Hemifield differences in perceived spatial frequency

Measurements of the perceived spatial frequency of stationary sinewave gratings were made with the gratings presented at the same eccentricity in the left, right, upper, and lower visual hemifields. Ten subjects performed the task binocularly with spatial frequencies of 1, 2, and 4 cycles deg-1. Two of these subjects also performed the task monocularly at 2 cycles deg-1. In the majority of cases, the spatial frequency of stimuli presented in the left and lower visual hemifields was overestimated relative to stimuli presented in the right and upper visual hemifields. The results were similar for all spatial frequencies tested, and the direction of the asymmetry was the same whether viewing was with the left eye, right eye or binocular, suggesting that the differences in perceived spatial frequency are not retinal in origin.     

Absence of binocular interaction between spatial and color attributes of visual stimuli

The hypothesis that induction of the McCollough effect (spatially selective color aftereffects) entails adaptation of monocularly driven detectors tuned to both spatial and color attributes of the visual stimulus was examined in four experiments. The McCollough effect could not be generated by displaying contour information to one eye and color information to the other eye during inspection, even in the absence of binocular rivalry. Nor was it possible to induce depth-specific color aftereffects following an inspection period during which random-dot stereograms were viewed, with crossed and uncrossed disparity seen in different colored light. Masking and aftereffect in the perception of stereoscopic depth were also nonselective to color; in both cases, perceptual distortion was controlled by stereospatial variables but not by the color relationship between the inspection and test stimuli. The results suggest that binocularly driven spatial detectors in human vision are insensitive to wavelength.     

Binocular summation of equal-energy flashes of unequal duration

To determine if binocular summation occurs when increment flashes are of equal energy (Bloch’s law) but unequal in luminance-duration parameters, three Ss made temporal forced-choice judgments: (1) monocularly, (2) binocularly when the flashes to each eye were identical, (3) binocularly when the flashes to each eye were of equal energy but different in terms of their luminance and duration parameters, and (4) binocularly when flashes to each eye were separated by 100 msec. Binocular detection rates were consistently superior to monocular detection rates. Similarity in performance between Conditions 2 and 3 indicates that the binocular visual system responds only to the total energy of each monocular flash. The data from two Ss reveal that binocular performance was greater than that predicted on the basis of probability summation.     

Lateralized cognition: asymmetrical and complementary strategies of pigeons during discrimination of the "human concept"

This study was aimed at revealing which cognitive processes are lateralized in visual categorizations of "humans" by pigeons. To this end, pigeons were trained to categorize pictures of humans and then tested binocularly or monocularly (left or right eye) on the learned categorization and for transfer to novel exemplars (Experiment 1). Subsequent tests examined whether they relied on memorized features or on a conceptual strategy, using stimuli composed of new combinations of familiar and novel humans and backgrounds (Experiment 2), whether the hemispheres processed global or local information, using pictures with different levels of scrambling (Experiment 3), and whether they attended to configuration, using distorted human figures (Experiment 4). The results suggest that the left hemisphere employs a category strategy and concentrates on local features, while the right hemisphere uses an exemplar strategy and relies on configuration. These cognitive dichotomies of the cerebral hemispheres are largely shared by humans, suggesting that lateralized cognitive systems already defined the neural architecture of the common ancestor of birds and mammals.     

Hitting the target: relatively easy, yet absolutely difficult

It is generally agreed that absolute-direction judgments require information about eye position, whereas relative-direction judgments do not. The source of this eye-position information, particularly during monocular viewing, is a matter of debate. It may be either binocular eye position, or the position of the viewing-eye only, that is crucial. Using more ecologically valid stimulus situations than the traditional LED in the dark, we performed two experiments. In experiment 1, observers threw darts at targets that were fixated either monocularly or binocularly. In experiment 2, observers aimed a laser gun at targets while fixating either the rear or the front gunsight monocularly, or the target either monocularly or binocularly. We measured the accuracy and precision of the observers' absolute- and relative-direction judgments. We found that (a) relative-direction judgments were precise and independent of phoria, and (b) monocular absolute-direction judgments were inaccurate, and the magnitude of the inaccuracy was predictable from the magnitude of phoria. These results confirm that relative-direction judgments do not require information about eye position. Moreover, they show that binocular eye-position information is crucial when judging the absolute direction of both monocular and binocular targets.     

Are monocular arrays discriminable from binocular arrays?

On each trial of this experiment, a subject was visually stimulated by one or two shadows on a translucent background in a Telebinocular. For each subject, there were 40 trials of monocular stimulation by one shadow, 40 trials of monocular stimulation by two shadows (one in each hemifield), and 40 trials of binocular stimulation by two shadows (one in the left hemifield of one eye and another in the right hemifield of the other eye). Across these randomly ordered trials, 52 subjects were unable to discriminate two right-eye shadows from two left-eye shadows and were unable to discriminate two monocularly perceived shadows from two binocularly perceived shadows. Generally, subjects tended to misidentify right-hemifield shadows as right-eye shadows and tended to misidentify two-hemifield shadows as two-eye shadows.     

“Extinction” of the McCollough effect does not transfer interocularly

A McCollough effect was induced in subjects by having them view typical adapting stimuli binocularly for 5 min. In the control condition, the strength of the McCollough effect was measured 20 min after the end of the adaptation. The strength was measured during monocular and binocular viewing of a test pattern via a color cancellation technique. Monocular strengths for the two eyes of a given subject were equal to each other and slightly weaker than the binocular strength. In the test condition, 15 min of the 20 min between adaptation and testing were spent monocularly viewing black and white gratings of the same orientation and spatial frequency as the adapting gratings. The strength of the effect as measured ipsilaterally was markedly decreased from that in the control condition. The strength of the effect as measured with the contralateral eye showed only a small decrease from that of the control condition. This finding is relevant to various models of the McCollough effect and related color aftereffects, especially those that posit a “learning” type of mechanism between achromatic spatial channels (which exhibit clear interocular transfer of various achromatic effects) and monocular color channels.     

Six-month-old infants perceive the hollow-face illusion

In this study we investigated infants' perception of the hollow-face illusion. 6-month-old infants were shown a concave mask under monocular and binocular viewing conditions and the direction of their reaches toward the mask was recorded. Adults typically perceive a concave mask as convex under monocular conditions but as concave under binocular conditions, depending on viewing distance. Based on previous findings that infants reach preferentially toward the parts of a display that are closest to them, we expected that, if infants perceive the hollow-face illusion as adults do, they would reach to the center of the mask when viewing it monocularly and to the edges when viewing it binocularly. The results were consistent with these predictions. Our findings indicated that the infants perceived the mask as convex when viewing it with one eye and concave when viewing it with two eyes. The results show that 6-month-old infants respond to the hollow-face illusion. Our finding suggests that, early in life, the visual system uses the constraint, or assumption, that faces are convex when interpreting visual input.     

Light and dark adaptation of visually perceived eye level controlled by visual pitch

The pitch of a visual field systematically influences the elevation at which a monocularly viewing subject sets a target so as to appear at visually perceived eye level (VPEL). The deviation of the setting from true eye level averages approximately 0.6 times the angle of pitch while viewing a fully illuminated complexly structured visual field and is only slightly less with one or two pitched-from-vertical lines in a dark field (Matin & Li, 1994a). The deviation of VPEL from baseline following 20 min of dark adaptation reaches its full value less than 1 min after the onset of illumination of the pitched visual field and decays exponentially in darkness following 5 min of exposure to visual pitch, either 30° topbackward or 20° topforward. The magnitude of the VPEL deviation measured with the dark-adapted right eye following left-eye exposure to pitch was 85% of the deviation that followed pitch exposure of the right eye itself. Time constants for VPEL decay to the dark baseline were the same for same-eye and cross-adaptation conditions and averaged about 4 min. The time constants for decay during dark adaptation were somewhat smaller, and the change during dark adaptation extended over a 16% smaller range following the viewing of the dim two-line pitched-from-vertical stimulus than following the viewing of the complex field. The temporal course of light and dark adaptation of VPEL is virtually identical to the course of light and dark adaptation of the scotopic luminance threshold following exposure to the same luminance. We suggest that, following rod stimulation along particular retinal orientations by portions of the pitched visual field, the storage of the adaptation process resides in the retinogeniculate system and is manifested in the focal system as a change in luminance threshold and in the ambient system as a change in VPEL. The linear model previously developed to account for VPEL, which was based on the interaction of influences from the pitched visual field and extraretinal influences from the body-referenced mechanism, was employed to incorporate the effects of adaptation. Connections between VPEL adaptation and other cases of perceptual adaptation of visual direction are described.     

Interocular transfer of orientation-contingent color aftereffects with external and internal adaptation

The issue of whether McCollough effects transfer interocularly was examined in two experiments. In Experiment 1, as in previous experiments, no interocular transfer of McCollough effects was obtained when subjects adapted monocularly to externally present patterns with their unused eyes fully occluded. However, when subjects adapted monocularly (with the unused eye fully occluded) to visual images of those patterns superimposed on physically present chromatic backgrounds, McCollough effects transferred interocularly. In Experiment 2, subjects adapted to either physically present patterns or to images of those patterns (as in Experiment 1), but the unused eye was exposed to unpatterned diffuse white light. In contrast to Experiment 1, the externally adapted subjects showed interocular transfer of McCollough effects. In Experiment 2, the magnitude of the interocular transfer effects produced by imagery was significantly larger than the magnitude of the effects produced by external adaptation, but the magnitude of the imagery-induced aftereffects did not differ from Experiment 1 to Experiment 2. These results extend earlier findings by Kunen and May (1980) and show that McCollough effects can be produced through adaptation to imagery, even though the direction of the imagery-induced aftereffects indicates adaptation to higher spatial frequencies whereas externally derived aftereffects indicate adaptation to lower spatial frequencies. It is concluded that failure of previous studies to obtain interocular transfer of McCollough effects may have resulted from complete occlusion of the contralateral eye. These data point up some interesting similarities and some important differences between imagery and actual vision. The implications for analogical models of visual imagery are discussed.     

Binocular rivalry and immediate memory

Three experiments examine features of a simple memory task on which right-handed, right eye dominant subjects have been reported to recall digits projected to the right eye more accurately than those projected simultaneously to the left eye. Superior recall by these subjects of information projected to the right eye was observed only when stimuli projected simultaneously to both eyes were seen as overlapped in the binocular percept. Under monocular presentations, accurracy of recall was not related to the eye with which stimuli were viewed. The binocular oveslap condition has a significance other than that of simply increasing the difficulty of identifying the elements in a visual display for there were no differences in accuracy of recall from each eye when overlapped stimuli were viewed monocularly. More accurate recall of right eye information appears to reflect the resolution of a conflict between inputs from each eye. The possible relation of this finding to cerebral dominance is also discussed. Order of recall in these experiments depended mainly on spatial cues provided by the experimental situation.     

Comparative neuropsychology of the dual brain: a stroll through animals' left and right perceptual worlds

Perceptual asymmetries in humans typically manifest themselves under quite unnatural settings (e.g., tachistoscopic viewing and dichotic listening) and this has put into question their real biological significance. In animals with laterally placed eyes, however, perceptual asymmetries are ubiquitous in the normal, everyday behavior, as revealed by the differential use of the lateral visual field of the left and right eye in a variety of tasks. Data are presented showing how preferential use of the left and right eyes influences visual discrimination learning and detour behavior in chicks; similarities with detour tests performed in fish and evidence for asymmetries in eye use in animals with larger binocular overlap (e.g., anuran amphibians) are discussed. Implications of these perceptual asymmetries on the formation and fate of memory traces are put forward, with examples from unihemispheric sleep and lateralization of spatial memory in chicks. Finally, speculations about the evolutionary origins and possible adaptive advantages of perceptual asymmetries in vertebrates are presented.     

Lateralized interhemispheric transfer of color cues: evidence for dynamic coding principles of visual lateralization in pigeons

Visual feature discrimination tasks in pigeons reveal a right eye/left hemisphere dominance at the population level. Anatomical studies and lesion data show this visual lateralization to be related to asymmetries of the tectofugal system, which ascends from the tectum over the n. rotundus to the forebrain. Anatomically, this system is characterized by numerous morphological and connectional asymmetries which result in a bilateral visual representation in the dominant left hemisphere and a mostly contralateral representation in the subdominant right hemisphere. Ontogenetically, visual lateralization starts with an asymmetrical embryonic position within the egg, which leads to asymmetries of light stimulation. Differences in exposure to light stimulation between the eyes result in activity differences between the ascending tectofugal pathways of the left and the right hemisphere, which are transcribed during a critical time span into morphological asymmetries. The asymmetries established after this transient period finally start to determine the lateralized processes of the visual system for the entire life span of the individual. We now can show that these anatomical lateralizations are accompanied by asymmetries of interocular transfer, which enable a faster shift of learned color cues from the dominant right to the left eye than vice versa. In summary, our data provide evidence that cerebral asymmetries are based both on "static" anatomical and on "dynamic" process-dependent principles.     

Monocular asymmetries in recognition after an eye movement: Sighting dominance and dextrality

Although monocular recognition scores for targets presented immediately after an eye movement do not differ, the two eyes show marked recognition asymmetries when both eyes are receiving inputs but a specific target is only presented to one. In general, the right eye performs better than the left, although there are interactions with sighting dominance and the direction of eye movement.     

Eye direction aftereffect

Three experiments using computer-generated human figures showed that after a prolonged observation of eyes looking to the left (or right), eyes looking directly toward the viewer appeared directed to the right (or left). Observation of an arrow pointing left or right did not induce this aftereffect on the perceived eye direction. Happy faces produced the aftereffect more effectively than surprised faces, even though the image features of the eyes were identical for both the happy and the surprised faces. These results suggest that the eye direction aftereffect may reflect the adaptation of relatively higher-level mechanisms analyzing the others eye direction.     

Don’t look! don’t touch! inhibitory control of eye and hand movements

Inhibitory control of eye and hand movements was compared in the stop-signal task. Subjects moved their eyes to the right or left or pressed keys on the right or left in response to visual stimuli. The stimuli were either central (angle brackets pointing left or right) or peripheral (plus signs turning into Xs left or right of fixation), and the task was either pro (respond on the same side as the stimulus) or anti (respond on the opposite side). Occasionally, a stop signal was presented, which instructed subjects to inhibit their responses to the go stimulus. Stop-signal reaction times (SSRTs) were faster overall for eye movements than for hand movements, and they were affected differently by stimulus conditions (central vs. peripheral) and task (pro vs. anti), suggesting that the eyes and hands are inhibited by different processes operating under similar principles (i.e., a race between stop and go processes).     

Rivalry with continuous and flashed stimuli as a measure of ocular dominance across the visual field

Two measures of sensory ocular dominance were compared. Both involved dichoptic presentation of orthogonal gratings--a situation which results in binocular rivalry. The gratings were presently briefly in experiment 1 and continuously in experiment 2 and by the predominance of one grating over the other a quantitative estimation of ocular dominance was obtained in both cases. Comparison of the results showed that (a) binocular rivalry suppression was present for exposures of 250 ms and (b) the briefly presented gratings were a more sensitive test for ocular dominance than conventional continuously presented stimuli. The variation of dominance over the horizontal meridian of the visual field was considered. For many subjects a consistent different in the ocular dominance in the two halves of the visual field, and therefore of the cortex, was found. Some showed dominance of the ipsilateral eye in each hemisphere while others showed dominance of the contralateral eye. It was found that there is, in fact, a continuum of types of dominance pattern amongst individuals.     

Monocular motion adaptation affects the perceived trajectory of stereomotion

Perceived stereomotion trajectory was measured before and after adaptation to lateral motion in the dominant or nondominant eye to assess the relative contributions of 2 cues: changing disparity and interocular velocity difference. Perceived speed for monocular lateral motion and perceived binocular visual direction (BVD) was also assessed. Unlike stereomotion trajectory perception, the BVD of static targets showed an ocular dominance bias, even without adaptation. Adaptation caused equivalent biases in perceived trajectory and monocular motion speed, without significantly affecting perceived BVD. Predictions from monocular motion data closely match trajectory perception data, unlike those from BVD sources. The results suggest that the interocular velocity differences make a significant contribution to stereomotion trajectory perception.     

What are you looking at? Acuity for triadic eye gaze

The authors measured observers' ability to determine direction of gaze toward an object in space. In Experiment 1, they determined the difference threshold for determining whether a live "looker" was looking to the left or right of a target point. Acuity for eye direction was quite high (approximately 30 s arc). Viewing the movement of the looker's eyes did not improve acuity. When one of the looker's eyes was occluded, the observers' acuity was disrupted and their point of subjective equality was shifted away from the exposed eye. Experiment 2 was a replication of Experiment 1, but digitized gaze displays were used. The results of Experiment 3 showed that the acuity for direction of gaze depended on the position of the looker's target. Overall, the results indicated that humans are highly sensitive to gaze direction and that information from both eyes is used to determine direction of regard.     

Where a person with a squint is actually looking: Gaze-cued orienting in crossed eyes

In this study, we investigated gaze-cued attention orienting when the perceived eyes are not looking in the same direction. This condition occurs in strabismus (squint). Participants were asked to detect laterally presented reaction signals preceded by schematic faces in which the direction (left, straight, or right) of the left and right eye was independently manipulated. Consistent with earlier studies, the results showed a reliable cuing effect by two eyes with parallel gaze direction. Gaze-cued orienting was also shown in a situation when one eye was averted and the other eye was looking straight ahead. The gaze cuing was not significantly stronger in the former than in the latter situation. When both eyes were either nasally or temporally averted, no shifts of visual attention were observed. The results suggest that, if both eyes are visible, the direction of both eyes is computed and integrated for the gaze-cued orienting.