Ординатура / Офтальмология / Английские материалы / Binocular Rivalry_Alais, Blake_2005

perception—was beset by methodological problems based, in part, on the definition of threshold. Moreover, it was difficult to envision a mechanism whereby a weak stimulus could, in effect, be smuggled into perceptual processing outside of awareness, yet then rise up and corrupt that processing.

Binocular rivalry, it seemed to me, could be an ideal phenomenon for evaluating the potency of subliminal perception. After all, rivalry effectively erases an otherwise easily perceived stimulus from awareness for durations on the order of seconds, thereby eliminating questions about the strength of the stimulus or its absence from awareness. Would it be possible to introduce during suppression a stimulus that induced an affective reaction—perhaps by making that stimulus the signal for a forthcoming electric shock? Even though the observer would report not seeing the stimulus, perhaps its unconscious registration would be revealed by autonomic responses. This kind of thinking led me to a survey of the literature on rivalry and to informal experiments that allowed me to observe rivalry’s characteristics for myself. First, it was obvious that all kinds of stimuli engaged in rivalry, the key feature being stimulation of corresponding retinal areas by patterns incapable of being fused. Moreover, larger stimuli seemed to rival in a piecemeal random fashion, while smaller stimuli tended to be suppressed as complete units. Contrary to expectations based on some published accounts, I observed that rivalry was not invariably a clear-cut switch between dominance and suppression states analogous to the changes in reversible figures. Rather, there were mixed or transitory phases with both stimuli being visible. Like others before me, I found that increasing the intensity of the stimulus seen by one eye made that stimulus more visible over time. Similarly, motion of one eye’s stimulus served to increase its total duration of visibility. Yet it was never possible for me to achieve complete suppression of a weaker stimulus, nor was I able to influence significantly the temporal course of rivalry by voluntary attention. This inability to suppress a given stimulus from awareness for an extended period of time thus put a crimp in my idea of exploiting suppression to completely erase an affectively charged stimulus from awareness.

This difficulty not withstanding, I was now convinced that rivalry represented a potentially useful tool for examining other questions in perception. For example, rivalry creates a significant dissociation between the total duration of physical stimulation and the total duration of phenomenal awareness, the two differing by 50% or even more under optimal conditions. It should be feasible, then, to identify visual phenomena

whose magnitude or persistence vary with the duration of stimulation and learn whether those phenomena are comparable in strength when the duration of stimulation includes significant periods of time during which the stimulus is invisible owing to rivalry. My original formulation of this idea centered around the figural aftereffect, a key phenomenon in Kohler and Wallach’s theory of visual cortical isomorphism, but it was years later before I got around to actually measuring aftereffects to suppressed inducing figures.

Another potentially useful application of rivalry was to index differences among individuals in terms of general organismic conditions. Already it was known that the rate of rivalry alternations could be modulated by drugs, with excitatory substances (e.g., amphetamines) increasing alternation rate and depressive substances (e.g., barbiturates) decreasing rate. Moreover, several published studies purported to show unique patterns of stimulus dominance dependent on specific personality types (e.g., scenes depicting violence being relatively more dominant in people psychometrically classified as more aggressive). Although I published studies on rivalry and individual differences myself, a nagging concern was the possible role of response bias especially when using large, complex rival figures prone to piecemeal dominance—a concern rendered more acute by the development of signal detection theory, which made the distinction between sensitivity and response bias explicit.

While fascinated with rivalry’s potential applications, I became convinced that its utility would remain unrealized until we better understood the underlying mechanisms. Toward that end, I decided to employ a psychophysical “probe” technique to assess sensitivity during suppression phases, similar to work being pursued to measure threshold changes associated with Mach-band edges and with metacontrast masking. An attractive feature of using probe stimuli to assess sensitivity during suppression is the considerable control that could be maintained over the stimulus conditions and over the responses of the observer. Using the dominant phase as the control condition and the suppression phase as the independent variable or experimental condition meant that the peripheral conditions of stimulation remained identical. Therefore, changes in sensitivity must be attributed to a central inhibitory process linked to suppression. These kinds of considerations contributed to my decision to pursue an analysis of suppression for my dissertation and in later work.

My initial explorations of rivalry were clearly incomplete and rather naïve, being pursued in large part without the guidance and stimulation that the results of other investigators invariably provide. It is gratifying to

see how our knowledge of rivalry has advanced over the years, as evidenced by the contents of the chapters in this volume. Yet at the same time, one can identify themes in this contemporary work that reflect prior conceptual developments. A significant factor in the emergence of interest in “set” was Hebb’s seminal ideas about neural organization. He emphasized the importance of understanding central control processes as exemplified by the concepts of set and attention and encouraged the quest for identifying relevant physiological mechanisms. This is a theme embedded in several of the chapters in this volume, including those by Leopold and colleagues (chapter 13) and by Pettigrew and Carter (chapter 15). A similar focus on central mechanisms is seen in the appearance of models of attention. One of the most influential of these was the filter model, couched in information processing constructs developed by Broadbent. The model renewed focus on the rejection of suprathreshold nonattended stimuli and drew a useful distinction between early and later stages of processing where rejection might occur. Once again, one finds the fruits of these ideas in current theorizing about rivalry (see, for example, chapter 7 by Ooi and He and chapter 3 by Freeman and colleagues). Sharper focus on central mechanisms also came from the discovery by Julesz of global stereopsis generated by large sets of random elements. Initially, this work revived interest in binocular vision, but more generally it focused attention on the statistical information conveyed by sets of stimuli as distinct from the physical attributes of the constituent elements.

The theoretical issues and conceptual orientations that began in the 1960s have continued to develop, and their influence is reflected in the present chapters. Two shifts in orientation are most notable. One is the interdisciplinary character of the work, as demonstrated by the varied background of the investigators and the institutions with which they are affiliated. The insularity produced by discipline boundaries is no longer present. Second, the explosive increase in information about neural processes makes it possible for investigators to pursue relationships among several levels of analysis. And as many of the chapters demonstrate, they have taken full advantage of that freedom. Upon examining individual chapters, other more specific connections with prior work can be identified in terms of issues and in approaches to rivalry.

The somewhat neglected yet enduring topic of individual differences in rivalry is addressed directly in the chapter by Pettigrew and Carter (chapter 15). In contrast to prior studies on individual differences, which were largely atheoretical, these investigators start from a combination of clinical observations and neurophysical data to outline a theory that makes

differences the critical variable. At the present state of development, it is not clear how well the theory can accommodate other characteristics of rivalry or the extent to which it is falsifiable. Nevertheless, one positive feature of any theory is to spark interest and inquiry.

Indeed, this effect is illustrated well by the chapter by O’Shea and Corballis (chapter 16). At the level of theory, they sought, perhaps in the spirit of strong inference, to provide a critical test of the idea that rivalry requires interaction between cortical hemispheres. The answer seems to be no, because they found normal rivalry in persons without interhemispheral connections. Apart from the theoretical implications of these results, the empirical contribution remains significant. It adds to the information about visual function from that small cadre of individuals with visual systems rendered unique by surgical separation of the callosal pathways.

Some attributes of the individual differences approach are reflected in chapter 6 by Kovács and Eisenberg, who obtained verbal reports of sequences of rivalry alternatives from children 4 to 7 years of age. They interpreted the results as suggesting developmental differences in maturation of the rivalry process; this may be the first study using this approach to rivalry in children with normal binocularity, as opposed to the extensive literature on orthoptics and visual training designed to promote binocular function. It is noteworthy in view of the substantial evidence indicating that other components of binocularity (e.g., stereopsis) and 3-D space perception (e.g., size constancy) become functional at much earlier stages of development.

The enduring interest in quantitative modeling implemented through simulations is represented in the contributions by Wilson (chapter 17) and by Crewther and colleagues (chapter 18). Wilson focuses on the network of inhibitory connections that would account for the temporal pattern of dominance and suppression, once rivalry begins, and in this formulation shows how the special form of “rivalry” based on rapid inter-eye stimulus exchange could be accommodated. In the former model, the process by which the conflict between stimuli induces rivalry is not addressed explicitly. That process, however, is incorporated into the network outlined by Crewther et al., which incorporates a stimulus classification scheme that compares the stimulus from each eye to determine whether the difference in stimuli warrants the initiations of suppression. The process itself is assigned to earlier stages in the system.

The psychophysical approach to rivalry, as described earlier, is exemplified in chapter 3 by Freeman, Nguyen, and Alais, who use probe stimuli to

assess differences in threshold between dominance and suppression phases. Consistent with prior research, they found an elevation of threshold during suppression that operated nonselectively on attributes of the probe stimulus. In addition, the dominance/suppression threshold difference increased with increases in the complexity of the probe stimulus, a result the authors interpret as suggesting that suppression occurs at several successive stages of processing.

The systematic manipulation of stimulus dimensions intrinsic to psychophysical analyses was applied by Rubin and Hupé (chapter 8) to determine whether the temporal parameters of rivalry alternations would also be present in the bistable percept induced by a plaid motion stimulus. This stimulus, which can be composed of overlapping arrays of contours moving orthogonally, often produces a unified single direction. But after prolonged viewing, as the authors note, motion direction shifts to a bistable mode, alternating between unified and separate motion phases (typically referred to as “coherent” and “transparent” motion). Applying analogs of the stimulus strength manipulation used in rivalry, they found parallels in the pattern of bistable percepts, leading them to suggest that the reciprocal inhibitory models developed within rivalry might be profitably extended to account for bistable phenomena.

The connection between binocular rivalry and binocular vision in general is made explicit in chapter 2 by Wade, who outlines in his historical review some of the major issues that have defined research in the area. He notes that rivalry traditionally has been regarded as the default mode that becomes manifest when fusion and stereopsis cannot be achieved.

The relationship between rivalry and stereopsis is integral to chapter 10 by Howard, who examines the function of monocular occlusion in contributing to the formation of stable stereoscopic percepts. Although such occluding stimuli should, on a local retinal point-by-point scale, induce rivalry, they do not when embedded in the context of a normal binocular view. This underscores the point that the monocular stimulus arrays must be compared at high resolution by some binocular mechanism preparatory to commencement of the rivalry mode.

An interest in the stimulus conditions involved in both rivalry and binocular vision led Ooi and He (chapter 7) to focus on occlusion and the perception of surfaces as critical variables. They suggest that rivalry may be part of an inhibitory network whose more general function is the perception of stable binocular surfaces and objects. In this, attention may play a significant role.

The role of stimulus organization in influencing the pattern of rivalry alternations is the key issue examined in chapter 9 by Papathomas, Kovács, and Conway. They presented arrays of stimuli that could be grouped either by eye of origin or by stimulus attributes and requested observers to report on changes in dominance over time. The incidence of reports indicating attribute groupings was greater than expected by chance and was interpreted as suggesting the operation of stimulus grouping processes in rivalry.

Visual awareness is the central theme of chapter 4 by Tong, who equates the dominance and suppression phases of rivalry with the presence and absence of awareness. Because he regards cortical activation as indexed by fMRI as a measure of awareness, Tong suggests that studies showing a substantial reduction of activity in primary visual cortex during suppression imply that awareness is localized at that stage.

Motion and time are the dimensions focused upon by He, Carlson, and Chen (chapter 5), who review the literature from the perspective provided by the distinction between the M and P visual pathways. They conclude that stimuli presumed to engage the P pathway are more likely to be more active in rivalry and that this issue is closely related to the question of the variables that influence the duration or “switching time” of successive rivalry events.

Psychophysical analysis, by definition, can bear only inferentially on neural mechanisms, but in the research described by Kreiman, Fried, and Koch (chapter 17), inference is replaced by direct observation of single neuron responses in human observers making psychophysical reports of dominance and suppression produced by flash suppression. Although such suppression is a transient event, it resembles rivalry both at the phenomenal level and in terms of the magnitude of the effect of suppression on probe stimuli. The differences in neuron responses observed correlated well with phenomenal reports and, in general, are consistent with assumptions about neural coding.

Evidence from psychophysical, electrophysiological and brain imaging lines of inquiry relevant to rivalry and bistable stimuli are reviewed by Andrews, Sengpiel, and Blakemore in chapter 11. They conclude that rivalry and bistable perception must involve multiple mechanisms that will be selectively engaged as a function of specific conditions of stimulation.

Rivalry and bistable perception are also examined by Fries, CasteloBranco, Engel, and Singer (chapter 14) within the context of their search for neural processes that can integrate and amplify perceptually relevant signals from distributed processing systems. Focusing on eye dominance

in strabismic cats as indexed by multiple cortical recordings and on percepts induced by bistable plaid stimuli in humans, they marshal evidence and arguments that suggest synchronized neural activity may reflect the emergence of perceptual dominance in rivalry and bistability.

CONCLUDING COMMENTS

Although each chapter makes its own unique contribution, together they demonstrate unequivocally that rivalry has moved into the mainstream of research on perception. No longer can it be dismissed as an artificial, optical curiosity limited to highly artificial conditions. The underlying reason is the relevance of rivalry to understanding two significant problems. One is the role of central cognitive processes in modifying perception. This, in a sense, is the older question about the effect of set. Now it arises with greater sophistication made possible by deeper knowledge of neural processes and is framed in terms of the concept of cognitive impenetrability. Here the issue is characterized as a conflict between early automatic processing of stimulus information and the modulating influences from higher-order processes linked to cognition.

Rivalry is the ideal phenomenon for investigating the issue because it has a foot in each camp. As a binocular process, the stimuli from both eyes reach awareness, hence potentially subject to all cognitive factors available for muster. Moreover, we know from stereopsis and fusion that a refined Vernier-like comparison process occurs in where differences measured in seconds of arc can be resolved. Finally, the conditions of rivalry would seem to provide the opportunity for a choice between competing stimulus representations. Yet, marshaled against these capacities are the attributes of suppression, which seem to operate in a much coarser automatic way. If one described spatial resolution of the binocular comparison level as akin to the precision of a scalpel, then meat axe would apply to suppression. More information about the way this paradox works would be of considerable interest. To that end, the time scale of rivalry (seconds) and its cyclic character facilitate the pursuit of relevant investigations. One other feature of the binocular comparison that should be noted, and which has not been emphasized in current research on rivalry, is that it can be regarded as a kind of stimulus categorization in terms of same versus different. The existence of fusion and rivalry implies the presence of a seemingly automatic process that measures stimuli in terms of their similarity. Rivalry occurs when stimuli are classed as dissimilar. Presumably, such a categorization is the product of well-established neural