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

A neurochemical signature of visual recovery after extrastriate cortical damage in the adult cat

In adult cats, damage to the extrastriate visual cortex on the banks of the lateral suprasylvian (LS) sulcus causes severe deficits in motion perception that can recover as a result of intensive direction discrimination training. The fact that recovery is restricted to trained visual field locations suggests that the neural circuitry of early visual cortical areas, with their tighter retinotopy, may play an important role in attaining perceptual improvements after damage to higher level visual cortex. The present study tests this hypothesis by comparing the manner in which excitatory and inhibitory components of the supragranular circuitry in an early visual cortical area (area 18) are affected by LS lesions and postlesion training. First, the proportion of LS-projecting pyramidal cells as well as calbindin- and parvalbumin-positive interneurons expressing each of the four AMPA receptor subunits was estimated in layers II and III of area 18 in intact animals. The degree to which LS lesions and visual retraining altered these expression patterns was then assessed. Both LS-projecting pyramidal cells and inhibitory interneurons exhibited long-term, differential reductions in the expression of glutamate receptor (GluR)1, -2, -2/3, and -4 following LS lesions. Intensive visual training post lesion restored normal AMPAR subunit expression in all three cell-types examined. Furthermore, for LS-projecting and calbindin-positive neurons, this restoration occurred only in portions of the ipsi-lesional area 18 representing trained visual field locations. This supports our hypothesis that stimulation of early visual cortical areas-in this case, area 18-by training is an important factor in restoring visual perception after permanent damage to LS cortex.

Feedback decoding of spatially structured population activity in cortical maps

A mechanism is proposed by which feedback pathways model spatial patterns of feedforward activity in cortical maps. The mechanism can be viewed equivalently as readout of a content-addressable memory or as decoding of a population code. The model is based on the evidence that cortical receptive fields can often be described as a separable product of functions along several dimensions, each represented in a spatially ordered map. Given this, it is shown that for an N-dimensional map, accurate modeling and decoding of x(N) feedforward activity patterns can be done with Nx fibers, N of which must be active at any one time. The proposed mechanism explains several known properties of the cortex and pyramidal neurons: (1) the integration of signals by dendrites with a narrow tangential distribution, that is, apical dendrites; (2) the presence of fast-conducting feedback projections with broad tangential distributions; (3) the multiplicative effects of attention on receptive field profiles; and (4) the existence of multiplicative interactions between subthreshold feedforward inputs to basal dendrites and inputs to apical dendrites.

A spatio-temporal interaction on the apparent motion trace

During the perception of apparent motion, activity along the apparent motion trace has been found in the primary visual cortex. It has been hypothesized that this activity interferes with stimuli presented on the apparent motion trace ("motion masking"). We investigated whether this perceptual interference varies with regard to the trajectory of a moving object token in a detection task. We found a general decrease of detectability of targets presented on the trace. Surprisingly, targets presented in time with the trajectory were detected significantly more often than targets which appeared out of time. We relate this finding to a spatio-temporally specific prediction of visual events along the apparent motion trace.

Feedback signals from cat's area 21a enhance orientation selectivity of area 17 neurons

We have studied the contribution of feedback signals originating from one of the "form-processing" extrastriate cortical areas, area 21a (A21a), to orientation selectivity of single neurons in the ipsilateral area 17 (A17). Consistent with previous findings, reversible inactivation (cooling to 5-10 degrees C) of area 21a resulted in a substantial reduction in the magnitude of the maximum response (R (max)) of A17 cells accompanied by some changes in the half-width at half-height of the R (max) (HWHH). By fitting model functions to the neurons' response profiles we found that in the vast majority of orientation-tuned A17 cells tested (30/39, 77%), inactivation of A21a resulted in a "flattening" of their orientation-tuning curves. It is characterised by a substantial reduction in the R (max) associated with either a broadening of the orientation-tuning curves (17 cells) or a relatively small reduction (12 cells) or no change (1 cell) in the HWHH. When the "flattening" effect was quantified using a simple ratio index or R/W, defined as R (max)/HWHH, we found that R/W was significantly reduced during inactivation of A21a. The change in R/W is strongly correlated with the change in the maximum slope of the orientation-tuning curves. Furthermore, analysis of response variability indicates that "signal-to-noise" ratio of the responses of A17 neurons decreases during inactivation of A21a. Our results suggest that the predominately excitatory feedback signals originating from A21a play a role in enhancing orientation selectivity of A17 neurons and hence are likely to improve overall orientation discriminability.

Layout of transcallosal activity in cat visual cortex revealed by optical imaging

The contribution of interhemispheric connections to functional maps in cat visual cortex was investigated by using optical imaging of intrinsic signals. In order to isolate the functional inputs arriving via the corpus callosum (CC) from other inputs, we used the split-chiasm preparation. The regions activated through the CC in visual areas 17 (A17) and 18 (A18) were localized and characterized by stimulating monocularly split-chiasm cats with moving, high contrast oriented gratings. We found that the CC mediates the activation of orientation selective domains in the transition zone (TZ) between A17 and A18 and occasionally within portions of both of these areas. We observed transcallosally activated orientation domains all along the TZ without any obvious interruption, and these domains were arranged around "pinwheel" centers. Interestingly, the TZ was divided in two parallel regions, which resemble A17 and A18 in their preferred temporal and spatial frequencies. Finally, we demonstrated that orientation maps evoked through the transcallosal and geniculo-cortical pathways were similar within the TZ, indicating a convergence of inputs of matching orientations in this region. These results contribute to a better understanding of the role of the CC in visual perception of orientations and shapes, at the level of the visual cortex.

Complex motion selectivity in PMLS cortex following early lesions of primary visual cortex in the cat

In the cat, the analysis of visual motion cues has generally been attributed to the posteromedial lateral suprasylvian cortex (PMLS) (Toyama et al., 1985; Rauschecker et al., 1987; Rauschecker, 1988; Kim et al., 1997). The responses of neurons in this area are not critically dependent on inputs from the primary visual cortex (VC), as lesions of VC leave neuronal response properties in PMLS relatively unchanged (Spear & Baumann, 1979; Spear, 1988; Guido et al., 1990b). However, previous studies have used a limited range of visual stimuli. In this study, we assessed whether neurons in PMLS cortex remained direction-selective to complex motion stimuli following a lesion of VC, particularly to complex random dot kinematograms (RDKs). Unilateral aspiration of VC was performed on post-natal days 7–9. Single unit extracellular recordings were performed one year later in the ipsilateral PMLS cortex. As in previous studies, a reduction in the percentage of direction selective neurons was observed with drifting sinewave gratings. We report a previously unobserved phenomenon with sinewave gratings, in which there is a greater modulation of firing rate at the temporal frequency of the stimulus in animals with a lesion of VC, suggesting an increased segregation of ON and OFF sub-regions. A significant portion of neurons in PMLS cortex were direction selective to simple (16/18) and complex (11/16) RDKs. However, the strength of direction selectivity to both stimuli was reduced as compared to normals. The data suggest that complex motion processing is still present, albeit reduced, in PMLS cortex despite the removal of VC input. The complex RDK motion selectivity is consistent with both geniculo-cortical and extra-geniculate thalamo-cortical pathways in residual direction encoding.

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.

How do functional maps in primary visual cortex vary with eccentricity?

It is important to understand whether functional maps of primary visual cortex (V1) are organized differently at the representation of different eccentricities. By using optical imaging of intrinsic signals, we compared the maps of orientation and spatial frequency (SF) preference between central (0-3 degrees ) and paracentral (4-8 degrees ) V1 in the prosimian bush baby (Otolemur garnetti). No differences related to eccentricity were found for orientation selectivity or magnitude between central and paracentral V1. We found, however, that cardinal orientations were overrepresented in central but not in paracentral V1 and that isoorientation domain size tended to be smaller in the central representation. We demonstrated that spatial frequency was represented continuously across V1, and that the map of SF preference exhibited eccentricity-dependent variations, with more territory devoted to higher SFs in central than in paracentral V1. Although there were no spatial relationships between orientation domains and cytochrome oxidase (CO) blobs or interblobs, CO blobs tended to prefer lower SFs than interblobs. Taken together with previous research, our data indicate that functional domains in V1 show eccentricity-related differences in organization and also support the idea that different maps (with or without specific geometrical relationships) are organized for adequate coverage of each feature in visual space.

Pattern motion selectivity in population responses of area 18

It is commonly believed that the complexity of visual stimuli represented by individual neurons increases towards higher cortical areas. However, even in early visual areas an individual neuron's response is influenced by stimuli presented outside its classical receptive field. Thus, it has been proven difficult to characterize the coding of complex stimuli at the level of single neurons. We therefore investigated population responses using optical imaging in cat area 18 to complex stimuli, plaids. Plaid stimuli are composed of two superimposed gratings moving in different directions. They may be perceived as either two separate surfaces or as a global pattern moving in intermediate direction to the components' direction of motion. We found that in addition to activity maps representing the individual components' motion, plaid stimuli produced activity distributions matching the predictions from a pattern-motion model in central area 18. Thereby, relative component- and pattern-like modulations followed the degree of psychophysical pattern bias in the stimulus. Thus, our results strongly indicate that area 18 exhibits a substantial response to pattern-motion signals at the population level suggesting the presence of intrinsic or extrinsic mechanisms that allow for integration of motion responses from far outside the classical receptive field.

Opposite impact on 14C-2-deoxyglucose brain metabolism following patterns of high and low frequency repetitive transcranial magnetic stimulation in the posterior parietal cortex

Repetitive transcranial magnetic stimulation (rTMS) appears capable of modulating human cortical excitability beyond the duration of the stimulation train. However, the basis and extent of this "off-line" modulation remains unknown. In a group of anesthetized cats, we applied patterns of real or sham focal rTMS to the visuo-parietal cortex (VP) at high (HF) or low (LF) frequency and recorded brain glucose uptake during (on-line), immediately after (off-line), or 1 h after (late) stimulation. During the on-line period LF and HF rTMS induced a significant relative reduction of (14)C-2DG uptake in the stimulated VP cortex and tightly linked cortical and subcortical structures (e.g. the superficial superior colliculus, the pulvinar, and the LPl nucleus) with respect to homologue areas in the unstimulated hemisphere. During the off-line period HF rTMS induced a significant relative increase in (14)C-2DG uptake in the targeted VP cortex, whereas LF rTMS generated the opposite effect, with only mild network impact. Moderate distributed effects were only recorded after LF rTMS in the posterior thalamic structures. No long lasting cortical or subcortical effects were detected during the late period. Our findings demonstrate opposite modulation of rTMS on local and distant effects along a specific network, depending on the pattern of stimulation. Such effects are demonstrated in the anesthetized animal, ruling out behavioral and non-specific reasons for the differential impact of the stimulation. The findings are consistent with previous differential electrophysiological and behavioral effects of low and high frequency rTMS patterns and provide support to uses of rTMS in neuromodulation.

Cortical algorithms for perceptual grouping

A fundamental task of vision is to group the image elements that belong to one object and to segregate them from other objects and the background. This review provides a conceptual framework of how perceptual grouping may be implemented in the visual cortex. According to this framework, two mechanisms are responsible for perceptual grouping: base-grouping and incremental grouping. Base-groupings are coded by single neurons tuned to multiple features, like the combination of a color and an orientation. They are computed rapidly because they reflect the selectivity of feedforward connections. However, not all conceivable feature combinations are coded by dedicated neurons. Therefore, a second, flexible form of grouping is required called incremental grouping. Incremental grouping enhances the responses of neurons coding features that are bound in perception, but it takes more time than does base-grouping because it relies also on horizontal and feedback connections. The modulation of neuronal response strength during incremental grouping has a correlate in psychology because attention is directed to those features that are labeled by the enhanced neuronal response.

Posteromedial lateral suprasylvian motion area modulates direction but not orientation preference in area 17 of cats

In visual cortices of cats there are two major, largely parallel, feedforward processing streams which conduct visual information from the primary visual cortices to the parietal and temporal visual cortices, processing motion and form information, respectively. In addition to the feedforward streams, there exist many feedback projections from higher-order visual cortices to lower-order visual cortices. Using the intrinsic signal optical imaging, this study has examined the influence of feedback signals originating from area posteromedial lateral suprasylvian (PMLS), the dominant motion-processing region of the parietal cortex, on responses of neurons, orientational maps, and directional maps in cats' area 17 (striate cortex). The inactivation of area PMLS by local application of GABA resulted in the reduction of the magnitude of responses of area 17 cells though area 17 of the cat is mainly involved in form information processing rather than motion. Furthermore, inactivation of area PMLS abolished the global layout of direction maps in area 17 but did not affect the basic structure of the orientation maps in area 17. Thus, it appears that higher-order cortical areas along one information processing stream may exert cross-stream modulatory effects on fundamental properties of neurons located in the lower-order areas along distinct information processing streams.

Overlapping visual response latency distributions in visual cortices and LP-pulvinar complex of the cat

The visual system of the cat is considered to be organized in both a serial and parallel manner. Studies of visual onset latencies generally suggest that parallel processing occurs throughout the dorsal stream. These studies are at odds with the proposed hierarchies of visual areas based on termination patterns of cortico-cortical projections. In previous studies, a variety of stimuli have been used to compute latencies, and this is problematic as latencies are known to depend on stimulus parameters. This could explain the discrepancy between latency and neuroanatomical based studies. Therefore, the first aim of the present study was to determine whether latencies increased along the hierarchy of visual areas when the same stimuli are used. In addition, the effect of stimulus complexity was assessed. Visual onset latencies were calculated for area 17, PMLS, AMLS, and AEV neurons. Latencies were also computed from neurons in the lateral posterior (LP)-pulvinar complex given the importance of this extrageniculate complex in cortical intercommunication. Latency distributions from all regions overlapped substantially, and no significant difference was present, regardless of the type of stimulus used. The onset latencies in the LP-pulvinar complex were comparable to those seen in cortical areas. The data suggest that the initial processing of information in the visual system is parallel, despite the presence of a neuroanatomical hierarchy. Simultaneous response onsets among cortical areas and the LP-pulvinar suggest that the latter is more than a simple relay station for information headed to cortex. The data are consistent with proposals of the LP-pulvinar as a center for the integration and distribution of information from/to multiple cortical areas.

Gap junctions among dendrites of cortical GABAergic neurons establish a dense and widespread intercolumnar network

Gap junctions are common between cortical GABAergic interneurons but little is known about their quantitative distribution along dendritic profiles. Here, we provide direct morphological evidence that parvalbumin-containing GABAergic neurons in layer 2/3 of the cat visual cortex form dense and far-ranging networks through dendritic gap junctions. Gap junction-coupled networks of parvalbumin neurons were visualized using connexin36 immunohistochemistry and confocal laser-scanning microscopy (CLSM). The direct correspondence of connexin36-immunopositve puncta and gap junctions was confirmed by examining the same structures in both CLSM and electron microscopy. Single parvalbumin neurons with large somata (> or =200 microm2) formed 60.3 +/- 12.2 (mean +/- SD) gap junctions with other cells whereby these contacts were not restricted to proximal dendrites but occurred at distances of up to 380 microm from the soma. In a Sholl analysis of large-type parvalbumin neurons, 21.9 +/- 7.9 gap junctions were within 50 microm of the soma, 21.7 +/- 7.6 gap junctions in a segment between 50 and 100 microm, 11.2 +/- 4.7 junctions between 100 and 150 microm, and 5.6 +/- 3.6 junctions were in more distal segments. Serially interconnected neurons could be traced laterally in a boundless manner through multiple gap junctions. Comparison to the orientation-preference columns revealed that parvalbumin-immunoreactive cells distribute randomly whereby their large dendritic fields overlap considerably and cover different orientation columns. It is proposed that this dense and homogeneous electrical coupling of interneurons supports the precise synchronization of neuronal populations with differing feature preferences thereby providing a temporal frame for the generation of distributed representations.

Impaired face discrimination in acquired prosopagnosia is associated with abnormal response to individual faces in the right middle fusiform gyrus

The middle fusiform gyrus (MFG) and the inferior occipital gyrus (IOG) are activated by both detection and identification of faces. Paradoxically, patients with acquired prosopagnosia following lesions to either of these regions in the right hemisphere cannot identify faces, but can still detect faces. Here we acquired functional magnetic resonance imaging (fMRI) data during face processing in a patient presenting a specific deficit in individual face recognition, following lesions encompassing the right IOG. Using an adaptation paradigm we show that the fMRI signal in the rMFG of the patient, while being larger in response to faces as compared to objects, does not differ between conditions presenting identical and distinct faces, in contrast to the larger response to distinct faces observed in controls. These results suggest that individual discrimination of faces critically depends on the integrity of both the rMFG and the rIOG, which may interact through re-entrant cortical connections in the normal brain.

Primary visual cortex activity along the apparent-motion trace reflects illusory perception

The illusion of apparent motion can be induced when visual stimuli are successively presented at different locations. It has been shown in previous studies that motion-sensitive regions in extrastriate cortex are relevant for the processing of apparent motion, but it is unclear whether primary visual cortex (V1) is also involved in the representation of the illusory motion path. We investigated, in human subjects, apparent-motion-related activity in patches of V1 representing locations along the path of illusory stimulus motion using functional magnetic resonance imaging. Here we show that apparent motion caused a blood-oxygenation-level-dependent response along the V1 representations of the apparent-motion path, including regions that were not directly activated by the apparent-motion-inducing stimuli. This response was unaltered when participants had to perform an attention-demanding task that diverted their attention away from the stimulus. With a bistable motion quartet, we confirmed that the activity was related to the conscious perception of movement. Our data suggest that V1 is part of the network that represents the illusory path of apparent motion. The activation in V1 can be explained either by lateral interactions within V1 or by feedback mechanisms from higher visual areas, especially the motion-sensitive human MT/V5 complex.

Using fMRI in humans, the authors reveal a clear role for V1 cortex in forming an illusory perception of motion when stationary stimuli are successively flashed in different locations.

The importance of being agranular: a comparative account of visual and motor cortex

The agranular cortex is an important landmark-anatomically, as the architectural flag of mammalian motor cortex, and historically, as a spur to the development of theories of localization of function. But why, exactly, do agranularity and motor function go together? To address this question, it should be noted that not only does motor cortex lack granular layer four, it also has a relatively thinner layer three. Therefore, it is the two layers which principally constitute the ascending pathways through the sensory (granular) cortex that have regressed in motor cortex: simply stated, motor cortex does not engage in serial reprocessing of incoming sensory data. But why should a granular architecture not be demanded by the downstream relay of motor instructions through the motor cortex? The scant anatomical evidence available regarding laminar patterns suggests that the pathways from frontal and premotor areas to the primary motor cortex actually bear a greater resemblance to the descending, or feedback connections of sensory cortex that avoid the granular layer. The action of feedback connections is generally described as "modulatory" at a cellular level, or "selective" in terms of systems analysis. By contrast, ascending connections may be labelled "driving" or "instructive". Where the motor cortex uses driving inputs, they are most readily identified as sensory signals instructing the visual location of targets and the kinaesthetic state of the body. Visual signals may activate motor concepts, e.g. "mirror neurons", and the motor plan must select the appropriate muscles and forces to put the plan into action, if the decision to move is taken. This, perhaps, is why "driving" motor signals might be inappropriate-the optimal selection and its execution are conditional upon both kinaesthetic and motivational factors. The argument, summarized above, is constructed in honour of Korbinian Brodmann's centenary, and follows two of the fundamental principles of his school of thought: that uniformities in cortical structure, and development imply global conservation of some aspects of function, whereas regional variations in architecture can be used to chart the "organs" of the cortex, and perhaps to understand their functional differences.

Impact of repetitive transcranial magnetic stimulation of the parietal cortex on metabolic brain activity: a 14C-2DG tracing study in the cat

Transcranial magnetic stimulation (TMS) is increasingly utilized in clinical neurology and neuroscience. However, detailed knowledge of the impact and specificity of the effects of TMS on brain activity remains unresolved. We have used 14C-labeled deoxyglucose (14C-2DG) mapping during repetitive TMS (rTMS) of the posterior and inferior parietal cortex in anesthetized cats to study, with exquisite spatial resolution, the local and distant effects of rTMS on brain activity. High-frequency rTMS decreases metabolic activity at the primary site of stimulation with respect to homologue areas in the unstimulated hemisphere. In addition, rTMS induces specific distant effects on cortical and subcortical regions known to receive substantial efferent projections from the stimulated cortex. The magnitude of this distal impact is correlated with the strength of the anatomical projections. Thus, in the anesthetized animal, the impact of rTMS is upon a distributed network of structures connected to the primary site of application.

Functional organization of visual cortex in the owl monkey

In this study, we compared the organization of orientation preference in visual areas V1, V2, and V3. Within these visual areas, we also quantified the relationship between orientation preference and cytochrome oxidase (CO) staining patterns. V1 maps of orientation preference contained both pinwheels and linear zones. The location of CO blobs did not relate in a systematic way to maps of orientation; although, as in other primates, there were approximately twice as many pinwheels as CO blobs. V2 contained bands of high and low orientation selectivity. The bands of high orientation selectivity were organized into pinwheels and linear zones, but iso-orientation domains were twice as large as those in V1. Quantitative comparisons between bands containing high or low orientation selectivity and CO dark and light bands suggested that at least four functional compartments exist in V2, CO dense bands with either high or low orientation selectivity, and CO light bands with either high or low selectivity. We also demonstrated that two functional compartments exist in V3, with zones of high orientation selectivity corresponding to CO dense areas and zones of low orientation selectivity corresponding to CO pale areas. Together with previous findings, these results suggest that the modular organization of V1 is similar across primates and indeed across most mammals. V2 organization in owl monkeys also appears similar to that of other simians but different from that of prosimians and other mammals. Finally, V3 of owl monkeys shows a compartmental organization for orientation selectivity that remains to be demonstrated in other primates.

Inferential constraint sets in the organization of visual expectation

Many lines of evidence indicate that considering visual perception as a passive, stimulus-driven, feedforward decoding process is no longer tenable. Visual perception naturally occurs within the context of an integrated array of ongoing cognitive processes involving memory, perception in other modalities, and motor control. In many situations, these processes allow expectations to be formed for likely visual events. This article explores the idea that the formation of visual expectations involves the active organization of visual cortical areas, providing a framework of contextual information within which expected events are interpreted. Retinal inputs are treated as constraints that feed into a complex system of interacting visual cortical areas and thalamic nuclei, which are concurrently imposing constraints on one another. Although the nature of expectational organization in the visual cortex is not well-understood, a reasonable hypothesis is that expectation involves the mutual constraint of spatiotemporal activity patterns in multiple visual cortical areas. In this scenario, expectation is instantiated by a set of activity patterns in high-level visual cortical areas that impose constraints on one another as well as on low-level areas according to the partial information that is available about expected retinal inputs. One approach to testing this proposal is through the analysis of simultaneously recorded local field potentials (LFPs) from local neuronal assemblies in multiple visual cortical areas. Analysis of LFPs by multivariate autoregressive modeling is showing promise in revealing the organization of expectation in visual cortex.

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.

THE HUMAN VISUAL CORTEX

The discovery and analysis of cortical visual areas is a major accomplishment of visual neuroscience. In the past decade the use of noninvasive functional imaging, particularly functional magnetic resonance imaging (fMRI), has dramatically increased our detailed knowledge of the functional organization of the human visual cortex and its relation to visual perception. The fMRI method offers a major advantage over other techniques applied in neuroscience by providing a large-scale neuroanatomical perspective that stems from its ability to image the entire brain essentially at once. This bird's eye view has the potential to reveal large-scale principles within the very complex plethora of visual areas. Thus, it could arrange the entire constellation of human visual areas in a unified functional organizational framework. Here we review recent findings and methods employed to uncover the functional properties of the human visual cortex focusing on two themes: functional specialization and hierarchical processing.

Spatial frequency-dependent feedback of visual cortical area 21a modulating functional orientation column maps in areas 17 and 18 of the cat

The feedback effect of activity of area 21a on orientation maps of areas 17 and 18 was investigated in cats using intrinsic signal optical imaging. A spatial frequency-dependent decrease in response amplitude of orientation maps to grating stimuli was observed in areas 17 and 18 when area 21a was inactivated by local injection of GABA, or by a lesion induced by liquid nitrogen freezing. The decrease in response amplitude of orientation maps of areas 17 and 18 after the area 21a inactivation paralleled the normal response without the inactivation. Application in area 21a of bicuculline, a GABAa receptor antagonist caused an increase in response amplitude of orientation maps of area 17. The results indicate a positive feedback from high-order visual cortical area 21a to lower-order areas underlying a spatial frequency-dependent mechanism.

Processing Deficits in Primary Visual Cortex of Amblyopic Cats

Early esotropic squint frequently results in permanent visual deficits in one eye, referred to as strabismic amblyopia. The neurophysiological substrate corresponding to these deficits is still a matter of investigation. Electrophysiological evidence is available for disturbed neuronal interactions in both V1 and higher cortical areas. In this study, we investigated the modulation of responses in cat V1 to gratings at different orientations and spatial frequencies (SFs; 0.1–2.0 cycles/°) with optical imaging of intrinsic signals. Maps evoked by both eyes were well modulated at most spatial frequencies. The layout of the maps resembled that of normal cats, and iso-orientation domains tended to cross adjacent ocular dominance borders preferentially at right angles. Visually evoked potentials (VEPs) were recorded at SFs ranging from 0.1 to 3.5 cycles/° and revealed a consistently weaker eye for the majority of squinting cats. At each SF, interocular differences in VEP amplitudes corresponded well with differences in orientation response and selectivity in the maps. At 0.7–1.3 cycles/°, population orientation selectivity was significantly lower for the weaker eye in cats with VEP differences compared with those with no VEP amplitude differences. In addition, the cutoff SF, above which gratings no longer induced orientation maps, was lower for the weaker eye (≥1.0 cycles/°). These data reveal a close correlation between the loss of visual acuity in amblyopia as assessed by VEPs and the modulation of neuronal activation as seen by optical imaging of intrinsic signals. Furthermore, the results indicate that amblyopia is associated with altered intracortical processing already in V1.

Long-lasting anticonvulsant effect of focal cooling on experimental neocortical seizures

PURPOSE

Previous clinical and experimental observations have demonstrated that cooling the brain can rapidly terminate focal seizures. We wished to determine whether cooling at regular intervals could prevent or attenuate the development of seizures in a model of focal epilepsy.

METHODS

We induced focal neocortical seizures in halothane-anesthetized rats by the microinjection of 4-aminopyridine (4-AP) into the motor cortex. With a small thermoelectric device, the site of the 4-AP injection was either cooled intermittently (PostCool) for 30 s every 2 min, starting 15 min after the 4-AP injection, or precooled (PreCool) for 30 min before 4-AP injection, by using the same 30-s cooling cycle. Seizures were quantified in 30-min observation periods for 1.5 h.

RESULTS

The average durations of PostCool and PreCool seizures were shorter than those of controls (p < 0.001). In addition, total seizure duration was significantly reduced in both groups, compared with controls (p < 0.01). The ratio of the root mean square power during a seizure to power in the immediate preseizure period was reduced in both PostCool and PreCool groups (p < 0.001). The number of seizures significantly declined over a 30- to 60-min period in both experimental groups, and by 60 min, no seizures were evident.

CONCLUSIONS

These experiments show that gentle cooling to 20 degrees C is capable of markedly reducing subsequent seizure frequency and intensity. The effects in our model, which generates very frequent and intense ictal activity, were robust, suggesting that prophylactic cooling might be even more beneficial in clinical situations. The physiologic mechanism for this preventive effect requires elucidation.

Cerebral areas mediating visual redirection of gaze: Cooling deactivation of 15 loci in the cat

In humans, damage to posterior parietal or frontal cortices often induces a severe impairment of the ability to redirect gaze to visual targets introduced into the contralateral field. In cats, unilateral deactivation of the posterior middle suprasylvian (pMS) sulcus in the posterior inferior parietal region also results in an equally severe impairment of visually mediated redirection of gaze. In this study we tested the contributions of the pMS cortex and 14 other cortical regions in mediating redirection of gaze to visual targets in 31 adult cats. Unilateral cooling deactivation of three adjacent regions along the posterior bend of the suprasylvian sulcus (posterior middle suprasylvian sulcus, posterior suprasylvian sulcus, and dorsal posterior ectosylvian gyrus at the confluence of the occipital, parietal, and temporal cortices) eliminated visually mediated redirection of gaze towards stimuli introduced into the contralateral hemifield, while the redirection of gaze toward the ipsilateral hemifield remained highly proficient. Additional cortical loci critical for visually mediated redirection of gaze include the anterior suprasylvian gyrus (lateral area 5, anterior inferior parietal cortex) and medial area 6 in the frontal region. Cooling deactivation of: 1) dorsal or 2) ventral posterior suprasylvian gyrus; 3) ventral posterior ectosylvian gyrus, 4) middle ectosylvian gyrus; 5) anterior or 6) posterior middle suprasylvian gyrus (area 7); 7) anterior middle suprasylvian sulcus; 8) medial area 5; 9) the visual portion of the anterior ectosylvian sulcus (AES); 10) or lateral area 6 were all without impact on the ability to redirect gaze. In summary, we identified a prominent field of cortex at the junction of the temporo-occipito-parietal cortices (regions pMS, dPE, PS), an anterior inferior parietal field (region 5L), and a frontal field (region 6M) that all contribute critically to the ability to redirect gaze to novel stimuli introduced into the visual field during fixation. These loci have several features in common with cortical fields in monkey and human brains that contribute to the visually guided redirection of the head and eyes.

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.

Single-trial variability in early visual neuromagnetic responses: an explorative study based on the regional activation contributing to the N70m peak

Cortical activity evoked by repeated identical sensory stimulation is extremely variable. The source of this variability is often assigned to "random ongoing background activity" which is considered to be irrelevant to the processing of the stimuli and can therefore be eliminated by ensemble averaging. In this work, we studied the single-trial variability in neuromagnetic responses elicited by circular checkerboard pattern stimuli with radii of 1.8 degrees, 3.7 degrees, and 4.5 degrees. For most of the MEG sensors over the occipital areas, the averaged signal showed a clear early (N70m) response following the stimulus onset and this response was modulated by the checkerboard size. A data-driven spatial filter was used to extract one of the many possible composite time courses of single-trial activity corresponding to the complex of N70m generators. Pattern analysis principles were then employed to analyze, classify, and handle the extracted temporal patterns. We explored whether these patterns correspond to distinct response modes, which could characterize the evoked response better than the averaged signal and over an extended range of latencies around N70m. A novel scheme for detecting and organizing the structure in single-trial recordings was utilized. This served as a basis for comparisons between runs with different checkerboard sizes and provided a causal interpretation of variability in terms of regional dynamics, including the relatively weak activation in primary visual cortex. At the level of single trial activity, the polymorphic response to a simple stimulus is generated by a coupling of polymodal areas and cooperative activity in striate and extrastriate areas. Our results suggest a state-dependent response with a wide range of characteristic time scales and indicate the ongoing activity as a marker of the responsiveness state.

Quantitative analyses of principal and secondary compound parieto-occipital feedback pathways in cat

The purpose of our study was to quantify the magnitude of principal and secondary pathways emanating from the middle suprasylvian (MS) region of visuoparietal cortex and terminating in area 18 of primary visual cortex. These pathways transmit feedback signals from visuoparietal cortex to primary visual cortex. (1) WGA-HRP was injected into area 18 to identify inputs from visual structures. In terms of numbers of neurons, feedback projections to area 18 from MS sulcal cortex (areas PMLS, AMLS and PLLS) comprise 26% of inputs from all visual structures. Of these neurons, between 21% and 34.9% are located in upper layers 2-4 and the dominant numbers are located in deep layers 5 and 6. Areas 17 (11.8%) and 19 (11.2%) provide more modest cortical inputs, and another eight areas provide a combined total of 4.3% of inputs. The sum of neurons in all subcompartments of the lateral geniculate nucleus (LGN) accounts for another 34.8% of the input to area 18, whereas inputs from the lateral division of the lateral-posterior nucleus (LPl) account for the final 11.9%. (2) Injection of tritiated-((3)H)-amino acids into MS sulcal cortex revealed substantial direct projections from MS cortex that terminated in all layers of area 18, but with a markedly lower density in layer 4. Projections from MS cortex to both areas 17 and 19 are of similar density and characteristics, whereas those to other cortical targets have very low densities. Quantification also revealed minor-to-modest axon projections to all components of LGN and a massive projection throughout the LP-Pul complex. (3) Superposition of the labeled terminal and cell fields identified secondary, compound feedback pathways from MS cortex to area 18. The largest secondary pathway is massive and it includes the LPl nucleus. Much more modest secondary pathways include areas 17 and 19, and LGN. The relative magnitudes of the secondary pathways suggest that the one through LPl exerts a major influence on area 18, whereas the others exert more modest or minor influences. MS cortex in the contralateral hemisphere also innervates area 18 directly. These data are important for interpreting the impact of deactivating feedback projections from visuoparietal cortex on occipital cortex.

Local Cooling: A Therapy for Intractable Neocortical Epilepsy

Therapy for many of the neocortical epilepsies remains unsatisfactory. Recent research has demonstrated that focal cooling, using thermoelectric (Peltier) devices, may be capable of terminating, or possibly even preventing, some types of seizures.