Biomedical subjects

V A Lamme

Publications and source records attributed to V A Lamme.

At least 19 recordsLinked to original sources

PubMed2001

A neural correlate of working memory in the monkey primary visual cortex.

The brain frequently needs to store information for short periods. In vision, this means that the perceptual correlate of a stimulus has to be maintained temporally once the stimulus has been removed from the visual scene. However, it is not known how the visual system transfers sensory information into a memory component. Here, we identify a neural correlate of working memory in the monkey primary visual cortex (V1). We propose that this component may link sensory activity with memory activity.

Animals↗

PubMed2001

Colour-related oscillations in the striate cortex of awake monkeys: "reverse" observations.

Gamma oscillations of 30-70 Hz in local electroencephalograms (EEGs) were observed in primary visual cortex (V1) of monkeys when they viewed coloured stimuli under conditions which were not part of a training paradigm. No oscillatory modulations were detected in simultaneously recorded spike trains, although the latter responded to the stimuli. The oscillations had much longer latencies than the spike responses. Proceeding in a "reverse" manner, the oscillations were taken as primary elements of the analyses, and relationships to the accompanying behaviour were sought. Besides colour stimulation, the oscillations were found to be related to a form of annoyance. Only two of four monkeys showed the phenomena. We conclude that the oscillations had a central origin.

Action Potentials↗

PubMed2001

Blindsight: the role of feedforward and feedback corticocortical connections.

When human subjects suffer from a lesion to the primary visual cortex, they lose all visual percepts in the region of space that corresponds to the site of the lesion. However, they are still capable of responding to stimuli in this region when asked to 'guess' or to execute forced-choice motor commands related to these stimuli. This phenomenon, termed blindsight, is still only partly understood. Here, the possible roles of feedforward and feedback corticocortical connections in the visual brain in the understanding of blindsight are reviewed. What emerges is substantial evidence in favor of the theory that unconscious visuo-motor transformations, as in blindsight, may be executed in an entirely feedforward processing cycle, while visual awareness is critically dependent on feedback connections to the primary visual cortex.

Awareness↗

PubMed2001

Two distinct modes of sensory processing observed in monkey primary visual cortex (V1).

Even salient sensory stimuli are sometimes not detected. What goes wrong in the brain in that case? Here we show that a late (> 100-ms) component of the neural activity in the primary visual cortex of the monkey is selectively suppressed when stimuli are not seen. As there is evidence that this activity depends on feedback from extrastriate areas, these findings suggest a specific role for recurrent processing when stimuli are reaching a perceptual level. Further results show that this perceptual level is situated between purely sensory and decision or motor stages of processing.

Action Potentials↗

PubMed2000

Modulations of primary visual cortex activity representing attentive and conscious scene perception.

In the visual cortex, information is transferred from one area to the next by means of feedforward connections. These connections shape the receptive field properties of neurons in subsequent visual areas. Horizontal and feedback connections modulate this neuronal activity, resulting in the phenomenon of contextual modulation. In area V1, where receptive field properties reflect only low level processing, contextual modulation can be observed that represents fully evaluated perceptual saliency of the features within the receptive field. Here, we discuss to what extent these modulations are related to high level visual processes like perceptual organization, attention and visual awareness. Contextual modulation appears to reflects a process very distinct from receptive field based processing. This process seems to integrate information from distant areas in visual cortex to neurophysiologically 'highlight' those neurons that represent image elements or features of objects that stand out perceptually. Moreover, similar modulations are observed in relation to whether objects are attended to or not. Finally, these modulations are only present when subjects are aware of the visual input.

Animals↗

PubMed2000

The implementation of visual routines.

Many visual tasks can be decomposed into a sequence of simpler subtasks. Ullman suggested that such subtasks are carried out by elemental operations that are implemented by specialized processes in the visual brain [Ullman, S. (1984). Visual routines. Cognition (18), 97-159]. According to this hypothesis, there are a limited number of elemental operations that, since they can be applied sequentially, may nevertheless give rise to a large number of visual routines. Examples of such elemental operations are visual search, texture segregation and contour grouping. Here we attempt to delineate how such elemental operations are implemented in the visual brain. When an image appears, feedforward processing rapidly leads to an activity pattern that is distributed across many visual areas. Thereafter, elemental operations come into play, and these are implemented by the modulation of firing rates. Firing rate modulations effectuate grouping of neural responses into coherent object representations. Moreover, they permit transfer of information from one operator to the next, which allows flexibility in the sequencing of operations. We discuss how the elemental operations provide a tool to relate cortical physiology to psychophysics, and suggest a reclassification of pre-attentive and attentive processes.

Animals↗

PubMed2000

The role of primary visual cortex (V1) in visual awareness.

In the search for the neural correlate of visual awareness, much controversy exists about the role of primary visual cortex. Here, the neurophysiological data from V1 recordings in awake monkeys are examined in light of two general classes of models of visual awareness. In the first model type, visual awareness is seen as being mediated either by a particular set of areas or pathways, or alternatively by a specific set of neurons. In these models, the role of V1 seems rather limited, as the mere activity of V1 cells seems insufficient to mediate awareness. In the second model type, awareness is hypothesized to be mediated by a global mechanism, i.e. a specific kind of activity not linked to a particular area or cell type. Two separate versions of global models are discussed, synchronous oscillations and spike rate modulations. It is shown that V1 synchrony does not reflect perception but rather the horizontal connections between neurons, indicating that V1 synchrony cannot be a direct neural correlate of conscious percepts. However, the rate of spike discharges of V1 neurons is strongly modulated by perceptual context, and these modulations correlate very well with aspects of perceptual organization, visual awareness, and attention. If these modulations serve as a neural correlate of visual awareness, then V1 contributes to that neural correlate. Whether V1 plays a role in the neural correlate of visual awareness thus strongly depends on the way visual awareness is hypothesized to be implemented in the brain.

Animals↗

PubMed2000

The distinct modes of vision offered by feedforward and recurrent processing.

An analysis of response latencies shows that when an image is presented to the visual system, neuronal activity is rapidly routed to a large number of visual areas. However, the activity of cortical neurons is not determined by this feedforward sweep alone. Horizontal connections within areas, and higher areas providing feedback, result in dynamic changes in tuning. The differences between feedforward and recurrent processing could prove pivotal in understanding the distinctions between attentive and pre-attentive vision as well as between conscious and unconscious vision. The feedforward sweep rapidly groups feature constellations that are hardwired in the visual brain, yet is probably incapable of yielding visual awareness; in many cases, recurrent processing is necessary before the features of an object are attentively grouped and the stimulus can enter consciousness.

Animals↗

PubMed1999

Separate processing dynamics for texture elements, boundaries and surfaces in primary visual cortex of the macaque monkey.

A visual scene is rapidly segmented into the regions that are occupied by different objects and background. Segmentation may be initiated from the detection of boundaries, followed by the filling-in of the surfaces between these boundaries to render them visible. Alternatively, segmentation may be based on grouping of surface elements that are similar, so that boundaries are (implicitly) identified as the borders between elements that are grouped into objects. Here, we present recordings from awake monkey primary visual cortex that show that in late (>80 ms) components of the neural responses a correlate of boundary formation is expressed, followed by a filling-in (also called colouring) between the edges. These data favour a model of segmentation where boundary formation initiates surface filling-in.

Animals↗

PubMed1998

Neuronal synchrony does not represent texture segregation.

The visual environment is perceived as an organized whole of objects and their surroundings. In many visual cortical areas, however, neurons are typically activated when a stimulus is presented over a very limited portion of the visual field, the receptive field of that neuron. To bridge the gap between this piecewise neuronal analysis and our global visual percepts, it has been postulated that neurons representing elements of the same object fire in synchrony to represent the perceptual organization of a scene. Experiments with stimuli such as moving bars or gratings have provided evidence for this hypothesis. We have further tested this by presenting monkeys with various textured scenes consisting of a figure on a background, and recorded neuronal activity in the primary visual cortex (area V1). Our results show no systematic relationship between the synchrony of firing of pairs of neurons and the perceptual organization of the scene. Instead, pairs of recording sites representing elements of the same figure most commonly showed equal amounts of synchrony between them as did pairs of which one site represented the figure and the other the background. We conclude that synchrony in V1 does not reflect the binding of features that leads to texture segregation.

Animals↗

PubMed1998

Object-based attention in the primary visual cortex of the macaque monkey.

Typical natural visual scenes contain many objects, which need to be segregated from each other and from the background. Present theories subdivide the processes responsible for this segregation into a pre-attentive and attentive system. The pre-attentive system segregates image regions that 'pop out' rapidly and in parallel across the visual field. In the primary visual cortex, responses to pre-attentively selected image regions are enhanced. When objects do not segregate automatically from the rest of the image, the time-consuming attentive system is recruited. Here we investigate whether attentive selection is also associated with a modulation of firing rates in area V1 of the brain in monkeys trained to perform a curve-tracing task. Neuronal responses to the various segments of a target curve were simultaneously enhanced relative to responses evoked by a distractor curve, even if the two curves crossed each other. This indicates that object-based attention is associated with a response enhancement at the earliest level of the visual cortical processing hierarchy.

Animals↗

PubMed1998

Figure-ground activity in primary visual cortex is suppressed by anesthesia.

By means of their small receptive fields (RFs), neurons in primary visual cortex perform highly localized analyses of the visual scene, far removed from our normal unified experience of vision. Local image elements coded by the RF are put into more global context, however, by means of modulation of the responses of the V1 neurons. Contextual modulation has been shown to follow closely the perceptual interpretation of the scene as a whole. This would suggest that some aspects of contextual modulation can be recorded only in awake and perceiving animals. In this study, multi-unit activity was recorded with implanted electrodes from primary visual cortex of awake, fixating monkeys viewing textured displays in which figure and ground regions were segregated by differences in either orientation or motion. Contextual modulation was isolated from local RF processing, by keeping RF stimulation identical across trials while sampling responses for various positions of the RF relative to figure and ground. Contextual modulation was observed to unfold spatially and temporally in a way that closely resembles the figure-ground percept. When recording was repeated, but with the animals anesthetized, the figure-ground related modulatory activity was selectively suppressed. RF tuning properties, however, remained unaffected. The results show that the modulatory activity is functionally distinct from the RF properties. V1 thus hosts distinct regimes of activity that are mediated by separate mechanisms and that depend differentially on the animal being awake or anesthetized.

Anesthesia, Inhalation↗

PubMed1998

The role of the primary visual cortex in higher level vision.

In the classical feed-forward, modular view of visual processing, the primary visual cortex (area V1) is a module that serves to extract local features such as edges and bars. Representation and recognition of objects are thought to be functions of higher extrastriate cortical areas. This paper presents neurophysiological data that show the later part of V1 neurons' responses reflecting higher order perceptual computations related to Ullman's (Cognition 1984; 18:97-159) visual routines and Marr's (Vision NJ: Freeman 1982) full primal sketch, 2 1/2D sketch and 3D model. Based on theoretical reasoning and the experimental evidence, we propose a possible reinterpretation of the functional role of V1. In this framework, because of V1 neurons' precise encoding of orientation and spatial information, higher level perceptual computations and representations that involve high resolution details, fine geometry and spatial precision would necessarily involve V1 and be reflected in the later part of its neurons' activities.

Action Potentials↗

PubMed1998

Feedforward, horizontal, and feedback processing in the visual cortex.

The cortical visual system consists of many richly interconnected areas. Each area is characterized by more or less specific receptive field tuning properties. However, these tuning properties reflect only a subset of the interactions that occur within and between areas. Neuronal responses may be modulated by perceptual context or attention. These modulations reflect lateral interactions within areas and feedback from higher to lower areas. Recent work is beginning to unravel how horizontal and feedback connections each contribute to modulatory effects and what the role of these modulations is in vision. Whereas receptive field tuning properties reflect feedforward processing, modulations evoked by horizontal and feedback connections may reflect the integration of information that underlies perception.

Animals↗

PubMed1998

Functional connectivity within the visual cortex of the rat shows state changes.

The aim of this study was to investigate the dynamics of the horizontal functional connectivity within the visual cortex during spontaneous activity or during visual stimulation. Two arrays of 16 electrodes were inserted in the visual cortex of a rat. From these electrodes a depth profile was obtained of the local spiking activity. The cross-correlations between all electrodes were estimated. Three types of cross-correlation peaks were identified and classified as; 'thin peaks', 'fast waves' and 'slow waves'. Partialization was applied, a mathematical method to reduce the amount of common input in correlations, and its effect on the three types of correlation peaks was studied. Slow waves were found to be the most vulnerable to partialization and thin peaks the least. From these observations it was concluded that the three types of peaks represent synchronous neuronal assemblies of different magnitude; slow waves large, fast waves intermediate and thin peaks assemblies composed of small numbers of neurons. Large changes were observed in the types of cross-correlations and their spatial distribution within the set of interarray combinations of electrodes. These changes were spontaneous, and could not be related to the visual stimulation. Two states were identified; the 'thin-peak' state and the 'slow-wave' state. The 'thin-peak' state is interpreted as occurring at a light level of anaesthesia and is characterized by the presence of thin peaks in all combinations of electrodes. Thin peaks with the largest strength were found in the upper interarray electrode combinations. The 'slow-wave' state is interpreted as occurring at a deep level of anaesthesia and is characterized by the presence of exclusively slow waves, which were limited mostly to the middle and lower interarray combinations of electrodes. Activation of the cortex is thus associated with the appearance of synchrony between small groups of neurons (thin peaks) which, in contrast to the slow-wave state, include the upper layers of the cortex.

Action Potentials↗

PubMed1996

Contextual modulation in primary visual cortex.

We studied extra-receptive field contextual modulation in area V1 of awake, behaving macaque monkeys. Contextual modulation was studied using texture displays in which texture covering the receptive field (RF) was the same in all trials, but the perceptual context of this texture could vary depending on the configuration of extra-RF texture elements. We found robust contextual modulation when disparity, color, luminance, and orientation cues variously defined a textured figure centered on the RF of V1 neurons. We found contextual modulation to have a spatial extent of approximately 8 to 10 degrees diameter parafoveally. Contextual modulation correlated with perceptual experience of both binocularly rivalrous texture displays and of displays with a simple example of surface occlusion. We found contextual modulation in V1 to have a characteristic latency of 80-100 msec after stimulus onset, potentially allowing feedback from extrastriate areas to underlie to this effect.

Animals↗

PubMed1995

The neurophysiology of figure-ground segregation in primary visual cortex.

The activity of neurons in the primary visual cortex of the awake macaque monkey was recorded while the animals were viewing full screen arrays of either oriented line segments or moving random dots. A square patch of the screen was made to perceptually pop out as a circumscribed figure by virtue of differences between the orientation or the direction of motion of the texture elements within that patch and the surround. The animals were trained to identify the figure patches by making saccadic eye movements towards their positions. Almost every cell gave a significantly larger response to elements belonging to the figure than to similar elements belonging to the background. The figure-ground response enhancement was present along the entire extent of the patch and was absent as soon as the receptive field was outside the patch. The strength of the effect had no relation with classical receptive field properties like orientation or direction selectivity or receptive field size. The response enhancement had a latency of 30-40 msec relative to the onset of the neuronal response itself. The results show that context modulation within primary visual cortex has a highly sophisticated nature, putting the image features the cells are responding to into their fully evaluated perceptual context.

Animals↗

PubMed1994

Organization of contour from motion processing in primate visual cortex.

A major objective of visual processing is the segmentation of the scene into separate objects. Relative motion is one of the most salient segmentation cues. In man and monkey, we recorded visually evoked potentials to a stimulus, designed to signal the presence of relative motion processing. Relative motion specific response components were only elicited when human observers perceive contours from relative motion. Equivalent dipole source localization of the responses indicated the involvement of primary visual cortex in man. This was corroborated by intracortical recordings in awake monkey, where sources of the specific components are located within the supra- and infragranular layers of primary visual cortex. It is concluded that V1 does not merely provide an input stage to contour from motion processing, but that segmentation information, based on relative motion, is present at this early cortical level.

Animals↗