State dependent activity in monkey visual cortex. I. Single cell activity in V1 and V4 on visual tasks

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.

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.

Visual representations during saccadic eye movements

In normal vision, shifts of attention are usually followed by saccadic eye movements. Neurons in extrastriate area V4 are modulated by focal attention when eye movements are withheld, but they also respond in advance of visually guided saccadic eye movements. We have examined the visual selectivity of saccade-related responses of area V4 neurons in monkeys making delayed eye movements to receptive field stimuli of varying orientation. This task did not require the monkey to attend to orientation per se but merely to foveate the receptive field stimulus. We present evidence that the presaccadic enhancement exhibited by V4 neurons, quite separate from the response at stimulus onset, is a resurgent visual representation that seems as selective as the response is when the stimulus first appears. The presaccadic enhancement appears to provide a strengthening of a decaying featural representation immediately before an eye movement is directed to visual targets. We suggest that this reactivation provides a mechanism by which a clear perception of the saccade goal can be maintained during the execution of the saccade, perhaps for the purpose of establishing continuity across eye movements.

Recognition memory: neuronal substrates of the judgement of prior occurrence

Recognition memory relies on two processes: (i) identification and (ii) judgement concerning prior occurrence. A system centred on perirhinal cortex appears to be responsible for judgement of prior occurrence based on discrimination of the familiarity of stimuli or their recency of occurrence; in contrast, a hippocampal system probably supplies information concerning the episodic, contextual aspects of recognition memory. This review chiefly concerns the perirhinal system and, in particular, neurones that signal the prior occurrence of stimuli by a decrease in response. Details concerning such decremental responses are given and it is argued that such responses in perirhinal cortex are adequate for and central to discrimination of stimulus familiarity and recency in a wide range of situations. Information is given of similar types of neuronal responses in anatomically related brain regions and what may be deduced about the operation of the recognition memory system. The possibility is discussed that the neuronal responses that signal information concerning the recent occurrence of stimuli may contribute to repetition priming as well as recognition memory. Other described changes in the activity of individual neurones such as response enhancements, or sustained (delay) activity may allow solution of specialised forms of recognition memory tasks where relatively short-term working memory is adequate. Implications of the multi-faceted nature of recognition memory for the interpretation of results are emphasised. Unsolved problems and avenues for future experimentation, including determining the nature of possible underlying synaptic plastic changes, are discussed.

Adult cortical dynamics

There are many influences on our perception of local features. What we see is not strictly a reflection of the physical characteristics of a scene but instead is highly dependent on the processes by which our brain attempts to interpret the scene. As a result, our percepts are shaped by the context within which local features are presented, by our previous visual experiences, operating over a wide range of time scales, and by our expectation of what is before us. The substrate for these influences is likely to be found in the lateral interactions operating within individual areas of the cerebral cortex and in the feedback from higher to lower order cortical areas. Even at early stages in the visual pathway, cells are far more flexible in their functional properties than previously thought. It had long been assumed that cells in primary visual cortex had fixed properties, passing along the product of a stereotyped operation to the next stage in the visual pathway. Any plasticity dependent on visual experience was thought to be restricted to a period early in the life of the animal, the critical period. Furthermore, the assembly of contours and surfaces into unified percepts was assumed to take place at high levels in the visual pathway, whereas the receptive fields of cells in primary visual cortex represented very small windows on the visual scene. These concepts of spatial integration and plasticity have been radically modified in the past few years. The emerging view is that even at the earliest stages in the cortical processing of visual information, cells are highly mutable in their functional properties and are capable of integrating information over a much larger part of visual space than originally believed.

Voluntary attention modulates fMRI activity in human MT-MST

How does voluntary attention to one attribute of a visual stimulus affect the neural processing of that stimulus? We used functional magnetic resonance imaging to examine the attentional modulation of neural activity in the human homolog of the MT-MST complex, which is known to be involved in the processing of visual motion. Using a visual stimulus containing both moving and stationary dots, we found significantly more MT-MST activation when subjects attended to the moving dots than when they attended to the stationary dots, even though the visual stimulus was identical during the two conditions.

Probing visual motion signals with a priming paradigm

The perceived motion of a vertical sine-wave luminance grating which undergoes an abrupt 180 deg phase shift (motion step) is ambiguous. The grating sometimes appears to move rightward; sometimes leftward. However, when the 180 deg step follows closely upon an unambiguous grating step, the 180 deg step appears to be in the same direction as the unambiguous step. This phenomenon is termed visual motion priming (VMP), and some of the characteristics of the phenomenon were investigated in a series of experiments. The main findings were that priming (1) lasted for hundreds of msec; (2) was at a maximum when the magnitude of the priming step was 90 deg; (3) was scarcely affected by spatial frequency in the range 0.7-2.8 c/deg; and (4) at suprathreshold contrasts depended upon the relative contrast, not the absolute contrasts, of the frames comprising the priming step. The experiments were conducted within the framework of a motion energy model (Adelson & Bergen, 1985) which possessed an extra stage which summed motion signals over time. Some of the results could be explained by the second-stage integrator. Other nonlinear relationships between VMP and contrast require some form of motion signal compression, and perhaps even a mechanism of dynamic contrast processing.

Human brain activity related to orientation discrimination tasks

In order to relate regional activity in the human brain to the different components of discrimination tasks, we compared regional cerebral blood flow, measured with positron emission tomography, under four conditions: successive orientation discrimination, orientation identification, detection and passive viewing. By adding successive discrimination and passive viewing at a second, lower rate we were able to investigate the main effects and interaction between task and presentation rate. Four occipital regions--the posterior calcarine region bilaterally, the right lingual gyrus and the right interior occipital cortex--displayed a main effect of presentation rate. Two regions--a right posterolateral occipital region and a right posterior fusiform region--displayed a significant main effect of task. The involvement of this posterior fusiform region in successive discrimination was also revealed by the subtraction of detection from successive discrimination, as was that of the right middle fusiform gyrus. Finally, a more anterior right middle fusiform region was differentially active in successive discrimination compared to identification, suggesting that activity in this region is related to the temporal comparison of orientation.

Learning pop-out detection: specificities to stimulus characteristics

Training induces dramatic improvement in the performance of pop-out detection. In this study, we examined the specificities of this improvement to stimulus characteristics. We found that learning is specific within basic visual dimensions: orientation, size and position. Accordingly, following training with one set of orientations, rotating target and distractors by 30 deg or more substantially hampers performance. Furthermore, rotation of either target or distractors alone greatly increases threshold. Learning is not transferred to reduced-size stimuli. Position specificity near fixation may be finer than 0.7 deg. On the other hand, learning transfers to the untrained eye, to expanded images, to mirror image transformations and to homologous positions across the midline (near fixation). Thus, learning must occur at a processing level which is early enough to maintain fine separability along basic stimulus dimensions, yet sufficiently high to manifest the described generalizations. We suggest that the site of early perceptual learning is one of the cortical areas which receive input from primary visual cortex, V1, and where top-down attentional control is present.

Functional MRI of lateral occipitotemporal cortex during pursuit and motion perception

We performed functional imaging with a conventional 1.5-T magnetic resonance scanner in 9 normal subjects. We used a gradient-echo technique to examine changes in signal between periods when subjects viewed a stationary black-and-white grating, a moving grating, and when they followed a moving spot. We located image pixels with significant differences between the viewing conditions. In 7 subjects, these occurred in the lateral occipitotemporal cortex, a region previously identified as a putative human homologue of the motion-sensitive middle temporal area (MT, or V5) of monkeys. Signal intensity was greater during pursuit of the moving dot than during viewing of the moving grating with the eyes still, despite the fact that the moving grating generated more retinal image motion. In contrast, signal intensity in striate cortex was least during pursuit of the moving dot. These findings suggest that the lateral occipitotemporal cortex has extraretinal signals during pursuit. Such signals may include attentional input, corollary eye movement information, or even a pursuit command. Extraretinal signals suggest that the lateral occipitotemporal cortex may contain a human homologue not only of MT but also of other components of the monkey V5 complex, such as the medial superior temporal area.

Dual Population Coding in the Neocortex: A Model of Interaction between Representation and Attention in the Visual Cortex

The text describes a model that extends the population coding principles to any multidimensional attribute. The model distinguishes between the distribution of cell activity and the overall activity of a population. The distribution of cell activity is assumed to encode attribute information, while overall activity is assumed to reflect the significance or pertinence of the encoded attribute in the cerebral cortex, according to the dual coding principle. Three basic mechanisms of interaction between the representation of attribute and pertinence are defined and are applied to the motion (MT-MST) cortical pathway in the visual cortex. This framework determines three sources of pertinence that model cognitive processing, including preattentive processing, spatial-selective attention, and object-selective attention. The model accommodates most of the published psychophysical, neurophysiological, and neuroanatomical data and makes several testable predictions about the representations of attribute and enhanced effects in these areas.

The primate temporal pole: its putative role in object recognition and memory

In this article, we consider both the ventral temporopolar cortex and the perirhinal cortex (areas 35 and 36) as the anterior ventromedial temporal (aVMT) cortex, and discuss its role based on recent data in monkeys and human subjects. In monkeys, the aVMT cortex receives its primary input from area TE, and only minor input from other cortical areas. Laminar patterns of connections suggest that the aVMT cortex is a hierarchically higher-order area than area TE. Lesions of this cortex produce deficits in the learning and performance of visual memory tasks. Neurons in the aVMT cortex respond selectively to complex stimuli and changes in activity related to visual memory tasks. In humans, damage of this cortex induces deficits in the recognition of familiar objects and faces. The aVMT cortex is activated during recognition of familiar faces. In addition, the aVMT cortex is one of the most vulnerable areas in Alzheimer's disease. All these data indicate that the aVMT cortex is a higher-order visual cortical area that is related to object recognition and memory. The anterior area TE has been implicated in both functions. We propose here that these areas and the anterior entorhinal cortex are designated as the temporal pole, a brain region which is specialized for both object recognition and memory.

On the specificity of neurons and visual areas

The dominant view during the past 40 years has been that the visual system analyzes the visual scene by breaking it down into basic attributes such as color, form, motion, depth and texture. Individual dedicated neurons and specific visual areas were believed to be devoted to the analysis of each of these attributes. Current research has challenged these views by emphasizing that neurons, especially in the cortex, have multifunctional properties and therefore serve as general-purpose analyzers rather than feature detectors. Consequently, it appears that most extrastriate visual areas, rather than each being devoted to the analysis of a specific basic visual attribute, perform several different tasks and thereby engage in more advanced and complex analyses than had been realized.

The effects of visual cortex lesions on the perception of rotated shapes

Monkeys with inferior temporal cortex lesions cannot discriminate between different shapes (e.g., + vs. O) but can discriminate between shapes that differ only in orientation (e.g., 6 vs 9). Lesions of the parietal cortex, on the other hand, impair the discrimination of rotated shapes but spare the ability to discriminate between different shapes. A similar dissociation is found between some visual agnosics who can match but not identify rotated views of objects and other patients who can identify and discriminate objects only if the view is conventional; any change in orientation disrupts performance. In this paper we argue that two mechanisms may be available for the perception of rotated shapes. Which mechanism is used depends on the degree of rotation. It is suggested that the different effects of parietal and temporal lesions reflect the relative contributions of the two areas to the task and disrupt different stages of the two strategies used. A framework for the cortical processing of rotated shapes in the non-human primate is presented.

Neuronal activity in monkey visual areas V1, V2, V4 and TEO during fixation task

We analyzed 577 neurons recorded from visual areas V1, V2, V4, and the inferotemporal area (TEO) of macaque monkeys, which performed a visual fixation task and a spot-off-on (blink) test during the fixation period. Among these neurons, 35% were defined as task-related cells, because they gave responses at the task-start, fixation, or task-end periods but were unresponsive to the spot blink, which was physically identical to these stimuli. Blink-responsive cells accounted for 29% and task-unresponsive cells for 30% of the neurons. The task-related response was large and frequent in V4 (34%) and TEO (41%), but small and less frequent in V1 (31%) and V2 (27%). Other observations further demonstrated nonsensory activities in these areas: In some cells, response to the fixation spot was inhibitory, whereas light stimulation on the fovea was excitatory; some V1 and V2 cells had color-irrelevant responses, and some cells responded to the spot-off only when the monkey regarded it as a task-end cue.

The brain's visual world: representation of visual targets in cerebral cortex

Microelectrode recordings from behaving monkeys have shown that neuronal responses in the visual cerebral cortex can depend greatly on which aspect of the scene is the target of the animal's attention. Accumulating evidence suggests that while the early stages of the visual pathway provide a faithful representation of the retinal image, later stages of processing in the visual cortex hold representations that emphasize the viewer's current interest. By filtering out irrelevant signals and adding information about objects whose presence is remembered or inferred, the cortex creates an edited representation of the visual world that is dynamically modified to suit the immediate goals of the viewer.

ERP signs of early selective attention effects to check size

In ERP literature on visual selective attention evidence has been provided that selectively directing attention to a spatial frequency affects the visual processing of the attended frequency, and of unattended frequencies within the same channel bandwidth, starting at a relatively late level of post-stimulus processing, i.e., after about 150 msec. Nevertheless, little knowledge is available about the topographic distribution of these attention effects. This study investigated attentional selection of stimulus relative size at occipital and latero-occipital sites, as well as at fronto-lateral sites. ERPs from posterior scalp electrode sites showed that attention to check sizes enhanced the early sensory components, thus indicating that feature-based attention may result in a modulation of sensory processing. Comparisons of the ERPs to relevant and irrelevant patterns showed an enhanced latero-occipital P90 positivity as well as an occipital N115 negativity to relevant patterns, thus also suggesting possible differential mechanisms of early attentional selectivity at these locations. Later effects of attention consisted of a selection negativity to relevant patterns at posterior electrodes, and a selection positivity at latero-frontal sites. A larger late positivity to irrelevant patterns at anterior sites also suggested an active suppression of attentional response to irrelevant information. Moreover, right-and left-sided asymmetries were found to be respectively consistent for the P90 and N115 with left hemispheric specialization for high, and right hemispheric specialization for low spatial frequencies. A stronger left-sided attentional selectivity has also been found.

Equivalent source estimation of scalp potential fields contaminated by heteroscedastic and correlated noise

The customary ordinary least squares (OLS) approach to the estimation of equivalent sources of scalp potential fields relies on the assumption that noise in the potential measurements has an equal variance and is uncorrelated over leads. It is shown that this assumption is likely to be violated in practice, for instance by the use of a common reference lead. We describe tests to detect these violations and we propose several versions of an alternative estimation method called iterated generalised least squares (IGLS), which accounts for heteroscedastic or correlated noise by incorporating an estimate of the covariance matrix of the noise derived from single trial OLS residuals. Simulation results indicate that these alternatives give a considerable increase in the accuracy of both the parameter and the standard error and confidence interval estimates. The proposed tests and methods are finally integrated into a stepwise approach to equivalent source estimation, which incorporates in addition a test on the goodness of fit of the model, an assessment of the confidence intervals of the parameters and a powerful test of differences between experimental conditions. This stepwise approach is applied to the modelling of equivalent sources of early visual potentials elicited in a spatial attention task.

Effect of lesions in visual cortical area V4 on the recognition of transformed objects

The primate visual system has a remarkable capability for recognizing objects irrespective of the multitude of images they form on the retinal surface by virtue of changes in size, perspective, contrast, colour and partial obstruction by other stimuli in the visual scene. There is increasing evidence that this remarkable capacity is brought about by processes that occur earlier in the visual system than had previously been thought. Here I show that after ablation of area V4 in the rhesus monkey, major deficits arise in the recognition of objects that have been transformed in size, in the degree of occlusion, and in the amount of contour information provided. The ability to detect these objects when presented individually was unaffected by these lesions.

Attentional modulation of adaptation to two-component transparent motion

We have studied the effects of voluntary attention on the induction of motion aftereffects (MAEs). While adapting, observers paid attention to one of two transparently displayed random dot patterns, moving concurrently in opposite directions. Selective attention was found to modulate the susceptibility to motion adaptation very substantially. To measure the strength of the induced MAEs we modulated the signal-to-noise ratio of a real motion signal in a random dot pattern that was used to balance the aftereffect. Results obtained for adapting to single motion vectors show that the MAE can be represented as a shift of the psychometric function for motion direction discrimination. Selective attention to the different components of transparent motion altered the susceptibility to adaptation. Shifting attention from one component to the other caused a large shift of the psychometric curves, about 70-75% of the shift measured for the separate components of the transparent adapting stimulus. We conclude that attention can differentiate between spatially superimposed motion vectors and that attention modulates the activity of motion mechanisms before or at the level where adaptation gives rise to MAEs. The results are discussed in light of the role of attention in visual perception and the physiological site for attentional modulation of MAEs.

Feature-specific effects of selective visual attention

Four experiments were conducted to quantify the effect of performing a foveal discrimination task on sensitivity for a peripheral grating. The observer's primary task was to discriminate either the spatial frequency or orientation of successive foveal Gabor patches. On a third of the trials they also performed a secondary task to detect the presence of a near-threshold grating in the periphery. We find that sensitivity for detection of the peripheral grating depends on the similarity of the spatial frequencies and orientations between the foveal and peripheral stimuli. Importantly, sensitivity is also affected by which feature is being discriminated in the central task. Because the detectability of the peripheral grating is different when different features of the central stimuli are discriminated, we suggest that the effects on sensitivity are due to feature-specific attention and not simply to passive interactions between filters with similar tuning properties.

Attention and memory trials during neuronal recording from the primate pulvinar and posterior parietal cortex (area PG)

Cynomolgus monkeys were trained on a delayed match-to-sample (DMS) task. Several controls were performed for the separation of memory effects from other factors such as attention, stimulus attributes and eye movements. One of the controls employed a standardised sequence of events ('window trial') during which a window in a black screen was opened and a face, an object or a picture was presented. Unit cell activity was recorded from 209 sites in the posterior parietal cortex (area PG) and 186 sites in the pulvinar. Some units responded during the presentation of the DMS stimuli, mostly those in that part of area PG located in the superior temporal sulcus. They often appeared to be related to the task-related state of attention. There was no indication of pure memory-related changes in activity such as sustained responses during the inter-stimulus interval or specific responses to the second stimulus that would indicate a dependence on the kind of the preceding stimulus. In the window trial, some parietal units, mostly in that part of area PG located in the intraparietal sulcus (a region termed area LIP), responded in a relatively specific manner during and sometimes even after, the presentation of a human face. The responses often seemed to be related to the animal's state of attention. Almost no pulvinar unit responded in this paradigm. It is concluded that area PG and, to a lesser extent, the pulvinar are involved in the modulation of attention in relation to behaviourally relevant changes in the environment. If these structures have a role in memory function, it must be secondary to a role in controlling or regulating attention.

Temporal dynamics of visual-evoked neuromagnetic sources: effects of stimulus parameters and selective attention

Results are reviewed from several neuromagnetic studies which characterize the temporal dynamics of neural sources contributing to the visual evoked response and effects of attention on these sources. Different types of pattern-onset stimuli (< or = 2 degrees) were presented sequentially to a number of field locations in the right visual field. Multiple dipole models were applied to a sequence of instantaneous field distributions constructed at 10 ms intervals. Best-fitting source parameters were superimposed on Magnetic Resonance images (MRI) of each subject to identify the anatomical structure(s) giving rise to the surface patterns. At least three sources, presumably corresponding to different visual areas, were routinely identified from 80-150 ms following the onset of visual stimulation. This observation was consistent across subjects and studies. The temporal sequence and strength of activation of these sources, however, were dependent upon the specific stimulus parameters used to evoke the response (e.g., eccentricity) and on the relevance of the stimulus to the subject. In addition, our results provide evidence for the recurrence of activity in striate and extrastriate regions, following the initial cycle of responses.

Defining the neural bases of visual selective attention: conceptual and empirical issues

This paper focuses on conceptual and empirical issues relevant to defining the neural bases of visual selective attention. At the most general level, it is held that the integration of human and monkey research is essential to developing a generally applicable, yet precise, understanding of attentional mechanisms. More specific issues that are considered here include: 1) the general definition of attention (as a process, state, etc.) and the operational definition of attention in experimental work, 2) the possibility that different forms of attention use distinctive neural circuits, 3) the levels of the system at which attention may modulate sensory inflow, 4) the degree to which circuits and levels addressed by attentional modulation depend on task (sensory and behavioral) variables and 5) the nature of cellular processes that may underlie attentional modulation. The available techniques for examining neural processes in humans and monkeys are considered in light of these issues and the need for direct comparability between human and monkey experiments.

Factors limiting large-scale localisation

The mechanisms mediating relative spatial localisation in the visual system are still unclear. There is a growing amount of evidence that this capability is not merely limited by the processing of the front-end visual system. Models of localisation should, therefore, include higher-level processing stages. A careful study of the sources of error in localisation tasks may further our understanding of the nature of these processes. A study is reported in which the possible role of higher-order processing in relative spatial localisation is explicitly addressed. For this purpose the error sources of threshold performance were investigated for two similar relative-spatial-localisation tasks: two-dot separation discrimination and two-dot orientation discrimination. Fovea-centered stimuli with large dot separations were used. The front-end processing for these stimuli is probably identical in both tasks. Hence, differential effects of the variation of the experimental parameters on threshold performance for both tasks may reveal the characteristics of the higher-level processing involved. The effects of dot separation, stimulus orientation, and experimental procedure (single-stimulus binary forced choice versus two-alternative forced choice) on threshold performance for both tasks are reported. The results show that thresholds for both tasks increase proportionally with dot separation. However, separation-discrimination thresholds are always significantly higher than orientation-discrimination thresholds. Thresholds for separation discrimination are independent of stimulus orientation. In contrast, orientation-discrimination thresholds show an oblique effect: thresholds are consistently lower for horizontal stimuli. Both tasks also show a different dependency of threshold behaviour on the experimental procedure. For a horizontal stimulus orientation, separation discrimination is better with an explicit (physical) reference standard, whereas orientation discrimination is better with an implicit referent. These differential effects cannot be explained by any of the known characteristics of the front-end visual system. They suggest that large-scale spatial-localisation performance is probably limited at a processing level at which spatial relations are explicitly represented.

Divergent feedback connections from areas V4 and TEO in the macaque

Extrastriate areas TEO and V4 have been associated with form and color vision. Area V4 has also been suggested to participate in processes concerned with attention, stimulus salience, and perceptual learning. In a continuing effort to elucidate the connectional interactions and microcircuitry of these areas, we describe in this report the pattern of feedback connections from TEO and V4. Connections were demonstrated by injections of the high-resolution anterograde tracers PHA-L or biocytin and further analyzed by reconstruction of 25 individual axons through serial sections. This analysis yielded several new results: (1) Both areas TEO and V4 have widespread feedback connections (defined by their preferential termination in layer 1 and avoidance of layer 4). From TEO, there are dense projections to area V4 and moderate ones to V2 and V1. From V4, there are dense projections to V2 and moderate ones to V3 and V1. (2) Terminal fields span large territories in area V1, up to 6.0 mm in the case of axons originating from TEO; up to 5.0 mm in the case of axons originating from V4. In V2, fields tend to be smaller, between 3.0-5.0 mm. (3) Many axons from TEO and some from V4 have terminations in both areas V1 and V2. (4) Because individual terminal clusters and segments are often larger than cytochrome oxidase compartments, especially in V1, we suggest they may not be correlated with this compartmental organization. These results are consistent with the hypothesis that feedback connections may contribute to processes other than perceptual discrimination.

An account of consciousness in physical and functional terms: a target for research in the neurosciences

The neurophysiology of mental events cannot be fully understood unless that of consciousness is understood. As the first step in a top-down approach to that problem, one needs to find an account of consciousness as a property of the biological organism that can be clearly defined as such. However, if it is to deliver what must be expected of it, it should address what is commonly meant by the word consciousness. Unless the last condition is satisfied, the theory will fail to deliver what must ultimately be expected of it. Although current interest lies mainly in the higher functions of consciousness, such as its role in language and social relationships, the common usage of the word relates to modes of awareness that are not denied to creatures lacking language or social relationships. The basic features to be covered include awareness of the surrounding world, of the self, and of one's thoughts and feelings; the subjective qualities of phenomenal experience (qualia); the conditions a brain event must satisfy to enter consciousness; and the main divisions of mental events, such as sensations, feelings, perceptions, desires, volitions, and mental images. In the first four chapters we argue that these basic features of consciousness can all be accounted for in terms of just three categories of internal representations, each supported by the empirical evidence and each accurately definable in physical and functional terms. In the fifth, and last, chapter we take a closer look at two of the categories and what these in particular suggest as the most relevant lines of research in the contemporary spectrum of the neurosciences.

The electrophysiological basis of colour processing in macaques with V4 lesions

Monkeys with lesions of visual area V4 have deficits in colour constancy, but are able to discriminate hues and segment the spectrum in a categorical manner. To investigate the nature of the processing mechanisms subserving the spared functions we recorded occipital visual evoked potentials (VEPs) of normal monkeys and monkeys with bilateral lesions of area V4. The stimuli used to elicit the potentials were chromatic and achromatic gratings of low spatial frequency. The waveforms from the two groups of animals were similar in all respects. VEPs for the onset of a chromatic grating were negative-going, indicative of the activity of sustained units, as opposed to those elicited by offset or reversal of the grating which were positive-going. The amplitude of the chromatic, 12.5 Hz reversal VEP went through a minimum at isoluminance, in accord with low temporal resolution of colour processing. The VEP waveforms were identical in character from three weeks to approximately four years post-operatively. These data indicate that chromatic processing in areas V1 and V2 is normal after V4 lesions and, together with the behavioural evidence, that these areas are sufficient for some basic aspects of colour perception.

Neuronal tuning and associative mechanisms in form representation

We examine the hypothesis that the form representation in the anterior inferotemporal (AIT) cortex is acquired through learning. According to this hypothesis, perceptual aspects of the temporal association area are closely related to its visual representation, in that the response selectivity of AIT neurons can be influenced by visual experience. On the basis of the neurophysiological evidence, we summarize two neuronal mechanisms that subserve the acquisition of form selectivity in AIT neurons. The first mechanism is neuronal tuning to particular stimuli that were learned in a cognitive task. The second mechanism is association, with which relevant information can be retrieved from other stored memories. On the grounds that long-term memory of objects is acquired and organized by at least these two neuronal mechanisms in the temporal association area, we further present a model of the cognitive memory system that unifies perception and imagery.

Color constancy. II. Results for two-stage linear recovery of spectral descriptions for lights and surfaces

Our analysis of color constancy in a companion paper [J. Opt. Soc. Am A 10, 2148 (1993)] provided an algorithm that lets one test how well linear color constancy schemes work. Here we present the results of applying the algorithm to a large parametric class of color constancy problems involving bilinear models that relate photoreceptoral spectral sensitivities, surface reflectance functions, and illuminant spectral power distributions. These results, supported by simulation and further analysis, provide a detailed classification of two-stage linear methods for recovering the spectral properties of reflectances and illuminants from reflected lights.

Dynamics of orientation coding in area V1 of the awake primate

To investigate the importance of feedback loops in visual information processing, we have analyzed the dynamic aspects of neuronal responses to oriented gratings in cortical area V1 of the awake primate. If recurrent feedback is important in generating orientation selectivity, the initial part of the neuronal response should be relatively poorly selective, and full orientation selectivity should only appear after a delay. Thus, by examining the dynamics of the neuronal responses it should be possible to assess the importance of feedback processes in the development of orientation selectivity. The results were base on a sample of 259 cells recorded in two monkeys, of which 89% were visually responsive. Of these, approximately two-thirds were orientation selective. Response latency varied considerably between neurons, ranging from a minimum of 41 ms to over 150 ms, although most had latencies of 50-70 ms. Orientation tuning (defined as the bandwidth at half-height) ranged from 16 deg to over 90 deg, with a mean value of around 55 deg. By examining the selectivity of these different neurons by 10-ms time slices, starting at the onset of the neuronal response, we found that the orientation selectivity of virtually every neuron was fully developed at the very start of the neuronal response. Indeed, many neurons showed a marked tendency to respond at somewhat longer latencies to stimuli that were nonoptimally oriented, with the result that orientation selectivity was highest at the very start of the neuronal response. Furthermore, there was no evidence that the neurons with the shortest onset latencies were less selective. Such evidence is inconsistent with the hypothesis that recurrent intracortical feedback plays an important role in the generation of orientation selectivity. Instead, we suggest that orientation selectivity is primarily generated using feedforward mechanisms, including feedforward inhibition. Such a strategy has the advantage of allowing orientation to be computed rapidly, and avoids the initially poorly selective neuronal responses that characterize processing involving recurrent loops.

Spatiotemporal dynamics of component processes in human working memory

Working memory (WM), the ability to momentarily maintain information in an active state, is central to higher cognitive functions. The processes involved in WM operate on a sub-second timescale, and thus evoked potential measures have an appropriate temporal resolution for studying them. In the experiment reported here, evoked potential covariances (EPC) between scalp recording sites were computed for a task requiring maintenance of numeric information in WM; these EPCs were compared to those observed in a control task which had the same stimuli and responses but less of a WM requirement. EPC patterns differed between conditions prior to the stimulus, and in an interval spanning the P300 peak in the match detection trials which required response inhibition. The pattern of prestimulus EPCs was more complex and left-sided in the WM task, when memory codes were being maintained and responses contingent on those codes were being prepared. P300 peak latency was 140 msec shorter in the WM task, and the P300 EPC pattern was more anterior and left-sided. In contrast, EPC patterns did not differ during early stages of stimulus processing or during response execution. These results suggest that distinct EPC patterns associated with WM only occur during intervals in which the information in an active state is being utilized for task performance.