The effect of orientation in binocular contour rivalry of real images and afterimages

Binocular rivalry was investigated using gratings of different orientations in three experiments. No consistent effects of orientation were found for predominance measures of rivalry between real images. Rivalrous afterimages, on the other hand, did exhibit orientation selectivity: vertical gratings were visible for longer than were 45-deg gratings. This effect was compared to the similar orientation selectivity found for monocular observation of grating afterimages. Comparisons of binocular rivalry between real images and afterimages were made in terms of the frequency distributions of the dominance periods.     

Orientation effects on line afterimages

Lines of various orientations were viewed as prolonged afterimages in six experiments. The duration of unitary appearance was not influenced by line orientation for monocular afterimages, but there was a marginal effect for afterimages generated binocularly; vertical and horizontal lines tended to be visible for longer than did 45-deg lines. Measures of fragmentation frequency and the latency to the first disappearance did not vary reliably with orientation under any conditions. Binocular afterimages lasted longer than did monocular ones, but generally showed the same pattern of fragmentations. These results are compared with those from experiments using optical stabilization and steady fixation, in which orientation differences have been reported.     

The effect of fear in the periphery in binocular rivalry

The perceived dominance of percepts within a rival pair of images can be influenced by emotional content, with emotional images dominating over neutral images. We investigated this effect in the periphery. Rival gratings and (fearful or neutral) face/house pairs were viewed centrally and with the near edge positioned 1 degrees and 4 degrees from the fixation. Both fearful and neutral faces were perceived as dominant for significantly longer than houses, with fearful faces dominating for significantly longer than neutral faces at all three eccentricities. There was no difference between dominances at 1 degrees and 4 degrees eccentricity, and there was no difference in the dominance of the gratings at any eccentricity. Our findings show that face stimuli, and in particular fearful faces, continue to dominate perception in binocular rivalry even when viewed in the periphery.     

The role of voluntary and involuntary attention in selecting perceptual dominance during binocular rivalry

When incompatible images are presented to corresponding regions of each eye, perception alternates between the two monocular views (binocular rivalry). In this study, we have investigated how involuntary (exogenous) and voluntary (endogenous) attention can influence the perceptual dominance of one rival image or the other during contour rivalry. Subjects viewed two orthogonal grating stimuli that were presented to both eyes. Involuntary attention was directed to one of the grating stimuli with a brief change in orientation. After a short period, the cued grating was removed from the image in one eye and the uncued grating was removed from the image in the other eye, generating binocular rivalry. Subjects usually reported dominance of the cued grating during the rivalry period. We found that the influence of the cue declined with the interval between its onset and the onset of binocular rivalry in a manner consistent with the effect of involuntary attention. Finally, we demonstrated that voluntary attention to a grating stimulus could also influence the ongoing changes in perceptual dominance that accompany longer periods of binocular rivalry Voluntary attention did not increase the mean dominance period of the attended grating, but rather decreased the mean dominance period of the non-attended grating. This pattern is analogous to increasing the perceived contrast of the attended grating. These results suggest that the competition during binocular rivalry might be an example of a more general attentional mechanism within the visual system.     

Orientation and spatial frequency effects on linear afterimages

Single lines and gratings oriented vertically or at 45 deg were observed as prolonged afterimages in two experiments. The duration of complete visibility exhibited consistent orientation selectivity for the gratings but not for single lines: vertical gratings were visible for longer than were 45-deg gratings. The results are discussed in relation to the hypothesized spatial frequency channels in the human visual system.     

Afterimages, binocular rivalry, and the temporal properties of dominance and suppression

When different contours are presented to the two eyes, an unstable percept, binocular rivalry, is the result. Parts of each set of contours may be seen but the two sets are not seen in the same place at the same time. The contours need not be physically present. Afterimages will produce binocular rivalry. Normal rivalry can be prevented if intermittent stimulation is used. Previous work has shown that orthogonal gratings, flashed for less than 150 ms and separated by more than 150 ms, will appear to fuse into a plaid or checkerboard pattern. In the present experiment this phenomenon is examined with afterimages used to produce rivalry. This abnormal 'fusion' is seen when negative afterimages are stroboscopically illuminated at less than 5 Hz. The results obtained with afterimages are predictable from the previous results obtained with stimuli external to the eye.     

Binocular rivalry with moving patterns

Binocular rivalry between a horizontal and a vertical grating was examined in six experiments. The gratings could be presented in a static form or dynamically so that either one or both gratings moved. The motion consisted of a symmetrical transformation of the gratings about their centers, so that the lines moved outwards or inwards. During rivalry, a moving pattern was visible for about 50% longer than an equivalently oriented static pattern (Experiments 1, 2, and 4). When both gratings were in motion (Experiments 3 and 5), the course of rivalry was similar to that found for two static gratings. The duration of dominance of the moving grating was influenced by its velocity (Experiment 6). The results are interpreted in terms of the stimulus strengths of the static and dynamic patterns.     

Temporal perturbations of binocular rivalry

Successive durations of binocular rivalry are sequentially independent, random variables. To explore the underlying control process, we perturbed the cycle during a 30-sec viewing period by immediately forcing an eye to return to dominance whenever it became suppressed. During this period of forced dominance, that eye's individual dominance durations were unusually brief, but immediately following the period of forced dominance that eye's suppression durations were unusually long. However, no long-term change in the sequential pattern of rivalry occurred, and the stochastic independence of successive durations was maintained during and following the period of forced dominance. The same pattern of results was obtained with even longer periods of forced dominance. These results are consistent with the existence of a short-term adaptation, or fatigue, process responsible for transitions from dominance to suppression.     

Illusory colors promote interocular grouping during binocular rivalry

When dissimilar monocular images are presented separately to each of a person’s eyes, these images compete for visual dominance, with dominance of one image or the other alternating over time. While this phenomenon, called binocular rivalry, transpires, local image features distributed over space and between the eyes can become visually dominant at the same time; the resulting global figure implicates interocular grouping. Previous studies have suggested that color tends to influence the incidence of global dominance; in this study, we assess whether illusory color can also influence interocular grouping. To test this, we exploited the McCollough effect, an orientation-contingent color aftereffect induced by prolonged adaptation to different colors paired with different orientations. Results show that during binocular rivalry, illusory colors induced by the McCollough adaptation enhance strong interocular grouping relative to preadaptation testing, to an extent comparable in strength with the enhancement induced by real colors. Thus, illusory colors that are present only in an observer’s mind are sufficiently potent to influence low-level visual processes such as binocular rivalry.     

Pattern and orientation effects on afterimage duration

Patterns composed of a pair of lines: vertical + horizontal, vertical + oblique, horizontal + oblique, and oblique + oblique, either centrally-aligned or edge-aligned, were shown at two positions of rotation. After a 1-s exposure to the pattern observers timed the duration of afterimages consisting of individual lines (fragmentary state) and the pattern as a whole (unitary state) for the ensuring 60 s. Summing unitary and fragmentary afterimage durations yielded the total afterimage duration for each pattern. Three hypotheses were confirmed by the results: (i) total afterimage duration is constant for all patterns when integrated spatiotemporal luminances are equal; (ii) unitary afterimage duration is also constant; (iii) fragmentary afterimages of vertically oriented lines have longer durations than either their horizontal or oblique pair members, regardless of alignment and, with one exception, rotation. For all patterns, the durations of the unitary and the fragmentary state represent a fixed portion of the total afterimage duration. The difference between afterimage duration for differently oriented lines in patterns which include a vertical is discussed in relation to the vertical-horizontal illusion, the function and structure of cortical orientation processes, and perceptual development.     

Monocular and binocular rivalry between contours.

The temporal characteristics of binocular and monocular rivalry between orthogonal gratings of the same or complementary colours were investigated. Rivalry was measured in terms of the dominance of either grating or the visibility of composites comprised of parts of both gratings. The total duration for which either grating was dominant was significantly longer in binocular rivalry between gratings of complementary colours. A comparison of binocular and monocular rivalry indicated considerable phenomenal differences between them. Dominance in binocular rivalry corresponds to the visibility of one grating alone; this occurs rarely in monocular rivalry, which is characterized by fluctuations in the distinctiveness of the gratings. The changes in distinctiveness are influenced by colour in a similar manner to that in binocular rivalry, and the frequencies of fluctuations are higher for gratings of complementary colours.     

Fragmentation of monocular afterimages in individuals with and without normal binocular vision

Monocular afterimages of a vertical line were generated in each eye of binocularly normal individuals and of those lacking binocular single vision or stereopsis as a consequence of childhood strabismus. Fragmentations and disappearances of the afterimages occurred less frequently in the strabismic than in the normal subjects; they were also less frequent in the dominant (nonamblyopic) eyes of the strabismics than in their amblyopic eyes. The afterimages remained visible for longer when presented to the dominant eyes of the strabismic subjects, and a similar effect was found between the sighting dominant and nondominant eyes of normal subjects.     

Perceptual filling-in of negative coloured afterimages

Filling-in of brightness, colour, and texture refers to the perceptual spreading of surround features onto the target area. This phenomenon has been extensively investigated in real images. Here, we study colour spreading in afterimages of disk-ring patterns, using a variety of colour combinations. We show that colour filling-in and filling-out in afterimages follow rules different from those found for real images, suggesting that the brain treats afterimages as genuine 'stimuli'.     

Orientation and spatial frequency effects on linear afterimages: The retinal reference for selectivity — A supplementary report

Afterimages of gratings oriented vertically or at 45 deg relative to gravity were generated with the head upright or tilted so that the retinal meridian was inclined 45 deg. The duration of visibility of the whole afterimage was longer for the vertical grating when the head was upright and for the 45-deg grating when the head was tilted. It was concluded that this orientation selectivity of afterimages is dependent upon the orientation of the grating with respect to the retinal meridian rather than to the direction of gravity.     

An empirical test of formal equivalence between Emmert's law and the size-distance invariance hypothesis

Emmert's law and the size-distance invariance hypothesis have been said to be formally equivalent, provided that Emmert's law means that the perceived size of an afterimage is proportional to the perceived distance of the projected surface of the afterimage. However, there have been very few studies that have attempted to verify this formal equivalence empirically. We measured both the perceived size and distance of afterimages and real objects with the same proximal size. Nineteen participants projected afterimages of 1 deg in visual angle on the wall located at distances of 1 to 23 meters from the participants. They also observed real objects, disc-shaped and made from a sheet of Styrofoam board, with the same proximal size as that of the afterimages, which were located at the same physical distances as those of the wall on which the afterimages were projected. Each participant reproduced the apparent sizes of the afterimages and real objects using the reproduction method and estimated the apparent distances using the magnitude estimation method. When the mean apparent sizes of the afterimages and real objects, represented as a function of apparent distance, were fitted to a linear function, the slopes for the afterimages and real objects did not differ significantly. These results are interpreted as evidence for the formal equivalence of Emmert's law and the size-distance invariance hypothesis.     

Figural effects in afterimage fragmentation

Afterimages consisting of between two and five lines were observed in five experiments. Measures were taken of both the duration and frequency of visibility of the lines comprising the configurations, and also of all line combinations. In Experiments I-III, lines forming angles, triangles, and a square were varied in orientation. Neither the absolute nor the relative orientation of the lines had any systematic effects on the measures. These results are compared with those that have been derived from steady fixation and optical stabilization, in which vertical-horizontal line pairs yield greater stability than other orientations. In Experiment IV, afterimages of the letters N and Z were found to be more stable than those of their mirror-image reversals, both in terms of the duration of visibility of the entire configurations and the obliques. This pattern was not produced using a complex configuration that could fragment into either a letter or a reversed letter (Experiment V). Similar effects have been found during steady fixation.     

Pattern of cortical activity, degree of synchronization, binocular fusion, and binocular rivalry

The synchronization hypothesis is the likely idea for the binding problem in the brain. Here we tested whether the theory is applicable for the occurrence of either binocular fusion or binocular rivalry. We first showed patterns of activated patches in V1 with proceeding from fusion to rivalry on the basis of Hubel and Wiesel's 1979 illustration. We then assumed that the strength of synchrony between the patches in the left-eye and right-eye ocular dominance columns is a crucial determinant for the divergence between fusion and rivalry. By using the strength of fusion between the paired images as a measure of degree of synchrony, we confirm the assumption about interocular vision and the synchronization hypothesis as well.     

The temporal course of suppression during binocular rivalry

Orthogonally oriented sinusoidal luminance gratings were dichoptically presented to the observers' left and right eyes. During the subsequent binocular rivalry, a small target was briefly presented (4AFC) to probe the strength of interocular suppression at various temporal latencies. Both stationary and moving rivalrous patterns were investigated. The purpose of experiment 1 was to compare the temporal characteristics of stationary and motion rivalry (0 and 1.2 deg s-1), while that of experiment 2 was to examine rivalry suppression for higher speeds (2 and 4 deg s-1). In all cases, it was found that the strength of suppression remained essentially constant throughout a single phase of binocular rivalry. The results of the investigation also revealed that moving rivalrous patterns lead to greater magnitudes of interocular suppression than static patterns. Despite these differences in the strength of suppression, the results of both experiments show that the temporal characteristics of motion and static rivalry are essentially identical.     

What causes alternations in dominance during binocular rivalry?

Several mechanisms have been proposed to account for perceptual alterations during binocular rivalry, including neural adaptation and neural noise. However, the importance of neural adaptation for producing perceptual alterations has been challenged in several articles (Y.-J. Kim, Grabowecky, & Suzuki, 2006; Moreno-Bote, Rinzel, & Rubin, 2007). We devised an “online” adaptation procedure to reexamine the role of adaptation in binocular rivalry. Periods of adaptation inserted into rivalry observation periods parametrically alter the dynamics of rivalry such that increased adaptation duration decreases dominance duration, which cannot be accounted for by neural noise. Analysis of the average dominance durations and their variance (coefficient of variation) provides evidence for an increasingly important role of noise in rivalry alternations as a given dominance period continues in time, consistent with recent computational models.     

A common oscillator for perceptual rivalries?

Perceptual rivalry is an oscillation of conscious experience that takes place despite univarying. if ambiguous, sensory input. Much current interest is focused on the controversy over the neural site of binocular rivalry, a variety of perceptual rivalry for which a number of different cortical regions have been implicated. Debate continues over the relative role of higher levels of processing compared with primary visual cortex and the suggestion that different forms of rivalry involve different cortical areas. Here we show that the temporal pattern of disappearance and reappearance in motion-induced blindness (MIB) (Bonneh et al, 2001 Nature 411 798-801) is highly correlated with the pattern of oscillation reported during binocular rivalry in the same individual. This correlation holds over a wide range of inter-individual variation. Temporal similarity in the two phenomena was strikingly confirmed by the effects of the hallucinogen LSD, which produced the same, extraordinary, pattern of increased rhythmicity in both kinds of perceptual oscillation. Furthermore. MIB demonstrates the two properties previously considered characteristic of binocular rivalry. Namely the distribution of dominance periods can be approximated by a gamma distribution and, in line with Levelt's second proposition of binocular rivalry, predominance of one perceptual phase can be increased through a reduction in the predominance time of the opposing phase. We conclude that (i) MIB is a form of perceptual rivalry, and (ii) there may be a common oscillator responsible for timing aspects of all forms of perceptual rivalry.