Effects of lateral suprasylvian visual cortex lesions on visual localization, discrimination, and attention in cats

Cross-modal connectivity effects in age-related hearing loss

Age-related sensorineural hearing loss (HL) leads to localized brain changes in the primary auditory cortex, long-range functional alterations, and is considered a risk factor for dementia. Nonhuman studies have repeatedly highlighted cross-modal brain plasticity in sensorial brain networks other than those primarily involved in the peripheral damage, thus in this study, the possible cortical alterations associated with HL have been analyzed using a whole-brain multimodal connectomic approach. Fifty-two HL and 30 normal hearing participants were examined in a 3T MRI study along with audiological and neurological assessments. Between-regions functional connectivity and whole-brain probabilistic tractography were calculated in a connectome-based manner and graph theory was used to obtain low-dimensional features for the analysis of brain connectivity at global and local levels. The HL condition was associated with a different functional organization of the visual subnetwork as revealed by a significant increase in global efficiency, density, and clustering coefficient. These functional effects were mirrored by similar (but more subtle) structural effects suggesting that a functional repurposing of visual cortical centers occurs to compensate for age-related loss of hearing abilities.

Auditory projections to extrastriate visual cortex: connectional basis for multisensory processing in 'unimodal' visual neurons

Neurophysiological studies have recently documented multisensory properties in 'unimodal' visual neurons of the cat posterolateral lateral suprasylvian (PLLS) cortex, a retinotopically organized area involved in visual motion processing. In this extrastriate visual area, a region has been identified where both visual and auditory stimuli were independently effective in activating neurons (bimodal zone), as well as a second region where visually-evoked activity was significantly facilitated by concurrent auditory stimulation but was unaffected by auditory stimulation alone (subthreshold multisensory region). Given their different distributions, the possible corticocortical connectivity underlying these distinct forms of crossmodal convergence was examined using biotinylated dextran amine (BDA) tracer methods in 21 adult cats. The auditory cortical areas examined included the anterior auditory field (AAF), primary auditory cortex (AI), dorsal zone (DZ), secondary auditory cortex (AII), field of the rostral suprasylvian sulcus (FRS), field anterior ectosylvian sulcus (FAES) and the posterior auditory field (PAF). Of these regions, the DZ, AI, AII, and FAES were found to project to the both the bimodal zone and the subthreshold region of the PLLS. This convergence of crossmodal inputs to the PLLS suggests not only that complex auditory information has access to this region but also that these connections provide the substrate for the different forms (bimodal versus subthreshold) of multisensory processing which may facilitate its functional role in visual motion processing.

Deficits of visual motion perception and optokinetic nystagmus after posterior suprasylvian lesions in the ferret (Mustela putorius furo)

We recently described an area in the ferret posterior suprasylvian (PSS) cortex characterized by a high proportion of direction selective neurons. To answer the question whether area PSS subserves functions similar to cat posteromediolateral suprasylvian area (PMLS) and monkey medial temporal area (MT) we investigated the contribution of area PSS to visual motion perception and optokinetic nystagmus. Ferrets were tested on global motion detection before and after bilateral lesions involving area PSS and control lesions of other extrastriate visual areas. Following PSS lesions motion coherence thresholds were significantly increased both in pigmented and albino ferrets, whereas control lesions sparing PSS did not affect visual motion perception. Optokinetic nystagmus was strongly reduced to absent after PSS lesions. These results indicate that area PSS is crucial for global motion processing in the ferret and in that sense may be functionally equivalent to PMLS in the cat and area MT in the monkey.

Graded classes of cortical connections: quantitative analyses of laminar projections to motion areas of cat extrastriate cortex

Current hierarchical models of the cerebral cortex are mainly based on qualitative connection studies. From wheatgerm-agglutinin-horseradish peroxidase injections, we examined the laminar patterns of projections to and between the three major subdivisions of the motion-processing lateral suprasylvian (LS) complex [areas posteromedial lateral suprasylvian area (PMLS), anteromedial lateral suprasylvian (AMLS), posterolateral lateral suprasylvian area (PLLS)] of cat extrastriate cortex and of an adjoining form-processing area, 21a. We counted approximately 145,000 labelled projection cells in 20 cortical areas in 11 cats, and applied various analyses to the data, expressed as the percent supragranular layer (%SG) origin of each connection. We report two main results. (i) A wide range of %SG values was obtained, both from each individual cat and across the 163 projections examined. Nonetheless, both hierarchical and non-parametric cluster analyses of the pooled connection origins suggested the presence of three distinct laminar projection classes, constrained by graded %SG values of 0-33%, 39-69% and 76-97%. These conformed, respectively, to feedback, lateral and feedforward laminar patterns seen qualitatively in our material. (ii) Hierarchical connectivity analyses suggested that PMLS, AMLS and PLLS are ordered in a hierarchical sequence. Macaque motion areas V5/MT, MST and FST are arranged in a similar sequence, and areas at equivalent levels of the two motion hierarchies have some analogous functional specializations. Our findings provide the first objective support for the three laminar projection classes that underpin existing theoretical models of hierarchical cortical organization, and they suggest that the implementation of higher-order motion processing evolved along similar lines in the cat and monkey visual cortex.

Functional circuitry underlying natural and interventional cancellation of visual neglect

A large body of work demonstrates that lesions at multiple levels of the visual system induce neglect of stimuli in the contralesional visual field and that the neglect dissipates as neural compensations naturally emerge. Other studies show that interventional manipulations of cerebral cortex, superior colliculus or deep-lying midbrain structures have the power to attenuate, or cancel, the neglect and reinstate orienting into a neglected hemifield, and even into a profound cortically blind field. These results, and those derived from experiments on the behavioral impacts of unilateral and bilateral lesions, lead us to evaluate the repercussions of unilateral and bilateral deactivations, neural compensations and cancellations of attentional deficits in terms of an overarching hypothesis of neglect. The cancellations can be both striking and enduring, and they suggest that therapeutic strategies can be developed to reverse or ameliorate neglect in human patients. Animal studies show that in many instances of neglect adequate representations and the accompanying motor mechanisms are present despite the lesion and they simply need to be unmasked and brought into use to effect a remedy.

Removal of two halves restores the whole: reversal of visual hemineglect during bilateral cortical or collicular inactivation in the cat

The purpose of the present study was to compare visual orienting behavior in the adult cat during (1) unilateral and bilateral cooling deactivation of posterior-middle suprasylvian (pMS) sulcal cortex, and (2) unilateral and bilateral deactivation of the superior colliculus. As expected, unilateral cooling deactivation of either pMS cortex or the superior colliculus resulted in a profound visual neglect of the contracooled hemifield. The addition of cooling the homotopic region in the opposite hemisphere largely reversed this deficit and restored visual orienting into the previously neglected hemifield. These results show that (1) pMS cortex and the superior colliculus are essential for normal detection and orienting to visual targets, and (2) unilateral visual neglect results from an imbalance of activities in the two hemispheres induced at either cortical or subcortical levels. These conclusions have implications for understanding neural bases of visual hemineglect following unilateral lesions in humans.

Extrinsic and intrinsic connections of the cat's lateral suprasylvian visual area

The lateral suprasylvian visual area (LS) is known to have numerous interconnections with visual cortical areas as well as with subcortical structures implicated in visually-guided behaviors. In contrast, little data is available regarding connections within the LS itself. In order to obtain information about intra-areal connections and to re-investigate LS connectivity with various cortical and subcortical areas, the traces (biocytin or WGA-HRP) was injected into various loci along the medial and lateral banks of the LS. The anterograde tracer, biocytin injections into both medial and lateral bank produced label contained within the respective bank that extended rostrally and caudally from the infection site. In addition, following medical bank injections, considerable label was distributed throughout the fundus and, to a lesser extent, in the lateral bank. In contrast, no label could be detected in the medial bank after lateral bank injections, and, although label was observed in the fundus, it was restricted to the most lateral aspects. Moderate labeling could be observed in the medial bank following the tracer injection into the most rostral aspect of the lateral bank. It is likely that input derived from various visual cortical areas which project to the medial bank of the LS has access to this intra-areal circuitry. This may provide a route by which visual cortical information can be relayed to other cortical and subcortical structures involved in visually-guided behaviors such as the anterior ectosylvian visual cortex, striatum, and the deep layers of the superior colliculus, despite the fact that these structures themselves do not receive substantial direct projections from the visual cortical areas that are associated with the medial bank. Examination of the laminar location of the cells-of-origin of striate and extrastriate projections to LS using retrograde trace, WGA-HRP, revealed that the supragranular laminae of areas 17, 18 and 19 were the source of LS afferents whereas afferents from the other cortical areas (e.g., 20a, 20b, 21a, 21b, 7 and anterior ectosylvian visual area) were from both supra- and infragranular laminae. In addition, all LS subregions received intra-areal afferent projections from all LS cortical laminae. Thus, although rather clear hierarchical relationship between LS and visual cortical areas appears to exist, the interconnections among LS subregions provide no clear evidence of simple hierarchical relationships between regions LS or may have feed-forward and feed-back pathways.

Reversible inactivation of visual processing operations in middle suprasylvian cortex of the behaving cat

Extrastriate visual areas on the banks of the middle suprasylvian sulcus were inactivated by cooling to assess the behavioral contribution of this cortical region to the extraction of a stationary figure from a moving mask. Cooling blocked figure-ground separation when the mask was moving but had no influence when the mask was static. This difference provides strong evidence that the areas bounding the middle suprasylvian sulcus contribute to the neural separation of stationary from moving visual stimuli.

The role of the lateral suprasylvian visual cortex of the cat in object-background interactions: permanent deficits following lesions

The contribution of the lateral suprasylvian cortex to pattern recognition was studied by behavioural detection experiments in combination with bilateral lesions of different parts of the lateral suprasylvian areas (LSA) and area 7 in seven cats. In a two-alternatives forced choice task the cats had to discriminate simple outline patterns which were additively superimposed on a structured visual background made up of broadband Gaussian noise. For various stimulus conditions (moving or stationary patterns and/or background) the detection probability (PD) of the cats was measured as a function of the signal-to-noise ratio (S/N). Each cat was tested before and after the lesion. Four different types of lesion could be distinguished depending on their extent: (1) lesion of parts of the (LSA); (2) lesion of parts of the LSA with undercutting of areas 17, 18 and 19; (3) lesion of area 7; (4) lesion of area 7 and parts of the LSA. 1. We found that a large bilateral lesion of the LSA led to significant deficits in all test situations which were dependent on the existence of relative velocity of moving patterns against a structured background. The ability of the cats to discriminate simple outline patterns which were kept stationary was not reduced. On the contrary, when they were tested with stationary and moving patterns on unfocused (empty) backgrounds, we found, to our great surprise, that the performance of the lesioned cats was significantly improved compared with intact animals. As these lesioned cats had no deficits with moving patterns on a uniformly grey background, we conclude that the deficits with the moving patterns must have been caused by interactions between patterns and background, and not by movement of a pattern per se. 2. As soon as the lesion of the LSA was extended by a bilateral undercutting of areas 17, 18 and 19 we found very severe deficits in all test situations, regardless of whether the patterns were moving or kept stationary, or whether they were superimposed on a background or not. The most substantial deficits occurred when the patterns were moving on a stationary background. In these situations the cats were no longer able to reach the 84% correct criterion. Again, the cats were able to reach criterion with moving patterns on a uniformly grey background indicating that this deficit is probably caused by the interaction of patterns and background and not by motion of the patterns per se.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison of magnification functions in area 19 and the lateral suprasylvian visual area in the cat

A retinotopic map can be described by a magnification function that relates magnification factor to visual field eccentricity. Magnification factor for primary visual cortex (V1) in both the cat and the macaque monkey is directly proportional to retinal ganglion cell density. However, among those extrastriate areas for which a magnification function has been described, this is often not the case. Deviations from the pattern established in V1 are of considerable interest because they may provide insight into an extrastriate area's role in visual processing. The present study explored the magnification function for the lateral suprasylvian area (LS) in the cat. Because of its complex retinotopic organization, magnification was calculated indirectly using the known magnification function for area 19. Small tracer injections were made in area 17, and the extent of anterograde label in LS and in area 19 was measured. Using the ratio of cortical area labeled in LS to that in area 19, and the known magnification factor for area 19 at the corresponding retinotopic location, we were able to calculate magnification factor for LS. We found that the magnification function for LS differed substantially from that for area 19: central visual field was expanded, and peripheral field compressed in LS compared with area 19. Additionally, we found that the lower vertical meridian's representation was compressed relative to that of the horizontal meridian. We also examined receptive field size in areas 17, 19, and LS and found that, for all three areas, receptive field size was inversely proportional to magnification factor.

How complete is physiological compensation in extrastriate cortex after visual cortex damage in kittens?

Previous studies indicate that neurons in the cat's posteromedial lateral suprasylvian (PMLS) visual area of cortex show physiological compensation after neonatal but not adult damage to areas 17, 18, and 19 of the visual cortex (collectively, VC). Thus, VC damage in adults produces a loss of direction selectivity and a decrease in response to the ipsilateral eye among PMLS cells, but these changes are not seen in adult cats that received VC damage as kittens. This represents compensation for early VC damage in the sense that PMLS neurons develop properties they would have had if there had been no brain damage. However, this is only a partial compensation for the effects of VC damage. A full compensation would involve development of properties of the VC cells that were removed in the damage. The present study investigated whether this type of compensation occurs for detailed spatial- and temporal-frequency processing. Single-cell recordings were made in PMLS cortex of adult cats that had received a VC lesion on the day of birth or at 8 weeks of age. Responses to sine-wave gratings that varied in spatial frequency, contrast, and temporal frequency were assessed quantitatively. We found that the spatial- and temporal-frequency processing of PMLS cells in adult cats that had neonatal VC damage were not significantly different from PMLS cells in normal cats. Therefore, there was no evidence that PMLS cells can compensate for VC damage by developing properties that are better than normal and like those of the striate cortex cells that were damaged. We also assessed the effects of long-term VC damage in adult cats to determine whether the normal properties seen in cats with neonatal VC damage represent a compensation for abnormalities in PMLS cortex present after adult damage. In a previous study, we found that acute VC damage in adult cats has small but reliable effects on maximal response amplitude, maximal contrast sensitivity, and spatial resolution (Guido et al. 1990b). In the present study, we found that long-term VC damage in adult cats does not increase these abnormalities as a result of secondary degenerative changes. In fact, the minor abnormalities that were present after an acute VC lesion were virtually absent following a long-term adult lesion, perhaps because they were due to transient traumatic effects. Therefore, there was little evidence for abnormalities in spatial- or temporal-frequency processing following long-term adult VC damage for which PMLS cells might show compensation following long-term neonatal damage.(ABSTRACT TRUNCATED AT 400 WORDS)

Visuomotor interactions in responses of neurons in the middle and lateral suprasylvian cortices of the behaving cat

We studied visuomotor processing in the middle (MS) and lateral suprasylvian (LS) cortices of the alert cat by making single cell recordings while the cat was working in a behavioral task requiring visual fixation and visually guided eye movements. We found responses with three different components: visual sensory, saccade-related motor, and fixation. Some cells exhibited purely visual responses and all of their activity during visuomotor tasks could be attributed to the sensory aspects of the task. Other cells showed no sensory response properties, but discharged in relation to the saccadic eye movements that the cat made to visual targets. A smaller number of fixation cells displayed increased discharge when the cat fixated a target light and usually only when that target was in a particular region of the visual field. These response components could be present in a variety of combinations in different cells, of which the largest proportion combined visuomotor responses and could take five general forms: simple visuomotor, saccadic enhanced, visually triggered movement (VTM), enhanced VTM, and disenhanced. Simple visuomotor responses had both a visual and saccade-related component. Saccadic enhanced responses had a visual response to the appearance of a spot in the cell's receptive field that became enhanced when the cat subsequently made a saccade to that spot. The VTM responses were synchronized better to the visual stimulus than to the saccade, but they also exhibited properties expected of motor responses. The last two classes of visuomotor responses were rare: one we termed enhanced VTM and the other disenhanced. Cells could combine different visuomotor response components or even sensory, saccade-related and fixation responses in different combinations for different directions of eye movements. Generally, the timing of the saccade-related responses occurred too late to play a role in the initiation of saccades: most (83%) saccade-related responses occurred between 40 ms before to 80 ms after the onset of the eye movement. Cells of all different types could be found in both the MS and LS areas, though in general the responses in LS were more sensory in nature while those in MS were more closely related to the eye movement. About a quarter of the cells were unresponsive during any aspect of our tasks.

Visual response properties of neurons in the middle and lateral suprasylvian cortices of the behaving cat

The visual response properties of cells in the middle (MS) and lateral (LS) suprasylvian cortices were studied in alert cats, which were trained to fixate a spot of light and maintain fixation when a second test light was introduced in the midst of fixation. This second light served to test for visual sensitivity, and it could be moved at different speeds in any direction under computer control. Over half of the cells exhibited a visual response. With a small spot of light, most cells were directionally selective and responded better to a moving spot than to a stationary one. In some cases movements of the spot in the non-preferred direction revealed an inhibitory process. The visual receptive fields were large and often extended into the ipsilateral hemifield, though the centers of the receptive fields were usually in the contralateral field. We used Fourier analysis to quantify directional selectivity and compared these results to other commonly used measures of directional selectivity. Compared to cells in MS, there was a higher incidence of visual cells in LS and the visual cells were more directional. We also made comparisons between our results and those found in anesthetized cats and awake monkeys.

Deficits in speed discrimination following lesions of the lateral suprasylvian cortex in the cat

We examined the role of the lateral suprasylvian (LS) cortex in motion perception by testing the ability of three cats to detect moving targets and to discriminate differences in stimulus direction and speed before and after making bilateral ibotenic acid lesions in LS. The lesions had little or no effect on contrast sensitivity for detecting moving sinusoidal gratings. Moreover, we found no deficits in discriminating opposite directions of motion: the cats discriminated grating directions at threshold contrasts. All three cats, however, showed permanent deficits in discriminating differences in speed and in flicker rate. The deficits were most pronounced at higher temporal and spatial frequencies and at lower contrasts. This result suggests that LS plays an important role in the analysis of stimulus speed. It appears that information needed for discriminating opposite directions of motion may be signalled by visual areas outside LS.

Small lateral suprasylvian cortex lesions produce visual neglect and decreased visual activity in the superior colliculus

Previous experiments in cats have shown that complete contralateral visual neglect is produced by removing all known visual cortex on one side of the brain, which can then be reversed by damaging the opposite superior colliculus. Presumably, descending facilitatory influences from the visual cortex to the ipsilateral superior colliculus are counterbalanced by intercollicular inhibition (Sprague: Science 153:1544-1546, '66). However, not all of visual cortex or all of the superior colliculus needs to be involved in this circuit. It is the deep rather than the superficial laminae of the superior colliculus that are primarily involved in visual attentive and orientation behaviors, and these laminae are largely independent of primary visual cortex. However, they do depend on corticotectal influences from a comparatively small extraprimary visual area of the posterior region of the lateral suprasylvian cortex (PSSC-Ogasawara et al: J. Neurophysiol. 52:1226-1245, '84). The present experiments demonstrate that lesions only a few millimeters in diameter in this corticotectal zone of the PSSC can produce profound visual neglect. While damage to this area has little, if any, effect on superficial laminae visual activity, it produces a dramatic decrease in the visual activity of the deep laminae. These cats with PSSC lesions fail to orient to a visual stimulus that is introduced suddenly into the contralateral visual field, yet they respond on nearly 100% of the trials to this same stimulus when it is presented in the ipsilateral visual field. The lesion-induced visual neglect produced by PSSC lesions is long-lasting but can be abruptly ameliorated by a midbrain lesion that primarily involves, or undercuts, the deep laminae of the contralateral superior colliculus. Thus, 1) visual neglect can be produced by depriving the deep laminae of the superior colliculus of visual inputs from the cortex, even when the principal visual cortical regions (17, 18, and 19) and their target structures are intact, and 2) visually guided behavior can be restored by eliminating afferents originating in, or passing through, the deep laminae of the contralateral superior colliculus.

Lesion size and recovery of function: some new perspectives

The present article discusses the possibility that functional recovery following brain damage may be to a large degree dependent on the amount of nervous tissue destroyed, such that more neuronal destruction may lead to more and not (as commonly suggested) to less recovery. This assumption may derive from the neuropsychological and neurological literature: many cases with circumscribed brain lesions are implicated with severe functional losses. However, patients with dramatic and severe brain destructions often show astonishingly normal behavior regarding cognition, speech, visuospatial, motor and sensory functions. Animal experimentation as well shows that an extensive lesion of a brain area may be associated with equal or less functional detriment than a small lesion of the same area. Along with the well-known variables of age, lesion growth, or personality and environmental factors, the amount of tissue destroyed should be considered as a potent mediator of functional recovery. At least for some functions and brain regions, the likeliness of recovery may increase with the extent of the lesion and thus the necessity of the brain to fulfill plastic changes.

Visual localization and discrimination after ibotenic lesion of the cat orbito-insular cortex

Behavioral tasks were used to investigate how the orbito-insular cortex (OIC) of the cat is involved in complex operations such as the orienting reaction towards a novel stimulus. Six cats were trained preoperatively on a perimetry test to assess their ability to orient the head and eyes to objects presented in restricted regions of the visual field, and on brightness, pattern and form discrimination tasks for food reward in a two-choice discrimination apparatus. Two animals then underwent unilateral chemical lesion of the OIC using injections of ibotenic acid, two others received bilateral lesions of this same area, and the remaining two cats were used as normal controls. Postoperative performance of brightness, pattern and form discrimination was normal following OIC lesions, and no lack of retention was observed. In contrast, the cats with OIC lesions had significant deficits in their visually guided behavior. The cats ignored objects presented in the monocular segment of both sides of the visual field, even after unilateral lesion, and there was an effect on the ability to attend and fixate the central preconditioned stimulus.

Connections of the parahippocampal cortex in the cat. III. Cortical and thalamic efferents

To study the distribution of the cortical and thalamic efferent projections from the parahippocampal cortex in the cat, a series of injections of anterogradely transported radioactively labeled amino acids were placed in different parts of the entorhinal and perirhinal cortices. Subsequently, some of the identified cortical and thalamic target areas were injected with retrograde tracers such as wheat germ-agglutinin conjugated with horseradish peroxidase (WGA-HRP) or with a fluorescent tracer--fast blue or nuclear yellow--in order to disclose the laminar origin of the parahippocampal efferent projections. The results indicate that the parahippocampal cortex gives rise to widespread projections to the association cortex, and, to a lesser extent, sends fibers to the limbic cortex and the primary sensory cortex. These projections arise mainly from the deep layers of the parahippocampal cortex and terminate predominantly in superficial layers of the cortex, with a preference for layer I. Within the cortical projections a medial-to-lateral topography could be observed such that the entorhinal cortex projects predominantly to the allocortical and periallocortical limbic areas, including parts of the subicular complex, the ventral retrosplenial and the infralimbic cortices, and olfactory related areas--i.e., the olfactory bulb, the anterior olfactory nucleus, the prepiriform cortex, and the ventral tenia tecta. The more lateral parts of the parahippocampal cortex, which surround the posterior rhinal sulcus, project in addition to extensive parts of the paralimbic association cortex that include the proisocortical cingular, prelimbic, orbitofrontal, and agranular and granular insular cortices. The most lateral portion of the parahippocampal cortex, the perirhinal cortex, furthermore issues projections to widespread neocortical areas on the lateral and medial aspects of the hemisphere that constitute part of the parasensory association cortex. Weak-to-moderate projections are found to the cortex of the middle suprasylvian and anterior ectosylvian sulci, as well as the cruciate and splenial sulci, all of which have been reported to constitute sensory convergence areas. The most marked projections from the perirhinal cortex reach a zone of neocortex directly lateral to the perirhinal cortex including ventral parts of the posterior sylvian, posterior ectosylvian, posterior suprasylvian, and lateral gyri. These projections appear to be topographically organized such that rostral parts of the perirhinal cortex project more rostrally, and more caudal parts of the perirhinal cortex project to more caudal parts of this cortical zone.(ABSTRACT TRUNCATED AT 400 WORDS)

Detection performance of normal cats and those lacking areas 17 and 18: a behavioral approach to analyse pattern recognition deficits

The ability of cats to discriminate between two geometrical outline patterns in the presence of superimposed Gaussian visual noise was tested before and after bilateral removal of cortical area 17 and parts of area 18. The detection probability PD was measured as a function of the signal-to-noise ratio for the parameters: noise bandwidth, spatial frequency content and rate of movement of patterns. In both normal and lesioned cats a broadband noise was found to be most effective in masking the large patterns while two other types of noise, a medium frequency noise and a high frequency noise had little or no masking effect. For recognition of the smaller patterns in normal cats the medium frequency noise was found to be more effective than the broadband noise. The performance of the lesional cats was disturbed severely at low signal-to-noise ratios and was significantly inferior to that of normal cats-especially for small patterns. However, at high S/N ratios and for large patterns the performance of the lesioned cats was comparable to that of normals while for the small patterns they reached PD values inferior to those of normal cats. It is concluded that although pattern recognition can be performed successfully by cats lacking areas 17 and 18, these cortical areas probably make an essential contribution to this function under natural conditions in two ways: because of the X-type input of area 17, they increase the acuity of the system by making it more sensitive to higher spatial frequencies, and they permit detection of patterns at much lower S/N ratios i.e. they lower the signal-to-noise ratio at which the system is able to detect the presence of a pattern in a background of statistical visual noise. The latter effect is not limited to the higher spatial frequencies but also affects the very low spatial frequencies which are normally used for pattern detection. Previous failures to demonstrate clear deficits in pattern discrimination after 17/18 lesions in cats may be attributed to the fact that the patterns presented for discrimination were not masked by visual noise. Movement of patterns led to a slight, but not significant improvement of the performance in both normal and lesioned cats, but the deficits found for stationary and moving patterns were more or less equal.

Visual field defects in cats with neonatal or adult immunological loss of retinal ganglion cells

Behavioral perimetry methods were used to assess the monocular visual fields of 12 cats that had received intraocular injections of antibodies against large (alpha/Y) retinal ganglion cells. The antibodies produced defects in head and eye orientation responses to stimuli presented in the nasal visual field of the treated eye; responses to stimuli in the temporal visual hemifield were normal. Similar results were seen in cats that received antibody injections at 4 weeks of age or as adults. In the context of previous results, these findings suggest that a loss of Y cells in the lateral geniculate nucleus is sufficient to reduce geniculocortical function for head and eye orientation to visual stimuli in the nasal visual field.

Immediate postoperative retention of visual discriminations following selective cortical lesions in the cat

Cats were trained preoperatively for brightness discrimination, and 7 pattern and form discriminations, and then retested for preoperative retention on each discrimination. Cortical lesions were then placed in areas 17 and 18 in one group (4 cats), in areas 17, 18 and 19 in another group (3 cats), and in suprasylvian cortex (areas 7, 21, and parts of 19, 5 and the lateral suprasylvian cortex) in a third group (4 cats). Results are also reported for a fourth group with extensive suprasylvian lesions, to which was added an unintended undercutting of areas 17 and 18 (4 cats). While during original preoperative learning the training continued until a fixed, stringent criterion of performance was attained, both preoperative and postoperative retention was tested in short sessions, involving a limited number of trials and a less stringent statistical criterion (significant run). After extensive removal of areas 17 and 18, all cats behaved as though following the cortical lesion they could immediately recognize the discriminative stimuli as efficiently as before, with no need for retraining. On the contrary, the group with areas 17, 18 and 19 lesions showed a substantial postoperative loss of all discriminations, and especially for the more difficult form discriminations, the reattainment of a significant level of performance was hard or impossible within the allotted number of trials. Also in the group with limited suprasylvian lesions, postoperative retention was generally impaired, but the reacquisition of efficient performance was superior to that of the previous group. Finally, large suprasylvian lesions encroaching on the white matter under areas 17, 18 and 19 proved disruptive for all discriminative capacities, both in retention and in relearning. The excellent retention of all discriminations following areas 17 and 18 lesions once again shows that these areas are by no means essential for complex vision in the cat. In addition, the results strongly indicate that the high-level visual capacities of destriate cats are not due to reorganization of readaptation processes occurring in extrastriate areas after a 17/18 removal. The clear-cut retention deficits which were present in cats with cortical lesions more extensive than areas 17 and 18 or outside of the latter areas prove the essential participation of extrastriate cortical areas in visual discrimination including form. However, the distribution of functions among the various visual cortical areas in visual discrimination remains poorly understood.(ABSTRACT TRUNCATED AT 400 WORDS)