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

Effects of Cortical Cooling on Activity Across Layers of the Rat Barrel Cortex

Moderate cortical cooling is known to suppress slow oscillations and to evoke persistent cortical activity. However, the cooling-induced changes in electrical activity across cortical layers remain largely unknown. Here, we performed multi-channel local field potential (LFP) and multi-unit activity (MUA) recordings with linear silicone probes through the layers of single cortical barrel columns in urethane-anesthetized rats under normothermia (38°C) and during local cortical surface cooling (30°C). During cortically generated slow oscillations, moderate cortical cooling decreased delta wave amplitude, delta-wave occurrence, the duration of silent states, and delta wave-locked MUA synchronization. Moderate cortical cooling increased total time spent in the active state and decreased total time spent in the silent state. Cooling-evoked changes in the MUA firing rate in cortical layer 5 (L5) varied from increase to decrease across animals, and the polarity of changes in L5 MUA correlated with changes in total time spent in the active state. The decrease in temperature reduced MUA firing rates in all other cortical layers. Sensory-evoked MUA responses also decreased during cooling through all cortical layers. The cooling-dependent slowdown was detected at the fast time-scale with a decreased frequency of sensory-evoked high-frequency oscillations (HFO). Thus, moderate cortical cooling suppresses slow oscillations and desynchronizes neuronal activity through all cortical layers, and is associated with reduced firing across all cortical layers except L5, where cooling induces variable and non-consistent changes in neuronal firing, which are common features of the transition from slow-wave synchronization to desynchronized activity in the barrel cortex.

Parietal Cortex Regulates Visual Salience and Salience-Driven Behavior

Unique stimuli stand out. Despite an abundance of competing sensory stimuli, the detection of the most salient ones occurs without effort, and that detection contributes to the guidance of adaptive behavior. Neurons sensitive to the salience of visual stimuli are widespread throughout the primate visual system and are thought to shape the selection of visual targets. However, a neural source of salience remains elusive. In an attempt to identify a source of visual salience, we reversibly inactivated parietal cortex and simultaneously recorded salience signals in prefrontal cortex. Inactivation of parietal cortex not only caused pronounced and selective reductions of salience signals in prefrontal cortex but also diminished the influence of salience on visually guided behavior. These observations demonstrate a causal role of parietal cortex in regulating salience signals within the brain and in controlling salience-driven behavior.

Macaque anterior cingulate cortex deactivation impairs performance and alters lateral prefrontal oscillatory activities in a rule-switching task

In primates, both the dorsal anterior cingulate cortex (dACC) and the dorsolateral prefrontal cortex (dlPFC) are key regions of the frontoparietal cognitive control network. To study the role of the dACC and its communication with the dlPFC in cognitive control, we recorded local field potentials (LFPs) from the dlPFC before and during the reversible deactivation of the dACC, in macaque monkeys engaging in uncued switches between 2 stimulus-response rules, namely prosaccade and antisaccade. Cryogenic dACC deactivation impaired response accuracy during maintenance of—but not the initial switching to—the cognitively demanding antisaccade rule, which coincided with a reduction in task-related theta activity and the correct-error (C-E) difference in dlPFC beta-band power. During both rule switching and maintenance, dACC deactivation prolonged the animals’ reaction time and reduced task-related alpha power in the dlPFC. Our findings support a role of the dACC in prefrontal oscillatory activities that are involved the maintenance of a new, challenging task rule.

Reversible deactivation of the dorsal anterior cingulate cortex — an area of the cognitive control network — impairs rule maintenance but not rule switching per se, and disrupts task-related oscillatory activities in the dorsolateral prefrontal cortex — another area of the same network.

Perturbation-driven paradoxical facilitation of visuo-spatial function: Revisiting the 'Sprague effect'

The 'Sprague Effect' described in the seminal paper of James Sprague (Science 153:1544-1547, 1966a) is an unexpected paradoxical effect in which a second brain lesion reversed functional deficits induced by an earlier lesion. It was observed initially in the cat where severe and permanent contralateral visually guided attentional deficits generated by the ablation of large areas of the visual cortex were reversed by the subsequent removal of the superior colliculus (SC) opposite to the cortical lesion or by the splitting of the collicular commissure. Physiologically, this effect has been explained in several ways-most notably by the reduction of the functional inhibition of the ipsilateral SC by the contralateral SC, and the restoration of normal interactions between cortical and midbrain structures after ablation. In the present review, we aim at reappraising the 'Sprague Effect' by critically analyzing studies that have been conducted in the feline and human brain. Moreover, we assess applications of the 'Sprague Effect' in the rehabilitation of visually guided attentional impairments by using non-invasive therapeutic approaches such as transcranial magnetic stimulation (TMS) and transcranial direct-current stimulation (tDCS). We also review theoretical models of the effect that emphasize the inhibition and balancing between the two hemispheres and show implications for lesion inference approaches. Last, we critically review whether the resulting inter-hemispheric rivalry theories lead toward an efficient rehabilitation of stroke in humans. We conclude by emphasizing key challenges in the field of 'Sprague Effect' applications in order to design better therapies for brain-damaged patients.

Inactivation of Medial Frontal Cortex Changes Risk Preference

Humans and other animals need to make decisions under varying degrees of uncertainty. These decisions are strongly influenced by an individual's risk preference; however, the neuronal circuitry by which risk preference shapes choice is still unclear [1]. Supplementary eye field (SEF), an oculomotor area within primate medial frontal cortex, is thought to be an essential part of the neuronal circuit underlying oculomotor decision making, including decisions under risk [2-5]. Consistent with this view, risk-related action value and monitoring signals have been observed in SEF [6-8]. However, such activity has also been observed in other frontal areas, including orbitofrontal [9-11], cingulate [12-14], and dorsal-lateral frontal cortex [15]. It is thus unknown whether the activity in SEF causally contributes to risky decisions, or whether it is merely a reflection of neural processes in other cortical regions. Here, we tested a causal role of SEF in risky oculomotor choices. We found that SEF inactivation strongly reduced the frequency of risky choices. This reduction was largely due to a reduced attraction to reward uncertainty and high reward gain, but not due to changes in the subjective estimation of reward probability or average expected reward. Moreover, SEF inactivation also led to increased sensitivity to differences between expected and actual reward during free choice. Nevertheless, it did not affect adjustments of decisions based on reward history.

Reversible Cooling-induced Deactivations to Study Cortical Contributions to Obstacle Memory in the Walking Cat

On complex, naturalistic terrain, sensory information about an environmental obstacle can be used to rapidly adjust locomotor movements for avoidance. For example, in the cat, visual information about an impending obstacle can modulate stepping for avoidance. Locomotor adaptation can also occur independent of vision, as sudden tactile inputs to the leg by an expected obstacle can modify the stepping of all four legs for avoidance. Such complex locomotor coordination involves supraspinal structures, such as the parietal cortex. This protocol describes the use of reversible, cooling-induced cortical deactivation to assess parietal cortex contributions to memory-guided obstacle locomotion in the cat. Small cooling loops, known as cryoloops, are specially shaped to deactivate discrete regions of interest to assess their contributions to an overt behavior. Such methods have been used to elucidate the role of parietal area 5 in memory-guided obstacle avoidance in the cat.

Effects of neonatal deafness on resting-state functional network connectivity

Normal brain development depends on early sensory experience. Behavioral consequences of brain maturation in the absence of sensory input early in life are well documented. For example, experiments with mature, neonatally deaf human or animal subjects have revealed improved peripheral visual motion detection and spatial localization abilities. Such supranormal behavioral abilities in the nondeprived sensory modality are evidence of compensatory plasticity occurring in deprived brain regions at some point or throughout development. Sensory deprived brain regions may simply become unused neural real-estate resulting in a loss of function. Compensatory plasticity and loss of function are likely reflected in the differences in correlations between brain networks in deaf compared with hearing subjects. To address this, we used resting-state functional magnetic resonance imaging (fMRI) in lightly anesthetized hearing and neonatally deafened cats. Group independent component analysis (ICA) was used to identify 20 spatially distinct brain networks across all animals including auditory, visual, somatosensory, cingulate, insular, cerebellar, and subcortical networks. The resulting group ICA components were back-reconstructed to individual animal brains. The maximum correlations between the time-courses associated with each spatial component were computed using functional network connectivity (FNC). While no significant differences in the delay to peak correlations were identified between hearing and deaf cats, we observed 10 (of 190) significant differences in the amplitudes of between-network correlations. Six of the significant differences involved auditory-related networks and four involved visual, cingulate, or somatosensory networks. The results are discussed in context of known behavioral, electrophysiological, and anatomical differences following neonatal deafness. Furthermore, these results identify novel targets for future investigations at the neuronal level.

Acute Inactivation of Primary Auditory Cortex Causes a Sound Localisation Deficit in Ferrets

The objective of this study was to demonstrate the efficacy of acute inactivation of brain areas by cooling in the behaving ferret and to demonstrate that cooling auditory cortex produced a localisation deficit that was specific to auditory stimuli. The effect of cooling on neural activity was measured in anesthetized ferret cortex. The behavioural effect of cooling was determined in a benchmark sound localisation task in which inactivation of primary auditory cortex (A1) is known to impair performance. Cooling strongly suppressed the spontaneous and stimulus-evoked firing rates of cortical neurons when the cooling loop was held at temperatures below 10°C, and this suppression was reversed when the cortical temperature recovered. Cooling of ferret auditory cortex during behavioural testing impaired sound localisation performance, with unilateral cooling producing selective deficits in the hemifield contralateral to cooling, and bilateral cooling producing deficits on both sides of space. The deficit in sound localisation induced by inactivation of A1 was not caused by motivational or locomotor changes since inactivation of A1 did not affect localisation of visual stimuli in the same context.

A quantitative comparison of the hemispheric, areal, and laminar origins of sensory and motor cortical projections to the superior colliculus of the cat

The superior colliculus (SC) is a midbrain structure central to orienting behaviors. The organization of descending projections from sensory cortices to the SC has garnered much attention; however, rarely have projections from multiple modalities been quantified and contrasted, allowing for meaningful conclusions within a single species. Here, we examine corticotectal projections from visual, auditory, somatosensory, motor, and limbic cortices via retrograde pathway tracers injected throughout the superficial and deep layers of the cat SC. As anticipated, the majority of cortical inputs to the SC originate in the visual cortex. In fact, each field implicated in visual orienting behavior makes a substantial projection. Conversely, only one area of the auditory orienting system, the auditory field of the anterior ectosylvian sulcus (fAES), and no area involved in somatosensory orienting, shows significant corticotectal inputs. Although small relative to visual inputs, the projection from the fAES is of particular interest, as it represents the only bilateral cortical input to the SC. This detailed, quantitative study allows for comparison across modalities in an animal that serves as a useful model for both auditory and visual perception. Moreover, the differences in patterns of corticotectal projections between modalities inform the ways in which orienting systems are modulated by cortical feedback. J. Comp. Neurol. 524:2623-2642, 2016. © 2016 Wiley Periodicals, Inc.

Effects of core auditory cortex deactivation on neuronal response to simple and complex acoustic signals in the contralateral anterior auditory field

Interhemispheric communication has been implicated in various functions of sensory signal processing and perception. Despite ample evidence demonstrating this phenomenon in the visual and somatosensory systems, to date, limited functional assessment of transcallosal transmission during periods of acoustic signal exposure has hindered our understanding of the role of interhemispheric connections between auditory cortical fields. Consequently, the present investigation examines the impact of core auditory cortical field deactivation on response properties of contralateral anterior auditory field (AAF) neurons in the felis catus. Single-unit responses to simple and complex acoustic signals were measured across AAF before, during, and after individual and combined cooling deactivation of contralateral primary auditory cortex (A1) and AAF neurons. Data analyses revealed that on average: 1) interhemispheric projections from core auditory areas to contralateral AAF neurons are predominantly excitatory, 2) changes in response strength vary based on acoustic features, 3) A1 and AAF projections can modulate AAF activity differently, 4) decreases in response strength are not specific to particular cortical laminae, and 5) contralateral inputs modulate AAF neuronal response thresholds. Collectively, these observations demonstrate that A1 and AAF neurons predominantly modulate AAF response properties via excitatory projections.

The visual callosal connection: a connection like any other?

Recent work about the role of visual callosal connections in ferrets and cats is reviewed, and morphological and functional homologies between the lateral intrinsic and callosal network in early visual areas are discussed. Both networks selectively link distributed neuronal groups with similar response properties, and the actions exerted by callosal input reflect the functional topography of those networks. This supports the notion that callosal connections perpetuate the function of the lateral intrahemispheric circuit onto the other hemisphere. Reversible deactivation studies indicate that the main action of visual callosal input is a multiplicative shift of responses rather than a changing response selectivity. Both the gain of that action and its excitatory-inhibitory balance seem to be dynamically adapted to the feedforward drive by the visual stimulus onto primary visual cortex. Taken together anatomical and functional evidence from corticocortical and lateral circuits further leads to the conclusion that visual callosal connections share more features with lateral intrahemispheric connections on the same hierarchical level and less with feedback connections. I propose that experimental results about the callosal circuit in early visual areas can be interpreted with respect to lateral connectivity in general.

Deactivation of the inferior colliculus by cooling demonstrates intercollicular modulation of neuronal activity

The auditory pathways coursing through the brainstem are organized bilaterally in mirror image about the midline and at several levels the two sides are interconnected. One of the most prominent points of interconnection is the commissure of the inferior colliculus (CoIC). Anatomical studies have revealed that these fibers make reciprocal connections which follow the tonotopic organization of the inferior colliculus (IC), and that the commissure contains both excitatory and, albeit fewer, inhibitory fibers. The role of these connections in sound processing is largely unknown. Here we describe a method to address this question in the anaesthetized guinea pig. We used a cryoloop placed on one IC to produce reversible deactivation while recording electrophysiological responses to sounds in both ICs. We recorded single units, multi-unit clusters and local field potentials (LFPs) before, during and after cooling. The degree and spread of cooling was measured with a thermocouple placed in the IC and other auditory structures. Cooling sufficient to eliminate firing was restricted to the IC contacted by the cryoloop. The temperature of other auditory brainstem structures, including the contralateral IC and the cochlea were minimally affected. Cooling below 20°C reduced or eliminated the firing of action potentials in frequency laminae at depths corresponding to characteristic frequencies up to ~8 kHz. Modulation of neural activity also occurred in the un-cooled IC with changes in single unit firing and LFPs. Components of LFPs signaling lemniscal afferent input to the IC showed little change in amplitude or latency with cooling, whereas the later components, which likely reflect inter- and intra-collicular processing, showed marked changes in form and amplitude. We conclude that the cryoloop is an effective method of selectively deactivating one IC in guinea pig, and demonstrate that auditory processing in the IC is strongly influenced by the other.

Cortical inactivation by cooling in small animals

Reversible inactivation of the cortex by surface cooling is a powerful method for studying the function of a particular area. Implanted cooling cryoloops have been used to study the role of individual cortical areas in auditory processing of awake-behaving cats. Cryoloops have also been used in rodents for reversible inactivation of the cortex, but recently there has been a concern that the cryoloop may also cool non-cortical structures either directly or via the perfusion of blood, cooled as it passed close to the cooling loop. In this study we have confirmed that the loop can inactivate most of the auditory cortex without causing a significant reduction in temperature of the auditory thalamus or other subcortical structures. We placed a cryoloop on the surface of the guinea pig cortex, cooled it to 2°C and measured thermal gradients across the neocortical surface. We found that the temperature dropped to 20–24°C among cells within a radius of about 2.5 mm away from the loop. This temperature drop was sufficient to reduce activity of most cortical cells and led to the inactivation of almost the entire auditory region. When the temperature of thalamus, midbrain, and middle ear were measured directly during cortical cooling, there was a small drop in temperature (about 4°C) but this was not sufficient to directly reduce neural activity. In an effort to visualize the extent of neural inactivation we measured the uptake of thallium ions following an intravenous injection. This confirmed that there was a large reduction of activity across much of the ipsilateral cortex and only a small reduction in subcortical structures.

Reciprocal modulatory influences between tonotopic and nontonotopic cortical fields in the cat

Functional and anatomical studies suggest that acoustic signals are processed hierarchically in auditory cortex. Although most regions of acoustically responsive cortex are not tonotopically organized, all previous electrophysiological investigations of interfield interactions have only examined tonotopically represented areas. The purpose of the present study was to investigate the functional interactions between tonotopically and nontonotopically organized fields in auditory cortex. We accomplished this goal by examining the bidirectional contributions between the cochleotopically organized primary auditory cortex (A1) and the noncochleotopically organized second auditory field (A2). Multiunit acute recording procedures in combination with reversible cooling deactivation techniques were used in eight mature cats. The synaptic activity of A1 or A2 was suppressed while the neuronal response to tonal stimuli of the noninactivated area (A1 or A2) was measured. Response strength, neuronal threshold, receptive field bandwidths, and latency measures were collected at each recorded site before, during, and after cooling deactivation epochs. Our analysis revealed comparable changes in A1 and A2 neuronal response properties. Specifically, significant decreases in neuronal response strength, increases in neuronal threshold, and shortening of response latency were found in both fields during periods of cooling deactivation. The weak anatomical connections between the two fields investigated make these findings unexpected. Furthermore, the observed neuronal changes suggest a model of corticocortical interaction among auditory fields in which neither differences in the magnitude of anatomical projections nor cortical representation of sensory stimuli are reliable determinants of modulatory functions.

Evidence for hierarchical processing in cat auditory cortex: nonreciprocal influence of primary auditory cortex on the posterior auditory field

The auditory cortex of the cat is composed of 13 distinct fields that have been defined on the basis of anatomy, physiology, and behavior. Although an anatomically based hierarchical processing scheme has been proposed in auditory cortex, few functional studies have examined how these areas influence one another. The purpose of the present study was to examine the bidirectional processing contributions between primary auditory cortex (A1) and the nonprimary posterior auditory field (PAF). Multiunit acute recording techniques in eight mature cats were used to measure neuronal responses to tonal stimuli in A1 or PAF while synaptic activity from PAF or A1 was suppressed with reversible cooling deactivation techniques. Specifically, in four animals, electrophysiological recordings in A1 were conducted before, during, and after deactivation of PAF. Similarly, in the other four animals, PAF activity was measured before, during, and after deactivation of A1. The characteristic frequency, bandwidth, and neuronal threshold were calculated at each receptive field collected and the response strength and response latency measures were calculated from cumulative peristimulus time histograms. Two major changes in PAF response properties were observed during A1 deactivation: a decrease in response strength and a reduction in receptive field bandwidths. In comparison, we did not identify any significant changes in A1 neuronal responses during deactivation of PAF neurons. These findings support proposed models of hierarchal processing in cat auditory cortex.

Differential modulatory influences between primary auditory cortex and the anterior auditory field

Neuroanatomical studies have revealed a vast network of corticocortical connections among the various fields that form cat auditory cortex. However, few studies have explored the functional communicative properties of these connections. The purpose of the present study was to examine the bidirectional processing contributions between the primary auditory cortex (A1) and the nonprimary anterior auditory field (AAF). Using acute recording techniques, multiunit neuronal activity was collected from the right hemisphere of nine mature cats. Cortical maps were generated, and the precise location of A1 and AAF was identified. Subsequently, the synaptic activity of A1 or AAF was suppressed with reversible thermal deactivation procedures while the neuronal response to tonal stimuli of the non-inactivated area (A1 or AAF) was measured. We examined response strength and latency, characteristic frequency, bandwidth, and neuronal threshold of A1 and AAF receptive fields before and during epochs of deactivation. Three major changes in A1 response properties were observed during AAF neuronal suppression: a decrease in response strength, an increase in neuronal thresholds, and a sharpening of receptive field bandwidths. In contrast, A1 deactivation did not produce any discernible changes in AAF neuronal responses. Collectively, these results suggest that the modulation of acoustic information between A1 and AAF in cat auditory cortex is dominated by a unidirectional AAF to A1 pathway.

'Top-down' influences of ipsilateral or contralateral postero-temporal visual cortices on the extra-classical receptive fields of neurons in cat's striate cortex

In anesthetized and immobilized domestic cats, we have studied the effects of brief reversible inactivation (by cooling to 10 degrees C) of the ipsilateral or contralateral postero-temporal visual (PTV) cortices on: 1) the magnitude of spike-responses of neurons in striate cortex (cytoarchitectonic area 17, area V1) to optimized sine-wave modulated contrast-luminosity gratings confined to the classical receptive fields (CRFs) and 2) the relative strengths of modulation of CRF-induced spike-responses by gratings extending into the extra-classical receptive field (ECRF). Consistent with our previous reports (Bardy et al., 2006; Huang et al., 2007), inactivation of ipsilateral PTV cortex (presumed homologue of primate infero-temporal cortex) resulted in significant reversible changes (almost all substantial reductions) in the magnitude of spike-responses to CRF-confined stimuli in about half of the V1 neurones. Similarly, in half of the present sample, inactivation of ipsilateral PTV cortex resulted in significant reversible changes (in over 70% of cases, reduction) in the relative strength of ECRF modulation of the CRF-induced spike-responses. By contrast, despite the fact that receptive fields of all V1 cells tested were located within 5 degrees of representation of the zero vertical meridian, inactivation of contralateral PTV cortex only rarely resulted in significant (yet invariably small) changes in the magnitude of spike-responses to CRF-confined stimuli or significant (again invariably small) changes in the relative strength of ECRF modulation of spike-responses. Thus, the ipsilateral, but not contralateral, 'higher-order' visual cortical areas make significant contribution not only to the magnitude of CRF-induced spike-responses but also to the relative strengths of ECRF-induced modulation of the spike-responses of V1 neurons. Therefore, the feedback signals originating from the ipsilateral higher-order cortical areas appear to make an important contribution to contextual modulation of responses of neurons in the primary visual cortices.

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.

The effects of reversible inactivation of postero-temporal visual cortex on neuronal activities in cat's area 17

'Spontaneous' and visually evoked action potentials were recorded from single neurons in cytoarchitectonic area 17 (striate cortex, area V1) of anaesthetized and immobilized cats, prior to, during and after brief reversible inactivation of the ipsilateral postero-temporal visual (PTV) cortex (presumed homologue of primate inferotemporal cortex). Inactivation of PTV cortex resulted: 1) in significant changes in the response magnitude (mostly a reduction) to optimal and/or sub-optimal visual stimuli in over 55% of area 17 cells and 2) significant changes (usually a reduction) in the 'spontaneous' (background) activity of about two-thirds of the cells in which inactivation of PTV cortex significantly affected the magnitude of responses to optimal stimuli. In over 85% of the significantly affected area 17 cells, rewarming PTV cortex to normal temperature (36 degrees C) resulted in the recovery of both the magnitude of responses and the background activity to levels not significantly different from pre-inactivation levels. Irrespective of the significance of changes in the magnitude of responses, in a substantial proportion of area 17 cells, inactivation of PTV cortex resulted in changes in some receptive field characteristics. Thus, there were substantial (20% or more) changes in orientation tuning widths (in over a quarter of the sample) and/or direction selectivity indices (in about a third of the sample). Thus, the feedback signals originating from PTV cortex, like signals originating from some other 'higher-order' visual cortical areas exert a clear modulatory influence on the responsiveness, background activity and some receptive field properties of neurons in the ipsilateral area 17.

Relocation of specific visual functions following damage of mature posterior parietal cortex

Many visual deficits have been reported following damage to specific cerebral sites within posterior parietal cortex. These deficits generally involve aspects of vision including spatial or motion perception and visuomotor control. One characteristic of many of these deficits is that they tend to attenuate over time. Presumably, other cortical regions possess adaptive neuroplastic mechanisms that allow them to accommodate functions that were previously dominated by the damaged region. This report summarizes a series of experiments that examined adaptive cortical plasticity following cerebral cortex damage sustained in maturity. Following bilateral lesions of posterior middle suprasylvian sulcal (pMSs) cortex in the cat, deficits were identified in both visual orienting and landmark discrimination tasks. However, the deficits on the visual orienting task were only profound for the first few days following the lesion and orienting abilities returned to normal levels within the first 2 weeks postlesion. In contrast, no such attenuation of the effect of the lesion was evident on the landmark discrimination task. Following recovery of function on the visual orienting task, individual cortical areas flanking the lesion were bilaterally deactivated with cooling. Reversible deactivation of anterior middle suprasylvian sulcal (aMSs) cortex, but none of the other adjacent cortices, yielded visual orienting deficits that are not found in intact animals during deactivation of aMSs cortex. Therefore, we concluded that the visual orienting functions normally mediated by pMSs cortex were able to relocate to aMSs cortex following lesion of pMSs cortex. Finally, bilateral lesion of both pMSs and aMSs cortices yielded visual orienting deficits that did not attenuate. Overall, this series of experiments demonstrates that certain visual functions may relocate to specific cortical loci following damage to discrete areas within posterior parietal cortex.

'Simplification' of responses of complex cells in cat striate cortex: suppressive surrounds and 'feedback' inactivation

In mammalian striate cortex (V1), two distinct functional classes of neurones, the so-called simple and complex cells, are routinely distinguished. They can be quantitatively differentiated from each other on the basis of the ratio between the phase-variant (F1) component and the mean firing rate (F0) of spike responses to luminance-modulated sinusoidal gratings (simple, F1/F0 > 1; complex, F1/F0 < 1). We investigated how recurrent cortico-cortical connections affect the spatial phase-variance of responses of V1 cells in the cat. F1/F0 ratios of the responses to optimally oriented drifting sine-wave gratings covering the classical receptive field (CRF) of single V1 cells were compared to those of: (1) responses to gratings covering the CRFs combined with gratings of different orientations presented to the 'silent' surrounds; and (2) responses to CRF stimulation during reversible inactivation of postero-temporal visual (PTV) cortex. For complex cells, the relative strength of the silent surround suppression on CRF-driven responses was positively correlated with the extent of increases in F1/F0 ratios. Inactivation of PTV cortex increased F1/F0 ratios of CRF-driven responses of complex cells only. Overall, activation of suppressive surrounds or inactivation of PTV 'converted' substantial proportions (50 and 30%, respectively) of complex cells into simple-like cells (F1/F0 > 1). Thus, the simple-complex distinction depends, at least partly, on information coming from the silent surrounds and/or feedback from 'higher-order' cortices. These results support the idea that simple and complex cells belong to the same basic cortical circuit and the spatial phase-variance of their responses depends on the relative strength of different synaptic inputs.

A motion-sensitive area in ferret extrastriate visual cortex: an analysis in pigmented and albino animals

In search of the neuronal substrate for motion analysis in the ferret (Mustela putorius furo), we extracellularly recorded from extrastriate visual cortex in five pigmented and two albino ferrets under general anaesthesia and paralysis. Visual stimulation consisted of large area random dot patterns moving either on a circular path in the frontoparallel plane or expanding and contracting radially. Strongly direction-selective neurons were recorded in a circumscribed area in and just posterior to the suprasylvian sulcus, thus named by us the posterior suprasylvian area (area PSS). Altogether, we recorded 210 (90%) and 95 (72%) PSS neurons in pigmented and albino ferrets, respectively, that were direction selective. In these neurons responses during random dot pattern stimulation in the preferred direction were at least twice as strong than stimulation in the non-preferred direction. Response strength in preferred direction and tuning sharpness of PSS neurons in albinos were significantly reduced when compared to pigmented animals (median values: 34.1 versus 14.8 spikes/s and 142 versus 165 degrees for pigmented and albino ferrets, respectively). Inter-spike-intervals during visual stimulation were significantly shorter in pigmented (median 9 ms) than in albino PSS neurons (median 14 ms). Our data indicate that area PSS may play a crucial role in motion perception in the ferret.

Directed interactions between visual areas and their role in processing image structure and expectancy

During sensory processing, cortical areas continuously exchange information in different directions along the hierarchy. The functional role of such interactions, however, has been the subject of various proposals. Here, we investigate the role of bottom-up and top-down interactions in processing stimulus structure and their relation to expected events. Applying multivariate autoregressive methods to local field potentials recorded in alert cats, we quantify directed interactions between primary (A17/18) and higher (A21) visual areas. A trial-by-trial analysis yields the following findings. To assess the role of interareal interactions in processing stimulus structure, we recorded in naïve animals during stimulation with natural movies and pink noise stimuli. The overall interactions decrease compared with baseline for both stimuli. To investigate whether forthcoming events modulate interactions, we recorded in trained animals viewing two stimuli, one of which had been associated with a reward. Several results support such modulations. First, the interactions increase compared with baseline and this increase is not observed in a context where food was not delivered. Second, these stimuli have a differential effect on top-down and bottom-up components. This difference is emphasized during the stimulus presentation and is maximal shortly before the possible reward. Furthermore, a spectral decomposition of the interactions shows that this asymmetry is most dominant in the gamma frequency range. Concluding, these results support the notion that interareal interactions are more related to an expectancy state rather than to processing of stimulus structure.

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.

Ultrastructure and synaptic targets of tectothalamic terminals in the cat lateral posterior nucleus

The recent appreciation of the fact that the pulvinar and lateral posterior (LP) nuclei receive two distinct types of cortical input has sparked renewed interest in this region of the thalamus. A key question is whether the primary or "driving" inputs to the pulvinar/LP complex originate in cortical or subcortical areas. To begin to address this issue, we examined the synaptic targets of tectothalamic terminals within the LP nucleus. Tectothalamic terminals were labeled using the anterograde transport of biotinylated dextran amine (BDA) or Phaselous leucoagglutinin placed in the superior colliculus or using immunocytochemical staining for substance P, a neurotransmitter found to be used by the tectothalamic pathway (Hutsler and Chalupa [ 1991] J. Comp. Neurol. 312:379-390). Our results suggest that most tectothalamic terminals are large and occupy a proximal position on the dendritic arbor of LP relay cells. In the medial LP, tectothalamic terminals labeled by the transport of neuronal tracers or substance P immunocytochemistry can form tubular clusters that surround the proximal dendrites of relay cells. In a rostral and lateral subdivision of the lateral LP nucleus (LPl-2), tectothalamic terminals form more typical glomerular arrangements. When compared with existing physiological data, these results suggest that a unique integration of tectal and cortical inputs may contribute to the response properties of LP neurons.

The role of feedback in shaping neural representations in cat visual cortex

In the primary visual cortex, neurons with similar response preferences are grouped into domains forming continuous maps of stimulus orientation and direction of movement. These properties are widely believed to result from the combination of ascending and lateral interactions in the visual system. We have tested this view by examining the influence of deactivating feedback signals descending from the visuoparietal cortex on the emergence of these response properties and representations in cat area 18. We thermally deactivated the dominant motion-processing region of the visuoparietal cortex and used optical and electrophysiological methods to assay neural activity evoked in area 18 by stimulation with moving gratings and fields of coherently moving randomly distributed dots. Feedback deactivation decreased signal strength in both orientation and direction maps and virtually abolished the global layout of direction maps, whereas the basic structure of the orientation maps was preserved. These findings could be accounted for by a selective silencing of highly direction-selective neurons and by the redirection of preferences of less selective neurons. Our data suggest that signals fed back from the visuoparietal cortex strongly contribute to the emergence of direction selectivity in early visual areas. Thus we propose that higher cortical areas have significant influence over fundamental neuronal properties as they emerge in lower areas.

The advantages and limitations of permanent or reversible deactivation techniques in the assessment of neural function

This review considers the different forms of reversible deactivation and how they differ from each other and from more conventional permanent deactivation methods. The different methods of chemical and cooling reversible inactivation and the limitations of each are discussed. Finally, future directions in the quest to localize brain function in the experimental animal are considered and how this work relates to emerging non-invasive imaging techniques presently being used with humans.

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.

Spatial and temporal parameters of cortical inactivation by GABA

Inactivation by GABA is a powerful tool for studying the function of specific cortical regions. It is especially useful in electrophysiology, because inactivation is reversible within short time periods, and because the extent of the inactivated region can be accurately controlled. Iontophoresis of GABA inactivates neurons up to 300 microm around the micropipette. Pressure injection of GABA inactivates neurons further away, but the spatial and temporal characteristics of inactivation by this method have been poorly studied. In order to address this question, we built devices made of micropipettes and microelectrodes glued at various distances. We experienced that repetition of small injections of 100 mM GABA inactivate cortex in a more homogenous way than bolus injections. Diffusion of GABA after pressure injection does not seem to follow a point spread diffusion model as in the case of iontophoresis: GABA probably goes up along the micropipette shaft, and the volume of inactivation has an ellipsoidal form. In order to precisely determine the extent of the inactivated region, we built a mathematical model to fit the experimental data of inactivations obtained above and below the pipette tip. The model provides estimates of the inactivated region for volumes smaller than 60 nl of GABA 100 mM. Limits of inactivation are between 250 and 500 microm lateral to the tip of the pipette. The geometry of inactivation is difficult to predict beyond 60 nl and it seems hazardous to try to inactivate neurons beyond 800 microm with pressure injections of GABA 100 mM.

Disinhibition of the superior colliculus restores orienting to visual stimuli in the hemianopic field of the cat

Following unilateral removal of all known visual cortical areas, a cat is rendered hemianopic in the contralateral visual field. Visual orientation can be restored to the blind hemifield by transection of the commissure of the superior colliculus or by destruction of the superior colliculus (SC) or the substantia nigra pars reticulata (SNpr) contralateral to the cortical lesion. It is hypothesized that a mechanism mediating recovery is disinhibition of the SC ipsilateral to the cortical lesion. The ipsilateral nigrotectal projection exerts a robust inhibitory tone onto cells in the SC. However, ibotenic acid destruction of SNpr neurons, which should decrease inhibition onto the SC, does not result in recovery. The failure of ipsilateral SNpr lesions to produce recovery puts into question the validity of SC disinhibition as a mechanism of recovery. We directly tested the disinhibition hypothesis by reversibly disinhibiting the SC ipsilateral to a visual cortical lesion with a gamma-aminobutyric acid (GABA)A antagonist, bicuculline methiodide. In accordance with the hypothesis, transient disinhibition of the SC restored visual orienting for several hours in three of eight animals. Recovery was not a volume or pH effect and was distinct from the release of irrepressible motor effects (i.e., approach and avoidance behaviors) seen within the first hour after injection. Thus, in the absence of all visual cortical areas unilaterally, disinhibition of the SC can transiently restore the ability of the cat to orient to visual stimuli in the previously "blind" hemifield.

Neuronal responses in extrastriate cortex to objects in optic flow fields

During locomotion, observers respond to objects in the environment that may represent obstacles to avoid or landmarks for navigation. Although much is known about how visual cortical neurons respond to stimulus objects moving against a blank background, nothing is known about their responses when objects are embedded in optic flow fields (the patterns of motion seen during locomotion). We recorded from cells in the lateral suprasylvian visual area (LS) of the cat, an area probably analogous to area MT. In our first experiments, optic flow simulations mimicked the view of a cat trotting across a plain covered with small balls; a black bar lying on the balls served as a target object. In subsequent experiments, optic flow simulations were composed of natural elements, with target objects representing bushes, rocks, and variants of these. Cells did not respond to the target bar in the presence of optic flow backgrounds, although they did respond to it in the absence of a background. However, 273/423 cells responded to at least one of the taller, naturalistic objects embedded in optic flow simulations. These responses might represent a form of image segmentation, in that cells detected objects against a complex background. Surprisingly, the responsiveness of cells to objects in optic flow fields was not correlated with preferred direction as measured with a moving bar or whole-field texture. Because the direction of object motion was determined solely by receptive-field location, it often differed considerably from a cell's preferred direction. About a quarter of the cells responded well to objects in optic flow movies but more weakly or not at all to bars moving in the same direction as the object, suggesting that the optic flow background modified or suppressed direction selectivity.

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.

A metabolic mapping study of orientation discrimination and detection tasks in the cat

Increasing evidence suggests that a large number of distinct cortical areas and associated subcortical structures participate in the processing of visual information and that different aspects of visual scenes are evaluated in different areas. This necessitates identification of cortical and subcortical regions cooperating in particular visual tasks. Using the 2-deoxyglucose technique, we monitored the differential activation of areas in the cat visual cortex participating in an orientation discrimination and a detection task. Concordant with previous lesion studies, we found increased activity levels in area 17 in the discrimination condition relative to the detection condition. In addition, the 2-deoxyglucose technique revealed discrimination-related increased activations in the claustrum, the putamen and in parts of the anteromedial, anterolateral and posterolateral lateral suprasylvian visual areas. Regions activated differentially with the detection task comprised subdivisions of areas 17, 18, 19 and 21, posterior area 7 (7p), several areas of the posterior part of the middle and posterior suprasylvian sulcus, the pulvinar complex and the superior colliculus. These results show that the 2-deoxyglucose technique is useful to investigate cognitive brain functions, and that different sets of cortical and subcortical regions are activated during two visual tasks with similar visual stimulation.

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

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

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.

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

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

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.

Parallel versus serial processing: new vistas on the distributed organization of the visual system

Recent functional studies question the validity of the hierarchical model of organization for processing visual information in cortical areas. The results of these studies suggest that beyond the primary visual cortex (V1), information is not serially processed through successive cortical areas, but that it is simultaneously processed in several areas. The idea that visual information is functionally segregated into different, parallel channels as it circulates through V1 and V2 towards V4 and the middle temporal visual area is also challenged by recent studies that report a smaller degree of functional specialization within the visual areas than previously thought.