Context-sensitive binding by the laminar circuits of V1 and V2: A unified model of perceptual grouping,attention, and orientation contrast

A detailed neural model is presented of how the laminar circuits of visual cortical areas V1 and V2 implement context-sensitive binding processes such as perceptual grouping and attention. The model proposes how specific laminar circuits allow the responses of visual cortical neurons to be determined not only by the stimuli within their classical receptive fields, but also to be strongly influenced by stimuli in the extra-classical surround. This context-sensitive visual processing can greatly enhance the analysis of visual scenes, especially those containing targets that are low contrast, partially occluded, or crowded by distractors. We show how interactions of feedforward, feedback, and horizontal circuitry can implement several types of contextual processing simultaneously, using shared laminar circuits. In particular, we present computer simulations that suggest how top-down attention and preattentive perceptual grouping, two processes that are fundamental for visual binding, can interact, with attentional enhancement selectively propagating along groupings of both real and illusory contours, thereby showing how attention can selectively enhance object representations. These simulations also illustrate how attention may have a stronger facilitatory effect on low contrast than on high contrast stimuli, and how pop-out from orientation contrast may occur. The specific functional roles which the model proposes for the cortical layers allow several testable neurophysiological predictions to be made. The results presented here simulate only the boundary grouping system of adult cortical architecture. However, we also discuss how this model contributes to a larger neural theory of vision that suggests how intracortical and intercortical feedback help to stabilize development and learning within these cortical circuits. Although feedback plays a key role, fast feedforward processing is possible in response to unambiguous information. Model circuits are capable of synchronizing quickly, but context-sensitive persistence of previous events can influence how synchrony develops. Although these results focus on how the interblob cortical processing stream controls boundary grouping and attention, related modelling of the blob cortical processing stream suggests how visible surfaces are formed, and modelling of the motion stream suggests how transient responses to scenic changes can control long-range apparent motion and also attract spatial attention.     

Perceptual organization of two-dimensional patterns

The perceptual organization of image patterns is considered from 2 standpoints. First, a theoretical framework is presented from which computational models of perceptual organization can be constructed and tested. Second, a specific computational model for perceptual organization of line images is described. In this model, input images are first processed by a dense array of neurons that have properties consistent with recent analyses of single-neuron responses in primary visual cortex. Then, complex image structure is discovered by interleaved pattern-matching and grouping processes constrained by a generalized uniqueness principle. A series of 3-pattern grouping experiments was performed to test a restricted version of the model and to estimate critical parameters. Using the estimated parameters, an extended version of the model was tested by generating predictions for a series of "textbook" perceptual organization demonstrations.     

Incremental grouping of image elements in vision

One important task for the visual system is to group image elements that belong to an object and to segregate them from other objects and the background. We here present an incremental grouping theory (IGT) that addresses the role of object-based attention in perceptual grouping at a psychological level and, at the same time, outlines the mechanisms for grouping at the neurophysiological level. The IGT proposes that there are two processes for perceptual grouping. The first process is base grouping and relies on neurons that are tuned to feature conjunctions. Base grouping is fast and occurs in parallel across the visual scene, but not all possible feature conjunctions can be coded as base groupings. If there are no neurons tuned to the relevant feature conjunctions, a second process called incremental grouping comes into play. Incremental grouping is a time-consuming and capacity-limited process that requires the gradual spread of enhanced neuronal activity across the representation of an object in the visual cortex. The spread of enhanced neuronal activity corresponds to the labeling of image elements with object-based attention.     

Neural Mechanisms of Visual Feature Binding Investigated with Microelectrodes and Models

A single object generally activates neurones in many visual cortical areas corresponding to a distributed representation of its features. Itis still under debate how the distributed representation of an objectis bound intoa coherent whole and how unrelated features are separated. Synchronization of neural signals has been proposed to code spatial feature binding, supported by the discovery of synchronized assemblies in visual cortex. Synchronizations are either fast oscillatory (30–90 Hz) cortically generated events, or non-rhythmical stimulus-locked responses, depending on the visual stimulation. The cortical range over which synchronizations occur, transformed to visual space, is generally several times larger than the classical receptive fields (CRF) of neurones in lower visual cortex areas. However, the cortical regions synchronized by fastoscillations do notspan the representational range of larger objects but only parts of it. To relate such restricted segments to perceptual processes the concept of the association field (AF) of local neural assemblies was introduced in accordance with CRFs of single neurones. Here an AF is composed of the aggregate of CRFs of an assembly engaged in a common synchronized state. Itis argued thatspatial continuity of an object is coded by a continuum of overlapping AFs, that is, by overlapping regions of phase coupled neurones. Hence, object continuity would be represented by phase continuity. Besides feature binding, feature separation is necessary for scene segmentation. Separation may be coded by functional decoupling causing uncorrelated activities or by mutual inhibition leading to alternating activations in assemblies of separate representations. A third temporal coding aspect is temporal segmentation by the shortactivation-inhibition cycles of fast oscillations orshorttransientstimulus-locked responses, which may preventperceptual “smearing” by interrupting the flow of visual information into precisely defined frames. The presentpaperalso aims atrelating signals of stimulus dependentsynchronization and desynchronization with basic neural mechanisms and circuits. Finally, the synchronization hypothesis is critically discussed with respect to contradictory psychophysical work and supportive new recording results, including evidence for perception-related synchronizations.     

Multiple Visual Latency

It has long been known that a dark visual stimulus is seen later than a bright one, with a delay up to several 10s of milliseconds. Systematic studies of various phenomena demonstrating this delay have revealed that the perceptual latency decreases monotonically as the stimulus intensity increases. Because latencies measured by psychological methods and cortical evoked responses are very similar to electroretinogram latencies, it has become a common belief that there is little in the intensity-dependent latency function that cannot be explained by retinal processes. In this study, we report evidence that there is no one absolute visual delay common to the whole visual system, but rather that the delay varies considerably in different perceptual subsystems. The relative visual latency was found to be considerably shorter in the task involving detecting the direction of movement than in other perceptual tasks that presume visual awareness of the beginning or temporal order of visual events.     

Latency facilitation in temporal-order judgments: time course of facilitation as a function of judgment type

The paper is concerned with two models of early visual processing which predict that priming of a visual mask by a preceding masked stimulus speeds up conscious perception of the mask (perceptual latency priming). One model ascribes this speed-up to facilitation by visuo-spatial attention [Scharlau, I., & Neumann, O. (2003a). Perceptual latency priming by masked and unmasked stimuli: Evidence for an attentional explanation. Psychological Research 67, 184-197], the other attributes it to nonspecific upgrading mediated by retino-thalamic and thalamo-cortical pathways [Bachmann, T. (1994). Psychophysiology of visual masking: The fine structure of conscious experience. Commack, NY: Nova Science Publishers]. The models make different predictions about the time course of perceptual latency priming. Four experiments test these predictions. The results provide more support for the attentional than for the upgrading model. The experiments further demonstrate that testing latency facilitation with temporal-order judgments may induce a methodological problem resulting in fairly low estimates. A method which provides a more exhaustive measure is suggested and tested.     

Why people see things that are not there: a novel Perception and Attention Deficit model for recurrent complex visual hallucinations

As many as two million people in the United Kingdom repeatedly see people, animals, and objects that have no objective reality. Hallucinations on the border of sleep, dementing illnesses, delirium, eye disease, and schizophrenia account for 90% of these. The remainder have rarer disorders. We review existing models of recurrent complex visual hallucinations (RCVH) in the awake person, including cortical irritation, cortical hyperexcitability and cortical release, top-down activation, misperception, dream intrusion, and interactive models. We provide evidence that these can neither fully account for the phenomenology of RCVH, nor for variations in the frequency of RCVH in different disorders. We propose a novel Perception and Attention Deficit (PAD) model for RCVH. A combination of impaired attentional binding and poor sensory activation of a correct proto-object, in conjunction with a relatively intact scene representation, bias perception to allow the intrusion of a hallucinatory proto-object into a scene perception. Incorporation of this image into a context-specific hallucinatory scene representation accounts for repetitive hallucinations. We suggest that these impairments are underpinned by disturbances in a lateral frontal cortex-ventral visual stream system. We show how the frequency of RCVH in different diseases is related to the coexistence of attentional and visual perceptual impairments; how attentional and perceptual processes can account for their phenomenology; and that diseases and other states with high rates of RCVH have cholinergic dysfunction in both frontal cortex and the ventral visual stream. Several tests of the model are indicated, together with a number of treatment options that it generates.     

Semantic and perceptual priming: how similar are the underlying mechanisms?

Both semantic priming and perceptual priming consist of facilitation of the identification of primed stimuli and inhibition of the identification of nonprimed stimuli. The similarities between the two phenomena suggest that a common attentional mechanism underlies both, and this has been explicitly proposed by several attention theorists. In this article it is argued that the phenomena of semantic and perceptual priming are qualitatively different, perceptual priming reflecting a sensitivity change in the recognition process brought about by attention, and semantic priming reflecting a bias change in the recognition process brought about by attention. Because different mechanisms are required to produce sensitivity and bias changes, this implies that the attentional mechanisms responsible for semantic and perceptual priming are distinct. In terms of recent discussions of the modularity versus the unity of cognitive architecture, the present conclusion supports a modular architecture for attentional processes.     

Electrophysiological markers of visual dimension changes and response changes

In cross-dimensional visual search tasks, target discrimination is faster when the previous trial contained a target defined in the same visual dimension as the current trial. The dimension-weighting account (DWA; A. Found & H. J. Müller, 1996) explains this intertrial facilitation by assuming that visual dimensions are weighted at an early perceptual stage of processing. Recently, this view has been challenged by models claiming that intertrial facilitation effects are generated at later stages that follow attentional target selection (K. Mortier, J. Theeuwes, & P. A. Starreveld, 2005). To determine whether intertrial facilitation is generated at a perceptual stage, at the response selection stage, or both, the authors focused on specific event-related brain potential components (directly linkable to perceptual and response-related processing) during a compound search task. Visual dimension repetitions were mirrored by shorter latencies and enhanced amplitudes of the N2-posterior- contralateral, suggesting a facilitated allocation of attentional resources to the target. Response repetitions and changes systematically modulated the lateralized readiness potential amplitude, suggesting a benefit from residual activations of the previous trial biasing the correct response. Overall, the present findings strengthen the DWA by indicating a perceptual origin of dimension change costs in visual search.     

Through the forest of motor representations

Following neuroscience, and using different labels, several philosophers have addressed the idea of the presence of a single representational mechanism lying in between (visual) perceptual processes and motor processes involved in different functions and useful for shaping suitable action performances: a motor representation (MR). MRs are the naturalized mental antecedents of action. This paper presents a new, non-monolithic view of MRs, according to which, contrarily to the received view, when looking at in between (visual) perceptual processes and motor processes, we find not only a single representational mechanism with different functions, but an ensemble of different sub-representational phenomena, each of which with a different function. This new view is able to avoid several issues emerging from the literature and to address something the literature is silent about, which however turns out to be crucial for a theory of MRs.     

Temporally unfolding neural representation of pictorial occlusion

The human visual system possesses a remarkable ability to reconstruct the shape of an object that is partly occluded by an interposed surface. Behavioral results suggest that, under some circumstances, this perceptual process (termed amodal completion) progresses from an initial representation of local image features to a completed representation of a shape that may include features that are not explicitly present in the retinal image. Recent functional magnetic resonance imaging (fMRI) studies have shown that the completed surface is represented in early visual cortical areas. We used fMRI adaptation, combined with brief, masked exposures, to track the amodal completion process as it unfolds in early visual cortical regions. We report evidence for an evolution of the neural representation from the image-based feature representation to the completed representation. Our method offers the possibility of measuring changes in cortical activity using fMRI over a time scale of a few hundred milliseconds.     

No evidence for overshadowing or facilitation of spatial pattern learning by visual cues

Two experiments were conducted to examine the effects of redundant and relevant visual cues on spatial pattern learning. Rats searched for hidden food items on the tops of poles that formed a square (Experiment 1) or a checkerboard (Experiment 2) pattern. The experimental groups were trained with visual cues that specified the locations of the baited poles. All groups were tested without visual cues so that any overshadowing or facilitation of spatial pattern learning by visual cues could be detected. Spatial choices were controlled by the spatial pattern and by the visual cues in both experiments. However, there was no evidence of overshadowing or facilitation of spatial pattern learning by visual cues in either experiment. The results are consistent with the idea that the representation of the spatial pattern that guides choices is not controlled by the same learning processes as those that produce associations between visual cues and food locations.     

Acetylcholine and olfactory perceptual learning

Olfactory perceptual learning is a relatively long-term, learned increase in perceptual acuity, and has been described in both humans and animals. Data from recent electrophysiological studies have indicated that olfactory perceptual learning may be correlated with changes in odorant receptive fields of neurons in the olfactory bulb and piriform cortex. These changes include enhanced representation of the molecular features of familiar odors by mitral cells in the olfactory bulb, and synthetic coding of multiple coincident odorant features into odor objects by cortical neurons. In this paper, data are reviewed that show the critical role of acetylcholine (Ach) in olfactory system function and plasticity, and cholinergic modulation of olfactory perceptual learning at both the behavioral and cortical level.     

When does grouping happen?

Recent research on perceptual grouping is described with particular emphasis on identifying the level(s) at which grouping factors operate. Contrary to the classical view of grouping as an early, two-dimensional, image-based process, recent experimental results show that it is strongly influenced by phenomena related to perceptual constancy, such as binocular depth perception, lightness constancy, amodal completion, and illusory contours. These findings imply that at least some grouping processes operate at the level of phenomenal perception rather than at the level of the retinal image. Preliminary evidence is reported showing that grouping can affect perceptual constancy, suggesting that grouping processes must also operate at an early, preconstancy level. If so, grouping may be a ubiquitous, ongoing aspect of visual organization that occurs for each level of representation rather than as a single stage that can be definitively localized relative to other perceptual processes.     

Climbing the cortical ladder from sensation to perception

A recent study using displays that are ambiguous for motion direction demonstrates that the current perceptual interpretation of such a stimulus is encoded in the highest areas of visual cortex whereas earlier areas encode only its sensory properties. This finding implies that cortical processing pathways perform a transition from a sensory representation to a representation that emphasizes the input's perceptual interpretation and ultimately the organism's behavioral state.     

EARLY CORTICAL INFLUENCES IN OBJECT SEGREGATION AND THE PERCEPTION OF SURFACE LIGHTNESS

Abstract— The apparent brightness of a surface is profoundly influenced by the brightness of an adjacent surface., but these contrast effects are reduced when the surface are perceived as separate three-dimensional entities. Previous work has suggested that high-level perceptual and cognitive processes involved in scence segmentation may be responsible for modifying a surface's appearance. We demonstrate large reductions in contrast effects when the sues available for segmentations are restricted to those that isolate separate groups of early cortical neurons in the visual system. Our data contradict standard contract-signaling models of brightness perception and imply that mechanisms of figure-ground segmentation are already available at low levels of visual processing.     

Learning strategy determines auditory cortical plasticity

Learning modifies the primary auditory cortex (A1) to emphasize the processing and representation of behaviorally relevant sounds. However, the factors that determine cortical plasticity are poorly understood. While the type and amount of learning are assumed to be important, the actual strategies used to solve learning problems might be critical. To investigate this possibility, we trained two groups of adult male Sprague-Dawley rats to bar-press (BP) for water contingent on the presence of a 5.0 kHz tone using two different strategies: BP during tone presence or BP from tone-onset until receiving an error signal after tone cessation. Both groups achieved the same high levels of correct performance and both groups revealed equivalent learning of absolute frequency during training. Post-training terminal "mapping" of A1 showed no change in representational area of the tone signal frequency but revealed other substantial cue-specific plasticity that developed only in the tone-onset-to-error strategy group. Threshold was decreased approximately 10 dB and tuning bandwidth was narrowed by approximately 0.7 octaves. As sound onsets have greater perceptual weighting and cortical discharge efficacy than continual sound presence, the induction of specific learning-induced cortical plasticity may depend on the use of learning strategies that best exploit cortical proclivities. The present results also suggest a general principle for the induction and storage of plasticity in learning, viz., that the representation of specific acquired information may be selected by neurons according to a match between behaviorally selected stimulus features and circuit/network response properties.     

Visual perception of force: comment on White (2012)

White (2012) proposed that kinematic features in a visual percept are matched to stored representations containing information regarding forces (based on prior haptic experience) and that information in the matched, stored representations regarding forces is then incorporated into visual perception. Although some elements of White's (2012) account appear consistent with previous findings and theories, other elements do not appear consistent with previous findings and theories or are in need of clarification. Some of the latter elements include the (a) differences between perception and impression (representation of force; relationship of force and resistance; role and necessity of stored representations and of concurrent simulation; roles of rules, cues, and heuristics), (b) characteristics of object motion and human movement (whether motion is internally generated or externally generated and whether motion is biological or nonbiological; generalization of human action and the extent to which perceived force depends upon similarity of object movement to human patterns of movement), (c) related perceptual and cognitive phenomena (representational momentum, imagery, psychophysics of force perception, perception of causality), and (d) scope and limitations of White's account (attributions of intentionality, falsifiability).     

Un modèle neurobiologique de la perception et de l'estimation du temps

Frontal-striatal circuits provide an important neurobiological substrate for timing and time perception as well as for working memory. In this review, we outline recent theoretical and empirical work to suggest that interval timing and working memory rely not only on the same anatomic structures, but also on the same neural representation of a specific stimulus. In the striatal beat-frequency model, cortical neurons fire in an oscillatory fashion to form representations of stimuli, and striatal medium spiny neurons detect those patterns of cortical firing that occur co-incident to important temporal events. Information about stimulus identity can be extracted from the specific cortical networks that are involved in the representation, and information about duration can be extracted from the relative phase of neural firing. The properties derived from these neurobiological models fit well with the psychophysics of timing and time perception as well as with information-processing models that emphasize the importance of temporal coding in a variety of working-memory phenomena.