Stimulus‐dependent augmented gamma oscillatory activity between the functionally connected cortical neurons in the primary visual cortex

Neuronal assemblies typically synchronise within the gamma oscillatory band (30–80 Hz) and are fundamental to information processing. Despite numerous investigations, the exact mechanisms and origins of gamma oscillations are yet to be known. Here, through multiunit recordings in the primary visual cortex of cats, we show that the strength of gamma power (20–40 and 60–80 Hz) is significantly stronger between the functionally connected units than between the unconnected units within an assembly. Furthermore, there is increased frequency coherence in the gamma band between the connected units than between the unconnected units. Finally, the higher gamma rhythms (60–80 Hz) are mostly linked to the fast‐spiking neurons. These results led us to postulate that gamma oscillations are intrinsically generated between the connected units within cell assemblies (microcircuits) in relation to the stimulus within an emergent ‘50‐ms temporal window of opportunity’.     

Quantitative analysis of the choline acetyltransferase-immunoreactive axonal network in the cat primary visual cortex: I. Adult cats

The laminar distribution of cholinergic axons was analyzed quantitatively in the visual cortex of adult cats by using immunocytochemistry with a monoclonal antibody against choline acetyltransferase (ChAT). ChAT(+) fibers and varicosities were counted at different locations within area 17 and the distribution patterns in various animals were compared. Choline acetyltransferase-immunoreactivity was localized in fine, varicose fibers, which were present in all layers of the visual cortex. The density of labeled fibers was highest in layer I, which contained 14% of all fibers and 19.5% of all varicosities, and decreased toward deeper layers. The number of varicosities decreased more markedly toward deeper layers than the frequency of fibers. These distribution patterns were very consistent, showing only slight intra- and interindividual variability.     

Direction selectivity of neurons in the visual cortex is non‐linear and lamina‐dependent

Neurons in the visual cortex are generally selective to direction of movement of a stimulus. Although models of this direction selectivity (DS) assume linearity, experimental data show stronger degrees of DS than those predicted by linear models. Our current study was intended to determine the degree of non‐linearity of the DS mechanism for cells within different laminae of the cat's primary visual cortex. To do this, we analysed cells in our database by using neurophysiological and histological approaches to quantify non‐linear components of DS in four principal cortical laminae (layers 2/3, 4, 5, and 6). We used a DS index (DSI) to quantify degrees of DS in our sample. Our results showed laminar differences. In layer 4, the main thalamic input region, most neurons were of the simple type and showed high DSI values. For complex cells in layer 4, there was a broad distribution of DSI values. Similar features were observed in layer 2/3, but complex cells were dominant. In deeper layers (5 and 6), DSI value distributions were characterized by clear peaks at high values. Independently of specific lamina, high DSI values were accompanied by narrow orientation tuning widths. Differences in orientation tuning for non‐preferred vs. preferred directions were smallest in layer 4 and largest in layer 6. These results are consistent with a non‐linear process of intra‐cortical inhibition that enhances DS by selective suppression of neuronal firing for non‐preferred directions of stimulus motion in a lamina‐dependent manner. Other potential mechanisms are also considered.     

Visual discrimination after bilateral removal of the visual cortex in the rabbit

Rabbits were trained on a brightness and a vertical vs horizontal discrimination. After bilateral removal of the visual cortex the threshold of brightness discrimination was significantly higher. In addition, there was a severe impairment of striated pattern discrimination.     

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

Experiments were carried out to begin to define the behavioral functions of the lateral suprasylvian (LS) visual area of the cat's cortex. Behavioral tasks were chosen for analysis on the basis of previous suggestions in the literature concerning possible functions of LS cortex and its afferent pathways. These tasks included the ability of cats to orient the head and eyes to a stimulus presented in particular locations in the visual field, the ability to learn successive reversals of a two-choice visual pattern discrimination, and the ability to maintain or shift attention between relevant or irrelevant visual form and brightness cues. Eight cats were trained on each of these tasks. Four of the cats then received bilateral lesions of LS cortex, including the AMLS and PMLS regions, and the remaining 4 cats were used to assess normal retention. The LS cortex lesions had no significant effect upon performance of any of the behaviors tested. Thus, this region of cortex appears to play no essential role in simple brightness, form, and pattern discrimination performance, visual reversal learning, maintaining and shifting visual attention, or orienting the head and eyes to stimuli in the visual field. These results are discussed in relation to previous lesion studies involving large regions of the cat's extrastriate cortex and studies in other species. Possible functions of LS cortex, based upon recent electrophysiological studies, are suggested.     

Gamma-frequency fluctuations of the membrane potential and response selectivity in visual cortical neurons

Fluctuations at frequencies of 25-70 Hz is an inherent property of cortical activity. These rapid, gamma-range fluctuations are apparent in the local field potentials, in spiking of cells and cell groups, and in the membrane potential of neurons. To investigate stimulus dependence of the gamma-frequency fluctuations of the membrane potential, we have recorded intracellularly responses of cells in cat visual cortex to presentation of moving gratings. We found gamma-range fluctuations of the membrane potential in both simple and complex cells. The strength of the gamma-frequency fluctuations correlated with the stimulus optimality. Furthermore, the amplitude of the gamma-frequency fluctuations correlated with the phase of stimulus-imposed slow changes of the membrane potential. The combination of these features makes cortical neurons capable of encoding the slow changes in the visual world in a kind of amplitude modulation of the high frequency fluctuations. This assures reliable transformation of the membrane potential changes into spike responses without compromising the temporal resolution of visual information encoding in the low frequency range.     

Is stimulus movement of particular importance in the functioning of cat visual cortex?

On average, cells in areas 17, 18 or 19 of normal and stroboscopically reared cats respond with equal maximum firing rates to a narrow bar of light whther moving at the optimal velocity in the preferred direction of movement or flashed at the optimal position within the receptive field. Firing rates in the non-preferred direction of movement are much lower, however, suggesting that direction selectivity is mostly based on inhibition.     

Resting‐state functional MRI: Functional connectivity analysis of the visual cortex in primary open‐angle glaucoma patients

Purpose: To analyze functional connectivity (FC) of the visual cortex using resting‐state functional MRI in human primary open‐angle glaucoma (POAG) patients. Materials and Methods: Twenty‐two patients with known POAG and 22 age‐matched controls were included in this IRB‐approved study. Subjects were evaluated by 3 T MR using resting‐state blood oxygenation level dependent and three‐dimensional brain volume imaging (3D‐BRAVO) MRI. Data processing was performed with standard software. FC maps were generated from Brodmann areas (BA) 17/18/19/7 in a voxel‐wise fashion. Region of interest analysis was used to specifically examine FC among each pair of BA17/18/19/7. Results: Voxel‐wise analyses demonstrated decreased FC in the POAG group between the primary visual cortex (BA17) and the right inferior temporal, left fusiform, left middle occipital, right superior occipital, left postcentral, right precentral gyri, and anterior lobe of the left cerebellum. Increased FC was found between BA17 and the left cerebellum, right middle cerebellar peduncle, right middle frontal gyrus, and extra‐nuclear gyrus (P < 0.05). In terms of the higher visual cortices (BA18/19), positive FC was disappeared with the cerebellar vermis, right middle temporal, and right superior temporal gyri (P < 0.05). Negative FC was disappeared between BA18/19 and the right insular gyrus (P < 0.05). Region of interest analysis demonstrated no statistically significant differences in FC between the POAG patients relative to the controls (P > 0.05). Conclusion: Changes in FC of the visual cortex are found in patients with POAG. These include alterations in connectivity between the visual cortex and associative visual areas along with disrupted connectivity between the primary and higher visual areas. Hum Brain Mapp 34:2455–2463, 2013. © 2012 Wiley Periodicals, Inc.     

Sensitivity to direction and orientation of random dot stereobars in the monkey visual cortex

We are able to judge the direction of movement and orientation of objects because they have contrast-defined edges. However, we are also able to perceive the orientation and direction of movement of stereobars made of random dot stereograms in the absence of contrast-defined edges. We recorded 207 disparity-sensitive cells from visual areas V1 and V2 of two Macaca mulatta monkeys while performing an attentive fixation task. Luminance defined bars and random-dot stereo-defined bars were used to assess direction and orientation selectivity of these cells. Orientation and direction preference for luminance bars and for stereobars showed a statistically significant relationship (r=0.83, P<0.01 for direction; r=0.63, P<0.01 for orientation). However, disparity-sensitive cells from these areas seem to be more sensitive to luminance than to stereobars regarding orientation and direction of movement. Similar results were obtained when the two areas were considered separately. Our results show that cells in areas V1 and V2 of the monkey visual cortex are able to detect the orientation and direction of movement of stereobars in a manner similar to those of luminance-defined bars. This finding is relevant because to detect the direction and orientation of stereobars a comparison between left and right eye inputs is required.     

A time-based stereoscopic depth mechanism in the visual cortex

We propose a new depth mechanism, operating in the visual cortex, which uses temporal as well as spatial cues. By varying the timing of input to the two eyes, and the position of stimuli on the two receptive fields, we show that both temporal and spatial disparities influence the binocular responses of single cortical cells.     

Impairments in visual discrimination after perirhinal cortex lesions: testing 'declarative' vs. 'perceptual-mnemonic' views of perirhinal cortex function

Two experiments tested the predictions of 'declarative' vs. 'perceptual-mnemonic' views of perirhinal cortex function. The former view predicts that perirhinal cortex lesions should impair rapidly learned, but not more slowly learned, visual discriminations, whereas the latter view predicts that impairments should be related not to speed of learning but to perceptual factors. It was found that monkeys with perirhinal cortex lesions were impaired in the acquisition and performance of slowly learned, perceptually difficult greyscale picture discriminations, but were not impaired in the acquisition of rapidly learned, perceptually easier discriminations. In addition, these same monkeys were not impaired in the acquisition or performance of difficult colour or size discriminations, indicating that the observed pattern of impairments was not due to ceiling effects or difficulty per se. These findings, taken together, are consistent with the 'perceptual-mnemonic' view that the perirhinal cortex is involved in both perception and memory, but are not consistent with the 'declarative' view that the perirhinal cortex is important exclusively for declarative memory, having little or no role in perception. Moreover, the results are consistent with the more specific proposal that the perirhinal cortex contributes to the solution of complex visual discriminations with a high degree of 'feature ambiguity', a property of visual discrimination problems that can emerge when features of an object are rewarded when part of one object, but not when part of another. These and other recent findings suggest the need for a revision of prevailing views regarding the neural organization of perception and memory.     

Developmentally degraded directional selectivity of the auditory cortex can be restored by auditory discrimination training in adults

Sound localization is one of the most important tasks performed by the auditory system. Studies have shown that intensive training can remediate deteriorated frequency representations and temporal information processing in the adult primary auditory cortex (A1) induced by early post-natal pulsed noise exposure. Here we demonstrate that intensive sound location discrimination training improved the dysfunctional sound azimuth selectivity degraded by early post-natal pulsed noise exposure. Rats exposed to pulsed white noise during a post-natal critical period were successfully trained to identify a target sound stimulus with specific azimuth angle that changed daily on a random schedule. Consistent with recovery of behavioral accuracy for sound-azimuth discriminations, we found that the average angular range (AR) of A1 neuronal azimuth selective curves in trained noise-raised rats was not significantly different from that measured in control rats, while the average AR of A1 neurons in untrained noise-raised rats was significantly higher, indicating that these neurons were less direction selective. Directional selectivity of A1 neurons was normalized by training, thus demonstrating the benefits of sensory discrimination training as a strategy for restoring auditory function in adult mammals damaged by sensory disruption during critical periods of cortical development.     

On the domain‐specificity of the visual and non‐visual face‐selective regions

What happens in our brains when we see a face? The neural mechanisms of face processing – namely, the face‐selective regions – have been extensively explored. Research has traditionally focused on visual cortex face‐regions; more recently, the role of face‐regions outside the visual cortex (i.e., non‐visual‐cortex face‐regions) has been acknowledged as well. The major quest today is to reveal the functional role of each this region in face processing. To make progress in this direction, it is essential to understand the extent to which the face‐regions, and particularly the non‐visual‐cortex face‐regions, process only faces (i.e., face‐specific, domain‐specific processing) or rather are involved in a more domain‐general cognitive processing. In the current functional MRI study, we systematically examined the activity of the whole face‐network during face‐unrelated reading task (i.e., written meaningful sentences with content unrelated to faces/people and non‐words). We found that the non‐visual‐cortex (i.e., right lateral prefrontal cortex and posterior superior temporal sulcus), but not the visual cortex face‐regions, responded significantly stronger to sentences than to non‐words. In general, some degree of sentence selectivity was found in all non‐visual‐cortex cortex. Present result highlights the possibility that the processing in the non‐visual‐cortex face‐selective regions might not be exclusively face‐specific, but rather more or even fully domain‐general. In this paper, we illustrate how the knowledge about domain‐general processing in face‐regions can help to advance our general understanding of face processing mechanisms. Our results therefore suggest that the problem of face processing should be approached in the broader scope of cognition in general.     

Role of primary visual cortex in the binocular integration of plaid motion perception

This study assessed the early mechanisms underlying perception of plaid motion. Thus, two superimposed gratings drifting in a rightward direction composed plaid stimuli whose global motion direction was perceived as the vector sum of the two components. The first experiment was aimed at comparing the perception of plaid motion when both components were presented to both eyes (dioptic) or separately to each eye (dichoptic). When components of the patterns had identical spatial frequencies, coherent motion was correctly perceived under dioptic and dichoptic viewing condition. However, the perceived direction deviated from the predicted direction when spatial frequency differences were introduced between components in both conditions. The results suggest that motion integration follows similar rules for dioptic and dichoptic plaids even though performance under dichoptic viewing did not reach dioptic levels. In the second experiment, the role of early cortical areas in the processing of both plaids was examined. As convergence of monocular inputs is needed for dichoptic perception, we tested the hypothesis that primary visual cortex (V1) is required for dichoptic plaid processing by delivering repetitive transcranial magnetic stimulation to this area. Ten minutes of magnetic stimulation disrupted subsequent dichoptic perception for approximately 15 min, whereas no significant changes were observed for dioptic plaid perception. Taken together, these findings suggest that V1 is not crucial for the processing of dioptic plaids but it is necessary for the binocular integration underlying dichoptic plaid motion perception.     

Multiple asynchronous stimulus‐ and task‐dependent hierarchies (STDH) within the visual brain's parallel processing systems

Results from a variety of sources, some many years old, lead ineluctably to a re‐appraisal of the twin strategies of hierarchical and parallel processing used by the brain to construct an image of the visual world. Contrary to common supposition, there are at least three ‘feed‐forward’ anatomical hierarchies that reach the primary visual cortex (V1) and the specialized visual areas outside it, in parallel. These anatomical hierarchies do not conform to the temporal order with which visual signals reach the specialized visual areas through V1. Furthermore, neither the anatomical hierarchies nor the temporal order of activation through V1 predict the perceptual hierarchies. The latter shows that we see (and become aware of) different visual attributes at different times, with colour leading form (orientation) and directional visual motion, even though signals from fast‐moving, high‐contrast stimuli are among the earliest to reach the visual cortex (of area V5). Parallel processing, on the other hand, is much more ubiquitous than commonly supposed but is subject to a barely noticed but fundamental aspect of brain operations, namely that different parallel systems operate asynchronously with respect to each other and reach perceptual endpoints at different times. This re‐assessment leads to the conclusion that the visual brain is constituted of multiple, parallel and asynchronously operating task‐ and stimulus‐dependent hierarchies (STDH); which of these parallel anatomical hierarchies have temporal and perceptual precedence at any given moment is stimulus and task related, and dependent on the visual brain's ability to undertake multiple operations asynchronously.     

Linearity of the fMRI response in category‐selective regions of human visual cortex

The goal of this study was to determine the linearity of the blood oxygen level‐dependent (BOLD) response, as measured by functional magnetic resonance imaging (fMRI), in category‐selective regions of human visual cortex. We defined regions of the temporal lobe that were selective to faces (fusiform face area, FFA) and places (parahippocampal place area, PPA). We then determined the linearity of the BOLD response in these regions to their preferred and nonpreferred stimuli. First, we tested the principle of scaling. As we increased the visibility of the stimulus, there was a corresponding linear increase in the fMRI signal in the FFA and PPA to their preferred stimulus (face and place, respectively). In contrast, responses in the FFA and PPA to the nonpreferred stimulus did not conform to the principle of scaling. Next, we asked whether the fMRI response in these regions of visual cortex conformed to the principle of additivity. To assess this, we determined whether the response to a long stimulus block could be predicted by adding the response to multiple shorter duration blocks. Although the fMRI response in the FFA and PPA was generally linear to the preferred stimulus, a more nonlinear response was apparent to the nonpreferred stimulus. In conclusion, the linearity of the BOLD response in the human ventral visual pathway varied across cortical region and stimulus category. This suggests that measures of linearity may provide a useful indication of neural selectivity in the brain. Hum Brain Mapp, 2009. © 2008 Wiley‐Liss, Inc.     

Endogenously generated gamma‐band oscillations in early visual cortex: A neurofeedback study

Human subjects were trained with neurofeedback (NFB) to enhance the power of narrow‐band gamma oscillations in circumscribed regions of early visual cortex. To select the region and the oscillation frequency for NFB training, gamma oscillations were induced with locally presented drifting gratings. The source and frequency of these induced oscillations were determined using beamforming methods. During NFB training the power of narrow band gamma oscillations was continuously extracted from this source with online beamforming and converted into the pitch of a tone signal. We found that seven out of ten subjects were able to selectively increase the amplitude of gamma oscillations in the absence of visual stimulation. One subject however failed completely and two subjects succeeded to manipulate the feedback signal by contraction of muscles. In all subjects the attempts to enhance visual gamma oscillations were associated with an increase of beta oscillations over precentral/frontal regions. Only successful subjects exhibited an additional marked increase of theta oscillations over precentral/prefrontal and temporal regions whereas unsuccessful subjects showed an increase of alpha band oscillations over occipital regions. We argue that spatially confined networks in early visual cortex can be entrained to engage in narrow band gamma oscillations not only by visual stimuli but also by top down signals. We interpret the concomitant increase in beta oscillations as indication for an engagement of the fronto‐parietal attention network and the increase of theta oscillations as a correlate of imagery. Our finding support the application of NFB in disease conditions associated with impaired gamma synchronization.     

Noninvasive extraction of microsecond‐scale dynamics from human motor cortex

State‐of‐the‐art noninvasive electromagnetic recording techniques allow observing neuronal dynamics down to the millisecond scale. Direct measurement of faster events has been limited to in vitro or invasive recordings. To overcome this limitation, we introduce a new paradigm for transcranial magnetic stimulation. We adjusted the stimulation waveform on the microsecond scale, by varying the duration between the positive and negative phase of the induced electric field, and studied corresponding changes in the elicited motor responses. The magnitude of the electric field needed for given motor‐evoked potential amplitude decreased exponentially as a function of this duration with a time constant of 17 µs. Our indirect noninvasive measurement paradigm allows studying neuronal kinetics on the microsecond scale in vivo.