Abstract: The posterior parietal cortex has long been considered an 'association' area that combines information from different sensory modalities to form a cognitive representation of space. However, until recently little has been known about the neural mechanisms responsible for this important cognitive process. Recent experiments from the author's laboratory indicate that visual, somatosensory, auditory and vestibular signals are combined in areas LIP and 7a of the posterior parietal cortex. The integration of these signals can represent the locations of stimuli with respect to the observer and within the environment. Area MSTd combines visual motion signals, similar to those generated during an observer's movement through the environment, with eye-movement and vestibular signals. This integration appears to play a role in specifying the path on which the observer is moving. All three cortical areas combine different modalities into common spatial frames by using a gain-field mechanism. The spatial representations in areas LIP and 7a appear to be important for specifying the locations of targets for actions such as eye movements or reaching; the spatial representation within area MSTd appears to be important for navigation and the perceptual stability of motion signals.

Abstract: Stochastic resonance can be used as a measuring tool to quantify the ability of the human brain to interpret noise contaminated visual patterns. Here we report the results of a psychophysics experiment which show that the brain can consistently and quantitatively interpret detail in a stationary image obscured with time varying noise and that both the noise intensity and its temporal characteristics strongly determine the perceived image quality.

Abstract: Following Hermann von Helmholtz, who described visual perceptions as unconscious inferences from sensory data and knowledge derived from the past, perceptions are regarded as similar to predictive hypotheses of science, but are psychologically projected into external space and accepted as our most immediate reality. There are increasing discrepancies between perceptions and conceptions with science's advances, which makes it hard to define 'illusion'. Visual illusions can provide evidence of object knowledge and working rules for vision, but only when the phenomena are explained and classified. A tentative classification is presented, in terms of appearances and kinds of causes. The large contribution of knowledge from the past for vision raises the issue: how do we recognize the present, without confusion from the past. This danger is generally avoided as the present is signalled by real-time sensory inputs-perhaps flagged by qualia of consciousness.

Abstract: Some researchers of studies of the incidence of early visual experience on spatial abilities have demonstrated profound spatial deficits in early blind participants, whereas others have not found evidence of deleterious effects of early visual deprivation. The aims of this article are to (a) consider the theoretical background of these studies, (b) take stock of the divergent data, and (c) propose new means of investigation. The authors examine the reasons why vision plays a critical role in spatial cognition. They review the literature data. They also review the factors that could account for the discrepant data and the effects of lack of early visual experience on brain functioning. They propose that the study of strategies is a valuable option to obtain insight into early blind persons' spatial impairment. The ability to move about independently in space, to localize places that cannot be directly perceived because they are hidden or remote, and to plan trajectories on the basis of this knowledge is of great importance in everyday human life activities. It is not necessary to refer to sophisticated experimental studies to assert that many of these spatial behaviors depend on to a great extent visual perception. In cases where an object or a place to reach is visible, the movement or trajectory is directly guided by the visual perception of the goal or of conspicuous landmarks associated with it. In many circumstances, however, spatial behavior takes place in larger environments where the goal is not visible. In that case, it is necessary that spatial knowledge takes the form of a representation. The latter may simply consist of remembering a specific route to follow, but this simple means to achieve accurate trajectories lacks adaptive properties (O'Keefe & Nadel, 1978). The most adequate form of spatial representation is that of the topography of the environment beyond perceptual reach. This representation is acquired either by one using symbolic supports (such as reading a map) or progressively constructing one's internal map on the basis of experience (as when one frequently goes shopping in the district in which one resides, e.g.).

Abstract: Saccadic eye movements, in which the eye moves rapidly between two resting positions, shift the position of our retinal images. If our perception of the world is to remain stable, the visual directions associated with retinal sites, and others they report to, must be updated to compensate for changes in the point of gaze. It has long been suspected that this compensation is achieved by a uniform shift of coordinates driven by an extra-retinal position signal, although some consider this to be unnecessary. Considerable effort has been devoted to a search for such a signal and to measuring its time course and accuracy. Here, by using multiple as well as single targets under normal viewing conditions, we show that changes in apparent visual direction anticipate saccades and are not of the same size, or even in the same direction, for all parts of the visual field. We also show that there is a compression of visual space sufficient to reduce the spacing and even the apparent number of pattern elements. The results are in part consistent with electrophysiological findings of anticipatory shifts in the receptive fields of neurons in parietal cortex and superior colliculi.

Abstract: The identification of brain regions that are associated with the conscious perception of visual stimuli is a major goal in neuroscience1. Here we present a test of whether the signals on neurons in cortical area V1 correspond directly to our conscious perception of binocular stereoscopic depth. Depth perception requires that image features on one retina are first matched with appropriate features on the other retina. The mechanisms that perform this matching can be examined by using random-dot stereograms2, in which the left and right eyes view randomly positioned but binocularly correlated dots. We exploit the fact that anticorrelated random-dot stereograms (in which dots in one eye are matched geometrically to dots of the opposite contrast in the other eye) do not give rise to the perception of depth3 because the matching process does not find a consistent solution. Anticorrelated random-dot stereograms contain binocular features that could excite neurons that have not solved the correspondence problem. We demonstrate that disparity-selective neurons in V1 signal the disparity of anticorrelated random-dot stereograms, indicating that they do not unambiguously signal stereoscopic depth. Hence single V1 neurons cannot account for the conscious perception of stereopsis, although combining the outputs of many V1 neurons could solve the matching problem. The accompanying paper4 suggests an additional function for disparity signals from V1: they may be important for the rapid involuntary control of vergence eye movements (eye movements that bring the images on the two foveae into register).

Abstract: This review focuses on advances in our understanding of the roles played by vision in the control of human locomotion. Vision is unique in its ability to provide information about near and far environment almost instantaneously: this information is used to regulate locomotion on a local level (step by step basis) and a global level (route planning). Basic anatomy and neurophysiology of the sensory apparatus. the neural substrate involved in processing this visual input, descending pathways involved in effecting control and mechanisms for controlling gaze are discussed. Characteristics of visual perception subserving control of locomotion include the following: (a) intermittent visual sampling is adequate for safe travel over various terrains; (b) information about body posture and movement from the visual system is given higher priority over information from the other two sensory modalities; (c) exteroceptive information about the environment is used primarily in a feedforward sampled control mode rather than on-line control mode; (d) knowledge acquired through past experience influences the interpretation of the exteroceptive information; (e) exproprioceptive information about limb position and movement is used on-line to fine tune the swing limb trajectory; (f) exproprioceptive information about self-motion acquired through optic flow is used on-line in a sampled controlled mode. Characteristics of locomotor adaptive strategies are: (a) most adaptive strategies can be implemented successfully in one step cycle provided the attention is biased towards the visual cues: only steering has to be planned in the previous step; (b) stability requirements constrain the selection of specific avoidance strategies: (c) response is not localized to a joint or limb: it is global, complex and task specific; (d) response characteristics are dependent upon available response time; (e) effector system dynamics are exploited by the control system to simplify and effectively control swing limb trajectory. Effects of various visual deficits on adaptive control are briefly discussed. Copyright 0 1997 Elsevier Science B.V.

Abstract: Introduction and historical overview a theory of hemispheric asymmetrics in perception visual perception - lateralization in simple and complex patterns attention and visual laterality auditory perception speech perception and language a computer implementation of the double filtering by frequency theory the DDF theory at work the two sides of perception.

Abstract: A central issue in cognitive neuroscience concerns the functional architecture of the prefrontal cortex and the degree to which it is organized by sensory domain. To examine this issue, multiple areas of the macaque monkey prefrontal cortex were mapped for selective responses to visual stimuli that are prototypical of the brain's object vision pathway-pictorial representations of faces. Prefrontal neurons not only selectively process information related to the identity of faces but, importantly, such neurons are localized to a remarkably restricted area. These findings suggest that the prefrontal cortex is functionally compartmentalized with respect to the nature of its inputs.

Abstract: Many theories of visual perception assume that before attention is allocated within a scene, visual information is parsed according to the Gestalt principles of organization. This assumption has been challenged by experiments in which participants were unable to identify what Gestalt grouping patterns had occurred in the background of primary-task displays (A. Mack, B. Tang, R. Tuma, S. Kahn, & I. Rock, 1992). In the present study, participants reported which of 2 horizontal lines was longer. Dots in the background, if grouped, formed displays similar to the Ponzo illusion (Experiments 1 and 2) or the Muller-Lyer illusion (Experiment 3). Despite inaccurate reports of what the patterns were, participants' responses on the line-length discrimination task were clearly affected by the 2 illusions. These results suggest that Gestalt grouping does occur without attention but that the patterns thus formed may not be encoded in memory without attention.

Abstract: Two experiments investigated the viewpoint dependence of spatial memories In Experiment 1, participants learned the locations of objects on a desktop from a single perspective and then took part in a recognition test, test scenes included familiar and novel views of the layout Recognition latency was a linear function of the angular distance between a test view and the study view In Experiment 2, participants studied a layout from a single view and then learned to recognize the layout from three additional training views A final recognition test showed that the study view and the training views were represented in memory, and that latency was a linear function of the angular distance to the nearest study or training view These results indicate that interobject spatial relations are encoded in a viewpoint-dependent manner, and that recognition of novel views requires normalization to the most similar representation in memory These findings parallel recent results in visual object recognition.

Abstract: Mental imagery is thought to play a key role in certain aspects of visual perception and to depend on neural activity in visual cortex. We asked whether tactile discrimination of grating orientation, which appears to involve visual mental imagery, recruits visual cortical areas. H215O positron emission tomography was performed in humans during presentation of gratings to the right index fingerpad. Selective attention to grating orientation significantly increased regional cerebral blood flow, relative to a control task involving selective attention to grating dimensions, in a region located in left parieto-occipital cortex. We propose that this activation reflects the use of imagery-related visuo-spatial processes to enable the tactile discrimination of orientation.

Abstract: It is more than half a century ago since Keffer H Hartline published his classical receptive fields studies of single optic nerve fibres in the frog. World War II intervened and the full impact of his work did not become apparent until the early fifties, when Horace Barlow extended Hartline's analysis in the frog and Stephen W Kuffler showed the on-centre and off-centre type ganglion cells in the cat retina. The next advances were made in the late fifties when Jerome Lettvin and Humberto Maturana described cells in the frog tectum with very complex response properties and when David Hubel and I discovered that cells in the cat striate cortex were sensitive to orientation of contours and binocular stimulation. Vision research has gone a long distance since that time—nonetheless we have just begun the long journey towards a detailed mechanistic understanding of the neural basis of visual perception. In this lecture I discuss the processing of visual information at the level of the striate cortex in the cat and monkey, and describe technical advances that have greatly facilitated the analysis of the neural mechanisms of visual perception.

Abstract: In extending our previous work, we addressed the question of whether different visual attributes are perceived separately when they belong to different objects, rather than the same one. Using our earlier psychophysical method, but separating the attributes to be paired in two different halves of the screen, we found that human subjects misbind the colour and the direction of motion, or the colour and the orientation of lines, because colour, form, and motion are perceived separately and at different times. The results therefore show that there is a perceptual temporal hierarchy in vision.

Abstract: Two triangulation methods for measuring perceived egocentric distance were examined. In the triangulation-by-pointing procedure, the observer views a target at some distance and, with eyes closed, attempts to point continuously at the target while traversing a path that passes by it. In the triangulation-by-walking procedure, the observer views a target and, with eyes closed, traverses a path that is oblique to the target; on command from the experimenter, the observer turns and walks toward the target. Two experiments using pointing and 3 using walking showed that perceived distance, averaged over observers, was accurate out to 15 m under full-cue conditions. For target distances between 15 and 25 m, the evidence indicates slight perceptual underestimation. Results also show that observers, on average, were accurate in imaginally updating the locations of previously viewed targets. The term visual space (or visually perceived space) refers to a perceptual representation of the immediate physical environment that exists independently of any of the particular spatial behaviors it helps to control. Much vision research has been devoted to establishing the functional properties of visual space and the mechanisms that underlie it. A major goal of such research has been to characterize the mapping from physical to visual space under different conditions of information availability, but ultimately the goal must be to predict visual space solely in terms of its sensory inputs and internal determinants (e.g., intrinsic noise, observer assumptions, etc.). Because visual direction is perceived accurately, most space perception research has examined the perception of egocentric distance (i-e., the distance from the object to the observer) and the perception of exocentric distance (i.e., the distance between two targets lying in the same visual direction or, more generally, the distance between any two locations). Because we believe that the perception of egocentric

Abstract: Neuropsychological evidence indicates that the global aspect of complex visual scenes is preferentially processed by the right hemisphere, and local aspects are preferentially processed by the left hemisphere. Using letter-based hierarchical stimuli (Navon figures), we recently demonstrated, in a directed-attention task, lateralized neural activity (assessed by positron emission tomography) in the left prestriate cortex during local processing, and in the right prestriate cortex during global processing. Furthermore, temporal-parietal cortex was critically activated bilaterally in a divided-attention task that involved varying the number of target switches between local and global levels of letter-based hierarchical stimuli. Little is known about whether such stimulus categories influence such hemispheric lateralization. We now present data on brain activity, derived from positron emission tomography, in normal subjects scanned during either local or global processing of object-based hierarchical stimuli. We again observe attentional modulation of neural activity in prestriate cortex. There is now greater right-sided activation for local processing and greater left-sided activation for global processing, which is the opposite of that seen with letter-based stimuli. The results suggest that the relative differential hemispheric activations in the prestriate areas during global and local processing are modified by stimulus category.

Abstract: Blindsight is the rare and paradoxical ability of some human subjects with occipital lobe brain damage to discriminate unseen stimuli in their clinically blind field defects when forced-choice procedures are used, implying that lesions of striate cortex produce a sharp dissociation between visual performance and visual awareness. Skeptics have argued that this is no different from the behavior of normal subjects at the lower limits of conscious vision, at which such dissociations could arise trivially by using different response criteria during clinical and forced-choice tests. We tested this claim explicitly by measuring the sensitivity of a hemianopic patient independently of his response criterion in yes-no and forced-choice detection tasks with the same stimulus and found that, unlike normal controls, his sensitivity was significantly higher during the forced-choice task. Thus, the dissociation by which blindsight is defined is not simply due to a difference in the patients’ response bias between the two paradigms. This result implies that blindsight is unlike normal, near-threshold vision and that information about the stimulus is processed in blindsighted patients in an unusual way.

Abstract: Research has shown that speaking in a deliberately clear manner can improve the accuracy of auditory speech recognition. Allowing listeners access to visual speech cues also enhances speech understanding. Whether the nature of information provided by speaking clearly and by using visual speech cues is redundant has not been determined. This study examined how speaking mode (clear vs. conversational) and presentation mode (auditory vs. auditory-visual) influenced the perception of words within nonsense sentences. In Experiment 1, 30 young listeners with normal hearing responded to videotaped stimuli presented audiovisually in the presence of background noise at one of three signal-to-noise ratios. In Experiment 2, 9 participants returned for an additional assessment using auditory-only presentation. Results of these experiments showed significant effects of speaking mode (clear speech was easier to understand than was conversational speech) and presentation mode (auditory-visual presentation led to better performance than did auditory-only presentation). The benefit of clear speech was greater for words occurring in the middle of sentences than for words at either the beginning or end of sentences for both auditory-only and auditory-visual presentation, whereas the greatest benefit from supplying visual cues was for words at the end of sentences spoken both clearly and conversationally. The total benefit from speaking clearly and supplying visual cues was equal to the sum of each of these effects. Overall, the results suggest that speaking clearly and providing visual speech information provide complementary (rather than redundant) information.

Abstract: To analyze the information provided about individual visual stimuli in the responses of single neurons in the primate temporal lobe visual cortex, neuronal responses to a set of 65 visual stimuli were recorded in macaques performing a visual fixation task and analyzed using information theoretical measures. The population of neurons analyzed responded primarily to faces. The stimuli included 23 faces and 42 nonface images of real-world scenes, so that the function of this brain region could be analyzed when it was processing relatively natural scenes. It was found that for the majority of the neurons significant amounts of information were reflected about which of several of the 23 faces had been seen. Thus the representation was not local, for in a local representation almost all the information available can be obtained when the single stimulus to which the neuron responds best is shown. It is shown that the information available about any one stimulus depended on how different (for example, how many standard deviations) the response to that stimulus was from the average response to all stimuli. This was the case for responses below the average response as well as above. It is shown that the fraction of information carried by the low firing rates of a cell was large - much larger than that carried by the high firing rates. Part of the reason for this is that the probability distribution of different firing rates is biased toward low values (though with fewer very low values than would be predicted by an exponential distribution). Another factor is that the variability of the response is large at intermediate and high firing rates. Another finding is that at short sampling intervals (such as 20 ms) the neurons code information efficiently, by effectively acting as binary variables and behaving less noisily than would be expected of a Poisson process.