Perception

This study addressed the ‘correspondence’ problem of apparent-motion (AM) perception in which parts of a scene must be matched with counterparts separated in time and space. Given evidence that AM correspondence can be mediated by two distinct processes—one based on a low-level motion-detection mechanism (the Reichardt process), the other involving the tracking of objects by visual attention (the attention-based process)—the present study explored how these processes interact in the perception of apparent motion between hierarchically structured figures. In three experiments, hierarchical figures were presented in a competition motion display so that, across frames, figures were identical at either the local or the global level. In experiment 1 it was shown that AM occurred between locally identical figures. Furthermore, with the Reichardt AM component eliminated in experiments 3 and 4, no preference was obtained for either level. While evidence from previous studies suggests that form extraction for hierarchically structured figures proceeds from the global to the local level, the present results indicate the irrelevance of such a global precedence in AM correspondence. In addition, it is suggested that Reichardt AM correspondence between local elements constrains attention-based AM correspondence between global figures so that both components move in the same direction. It is argued that this constraining process represents an elegant means of achieving AM correspondence between objects undergoing complex transformations.

Motion is perceived whenever a subject is presented with an appropriate spatiotemporal visual pattern. Like many other visual tasks, motion perception involves both local and global processing, and thus might be subject to the well-known paradox that arises from the fact that local features and observations form the basis for global perception, but sometimes this global percept can not be easily derived from any single local observation, as is best exemplified by the aperture problem. Globally, dual (transparent) motion can be readily perceived. Spatial limits on the local ability to perceive multiple motion are sought. By using the framework of apparent motion, it is found that dual, orthogonally oriented motion can be perceived only when the dots that constitute the two motions are separated by some spatial limit. For short-range apparent motion, the limit is found to be comparable to D_max, and the visual system cannot perceive more than a single coherent motion in a local ‘patch’ of radius D_max. It was also found that this spatial limit on local-motion perception is not constant, but depends linearly on the spatial organisation of the stimuli, and vanishes for stimuli having reverse contrast. The lower bound on the ability to perceive multiple motion is compared with some well-known bounds in stereopsis, and a cortical columnar architecture that might account for it is proposed.

Three experiments on temporal-discontinuity detection were carried out. In experiment 1, temporal-discontinuity thresholds were measured for sinusoidal gratings by the use of the double-staircase method. A sinusoidal grating was presented twice successively. The subject judged whether or not an interval was present. The temporal-discontinuity threshold increased as the spatial frequency of the grating increased, but decreased as the contrast of the grating increased. In experiment 2, contrast-modulated gratings were used instead of the sinusoidal grating. The temporal-discontinuity threshold increased as the carrier frequency increased, and the threshold for each contrast-modulated grating was similar to that for the no-modulation (sinusoidal) grating whose contrast was the same as the maximum local contrast of the contrast-modulated grating. In experiment 3, temporal-discontinuity thresholds were measured for low-contrast (3%) sinusoidal gratings. The thresholds were very low, even for such low-contrast gratings. These results suggest that the low-spatial-frequency channels are not involved in detecting the modulation frequency of the contrast-modulated grating. Rather, the local contrast seems to be the determinant of the detection of the contrast-modulated grating itself.

It has been widely acknowledged that the generation of an anti saccade, ie a saccade towards the direction opposite to that of a visual stimulus, requires the correct function of special brain structures. In the present study attempts were made to measure the time consumption of brain processes preceding the execution of pro and anti saccades. The saccadic eye movements of five adult human subjects were investigated in a series of combined pro/anti saccade tasks with the aid of the gap and the overlap paradigms. The type of trial—pro saccade and anti saccade—was defined by the structure of the stimulus. In some sessions the subjects were, in addition, preinformed about the actual command by a cue at different lead times before stimulus onset. Pro-saccade and anti-saccade trials were randomly intermixed in equal proportions. High error rates (>30% of all trials in some subjects) occurred in the test sessions without preinformative cues. These errors had long reaction times (∼200 ms), whereas the latencies of correct pro or correct anti saccades were even longer (∼350 ms). Analysis of the errors revealed that they were related to the situation in the previous trial: a correct response in the previous trial enhanced the chance of making a saccade of the same type in the actual trial by up to 30%. This pretrial effect occurred whether the actual trial was a pro-saccade or an anti-saccade command. With a cue lead time of 100 ms the numbers of errors decreased, but the latencies of the correct pro or anti saccades were about 70 ms longer than those obtained in the nonrandom control. With a 200 ms cue lead time the reaction times corresponded to those in the control condition. The results suggest that the situation in a given trial creates a kind of default program for the saccade preparation in the next trial. When a cue about the actual command is given early enough, the default program is overridden correspondingly. The perception of the cue and the programming of the corresponding saccade takes an additional 150 to 200 ms.

The purpose of this study was to determine whether the Müller-Lyer illusion is produced by a mechanism which uses information defined in the retinal coordinates, or by a mechanism taking into account the three-dimensional (3-D) shape of the illusion figure. The classical Müller-Lyer figure could not be used to address this question since it is two-dimensional. Three-dimensional Müller-Lyer figures were created to see if the illusion they produce is correlated with the shape of the projected retinal image, or with the shape of these figures defined in a 3-D coordinate frame. In the experiments retinal image shape was juxtaposed against the 3-D shape of the illusion displays. For some displays the direction in which the fins pointed, relative to the shafts, in the 3-D frame was the ‘opposite’ of the direction in which they pointed in the retinal images. For such displays, the illusion predicted on the basis of the 3-D structure was the opposite of that predicted on the basis of retinal image shapes. For another 3-D display the fins were oriented such that each projected a single straight line in the retinal image, thus the typical retinal image (< >, > <) was replaced by straight lines (‖, ‖). For all the displays the observed illusion was consistent with how the fins were oriented relative to the shaft in the 3-D coordinate frame, ie with the 3-D shape of the illusion displays. The retinal image shape appeared to play little, if any, role. One conclusion that emerges is that the specific retinal image shape projected by the classical line-drawn pattern is neither necessary nor sufficient for producing the illusion. The present findings are inconsistent with two well known theories of the Müller-Lyer illusion: inappropriate constancy scaling and selective filtering.

A visual-search paradigm was used to explore the relative ease with which the direction of gaze can be detected. Straight-gaze stimuli were presented as targets within a variable number of distractors with left-averted or right-averted gaze. Reaction time in this case was compared with that when either the left-averted or right-averted gaze stimuli were the targets among distractors of the two remaining gaze directions. The data were examined for the existence of a search asymmetry favoring the straight-gaze targets. Such an asymmetry was found with stimuli that were realistically drawn renditions of pairs of human eyes, as well as with similar schematic stimuli representing pairs of human eyes. The asymmetry, however, was not found with geometric control stimuli, which also presented the critical feature in the central, the left-lateral, or the right-lateral position within the stimulus, but were not eyelike. It was also not found for schematic stimuli consisting of only one eye. It was concluded that the straight gaze direction is a special stimulus with eyelike stimuli, which the visual system is set up to process faster and with fewer errors than averted gaze directions. The results are discussed in terms of the evolutionary significance of the straight gaze direction.

Trajectory mapping is a new scaling technique designed to recover the parameterizations, axes, and paths used to traverse a feature space. Unlike with multidimensional scaling, there is no assumption that the space is homogeneous or metric. Although some metric ordering information is obtained with trajectory mapping, the main output is the feature parameterizations that partition the given domain of object samples into different categories. After an introductory example, the technique is further illustrated by using first a set of colors and then a collection of textures.

The study of illusory brightness and contour phenomena has become an important tool in modern brain research. Gestalt, cognitive, neural, and computational approaches are reviewed and their explanatory powers are discussed in the light of empirical data. Two well-known phenomena of illusory form are dealt with, the Ehrenstein illusion and the Kanizsa triangle. It is argued that the gap between the different levels of explanation, bottom—up versus top—down, creates scientific barriers which have all too often engendered unnecessary debate about who is right and who is wrong. In this review of the literature we favour an integrative approach to the question of how illusory form is derived from stimulus configurations which provide the visual system with seemingly incomplete information. The processes that can explain the emergence of these phenomena range from local feature detection to global strategies of perceptual organisation. These processes may be similar to those that help us restore partially occluded objects in everyday vision. To understand better the Ehrenstein and Kanizsa illusions, it is proposed that different levels of analysis and explanation are not mutually exclusive, but complementary. Theories of illusory contour and form perception must, therefore, take into account the underlying neurophysiological mechanisms and their possible interactions with cognitive and attentional processes.