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Crowding limits peripheral visual discrimination and recognition: a target easily identified in isolation becomes impossible to recognize when surrounded by other stimuli, often called flankers. Most accounts of crowding predict less crowding when the target-flanker distance increases. On the other hand, the importance of perceptual organization and target-flanker coherence in crowding has recently received more attention. We investigated the effect of target-flanker spacing on crowding in multi-element stimulus arrays. We show that increasing the average distance between the target and the flankers does not always decrease the amount of crowding but can even sometimes increase it. We suggest that the regularity of inter-element spacing plays an important role in determining the strength of crowding: regular spacing leads to the perception of a single, coherent, texture-like stimulus, making judgments about the individual elements difficult.

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Human perception of a stimulus varies depending on the context in which the stimulus is presented. Such contextual modulation has often been explained by two basic neural mechanisms: lateral inhibition and spatial pooling. In the present study, we presented observers with a vernier stimulus flanked by single lines; observers' ability to discriminate the offset direction of the vernier stimulus deteriorated in accordance with both explanations. However, when the flanking lines were part of a geometric shape (i.e., a good Gestalt), this deterioration strongly diminished. These findings cannot be explained by lateral inhibition or spatial pooling. It seems that Gestalt factors play an important role in contextual modulation. We propose that contextual modulation can be used as a quantitative measure to investigate the rules governing the grouping of elements into meaningful wholes.

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The historical roots of the Fourier theory of spatial visual perception are traced. The development of the underlying concepts and the psychophysical experiments that led to them, and that they in turn spawned, are examined, as well as their relation to the current knowledge of neural substrates in the retina and primary visual cortex. Allowing nonlinearities or even substituting other types of basis functions does not eliminate the difficulties faced by any theory of visual perception that is built on the notion of fixed spatial filters.

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A distinction is drawn between two intrinsic directions within a simple spatial configuration. The line joining two elements is the radial direction and orthogonal to it is the tangential direction. Separation and bisection discrimination and just-detectable differences in line length are examples of radial thresholds. Vernier and alignment detection are tangential thresholds. Neural processing along these two intrinsic directions differs. There is a strong 'oblique effect' for tangential thresholds and virtually none for radial thresholds. Flank interaction impairs tangential but not radial thresholds.

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Crucial for the perception of form are the spatial relationships between the elements of a visual stimulus. To investigate the mechanisms involved in coding the distance between visual stimuli, thresholds for detecting whether a central marker accurately bisects a spatial interval were compared for a variety of configurations. Thresholds are best when all three members of the bisection configuration are identical. Performance is impaired, often by as much as a factor of two, when the outer delimiters of the spatial interval differ from the central marker in either length, orientation or contrast polarity. Illusory contours act poorly as borders for bisection by a central line. Disparity thresholds are not affected by orientation differences between test and flanking lines. Because in peripheral vision bisection acuity improves with practice, transfer of training between configurations can be used to gauge overlap of neural processing mechanisms. Transfer is complete only between patterns where all markers are similar, reduced when the outer markers differ by 20 degrees in orientation and absent when they are orthogonal. The dependence of bisection discrimination on similarity between the elements of the stimulus demonstrates that the encoding of spatial location and spatial extent are coupled to the coding of other stimulus properties.

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While it is generally accepted that foveal visual acuity in the adult has reached an optimal value, claims for improvement of peripheral acuity with training in the adult persist in the literature. Practice effects in peripheral hyperacuity have been amply documented. A carefully controlled test is here reported to examine the influence of training on the resolution thresholds for two lines and on Landolt C acuity measurements in the retinal periphery in eight normal adults. It involved 11-30 daily sessions of 300 responses with feedback. In some observers the first day's results were somewhat poorer, but otherwise the threshold curves were essential flat. Yet in the same location vernier acuity could be improved by 50% in six training sessions. Sustained and lasting neural modifications in peripheral vision can take place in stereoscopic, orientation, vernier, bisection and time discriminations, but not in resolution and Landolt C acuities.

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To examine the role of primary visual cortex in visuospatial integration, we studied the spatial arrangement of contextual interactions in the response properties of neurons in primary visual cortex of alert monkeys and in human perception. We found a spatial segregation of opposing contextual interactions. At the level of cortical neurons, excitatory interactions were located along the ends of receptive fields, while inhibitory interactions were strongest along the orthogonal axis. Parallel psychophysical studies in human observers showed opposing contextual interactions surrounding a target line with a similar spatial distribution. The results suggest that V1 neurons can participate in multiple perceptual processes via spatially segregated and functionally distinct components of their receptive fields.

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Thresholds for the detection of differences in the duration of visual stimuli were determined for a variety of programs of stimulus onset and offset. Performance suffers when a time interval begins with an ON step and ends with another ON stimulus, compared to the standard ON-OFF stimulation, but the decrement is reversed when the light is ramped down to background during the interval. Neither the magnocellular nor the parvocellular streams can be excluded because there is relatively little impairment of duration discrimination when the stimulus has low contrast or is heterochromatic at isoluminance. Performance at a variety of intensity levels suggests that sustained neural firing in an early stage of visual processing provides a background activity, which prevents good temporal precision of signals.

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Attention in early visual processing engages the higher order, context dependent properties of neurons. Even at the earliest stages of visual cortical processing neurons play a role in intermediate level vision - contour integration and surface segmentation. The contextual influences mediating this process may be derived from long range connections within primary visual cortex (V1). These influences are subject to perceptual learning, and are strongly modulated by visuospatial attention, which is itself a learning dependent process. The attentional influences may involve interactions between feedback and horizontal connections in V1. V1 is therefore a dynamic and active processor, subject to top-down influences.

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Human observers can discriminate the orientation of a stimulus configuration composed of a pair of collinear visual patterns much better than that of a single component pattern alone. Previous investigations of this type of orientation signal integration and of other similar visual integrative functions have shown that, for closely spaced elements, there is integration only for stimuli with the same contrast polarity (i.e., both lighter or both darker than the background) but, at greater separations, integration is independent of contrast polarity. Is this effect specific to differences in contrast polarity, which is known to be an important parameter in the organization of the visual system, or might there be a cluster of other stimulus dimensions that show similar effects, indicating a more widespread distinction between the processes limiting integration at local and long-range spatial scales? Here, we report a similar distance dependence for orientation signal integration across stimulus differences in binocular disparity, direction of motion, and direction of figure-ground assignment. We also demonstrate that the selectivity found at short separations cannot be explained only by "end-cuts," the small borders created at the junction of abutting contrasting patterns. These findings imply the existence of two distinct spatial domains for the integration of foveal orientation information: a local zone in which integration is highly selective for a number of salient stimulus parameters and a long-range domain in which integration is relatively unselective and only requires that patterns be roughly collinear.

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For time intervals in the 150-1500 ms range, the difference-discrimination thresholds are about 5%. The value of this Weber fraction varies somewhat depending whether the stimulus modality is vision, hearing or touch. Thresholds are higher when a time interval signaled in one modality has to be compared with one in another, and also when two different modalities are used to delineate a single time interval, as well as when onset and offset are in the same modality but signaled to opposite cortical hemispheres. There is a prominent practice effect. This effect was used to show that there is complete transfer of training between the two visual hemispheres. These findings imply that the time-discrimination mechanism is not located at an early stage of visual processing. If there is a single central time-discrimination apparatus, the observed intermodal differences must relate to the relative ease of access to it via different modalities. The mechanism involved needs elucidating. Counting of spikes or internal time modules would seem to be too simplistic a concept; there is still a need for a process in which the duration of a just concluded presentation and an internally stored interval duration can be compared.

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One of the fundamental tasks of the visual cortex is to integrate input from different parts of the retina, parsing an image into contours and surfaces, and then assembling these features into coherent representations of objects. To examine the role of the primary visual cortex in the integration of visual information, we measured the response properties of neurons under different stimulus conditions. Surprisingly, we found that even the most conventional measures of receptive field (RF) size were not fixed, but could vary depending on stimulus contrast and foreground-background relationships. On average, the length of the excitatory RF was 4-fold greater for a low-contrast stimulus than for a stimulus at high contrast. Embedding a high-contrast stimulus in a textured background tended to suppress neuronal responses and produced an enlargement in RF size similar to that observed by decreasing the contrast of an isolated stimulus. The results show that RF dimensions are regulated in a dynamic manner that depends both on local stimulus characteristics, such as contrast, and on global relationships between a stimulus and its surroundings.

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To examine the effect of reducing luminance contrast in human foveal vision, discrimination thresholds were measured in four tasks and also a numerical measure of two visual illusions were obtained by a nulling technique. The patterns used for all tasks were made very similar to facilitate comparison between them--all featured luminance step edges whose contrast could be varied from near unity down to the detection threshold. Orientation, vernier and blur discrimination thresholds rise on average 5-6-fold when the contrast is reduced from near unity to a Michelson value of 0.03. Jump displacement thresholds are somewhat more robust to contrast reduction, and the curve of separation discrimination versus contrast is much shallower, rising by a factor of about 2. The magnitude of the Poggendorff and tilt illusions changes very little until the inducing contours are barely detectable.

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In the 1920s Max Wertheimer enunciated a credo of Gestalt theory: the properties of any of the parts are governed by the structural laws of the whole. Intense efforts at the time to discover these laws had only very limited success. Psychology was in the grips of the Fechnerian tradition to seek exact relationships between the material and the mental and, because the Gestalt movement could not deliver these, it never attained a major standing among students of perception. However, as neurophysiological research into cortical processing of visual stimuli progresses the need for organizing principles is increasingly making itself felt. Concepts like contour salience and figure segregation, once the province of Gestalt psychology, are now taking on renewed significance as investigators combine neural modeling and psychophysical approaches with electrophysiological ones to characterize neural mechanisms of cognition. But it would be perilous not to take heed of some of the lessons that the history of the Gestalt movement teaches.

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Thresholds were measured for five tasks: line detection, intensity discrimination, two-line resolution, vernier acuity and line-orientation discrimination. For each task, 30 arcmin lines were presented foveally in eight retinal meridians to assess similarities in orientation anisotropies across tasks in the same observer. Three observers were tested. The pattern of the orientation anisotropy differs across tasks. Meridional anisotropies exist in detection, increment discrimination thresholds, and vernier acuity but the classical oblique effect is consistently found only in orientation discrimination.

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Brightness discrimination thresholds and facilitation by lateral interaction were measured in five human observers and two monkeys. The subjects judged the brightness of one of four peripherally seen lines against a reference. This experiment was performed both when the observer was cued to the position of the test line (focused attention) and when there was no cue (distributed attention). Discrimination was better with focused than with distributed attention. When the test line had a collinear flank, its brightness was enhanced; this enhancement was four times more prominent with distributed than with focused attention. After training, thresholds improved and collinear facilitation decreased under distributed but not under focused attention. The findings show that there are fewer benefits from contextual interaction once attention is directed toward a visual location, and that the attentional effects are subject to training.

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