a selective loss of low-spatial-frequency cortical responses to the remaining eye

During early development, interactions between the two eyes are critical in the formation of eye-specific domains within the lateral geniculate nucleus and the visual cortex. When monocular enucleation is done early in prenatal life, it induces remarkable anatomical and functional reorganizations of the visual pathways. Behavioral data have shown a loss in sensitivity to low-spatial-frequency gratings in cats. To correlate the behavioral observations with a possible change in the analysis of contrast at the level of primary visual areas we recorded visual evoked potentials at the 17/18 border in two cats enucleated prenatally (gestational age at enucleation, 39-42 days), three neonatal, two control animals, and one animal with a surgical removal of Y-ganglion fibers. Our results show a strong attenuation in the amplitude of response at all contrast values for gratings of low spatial frequency in prenatally enucleated cats, whereas neonatally enucleated and control animals present responses of comparable amplitude. We conclude that the behavioral results reflect the reduced sensitivity for low frequencies of visual cortical neurons. In addition, we define a critical period for the development of the contrast-sensitivity function that seems to be limited to the prenatal gestation period. We suggest that the prenatal interruption of binocular interactions leads to a functional elimination of the Y-ganglion system.

Processing of form and motion in area 21a of cat visual cortex

Extracellular recordings from single neurons have been made from presumed area 21a of the cerebral cortex of the cat, anesthetized with N2O/O2/sodium pentobarbitone mixture. Area 21a contains mainly a representation of a central horizontal strip of contralateral visual field about 5 deg above and below the horizontal meridian. Excitatory discharge fields of area 21a neurons were substantially (or slightly but significantly) larger than those of neurons at corresponding eccentricities in areas 17, 19, or 18, respectively. About 95% of area 21a neurons could be activated through either eye and the input from the ipsilateral eye was commonly dominant. Over 90% and less than 10% of neurons had, respectively, C-type and S-type receptive-field organization. Virtually all neurons were orientation-selective and the mean width at half-height of the orientation tuning curves at 52.9 deg was not significantly different from that of neurons in areas 17 and 18. About 30% of area 21a neurons had preferred orientations within 15 deg of the vertical. The mean direction-selectivity index (32.8%) of area 21a neurons was substantially lower than the indices for neurons in areas 17 or 18. Only a few neurons exhibited moderately strong end-zone inhibition. Area 21a neurons responded poorly to fast-moving stimuli and the mean preferred velocity at about 12.5 deg/s was not significantly different from that for area 17 neurons. Selective pressure block of Y fibers in contralateral optic nerve resulted in a small but significant reduction in the preferred velocities of neurons activated via the Y-blocked eye. By contrast, removal of the Y input did not produce significant changes in the spatial organization of receptive fields (S or C type), the size of the discharge fields, the width of orientation tuning curves, or direction-selectivity indices. Our results are consistent with the idea that area 21a receives its principal excitatory input from area 17 and is involved mainly in form rather than motion analysis.