Contextual modulation of orientation discrimination is independent of stimulus processing time.

Contextual inhibition of neural activity in the primary visual cortex begins immediately and is most pronounced in the early transient response component. Using backward masking to control available processing time, we investigated whether the interaction between perceptual contextual modulation and processing time reflects the neural dynamics of contextual inhibition. We found that the threshold elevation due to contextual inhibition in an orientation-discrimination task is essentially independent of the available processing time and that it is closely related to contextual inhibition of the early transient response component of orientation-selective neurons in the primary visual cortex.

Medial-point description of shape: a representation for action coding and its psychophysical correlates.

We describe a region-based shape representation that might be particularly useful from a biological perspective because it promotes the localization of objects, and object parts relative to each other. The proposed medial-point representation is similar to medial-axis type representations, but it is more compact. The medial points are those points along the medial axis that are equidistant from the longest segments of the boundary, therefore they represent the largest amount of edge information. The main advantage is that the original image can be reduced to a small number of points. We also provide psychophysical correlates of the representation for shapes with increasing complexity. Using a reverse mapping technique, we find that variations of contrast sensitivity within figures are defined by the shape of the bounding contour, and the peaks in the sensitivity maps correspond to the medial points of the proposed representation.

Withdrawing attention at little or no cost: detection and discrimination tasks.

We used a concurrent-task paradigm to investigate the attentional cost of simple visual tasks. As in earlier studies, we found that detecting a unique orientation in an array of oriented elements ("pop-out") carries little or no attentional cost. Surprisingly, this is true at all levels of performance and holds even when pop-out is barely discriminable. We discuss this finding in the context of our previous report that the attentional cost of stimulus detection is strongly influenced by the presence and nature of other stimuli in the display (Braun, 1994b). For discrimination tasks, we obtained a similarly mixed outcome: Discrimination of letter shape carried a high attentional cost whereas discrimination of color and orientation did not. Taken together, these findings lead us to modify our earlier position on the attentional costs of detection and discrimination tasks (Sagi & Julesz, 1985). We now believe that observers enjoy a significant degree of "ambient" visual awareness outside the focus of attention, permitting them to both detect and discriminate certain visual information. We hypothesize that the information in question is selected by a competition for saliency at the level of early vision.

A simple integrative method for presenting head-contingent motion parallax and disparity cues on intel x86 processor-based machines.

Rogers and Graham (1979) developed a system to show that head-movement-contingent motion parallax produces monocular depth perception in random dot patterns. Their display system comprised an oscilloscope driven by function generators or a special graphics board that triggered the X and Y deflection of the raster scan signal. Replication of this system required costly hardware that is no longer on the market. In this paper the Rogers-Graham method is reproduced with an Intel processor based IBM PC compatible machine with no additional hardware cost. An adapted joystick sampled through the standard game-port can serve as a provisional head-movement sensor. Monitor resolution for displaying motion is effectively enhanced 16 times by the use of anti-aliasing, enabling the display of thousands of random dots in real-time with a refresh rate of 60 Hz or above. A color monitor enables the use of the anaglyph method, thus combining stereoscopic and monocular parallax on a single display without the loss of speed. The power of this system is demonstrated by a psychophysical measurement in which subjects nulled head-movement-contingent illusory parallax, evoked by a static stereogram, with real parallax. The amount of real parallax required to null the illusory stereoscopic parallax monotonically increased with disparity.

Interactions of monocular and cyclopean components and the role of depth in the Ebbinghaus illusion.

A study of size interactions of objects in three-dimensional space is reported. The canonical form of the Ebbinghaus illusion-test circles surrounded by large or small inducers-was used. Both monocularly visible (M) and purely cyclopean (C) objects were displayed stereoscopically to isolate the monocular and cyclopean components of the illusion. The results of two experiments indicate that: (i) depth plays a significant role when the test circles are cyclopean, but not when they are monocularly visible; and (ii) the size of C objects is affected equally by C and M inducers, but the size of M objects is affected much more strongly by M than by C inducers. In conclusion, possible explanations are offered for the main trends in the data, the most interesting of which is that cyclopean tests seem to be interacting only with the cyclopean component of monocularly visible inducers.

Context effect of common objects on visual processing.

In previous experiments on visual processing it was shown that correct identification of sequentially and simultaneously presented numerals declined as the presentation interval was decreased from 50.0 to 16.7 ms/numeral. To determine how performance was influenced in context, common everyday objects were used in place of the numerals. These objects were grouped into 36 different categories with 4 objects per category. An example of a category was fruit: apple, banana, pear, pineapple. For all experimental trials, objects (ranging in number from one to four) were presented randomly at four possible positions 1.5 degrees from a central fixation point. In a sequential trial, the objects were displayed in succession at different locations for a stimulus onset asynchrony (SOA) of either 16.7, 33.3, or 50.0 ms. The masking of a previous object coincided with the onset of the object that followed in the sequence. A simultaneous trial consisted of 2, 3, or 4 objects presented concurrently. For these trials, simultaneous masking occurred at SOA's of 16.7, 33.3, or 50.0 ms. For both experimental paradigms, one-half of the trials consisted of objects from the same category, and one-half from mixed categories. Results taken from four observers showed that performance for all three SOA's was consistently better when the objects were related as opposed to when the objects were not for both presentation protocols. This lent support to the theory that there was simultaneous interaction of top-down and bottom-up processes in visual processing. In addition, as was found in the numerals study, performance was better when the objects were presented simultaneously as opposed to sequentially.

Simultaneous better than sequential for brief presentations.

During perceptually intensive tasks such as reading, there is a bottleneck in the information transfer between the large number of alphanumeric characters available and the acquiring of these characters. This is due mainly to the limited number of characters that one can report at a glance (also known as the "magic number 7 +/- 2") [Psychol. Rev. 63, 81 (1956)]. To examine where in the perceptual pathway this bottleneck occurred, several investigators tested and compared performance with simultaneous and with sequential target presentations [J. Exp. Psychol. 79, 1 (1969); 93, 72 (1972); Percept. Psychophys. 14, 231 (1973)]. They found that performance was nearly equal in the two cases and concluded that the bottleneck must be due to the limitation of short-term memory. However, these studies were limited either by a long stimulus-onset asynchrony (SOA), or time interval between onsets of icon presentations, or by a lack of poststimulus masking. We report on experiments designed to overcome these limitations. We used shorter SOA's than did previous investigators, and we removed persistence effects by poststimulus masking. Our stimuli were presented either sequentially or simultaneously. For the sequential presentation a numeral ranging from 0 to 9 was displayed at any one of eight positions 1.5 deg from a central fixation cross. The appearance of the next numeral in another part of the display coincided with the masking of the previous numeral. This was done for trials of one to four numerals and SOA's of 16.7, 33.3, and 50.0 ms.(ABSTRACT TRUNCATED AT 250 WORDS)

Perceptual sensitivity maps within globally defined visual shapes.

An unsolved problem of biology is the processing of global shape in natural vision. The known processes of early vision are spatially restricted (or local) operations, and little is known about their interactions in organizing the visual image into functionally coherent (or global) objects. Here we introduce a human psychophysical method which allows us to measure the effect of perceptual organization on the activity pattern of local visual detectors. We map differential contrast sensitivity for a target across regions enclosed by a boundary. We show that local contrast sensitivity is enhanced within the boundary even for large distances between the boundary and the target. Furthermore, the locations of maximal sensitivity enhancement in the sensitivity maps are determined by global shape properties. Our data support a class of models which describe shapes by the means of a medial axis transformation, implying that the visual system extracts 'skeletons' as an intermediate-level representation of objects. The skeletal representation offers a structurally simplified shape description which can be used for higher-level operations and for coding into memory.

A closed curve is much more than an incomplete one: effect of closure in figure-ground segmentation.

Detection of fragmented closed contours against a cluttered background occurs much beyond the local coherence distance (maximal separation between segments) of nonclosed contours. This implies that the extent of interaction between locally connected detectors is boosted according to the global stimulus structure. We further show that detection of a target probe is facilitated when the probe is positioned inside a closed circle. To explain the striking contour segregation ability found here, and performance enhancement inside closed boundaries, we propose the existence of a synergetic process in early vision.

Depth, motion, and static-flow perception at metaisoluminant color contrast.

Many experiments concerned with the role of color in depth and motion perception have applied isoluminant random-dot stereograms and cinematograms. The poor performance in the absence of luminance contrast has been associated with color-blindness of stereopsis and motion perception (Livingstone, M.S. & Hubel, D.H. (1987) J. Neurosci. 7, 3416-3468). Nevertheless, isoluminant stimuli are not fully accepted as appropriate tools in isolating central mechanisms (Logothetis, N.K., Schiller, P.H., Charles, E.R. & Hurlbert, A.C. (1990) Science 247, 214-217). In our experiments we use a broad luminance range to test whether color can contribute to a given mechanism when luminance contrast is present but has a strong "veto" effect from opposite luminance contrast, a condition we named "metaisoluminance." There is no fusion in stereopsis under polarity reversal, when only luminance information is given, and reversed-phi phenomenon is experienced for motion. As a third "matching" task, we included polarity-reversed random-dot Glass-patterns, which exhibit "static flow" and also show pattern reversal. We found that color can counteract the effects of polarity reversal by restoring stereoscopic fusion and reversed phi motion and does it with increased efficiency as the hue contrast increases. We found no such effect of color in Glass-patterns. Thus, we showed that the visual system for binocular depth and motion perception is not color-blind, although correlated hue information under metaisoluminance does not appear to yield shape perception.

Perceptual asymmetry in texture perception.

A fundamental property of human visual perception is our ability to distinguish between textures. A concerted effort has been made to account for texture segregation in terms of linear spatial filter models and their nonlinear extensions. However, for certain texture pairs the ease of discrimination changes when the role of figure and ground are reversed. This asymmetry poses a problem for both linear and nonlinear models. We have isolated a property of texture perception that can account for this asymmetry in discrimination: subjective closure. This property, which is also responsible for visual illusions, appears to be explainable by early visual processes alone. Our results force a reexamination of the process of human texture segregation and of some recent models that were introduced to explain it.

The speed of attentional shifts in the visual field.

The scanning speed of focal visual attention was measured directly by flashing a sequence of two, three, or four numerals one by one at random retinal positions and at a distance from each other to avoid interference between the numerals. Each numeral was followed by a mask pattern so the observers had to move their focal attention in the visual field in synchrony and at the same speed as the presentation rate of the numerals in order to recognize every numeral in the stimulus sequence. Observers could recognize the numerals orders of magnitude above the theoretical chance level of performance even when the presentation rate was as fast as 33 ms per numeral. However, the temporal order of the numerals was perceived rather poorly at the fast presentation rates and for the sequences of four numerals.

Two carriers for motion perception: color and luminance.

Starting with the experiments of Ramachandran and Gregory (Nature, 275, 55-56, 1978), several psychophysical studies in apparent motion (AM) have established that the perception of motion is significantly impaired at equiluminance. Still debated, however, is whether color alone can resolve ambiguities in AM. We report here on several psychophysical experiments, the quantitative results of which indicate that color does play a substantial role in AM. These findings seem to support recently proposed neurophysiological frameworks according to which there exist significant interactions among the neuronal pathways mediating the perception of basic visual attributes such as color, motion, form and depth.

Context superiority in a detection task with line-element stimuli: a low-level effect.

Detection and identification performances for vertical and horizontal target elements embedded within an array of oriented noise elements were measured as a function of the orientation difference between the target and noise elements. Detection performances obtained with one vertical and three horizontal target elements clustered together and displayed such that they formed a schematic face-like pattern were significantly better than those obtained with the same clustered target elements displayed in an arbitrary, symmetrical or asymmetrical, configuration. This was so even though the identification of the face and nonface stimuli was well below the detection threshold of their parts. Detection thresholds for the clustered nonface patterns were slightly but significantly lower than those for the same target elements dispersed among the noise elements. Probability summation calculations based on the detection results obtained with one single target element predict detection thresholds which are intermediate between those of the clustered and dispersed targets, suggesting that inhibitory and facilitatory spatial interactions respectively are at work for the two types of stimuli. The existence of a context(face)-superiority effect at the detection level indicates top-down/bottom-up interactions between remote visual processing stages.

Stereoscopic illusion based on the proximity principle.

A class of ambiguous random-dot stereograms were created that share the following interesting property: Although the binocular disparity forms a periodic 'sawtooth' waveform as a function of row number (the disparity is constant for a given row), these stimuli yield a monotonically increasing depth percept along the rows. The random-dot pattern of each row is periodic along the horizontal direction for the purpose of producing an ambiguous depth percept. It is this ambiguity that makes it possible for the periodic stimulus to give rise to a monotonic percept. This monotonic percept is substantially enhanced when the rows are shown in temporal sequence instead of all being displayed together. Experiments are reported which indicate that this illusion is due to the proximity, or pulling, effect in stereopsis.

The control and speed of shifts of attention.

We measured the detectability of a target pattern in a display consisting of 12 elements in a circle around the central fixation point. The display was presented briefly and followed after a variable amount of time by a mask. We found that presenting a pre-cue, designating the target position, facilitated target detectability. Attention is directed to the cued location. When the observer has to detect a (second) target among the non cued elements, performance for locations close to the cue is not significantly different from performance for locations further away. This suggests that there is no "scan-path" or proximity effect. We also found that the identification of the cued element delayed the detectability of the subsequent target by more than 160 msec. In another series of experiments we studied the control of attentional shifts. We found that, for short mask delays (100, 160, and 260 msec) the observer is unable to selectively process elements which are not physically cued but only verbally defined by their position relative to the cue. When we increase the positional uncertainty of the target by increasing the number of physical cues, performance drops until it reaches an asymptote with 5 elements. We infer that, even though the target is very similar to the background, a parallel mechanism, used for the extraction of stimulus features, designates prospective target locations which may be subsequently checked by a (slow) attentional process.

Globally perceived directional flow in static images.

Visual sensitivity to spatial direction has classically been associated with motion perception. Yet humans are adept at deriving directional information in the absence of motion, as when they read maps, or follow arrows or animal tracks. Experiments are reported on the perception of parallel arrow-like forms in which a specific visual sensitivity to static direction is demonstrated. Global processing is operationally defined in terms of the relative discriminability of sets and subsets of stimulus elements; a set of parallel elements and a set in which one element is antiparallel to the rest are shown to be processed globally. The result of this global processing is a static analog of unidirectional optic flow. Global spatial direction differs fundamentally from other perceptions derived from static image processing. It involves long-range interactions in texture arrays, it does not carry information about stimulus location, and it is not reducible to the perception of component stimulus elements. Its likely function is in the construction of the layout of visual space.

Short-range limitation on detection of feature differences.

We studied the ability of observers to detect the presence of a clearly visible line segment against a background of line segments of different orientation. As we increase the number (density) of these background lines, we find that detectability does not behave monotonically. Adding a small number of background lines decreases detectability but if adjacent line segments are permitted to fall in close range, a further increase of background lines improves performance which eventually reaches a constant level. This suggests that detection of feature differences involves a short-range process. The range of this process is about two degrees or twice the length of the line segments used. Thus texture-gradients between different elements are only formed if the distance between these elements is not much larger than the average element size.