Low frequency transcranial magnetic stimulation on the posterior parietal cortex induces visuotopically specific neglect-like syndrome

The visuo-parietal (VP) region of the cerebral cortex is critically involved in the generation of orienting responses towards visual stimuli. In this study we use repetitive transcranial magnetic stimulation (rTMS) to unilaterally and non-invasively deactivate the VP cortex during a simple spatial visual detection task tested in real space. Adult cats were intensively trained over 4 months on a task requiring them to detect and orient to a peripheral punctuate static LED presented at a peripheral location between 0 degrees and 90 degrees , to the right or left of a 0 degrees fixation target. In 16 different interleaved sessions, real or sham low frequency (1 Hz) rTMS was unilaterally applied during 20 min (1,200 pulses) to the VP cortex. The percentage of mistakes detecting and orienting to contralateral visual targets increased significantly during the 15-20 min immediately following real but not sham rTMS. Behavioral deficits were most marked in peripheral eccentricities, whereas more central locations were largely unaffected. Performance returned to baseline (pre-TMS) levels when animals were tested 45 min later and remained in pre-TMS levels 24 h after the end of the stimulation. Our results confirm that the VP cortex of the cat is critical for successful detection and orienting to visual stimuli presented in the corresponding contralateral visual field. In addition, we show that rTMS disrupts a robust behavioral task known to depend on VP cortex and does so for the far periphery of the visual field, but not for more central targets.

System-wide repercussions and adaptive plasticity: the sequelae of immature visual cortex damage

Damage of primary visual cortex in mature mammals severely disrupts vision by disconnecting much of the cognitive processing machinery of extrastriate cortex from its source of visual signals in the retina. However, equivalent lesions incurred early in postnatal life unmask a substantial latent flexibility of the brain to minimize the disruption by specific and ordered pathway expansions that bypass the lesion. The expansions shape pathways from retina through thalamus to extrastriate cortex and onto the midbrain into new, useful forms that are retained into adult life. These useful modifications support relatively normal signal processing in a variety of structures and the sparing of certain visually guided behaviors, such as aspects of complex-pattern vision and localizing objects introduced into the visual field. Thus, both the brain and the individual are optimized, in the absence of primary visual cortex, by adaptations for useful interactions with the environment. So far, the repercussions of early visual cortex lesions have been most thoroughly documented in cats, although it is likely on the basis of known repercussions and similarity of visual system organization and developmental sequence, that broadly equivalent repercussions and adaptations occur in monkeys and humans following early lesions of primary visual cortex. The knowledge gained has implications for devising therapeutic strategies to attenuate defects in vision induced by cortical lesions.

Role of the superior colliculus in analyses of space: superficial and intermediate layer contributions to visual orienting, auditory orienting, and visuospatial discriminations during unilateral and bilateral deactivations

The superior colliculus (SC) has been implicated in spatial analyses of the environment, although few behavioral studies have explicitly tested this role. To test its imputed role in spatial analyses, we used a battery of four spatial tasks combined with unilateral and bilateral cooling deactivation of the upper and intermediate layers of the superior colliculus. We tested the abilities of cats to orient to three different stimuli: (1) moving visual, (2) stationary visual, (3) stationary white-noise aural. Furthermore, we tested the ability of the cats to discriminate the relative spatial position of a landmark. Unilateral cooling deactivation of the superficial layers of the SC induced a profound neglect of both moving and stationary visual stimuli presented in, and landmark objects located within, the contralateral hemifield. However, responses to auditory stimuli were unimpaired. Unilateral cooling deactivation of both the superficial and intermediate layers induced a profound contralateral neglect of the auditory stimulus. Additional and equivalent deactivation of the opposite SC largely restored orienting to either moving visual or auditory stimuli, and restored landmark position reporting to normal levels. However, during bilateral SC deactivation, orienting to the static visual stimulus was abolished throughout the entire visual field. Overall, unilateral SC deactivation results show that the upper and intermediate layers of the SC contribute in different ways to guiding behavioral responses to visual and auditory stimuli cues. Finally, bilateral superior colliculus deactivations reveal that other structures are sufficient to support spatial analyses and guide visual behaviors in the absence of neural operations in the superior colliculus, but only under certain circumstances.

Task-specific reversal of visual hemineglect following bilateral reversible deactivation of posterior parietal cortex: a comparison with deactivation of the superior colliculus

The purpose of the present study was to compare and contrast behavioral performance on three different tasks of spatial cognition during unilateral and bilateral reversible deactivation of posterior parietal cortex. Specifically, we examined posterior middle suprasylvian (pMS) sulcal cortex in adult cats during temporary and reversible cooling deactivation. In Task 1, the cats oriented to a high-contrast, black visual stimulus moved into the visual field periphery. In Task 2, the cats oriented to a static light-emitting diode (LED). Task 3 examined the cats' ability to determine whether a black-and-white checkered, landmark box was closer to the right or left side of the testing apparatus. Following training on all tasks, cryoloops were implanted bilaterally within the pMS sulcus. Unilateral deactivation of pMS sulcal cortex resulted in virtually no responses to either moved or static stimuli and virtually no responses to landmarks presented in the contralateral hemifield, and a profound contralateral hemifield neglect was induced. Responses to stimuli and landmarks presented in the ipsilateral hemifield were unimpaired. Additive, bilateral cooling of the homotopic region in the contralateral hemisphere, but not an adjacent region, resulted in reversal of the initial hemineglect for the moved stimulus, yet induced a complete failure to orient to peripheral static LED stimuli. Bilateral cooling also reversed the contralateral neglect of the landmark, but then cats could not accurately determine position of the landmark anywhere in the visual field because performance was reduced to chance levels for all landmark loci in both hemifields. In this instance, as the contralateral neglect disappeared during bilateral cooling of pMS cortex, a new spatial discrimination deficit was revealed across the entire visual field. We conclude that pMS cortex contributes in multiple ways to the analyses of space, and that these contributions cannot be safely predicted from analyses of unilateral deactivations or from one task to another. Moreover, it is clear that other structures are capable of guiding orienting to high contrast, moved targets when pMS cortex is eliminated from brain circuitry. However, these same structures are incapable of supporting either orienting to static stimuli or analyses of spatial relations as tested with the landmark task. The impact of reversible deactivation of the superior colliculus on these same tasks is discussed.

Perinatal-lesion-induced reorganization of cerebral functions revealed using reversible cooling deactivation and attentional tasks

We tested the concept that lesions of primary visual cortical areas 17 and 18 sustained on the day of birth induce a redistribution of cerebral operations underlying the ability to disengage visual attention and then redirect it to a new location. In cats, these operations are normally highly localizable to posterior middle suprasylvian (pMS) cortex. Three stimulation paradigms were used: (i) movement of a high contrast visual stimulus into the visual field; (ii) illumination of a static light-emitting diode (LED) stimulus; and (iii) a control static auditory stimulus. To test for the redistribution of critical neural operations, cryoloops were implanted bilaterally in the pMS sulcus and in contact with ventral posterior suprasylvian (vPS) cortex. Separate and combined deactivations of pMS and vPS cortices in cats with early lesions of primary visual cortex showed that full, unilateral deactivation of pMS cortex only partially impaired the ability to detect and orient to stimuli moved into the contracooled hemifield. Much more complete impairment required the additional deactivation of ipsilateral vPS cortex. Bilateral pMS deactivation alone, or in combination with bilateral vPS deactivation, largely reversed the unilateral contracooled neglect. For the orienting to static, illuminated LED stimuli, unilateral deactivation of pMS cortex was sufficient to fully impair orienting to stimuli presented in the contracooled hemifield. Bilateral pMS deactivation induced an almost complete visual-field-wide neglect of stimuli. On its own, unilateral deactivation of vPS cortex was without effect on either task, although bilateral vPS deactivations introduced inconsistencies into the performance. Termination of cooling reversed all deficits. Finally, neither the initial lesion of areas 17 and 18 nor cooling of either the MS or vPS cortex alone, or in combination, interfered with orienting to sound stimuli. Overall, our results provide evidence that at least one highly localizable visual function of normal cerebral cortex is remapped across the cortical surface following the early lesion of primary visual cortical areas 17 and 18. Moreover, the redistribution has spread the essential neural operations from the visual parietal cortex to a normally functionally distinct type of cortex in the visual temporal system.

Feedback connections act on the early part of the responses in monkey visual cortex

We previously showed that feedback connections from MT play a role in figure/ground segmentation. Figure/ground coding has been described at the V1 level in the late part of the neuronal responses to visual stimuli, and it has been suggested that these late modulations depend on feedback connections. In the present work we tested whether it actually takes time for this information to be fed back to lower order areas. We analyzed the extracellular responses of 169 V1, V2, and V3 neurons that we recorded in two anesthetized macaque monkeys. MT was inactivated by cooling. We studied the time course of the responses of the neurons that were significantly affected by the inactivation of MT to see whether the effects were delayed relative to the onset of the response. We first measured the time course of the feedback influences from MT on V1, V2, and V3 neurons tested with moving stimuli. For the large majority of the 51 neurons for which the response decreased, the effect was present from the beginning of the response. In the responses averaged after normalization, the decrease of response was significant in the first 10-ms bin of response. A similar result was found for six neurons for which the response significantly increased when MT was inactivated. We then looked at the time course of the responses to flashed stimuli (95 neurons). We observed 15 significant decreases of response and 14 significant increases. In both populations, the effects were significant within the first 10 ms of response. For some neurons with increased responses we even observed a shorter latency when MT was inactivated. We measured the latency of the response to the flashed stimuli. We found that even the earliest responding neurons were affected early by the feedback from MT. This was true for the response to flashed and to moving stimuli. These results show that feedback connections are recruited very early for the treatment of visual information. It further indicates that the presence or absence of feedback effects cannot be deduced from the time course of the response modulations.

Graded sparing of visually-guided orienting following primary visual cortex ablations within the first postnatal month

We compared the abilities of intact cats and cats that incurred lesions of areas 17 and 18 in adulthood, at one month of age (P28), or on the day of birth (P1), to detect and orient towards visual stimuli either moved into or illuminated in the periphery of the visual field, and to detect and orient towards a stationary, broad-band white-noise auditory stimulus. For all groups of cats, movement of a stimulus into the visual field was a more potent stimulus for evoking visually-guided orienting movements than illumination of a static light-emitting diode (LED). The potency of the auditory stimulus was also extremely high. Proficiency on both visual tasks was graded according to the age at which areas 17 and 18 were ablated in the sequence: adult, P1, P28 and intact in the sequence worst-->best performance. The superior performance of the P1- and P28-groups provided evidence for sparing of visually-guided orienting, but the sparing was incomplete because it did not match performance of intact cats. Lesions of areas 17 and 18 incurred in adulthood had no significant impact on orienting to auditory white-noise stimuli. However, orienting performance to auditory stimuli presented in the peripheral quadrants was slightly superior in the P28 group and reduced in the P1 group. Thus, the visual sparing exhibited by the P1 group may be at the expense of highly proficient orienting to auditory cues. Overall, these results extend our knowledge by showing that in addition to P1-cats, cats that incur lesions of areas 17 and 18 at one month-of-age also exhibit sparing of visually-guided orienting, and that the sparing is not confined to a single stimulation paradigm. Finally, the covariation in the magnitude of pathway modifications with the scale of the orienting proficiency in P1- and P28 cats helps to solidify the linkage between rewired brain pathways and spared visually-guided behaviors.

Translaminar differentiation of visually guided behaviors revealed by restricted cerebral cooling deactivation

The purpose of the present study was to test the hypothesis that superficial and deep layers within a single cerebral region influence cerebral functions and behaviors in different ways. For this test, we selected posterior middle suprasylvian (pMS) sulcal cortex of the cat, a suspected homolog of the area V5 complex of primates, because the region has been implicated in several visually guided behaviors. Cats were trained on three tasks: (1) discrimination of direction of motion; (2) discrimination of static patterns partially obscured by static or moving masks; and (3) visual detection and orienting. Cooling of cryoloops in contact with pMS sulcal cortex to 8+/-1 degrees C selectively and completely impaired performance on the two motion discrimination tasks (1 and 2), while leaving the detection and orienting task (task 3) unimpaired. Further cooling to 3 degrees C resulted in an additional complete impairment of task 3. The 8 degrees C temperature resulted in silencing of neuronal activity in the supragranular layers (I-III) and the 3 degrees C temperature silenced activity throughout the thickness of pMS sulcal cortex. The variation in behavioral performance with covariation of cryoloop temperature and vertical, but not lateral, spread of deactivation shows that deactivation of superficial cerebral layers alone was sufficient to completely impair performance on the two motion discrimination tasks, whereas additional deactivation of the deep layers was essential to block performance on the detection and orienting task. Thus, these results show a functional bipartite division of labor between upper and lower cortical layers that is supported by efferent connectional anatomy. Similar bipartite division into upper and lower layers may be a general feature of cerebral cortical architecture, signal processing and guidance of behavior.

Gamma surgery for hemangiopericytomas

A retrospective analysis of a consecutive series of 12 patients with 15 intracranial hemangiopericytomas treated at the University of Virginia using Gamma surgery is presented. Clinical and radiographic follow up of 3 to 56 months is available for 10 patients with 12 tumors. There was one tumor present at the time of initial Gamma surgery in each patient. Two new tumors occurred in patients previously treated. Nine of the tumors decreased in volume and three remained stable. Four of the nine tumors that shrank later progressed at an average of 22 months after treatment. Of the tumors that decreased in volume and have not progressed, the response has been for an average of 11 months. The follow-up for two tumors that remained unchanged was 10 and 34 months (average 22 months). A third tumor was unchanged at 42 months but the patient died of new disease adjacent to the treated area in the anterior skull base. There were no complications and the quality of life following the procedure was maintained or improved in every case. Gamma surgery is effective in palliating the patients by decreasing tumor volume and delaying recurrence.

Contributions of cat posterior parietal cortex to visuospatial discrimination

The purpose of the present study was to examine the contributions made by cat posterior parietal cortex to the analyses of the relative position of objects in visual space. Two cats were trained on a landmark task in which they learned to report the position of a landmark object relative to a right or left food-reward chamber. Subsequently, three pairs of cooling loops were implanted bilaterally in contact with visuoparietal cortices forming the crown of the middle suprasylvian gyrus (MSg; architectonic area 7) and the banks of the posterior-middle suprasylvian sulcus (pMS sulcal cortex) and in contact with the ventral-posterior suprasylvian (vPS) region of visuotemporal cortex. Bilateral deactivation of pMS sulcal cortex resulted in a profound impairment for all six tested positions of the landmark, yet bilateral deactivation of neither area 7 nor vPS cortex yielded any deficits. In control tasks (visual orienting and object discrimination), there was no evidence for any degree of attentional blindness or impairment of form discrimination during bilateral deactivation of pMS cortex. Therefore, we conclude that bilateral cooling of pMS cortex, but neither area 7 nor vPS cortex, induces a specific deficit in spatial localization as examined with the landmark task. These observations have significant bearing on our understanding of visuospatial processing in cat, monkey, and human cortices.

Gamma surgery for vein of Galen malformations

OBJECT

The goal of this study was to evaluate the results of gamma surgery in nine patients treated for vein of Galen malformations (VGMs).

METHODS

A consecutive series of nine VGMs in eight children aged 4 to 14 years and in one adult were treated with gamma surgery. Six of the patients were male, including the adult, and three were female. Among these patients there were three Yasargil Type I, one Type II, two Type III, and three Type IV malformations. Previous embolization had failed in four cases. Three VGMs were treated with gamma surgery twice. An additional patient with a Type III VGM underwent stereotactic angiography in preparation for gamma surgery but was judged to be suitable for direct embolization. Follow-up angiograms were obtained in eight of the VGMs treated. Four no longer filled; one has probably been obliterated, but this cannot be confirmed because the patient refused to undergo final angiography; one patient has residual fistulas not included in the initial treatment field, which were retreated recently; and two other patients have marked reduction of flow through their VGMs.

CONCLUSIONS

Gamma surgery is a viable option in the treatment of VGMs in clinically stable patients. Combined endovascular therapy and gamma surgery is of benefit in complex malformations.

Gamma surgery for intracranial metastases from renal cell carcinoma

OBJECT

The goal of this study was to evaluate the effectiveness and limitations of gamma surgery (GS) in the treatment of renal cell carcinoma that has metastasized to the brain.

METHODS

The authors performed a retrospective analysis of a consecutive series of 21 patients with 37 metastatic brain deposits from renal cell carcinoma who were treated with GS at the University of Virginia from 1990 to 1999. Clinical data were available in all patients. No patient died of progression of intracranial disease or deteriorated neurologically following GS. Eight patients clinically improved. Follow-up imaging studies were available for 23 tumors in 12 patients. Nine patients did not undergo follow-up imaging. One patient lived 17 months and succumbed to systemic disease: no brain imaging was performed in this case. Another patient refused further imaging and lived 7 months. Seven patients lived up to 4 months after the procedure; however, their physicians did not require these patients to undergo follow-up imaging examinations because of their general conditions-all had systemic progression of disease. Of the 23 tumors that were observed posttreatment, one remained unchanged in volume, 16 decreased in volume, and six disappeared. No tumor progressed at any time, and there were no radiation-induced changes on follow-up imaging an average of 21 months after GS (range 3-63 months).

CONCLUSIONS

Gamma surgery provides an alternative to surgical resection of metastatic brain deposits from renal cell carcinoma. Neurological side effects were seen in only one case; freedom from progression of disease was achieved in all cases.

On variability in the density of corticocortical and thalamocortical connections

Variability is an important but neglected aspect of connectional neuroanatomy. The quantitative density of the 'same' corticocortical or thalamocortical connection may vary by over two orders of magnitude between different injections of the same tracer. At present, however, the frequency distribution of connection densities is unknown. Therefore, it is unclear what kind of sampling strategies or statistical methods are appropriate for quantitative studies of connectivity. Nor is it clear if the measured variability represents differences between subjects, or if it is simply a consequence of intra-individual differences resulting from experimental technique and the exact placement of tracers relative to local spatial and laminar variation in connectivity. We used quantitative measurements of the density of a large number of corticocortical and thalamocortical connections from our own laboratories and from the literature. Variability in the density of given corticocortical and thalamocortical connections is high, with the standard deviation of density proportional to the mean. The frequency distribution is close to exponential. Therefore, analysis methods relying on the normal distribution are not appropriate. We provide an appendix that gives simple statistical guidance for samples drawn from exponentially distributed data. For a given corticocortical or thalamocortical connection density, between-individual standard deviation is 0.85 to 1.25 times the within-individual standard deviation. Therefore, much of the variability reported in conventional neuroanatomical studies (with one tracer deposited per animal) is due to within-individual factors. We also find that strong, but not weak, corticocortical connections are substantially more variable than thalamocortical connections. We propose that the near exponential distribution of connection densities is a simple consequence of 'patchy' connectivity. We anticipate that connection data will be well described by the negative binomial, a class of distribution that applies to events occurring in clumped or patchy substrates. Local patchiness may be a feature of all corticocortical connections and could explain why strong corticocortical connections are more variable than strong thalamocortical connections. This idea is supported by the columnar patterns of many corticocortical but few thalamocortical connections in the literature.

A method to assess the functional impact of cerebral connections on target populations of neurons

We describe an innovative and tested approach combining two individually potent techniques to visualize simultaneously the functional impact of multiple projections on target populations of neurons in the brain. The rationale is simple: silence a defined set of efferent projections from one cortical region using cooling deactivation and then measure the impact of the deactivation on activities in multiple target regions using 2-deoxyglucose (2DG). This is a straightforward and sound approach because 2DG uptake by neurons reflects levels of underlying neural activity. All distant modifications evoked by the silencing of the set of efferent projections are examined in anatomical tissue and simultaneously for the multiple target sites to provide a global view of the functional impacts of the set of projections on the targets. With this method, downward adjustments of 2DG uptake levels identify removals of net excitatory signals, whereas upward adjustments identify net removals of suppressive influences. Future possible uses and modifications of the technique, including optical imaging, are discussed. Overall, the technique has the potential to provide fundamental, new measures on cerebral network interactions that both complement and extend current static models of cerebral networks and electrophysiological measures of functional impacts on individual neurons.

The cryoloop: an adaptable reversible cooling deactivation method for behavioral or electrophysiological assessment of neural function

We describe a very adaptable reversible inactivation technique for the behavioral or electrophysiological analysis of neural circuits. The cryoloop device can be permanently implanted or topically applied in an acute preparation to apply cold to discrete surface regions of the central nervous system (e.g. cerebral cortex or midbrain). The cryoloop consists of a custom shaped, stainless steel, hypodermic tubing and cooling is effected by passing chilled methanol through the lumen of the tubing. Cryoloop temperature is monitored by a microthermocouple attached to the union of the loop, and can be maintained within +/- 1 degrees C of a desired temperature. In chronic preparations, implanted cryoloops have been maintained in cats and monkeys for periods in excess of 2 years. After this period there are no structural, metabolic of functional changes in the deactivated tissue, and full reversibility of cooling-induced effects is maintained. Operation of multiple cryoprobes provides great flexibility of experimental protocols, permits double and triple functional dissociations to be made, and strengthens experimental design considerably.

Cortical feedback improves discrimination between figure and background by V1, V2 and V3 neurons

A single visual stimulus activates neurons in many different cortical areas. A major challenge in cortical physiology is to understand how the neural activity in these numerous active zones leads to a unified percept of the visual scene. The anatomical basis for these interactions is the dense network of connections that link the visual areas. Within this network, feedforward connections transmit signals from lower-order areas such as V1 or V2 to higher-order areas. In addition, there is a dense web of feedback connections which, despite their anatomical prominence, remain functionally mysterious. Here we show, using reversible inactivation of a higher-order area (monkey area V5/MT), that feedback connections serve to amplify and focus activity of neurons in lower-order areas, and that they are important in the differentiation of figure from ground, particularly in the case of stimuli of low visibility. More specifically, we show that feedback connections facilitate responses to objects moving within the classical receptive field; enhance suppression evoked by background stimuli in the surrounding region; and have the strongest effects for stimuli of low salience.

Neuroplasticity in the cat's visual system. Origin, termination, expansion, and increased coupling of the retino-geniculo-middle suprasylvian visual pathway following early ablations of areas 17 and 18

We used anterograde and retrograde transsynaptic pathway tracing techniques to reveal the retinal origin and the cortical termination of the expanded retino-geniculo-middle suprasylvian (MS) cortex pathway in adult cats which sustained lesions of areas 17 and 18 on the day of birth (P1) or at 1 month of age (P28). Following anterograde transsynaptic transport of tritiated amino acids from the eye, four major results were obtained: (1) a strong and specific pathway from retina through dorsal lateral geniculate nucleus (dLGN) to the posterior half of MS cortex was identified; this pathway is a substantial expansion of an insignificant pathway present in intact cats; (2) the terminus of the pathway was lower layer III and layer IV; (3) contralateral projections were stronger than ipsilateral projections; (4) projections in P28 cats were stronger than those in P1 cats. Following retrograde transsynaptic transport of WGA-HRP from posterior MS cortex, four additional results were obtained: (1) the pathway was enlarged and visuotopically organized; (2) the pathway arose primarily from alpha- and gamma-retinal ganglion cells; (3) a small number of beta-cells in P1 cats and a modest number in P28 cats also contribute to the pathway; (4) the combined numbers of gamma- and beta-cells relative to alpha-cells was greater in temporal retina than in nasal retina. The combined demonstration of both origin and terminus of the pathway with transsynaptic tracers argued strongly for high levels of coupling between primary and secondary pathway limbs in both P1 and P28 cats. This level of coupling, as well as other features of the pathway, have much in common with the retino-geniculo-17/18 pathway of intact cats. However, the retino-geniculo-MS system in P1 cats transmits primarily Y and W signals, in P28 cats X, Y, and W signals; whereas the retino-geniculo-17/18 pathway transmits primarily X and Y signals. These results have implications for understanding the repercussions of early visual cortex lesions in monkeys and humans.

Dissociation of visual and auditory pattern discrimination functions within the cat's temporal cortex

In ablation-behavior experiments performed in adult cats, a double dissociation was demonstrated between ventral posterior suprasylvian cortex (vPS) and temporo-insular cortex (TI) lesions on complex visual and auditory tasks. Lesions of the vPS cortex resulted in deficits at visual pattern discrimination, but not at a difficult auditory discrimination. By contrast, TI lesions resulted in profound deficits at discriminating complex sounds, but not at discriminating visual patterns. This pattern of dissociation of deficits in cats parallels the dissociation of deficits after inferior temporal versus superior temporal lesions in monkeys and humans.

Functional impact of cerebral projection systems

Typically, anatomical connections have been traced by injecting pathway-tracing chemicals into restricted portions of the brain. After a few days, the brains are fixed and the transported chemicals identified in histological sections. Orthograde tracers move forward along axons from cell body to axon terminals and retrograde tracers move backwards along axons from terminals to parent cell body. The use of both types of tracers has revealed origins and terminations of pathways and a massively complex network of connections between numerous functionally and anatomically distinct cerebral and subcortical regions. In the monkey visual system alone more than 300 connections have been described between the 32 visual cortical areas. Even so, in the network descriptions neither anatomical strengths nor functional impacts of individual connections are identified. Yet, there is no doubt that knowledge about both aspects of connectivity is essential for developing accurate descriptions of network operations. We describe a new combination of a metabolic mapping and a reversible deactivation technique in an animal model to assess the functional impact of cerebral connections.

Functional impact of cerebral connections

Cerebral networks are complex sets of connections that resemble a ladder-like web of multiple parallel feedforward, lateral, and feedback connections. This static anatomical description has been pivotal in guiding our understanding of signal processing within cerebral networks. However, measures on both magnitude and functional significance of connections are extremely limited. Here, we compare the anatomically defined strengths of a set of cerebral pathways emerging from the visual middle suprasylvian (MS) cortex of the cat with measures of the functional impact the same region has over distant sites. These functional measures were obtained by analyzing the local and distant effects of MS cooling deactivation on deoxyglucose uptake. Relative to major efferent projections from MS cortex that have a strong influence, projections to early visual processing stages have weaker functional influences than predicted from the anatomy. For higher processing stages, the converse holds: projections from MS cortex have stronger functional influence than predicted from the anatomy. We conclude that these and future functional measures, obtained using the same combination of techniques, will furnish fundamental, new information that complements and extends current models of static cerebral networks, and lead to more realistic models of cerebral network function and component interactions.

Thalamic and cortical projections to middle suprasylvian cortex of cats: constancy and variation

We investigated the constancy and variability in the numbers of thalamic and cortical neurons projecting to cat middle suprasylvian (MS) visual cortex. Retrograde pathway tracers were injected at a single anatomically and physiologically defined locus in MS cortex. Counts of labeled neurons showed that the visual thalamic projections to MS cortex consistently arose from a fixed set of nuclei in relatively constant proportions. In contrast, counts of cortical neurons revealed that transcortical inputs to MS cortex were much more variable. This differential variability may be linked to the developmental program, which affords greater influence of experiential factors on cortical pathway development than on thalamocortical pathway development. These results have implications for the development of models of cerebral connectivity that include measures of pathway variability.

Transgeniculate signal transmission to middle suprasylvian cortex in intact cats and following early removal of areas 17 and 18: a morphological study

Removal of cat areas 17 and 18 early, but not late, in postnatal development results in the sparing of certain reflexive and nonreflexive visually guided behaviors. These spared behaviors are accompanied by an expansion of geniculocortical projections to middle suprasylvian (MS) cortex. However, little is known about the types of visual signals relayed along these pathways. The purpose of our study was to reveal the morphologies of the neurons participating in the rewired circuits and, by relating them to the morphologies of functionally characterized neurons described by others, infer the types of visual signals transmitted via the lateral geniculate nucleus (LGN) to MS cortex. To do this, we retrogradely labeled LGN neurons from MS cortex with fluorescent microspheres, and subsequently intracellularly filled them with Lucifer Yellow. We then classified well-filled neurons according to a battery of morphological parameters (such as soma size and shape, and dendritic field-form and specializations), and compared them with already defined structure/function relationships. By doing this, we found that the large majority of visual thalamic relay neurons to MS cortex of both normal cats and cats that incurred removal of areas 17 and 18 were types I and IV. These results indicate that visual Y and W signals, respectively, are relayed directly from LGN to MS cortex in both types of cats. Following the early lesions, some of the MS-projecting type I neurons were found in layers A and A1, where they are never found in intact cats. Thus, some layer A and A1 type I neurons redirect axons to MS cortex following early removal of areas 17 and 18. For the type IV MS-projecting neurons in early lesioned cats, the somas were hypertrophied and they had more profuse and broader dendritic arbors than equivalent neurons in intact cats. These results suggest that dynamic interactions take place between inputs and outputs of LGN neurons during development that influence final LGN neuron morphology. Moreover, they suggest that signals transferred to MS cortex by type IV neurons may be modified by early lesions of areas 17 and 18. Overall, these results contribute to our understanding of the types of behaviors that may be spared by early lesions of areas 17 and 18.

Reversible deactivation of cerebral network components

Reversible deactivation techniques have shown that the cerebral network: (1) is dynamic, its functions depending on contemporaneous processing elsewhere in the network; (2) is composed of single nodes that contribute to several behaviors; (3) possesses an inherent plasticity that tends to minimize lesion-induced deficits; and (4) comprises feedforward and lateral connections that contribute in different ways to network operations. The next major advances in understanding network operations will probably be made by applying a combination of behavioral, neuron-recording and deactivation techniques. The greatest near-term gains are likely to be made in understanding the contributions that feedback projections make to cerebral network function.

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.

Perceptual and cognitive visual functions of parietal and temporal cortices in the cat

We used reversible cooling deactivation to compare the functions of cortices lining the middle suprasylvian (MS) sulcus and forming the ventral portion of the posterior suprasylvian (vPS) gyrus. A battery of attentional, motion and mnemonic processing tasks were used and performance was examined during deactivation of each region. The results show a clear dissociation of functions. Deactivation of MS cortex resulted in profound deficits on a visual orienting task and on the discrimination of direction of motion, whereas deactivation of vPS cortex severely impaired both retention and learning of novel and overlearned object discriminations. In addition, deactivation of either MS or vPS cortex impaired discrimination of learned patterns when components of the patterns were in motion, whereas only deactivation of vPS cortex impaired the discrimination when all components were static. Together, these results show that a region of parietal cortex contributes to the processing of visual motion and to attentional processes, whereas a region of temporal cortex contributes to the learning and recognition of three-dimensional objects and two-dimensional patterns. This functional dissociation is linked to differences in underlying visual pathways, which have many features in common with the parietal and temporal visual processing streams previously identified in monkeys and humans. Furthermore, the broad similarity in neural operations carried out in parietal and temporal cortices of cats, monkeys and humans suggests the existence of a common plan for cortical processing machinery within mammals with well developed cerebral cortices.

Age dependent modification of cytochrome oxidase activity in the cat dorsal lateral geniculate nucleus following removal of primary visual cortex

The purpose of the present study was to assess changes in the levels of cytochrome oxidase (CO) activity in the dorsal lateral geniculate nucleus (dLGN) of the adult cat following removal of primary visual cortical areas 17 and 18 on the day of birth (PI), P28, or in adulthood (> or = 6 months). Cytochrome oxidase activity was measured in histological sections 9 or more months after the cortical ablation. Control measures obtained from intact cats show that CO activity is normally highest in the A-laminae of dLGN, and slightly lower in the C-complex. Following visual cortex ablations incurred at any age, CO activity levels are reduced in the A-laminae. This reduction is most profound following ablations incurred on P28 or in adulthood. In contrast, CO activity in the C-complex of dLGN is at nearly normal levels following ablations on P1 or P28, but not in adulthood. These findings contribute to our understanding of the role played by the dLGN in the transfer of visual signals along retino-geniculo-extrastriate pathways that expand following early removal of areas 17 and 18. Moreover, they have implications for our understanding of spared behavioral functions attributed to the extrastriate cortex in cats which incurred early damage of areas 17 and 18.

Evidence for greater sight in blindsight following damage of primary visual cortex early in life

This review compares the behavioral, physiological and anatomical repercussions of lesions of primary visual cortex incurred by developing and mature humans, monkey and cats. Comparison of the data on the repercussions following lesions incurred earlier or later in life suggests that earlier, but not later, damage unmasks a latent flexibility of the brain to compensate partially for functions normally attributed to the damaged cortex. The compensations are best documented in the cat and they can be linked to system-wide repercussions that include selected pathway expansions and neuron degenerations, and functional adjustments in neuronal activity. Even though evidence from humans and monkeys is extremely limited, it is argued on the basis of known repercussions and similarity of visual system organization and developmental sequence, that broadly equivalent repercussions most likely occur in humans and monkeys following early lesions of primary visual cortex. The extant data suggest potentially useful directions for future investigations on functional anatomical aspects of visual capacities spared in human patients and monkeys following early damage of primary visual cortex. Such research is likely to have a substantial impact on increasing our understanding of the repercussions that result from damage elsewhere in the developing cerebral cortex and it is likely to contribute to our understanding of the remarkable ability of the human brain to adapt to insults.

Increased oxidative metabolism in middle suprasylvian cortex following removal of areas 17 and 18 from newborn cats

We measured changes in metabolic activity in middle suprasylvian (MS) cortex of cats subjected to early or late removal of areas 17 and 18 to localize shifts in activity possibly indicative of regions within MS cortex that may receive expanded inputs and be involved in the sparing of some visual behaviors following early primary visual cortex damage. Cytochrome oxidase (CO) activity was measured in MS cortex of mature, intact cats and of others with areas 17 and 18 removed in adulthood (P180), or on postnatal day 28 (P28) or postnatal day 1 (P1). Not less than 9 months after the ablation, brain sections were prepared and reacted for the presence of CO. The density of CO reactivity in each of the six cortical layers in MS cortex was measured and standardized against densities from ventral periaqueductal gray or hypothalamus on the same section. Following lesions on P1, significant increases in CO activity occurred in deep layer III and in layer IV of the medial bank of the MS sulcus, including all of area PMLS and the posterior portion of AMLS. In contrast, there were no significant differences in the level of CO activity among P28, P180, or intact cats for any of the cortical layers, and all had lower levels than the P1 cats. This metabolic change provides an anatomical marker for localizing adjustments in MS cortex and can be linked to amplified projections into MS cortex from the thalamus (LPm and A and C laminae of the dorsal lateral geniculate nucleus) and ventral posterior suprasylvian cortex following P1 ablations. Furthermore, this neurochemical analysis implicates a distinct region of MS cortex as the cortical locus of some spared visual functions following early primary visual cortex damage.

Rewiring of transcortical projections to middle suprasylvian cortex following early removal of cat areas 17 and 18

Retrograde tracers were injected into middle suprasylvian (MS) cortex of two groups of experimental adult cats that had incurred removal of visual areas 17 and 18 on either the day of birth (P1), or at 1 month of age (P28). Tracers were also injected into the same region of intact and adult ablated control cats. The locations and numbers of labeled neurons in the experimental and control groups were compared. Following lesions on P1, but at no other age, increased numbers of neurons projected to MS cortex. Virtually all of the additional neurons were located in the superficial layers of the ventral posterior suprasylvian (vPS) cortex. These results demonstrated that (1) neurons with ipsilateral transcortical axons have the potential to reconfigure their projections after early, localized cortical damage elsewhere in the cortex of the same hemisphere; (2) this reconfiguration involves expansion of specific projections and is not a generalized capacity of all cortical neurons; (3) the expansion is modality specific; and finally, (4) the ability of cortical neurons to reorganize projections is limited in time. The expanded projection from vPS to MS cortex may contribute to neuronal compensations and the sparing of visually guided behaviors previously demonstrated in cats with neonatal visual cortex damage, and is a testament to the latent capacities immature cerebral cortex neurons possess to establish new projections following restricted damage to the cerebral cortex early in life.

Learning and recall of form discriminations during reversible cooling deactivation of ventral-posterior suprasylvian cortex in the cat

Extrastriate visual cortex of the ventral-posterior suprasylvian gyrus (vPS cortex) of freely behaving cats was reversibly deactivated with cooling to determine its role in performance on a battery of simple or masked two-dimensional pattern discriminations, and three-dimensional object discriminations. Deactivation of vPS cortex by cooling profoundly impaired the ability of the cats to recall the difference between all previously learned pattern and object discriminations. However, the cats' ability to learn or relearn pattern and object discriminations while vPS was deactivated depended upon the nature of the pattern or object and the cats' prior level of exposure to them. During cooling of vPS cortex, the cats could neither learn the novel object discriminations nor relearn a highly familiar masked or partially occluded pattern discrimination, although they could relearn both the highly familiar object and simple pattern discriminations. These cooling-induced deficits resemble those induced by cooling of the topologically equivalent inferotemporal cortex of monkeys and provides evidence that the equivalent regions contribute to visual processing in similar ways.

Reversible visual hemineglect

We have identified a limited region in the posterior, but not anterior, half of the cat's middle suprasylvian region which, when cooled and inactivated unilaterally, results in a profound visual neglect of stimuli introduced into the contracooled hemifield. The severity of the deficit matches that induced by unilateral cooling of the superior colliculus. The cortical region is located at the temporo-occipito-parietal junction and is believed to be equivalent to a region centered on or close to the area V5 complex of primates.

Amplification of thalamic projections to middle suprasylvian cortex following ablation of immature primary visual cortex in the cat

The purpose of the present study was to identify expansions in thalamic projections to middle suprasylvian (MS) cortex that could be linked to the sparing of visually guided behaviors that follow the removal of visual cortex early in postnatal life. Injections of retrograde tracers were made into the medial bank of the middle suprasylvian sulcus in intact, adult cats and in adult cats that had incurred ablations of areas 17 and 18 on the day of birth (P1), P28, or > or = 6 months of age, and the numbers of labeled neurons in the thalamus were counted. In the thalamus of the intact cat, the greatest number of labeled neurons are located in the lateral division of the lateral posterior nucleus and there are intermediate numbers in the medial division of the lateral posterior nucleus (LPm); and smaller numbers within the medial interlaminar nucleus, the C-complex of the dorsal lateral geniculate nucleus (dLGN), the geniculate wing, and the pulvinar nucleus. Following the removal of areas 17 and 18 at different ages, thalamic projections to MS cortex exhibit an age-dependent reorganization. Removals on P1, induce twice the normal number of neurons in LPm and three times the normal number of neurons in the C-complex of dLGN to project to MS cortex. Removals on P28 induce five times the normal number of neurons in the C-complex to project to MS cortex. In addition, removals at both ages resulted in projections from the A-laminae to MS cortex becoming permanently established. No changes in the pattern or number of neurons that project to MS cortex were detected when areas 17 and 18 were removed in adulthood. These results show that pathways through the C-complex of dLGN and through LPm expand substantially following ablation of immature areas 17 and 18. These expanded pathways are linked to Y- and W-functional streams of visual signals that are relayed from the retina to extrastriate cortex either directly through dLGN or indirectly via the superior colliculus and LPm. These signals may be critical for the sparing of neural operations following ablation of areas 17 and 18 early in development.

Neuronal interactions in cat visual cortex mediated by the corpus callosum

The article summarizes three sets of physiological and anatomical studies carried out to investigate the structural basis of the functional interactions between visual cortical areas 17 and 18 in the two cerebral hemispheres of cats. (1) The visual field representations in the transcallosal sending and receiving zones are defined. (2) The consequences of severing callosal fibers on the visual field representation at the area 17/18 border are described. (3) Lastly, experiments using cooling to reversibly inactive transcallosal inputs are reported. The observations on the transcallosal sending and receiving zones show that callosal connections of area 17 are concerned with a vertical hour-glass shaped region of the visual field centered on the midline, and this region is doubly represented, once in each hemisphere. The zone represents azimuths within +/- 4 degrees of the midline at the 0 degree horizontal meridian, and azimuths out to +/- 15 to +/- 25 degrees at positions distant from the horizontal meridian. The observations suggest that, in addition to interactions between neurons concerned with positions immediately adjacent to the midline, there are positions, especially high and low in the visual field, where interactions can occur between neurons which have receptive fields displaced some distance from the midline. The extent of this double representation is reduced by approximately 2/3 when the corpus callosum is cut. The retention of some bilateral representation in these animals suggests that there are alternate routes for across-the-midline transmission of visual signals. Or, more likely, there are ganglion cells in temporal retina with crossed projections that make significant contributions to the remaining double representation of the visual field. Lastly, the results obtained using cooling inactivation of transcallosal fibers show that many excitatory and inhibitory circuits are under the direct control of transcallosal fibers in the normally functioning brain. These connections appear to be no different from intrinsic connections of area 17, and they undoubtedly contribute to the binding of the two half-field representations, one in each hemisphere, and perceptual unity across the midline.

The spatial relationship between the cerebral cortex and fiber trajectory through the corpus callosum of the cat

We related fiber trajectory through the feline corpus callosum to the site of fiber origin in the cortical mantle and to functional modality. The cortical fields which contribute axons to the different portions of the corpus callosum were revealed by applying horseradish peroxidase (HRP) to the cut ends of selected groups of callosal axons in twelve adult cats. Overall, the application of HRP at progressively more caudal positions in the corpus callosum labels fields of neurons at successively more caudal positions in the cerebral cortex. Comparison of these data to functionally distinct cortical zones shows that the callosal body conveys a mixture of fibers arising from functionally diverse regions of the cerebrum, whereas portions of the rostral and caudal ends appear to be essentially unimodal, conveying motor and visual signals, respectively.

A numerical analysis of the geniculocortical input to striate cortex in the monkey

Using data that are available in various publications, a quantitative analysis has been made of the geniculocortical input to layer IVC of the macaque striate cortex. The data suggest that only 1.3-1.9% of the excitatory, or asymmetric synapses in layer IVC alpha of striate cortex are provided by the neurons of the magnocellular layers of the LGN. This amounts to only 18-40 of the 1000-2100 asymmetric synapses that the average layer IVC alpha neuron receives. The parvicellular afferents to layer IVC beta, on the other hand, provide 3.7-8.7% of the asymmetric synapses formed by the average layer IVC beta neuron, or 37-191 synapses to each neuron. If it is assumed that the boutons in the geniculocortical axonal plexuses are evenly spread, it can be calculated that the input to an individual layer IVC neuron is provided by some 24 axonal plexuses. This is regardless of whether the neuron lies in layer IVC alpha or in IVC beta. This calculation suggests that a single axonal plexus provides not more than one or two of the excitatory synapses received by an individual layer IVC alpha neuron, and between one and eight excitatory synapses for a layer IVC beta neuron. Consequently, it is unlikely that the response properties of a particular cortical neuron are dominated by its input from a single geniculate neuron. Since the geniculocortical input essentially determines the response properties of neurons in layer IV of macaque striate cortex, it is surprising that this input amounts to such a small number of synapses to an individual neuron, although we obtained a somewhat similar result in our earlier quantitative analysis of the geniculate input to the striate cortex of the cat (Peters and Payne, 1993). But it has to be questioned whether the low values obtained are correct. Interestingly, the geniculocortical input to cortex has been largely neglected in favor of analyses of intracortical circuitry, but in view of the basic importance of this afferent input, it is suggested that more quantitative data about it should be generated, so that a better assessment can be made of its extent.

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.

System-wide repercussions of damage to the immature visual cortex

Damage of the primary visual cortex in mammals, including humans, severely disrupts vision by disconnecting much of the cognitive-processing machinery of extrastriate cortex from its source of visual signals in the retina. Studies of the anatomical consequences of damage to the immature primary visual cortex in cats reveal system-wide repercussions on neural circuitry that includes the retina, thalamus, midbrain and extrastriate cortex. The repercussions modify circuits that support relatively normal signal processing and the sparing of certain visually guided behaviors such as aspects of complex-pattern recognition and orienting to novel stimuli introduced into the visual field. These studies have implications for understanding the consequences of damage to the visual cortex in infant monkeys and humans, and for devising therapeutic strategies to attenuate defects in vision induced by cortical lesions.

Expansion of suprasylvian cortex projection in the superficial layers of the superior colliculus following damage of areas 17 and 18 in developing cats

Tritiated proline and leucine were injected into areas 17 and 18 of intact cats and into the medial bank of the lateral suprasylvian (LS) cortex of intact cats and cats from which areas 17 and 18 had been removed on postnatal day 1 (P1), P28, or in adulthood (A). The density of label transported to the superior colliculus was quantified using image-analysis equipment. The results from the intact cats confirmed previous reports that areas 17 and 18 project most heavily to stratum zonale (SZ) and stratum griseum superficiale (SGS) and LS cortex projects most heavily to stratum opticum (SO) of the superior colliculus. However, in cats with lesions of areas 17 and 18, the projections from LS cortex showed an age-dependent reorganization. LS projections to SGS and SZ were enhanced following ablation of areas 17 and 18 on P1, and projections to SGS were enhanced following an ablation on P28. The pattern of LS-collicular projection following ablations incurred in adulthood was indistinguishable from the pattern presented by intact cats. This study demonstrates that the LS corticocollicular projection expands in SGS and possibly substitutes for inputs eliminated by the removal of areas 17 and 18 from the immature brain. This enhanced pathway may contribute to compensatory neuronal changes and to spared behaviors that accompany damage of immature cortex.

Microglial invasion and activation in response to naturally occurring neuronal degeneration in the ganglion cell layer of the postnatal cat retina

Retinae of kittens between postnatal (P) days 2 and 10 were examined for the presence of degenerating neuronal profiles, normal nucleoli and microglia. Comparison of the numbers of degenerating profiles with numbers of axons lost from the optic nerve suggest that the majority of these profiles result from the degeneration of retinal ganglion cells. Analysis of local densities of the different profiles revealed different rates of cell loss, occurring at different times in central and peripheral retina. The period of rapid cell loss occurred between P2 and P3 in central retina compared to between P8 and P10 in peripheral retina. At both locations, these periods of rapid cell loss were accompanied by a decrease in the ratio of microglia to dying cells even though the absolute densities of microglia increased. However, calculation of the clearance times of cellular debris indicate that the speed of removal of degeneration products is greater during rapid cell loss, which suggests that cellular degeneration serves to activate the phagocytic process.

Capacity of the retinogeniculate pathway to reorganize following ablation of visual cortical areas in developing and mature cats

The purpose of the present study was to determine the pattern and density of retinal projections to the dorsal lateral geniculate nucleus (dLGN) following ablation of visual cortical areas in developing cats of different postnatal ages and in mature cats. The terminations of retinal projections to the dLGN were evaluated following the injection of tritiated amino acids into one eye. Regardless of age, a visual cortical ablation of areas 17 and 18 induces massive death of neurons within the regions of the dLGN that are linked topographically to the cortical areas removed. However, the pattern of retinal projections to these degenerated regions of the dLGN differs depending upon whether the cortical lesion is incurred early in postnatal life or in adulthood. Following ablation on the day of birth (P1), virtually all surviving cells were found in the C-complex of dLGN with only a token number in the A-laminae. Correspondingly, retinal projections were maintained to the C-complex of the nucleus and were barely detectable in the degenerated A-laminae. However, in cats in which areas 17 and 18 had been removed in adulthood (> or = 6 months of age) retinal projections were maintained to the A-laminae even though nearly all neurons in those laminae had degenerated. Moreover, a subgroup of animals that incurred area 17 and 18 ablations at P1 showed that the modification of retinal projections to the A-laminae occurs within the first postnatal month, and an additional subgroup showed that retinal projections become increasingly resistant to the degenerative events in the dLGN that follow ablation of areas 17 and 18 at progressively older ages during the first postnatal month. Furthermore, retinal inputs also respond, in an age-dependent way, to degeneration of neurons in the C-complex induced by extension of the cortical ablation to include extrastriate visual areas.

Direction selectivity and physiological compensation in the superior colliculus following removal of areas 17 and 18

Previous studies indicate that cortical areas 17 and 18 play a prominent role in generating the direction selectivities of neurons in the superior colliculus of the cat. This hypothesis was tested by quantifying the activities of neurons in the superficial collicular layers in intact cats and cats which incurred ablation of areas 17 and 18 and part of area 19. In addition, since behavioral and anatomical studies suggest a functional adjustment in the superior colliculus following removal of inputs from areas 17, 18, and 19 in the neonatal cat, we included a group of neonatally lesioned cats. Computation of an index of directionality indicated that the majority of neurons in intact cats preferred movement in one direction, thus confirming reports of others. Following ablation of areas 17 and 18 and part of area 19 in both groups of lesioned cats, only modest changes in the population indices were detected when poorly responsive neurons were eliminated from the analyses. Based upon levels of visually evoked neuronal activity, our data suggest a physiological compensation by neurons in stratum griseum superficiale following removal of areas 17, 18, and 19 inputs. In the intact and neonatally operated groups, activity in stratum griseum superficiale is high, whereas in the adult lesioned group activity is low. In stratum opticum, neuronal activity was similar in all three groups of cats. These results show that neurons in stratum griseum superficiale undergo a physiological compensation following removal of immature areas 17 and 18.

Visual cortex damage-induced growth of retinal axons into the lateral posterior nucleus of the cat

Ablation of visual cortical areas 17 and 18 in neonatal and young adult cats induces novel retinal projections to terminate bilaterally in the lateral posterior nucleus (LP) at a position ventromedial from the medial interlaminar nucleus. Comparison with the visual-field maps of LP indicate that the terminations are focussed on the representation of the visual-field center.