Binocular rivalry's characteristics

Experiencing Rivalry

You can experience rivalry for yourself using each of the three pairs of pictures in Figure 88.1. You will need to follow the instructions in the caption to achieve the conditions for rivalry, and when you do, you will notice several of rivalry's hallmark characteristics. First, note that your visual experience is highly dynamic, with one figure dominating perception for several seconds, only to become phenomenally suppressed in favor of the competing figure. Sometimes you may see bits and pieces of both figures, creating an impression of a dynamic patchwork. But often you'll see only one figure or the other for several seconds at a time, and neither figure will stay exclusively visible indefinitely (unless you have a strongly dominant eye). Note, too, that you cannot hold one eye's view dominant at will, even when one of the competing figures is a potentially interesting picture such as a female face; the alternations in dominance and suppression evidently are obligatory and may stem from neural adaptation of the currently dominant image (Lehky, 1988; Matsuoka, 1984; Mueller, 1990). You can, however, rescue one eye's figure from suppression prematurely by introducing an abrupt “transient” in the suppressed figure. In the laboratory, this can be accomplished by abruptly incrementing the contrast of a suppressed target (Blake et al., 1990; Wilson et al., 2001) or by introducing motion into a previously stationary, suppressed target (Walker and Powell, 1979). To demonstrate how these transient events can disrupt suppression, view rivalry between a pair of the rival figures in Figure 88.1. When one of the targets becomes suppressed, simply flick your finger in front of that figure; this maneuver will immediately restore the figure to dominance. If you were to repeatedly break suppression of that figure in this way—artificially forcing it to remain dominant—you would also find that it would stay dominant for shorter and shorter periods of time (Blake et al., 1990).

Figure 88.1..  

Provided here are three pairs of rival targets, each of which illustrates the alternations in dominance and suppression characterizing binocular rivalry. To achieve dichoptic stimulation (i.e., presentation of each figure to separate eyes), you must view these pairs either by crossing your eyes or by diverging your eyes, so that the fovea of each eye is seeing one image or the other. (Readers unable to “free-fuse” the targets may see anaglyphic examples of rivalry presented on the author's web page: http://www.psy.vanderbilt.edu/faculty/blake/rivalry/BR.html.) While viewing the upper pair of rival targets—faces of two different people—see if you can hold one face perceptually dominant indefinitely. Most people viewing rival targets find this to be impossible. The middle pair of rival targets were used by Wilson et al. (2001) to study dominance waves experienced as one portion of a previously suppressed figure emerges into dominance, quickly spreading to encompass the rest of the figure. To see these waves, fixate the central “bull's-eye” portion of the figure and see what happens as the radial grating assumes dominance. The bottom pair of rival targets are computer-generated images created by David Bloom and used here with permission of the artist. (See color plate 74.)

When considering binocular rivalry, it is important not to confuse it with Troxler's effect, the spontaneous fading of a visual figure that can occur when maintaining strict visual fixation. Troxler's effect does not require discordant stimulation to the two eyes, and it occurs readily in peripheral parts of the visual field and can be observed in central vision with targets of a few degrees in visual angle or larger. Troxler's effect is usually attributed to local retinal adaptation, whereas rivalry almost certainly arises from central neural events. It is important to distinguish Troxler's effect from rivalry suppression when studying the disappearance of rival targets viewed in the periphery (Levelt, 1965).

What Triggers Rivalry

Binocular rivalry can be instigated by differences between left- and right-eye views along almost any stimulus dimension, including contour orientation (Wade, 1974), spatial frequency (Fahle, 1982), and motion velocity (Breese, 1899; van de Grind et al., 2001). There are several conditions, however, in which dissimilar stimulation to corresponding areas of the two eyes does not seem to trigger rivalry. One of these is when the two eyes view rapidly flickering rival targets (O'Shea and Blake, 1986; O'Shea and Crassini, 1984), and another is when the two rival targets are very low in contrast, near their threshold for visibility (Liu et al., 1992). In both instances, people describe seeing the binocular superimposition of the two dissimilar targets. It has been noted that these two stimulus conditions—rapid flicker and low contrast—both favor activation of the magnocellular pathway (see Chapter 30), which could mean that rivalry is more tightly coupled to neural activity in the parvocellular pathway (Carlson and He, 2000; Ramachandran, 1991).

Another interesting situation where dissimilar monocular stimulation does not yield rivalry has to do with the geometry of 3D space (Shimojo and Nakayama, 1990). Consider the two monocular images produced when you view a 3D scene in which one object (a gray rectangle in this example) partially occludes another (a striped rectangle), as illustrated by the drawing in the top part of Figure 88.2. In this situation, your right eye would be seeing parts of the occluded object that are invisible to your left eye. This means, in other words, that a region of the right eye's image will contain a pattern of optical stimulation that is different from the optical stimulation falling on the corresponding region of the left eye's image. (By the way, this geometrical consequence of looking at the world from two different perspectives was realized and discussed by Leonardo da Vinci.) Despite this conflicting stimulation to corresponding retinal areas, observers report that vision is relatively more stable when viewing pairs of images that mimic this situation, implying that the brain “understands” that the “conflict” is attributable to the 3D layout of objects in the world (Nakayama and Shimojo, 1990). Interestingly, when the images going to the two eyes are exchanged, observers experience more instances of binocular rivalry within this region of conflict. (Readers may experience the dependence of rivalry on scene interpretation using the pair of stereo images in the bottom part of Fig. 88.2.) These observations imply that the brain knows which eye is receiving which figure, which in turn determines whether one experiences stable fusion or unstable rivalry (see also Blake and Boothroyd, 1985).

Figure 88.2..  

When one views binocularly a scene in which one object partially occludes another nearby object, one eye will see portions of the occluded object that are invisible to the other eye. This viewing situation—illustrated schematically in the upper part of this figure—creates a local region where dissimilar images stimulate corresponding retinal areas, conditions normally sufficient for binocular rivalry. When this condition is simulated in the laboratory, the incidence of rivalry is reduced (Shimojo and Nakayama, 1990). The pair of half-images in the lower part of the figure may be used to compare binocular rivalry for a “valid” binocular configuration and an “invalid” binocular configuration. If you combine the two half-images by crossing your eyes, use the middle and left-hand figures; if you combine the two half-images by uncrossing your eyes, use the middle and right-hand figures. When the two half-images are appropriately fused, the gray squares will stand out in depth from the textured background, and the partially occluded striped object will be valid in the upper part of the figure and invalid in the lower part. Compare how frequently the striped region disappears in the upper versus the lower field.

Finally, there is some evidence that different “colors” viewed by the left and right eyes do not necessarily rival each other in the same way that different forms do. This can be seen in Figure 88.3, which presents a version of one of the stimulus conditions studied by Creed (1935) in his widely cited (and sometimes misinterpreted) study of binocular fusion and binocular rivalry. Note that the two stamps are identical in form except for the numerals and words signifying the stamps' value; the colors, however, are distinctly different. When these two images are viewed in binocular combination, however, the color differences do not alternate in dominance; instead, people describe seeing a “washed-out” brown. Whatever the perceived color, it is clear—to put it in Creed's words—that “we are not dealing with the common type of binocular rivalry in which the form that prevails brings with it the colour of its own object” (p. 383). Using these stamps, you might also try to compare the color of the numeral 1, when it is dominant, with the numeral 2, when it is dominant. Here, too, there is no sense that color changes over time, even though the form does. Again, to quote Creed, “Successive rivalry of forms is therefore not necessarily accompanied by successive rivalry of the corresponding colours” (p. 384).

Figure 88.3..  

Two stamps that are nearly identifcal in form but different in color. Free-fuse these two rival stamps and note the appearance of color. (See color plate 75.)

Temporal and Spatial Dynamics

The alternations in dominance and suppression during rivalry are not periodic, like the oscillations of a metronome; instead, successive durations of visibility are independent, random events that collectively conform to a gamma distribution (Fig. 88.4). In other words, one cannot predict exactly how long a given duration of dominance will last (Fox and Herrmann, 1967; Lehky, 1995; Levelt, 1965). However, it is possible to influence the overall predominance of one figure over another, where predominance is defined as the total percentage of time that a given figure is dominant during an extended viewing period. Thus, for example, a high-contrast rival figure will tend to predominate over a low-contrast one (Mueller and Blake, 1989), and this increased predominance comes about mostly because the durations of suppression of a high-contrast figure are shorter, on average, than those of a low-contrast figure. It is as if a “stronger” stimulus manages to overcome suppression more quickly than does a “weaker” one (Blake, 1989; Fox and Rasche, 1969). When you stop and think about it, this also means that rivalry alternations should be faster, on average, between a pair of high-contrast rival targets than between a pair of low-contrast rival targets; this is, in fact, the case (Levelt, 1965). Besides contrast, other stimulus variables that “strengthen” a rival target and thereby enhance its predominance include spatial frequency (Fahle, 1982), motion velocity (Blake et al., 1998; Breese, 1899), and luminance (Levelt, 1965).

Figure 88.4..  

When an observer presses buttons to indicate successive periods of dominance of two rival targets, those successive durations are randomly distributed, independent variables (Levelt, 1965). The upper diagram shows a representative time line of successive periods of exclusive dominance of a vertical rival grating (viewed by the left eye in this case) and periods of exclusive dominance of a horizontal rival grating (viewed by the right eye). Intermixed among periods of exclusive dominance are occasional periods of “mixed” dominance during which portions of both rival gratings are visible in a patchwork mosaic pattern. The incidence of mixed dominance tends to be greater with rival targets larger in angular subtense (Meenes, 1930). When dominance durations measured during extended tracking periods are plotted as a frequency histogram (lower portion of the figure), that distribution is well fit by a gamma distribution (solid line in the histogram). In general, the gamma distribution provides a close fit to rivalry alternation data when parameters are adjusted for individual differences (Fox and Herrmann, 1967; Logothetis, 1998).

A rival figure embedded in a larger meaningful context also tends to predominate over one viewed in isolation, but in this case increased predominance arises from a lengthening of the durations of dominance, not an abbreviation of suppression durations (Sobel and Blake, 2001). This dissociation between the effect of a rival figure's context and the effect of the strength of the figure itself suggests that rivalry dynamics have multiple determinants (Blake, 2001). There are also reports in the literature that meaningful or familiar rival targets predominate in rivalry over less meaningful or unfamiliar ones (for a review of this literature, see Walker, 1978; for an alternative interpretation of the role of meaning in rivalry, see Blake, 1988). It would be informative to replicate these results and, moreover, to learn whether increases in predominance with meaning, if reliably demonstrable, come about through lengthened dominance durations. To the extent that meaning influences other aspects of visual perception (Raftopoulos, 2001), it would not be surprising to find that meaning influences dominance periods of rivalry, for the dominance phases of rivalry appear to be functionally equivalent to normal monocular vision (Fox, 1991).

The potency of global context to influence the temporal dynamics of rivalry could reflect the involvement of higher-level visual processes in rivalry. Is there other evidence for the involvement of such processes? There are several published studies pointing to a role for attention in the control of rivalry dynamics. Several decades ago, Lack (1978) showed that observers could be trained to prolong one eye's view during rivalry without resorting to tricks such as moving the eyes in a manner that would favor that view. Instead, Lack's observers purportedly directed attention to the favored target and thereby prolonged its visibility. It should be noted that rivalry alternations were not abolished under these conditions; despite focused attention, spontaneous reversals in dominance still occurred, albeit less frequently. More recently, Ooi and He (1999) showed that directing attention to a region of the visual field where a rivalry target is currently suppressed boosts the potency of visual motion at that region of the field to break suppression of that target.

What happens when one views multiple rival patterns distributed throughout the visual field? Take a look at the rival display presented in Figure 88.5, a pair of arrays of black and white Gaussian (i.e., blurred) blobs, with each white blob in one array pitted against a black blob in the other array. View the two arrays dichoptically using the free-fusion technique, hold your gaze steadily on the small fixation cross, and concentrate on the pattern of dominance throughout the visual field. Note how often all the black blobs or all the white blobs are simultaneously dominant. When observers actually track these periods of simultaneous dominance, the total incidence turns out to be greater than what would be predicted if the individual blobs were rivaling independently on their own (Logothetis, 1998). There is a tendency, in other words, for dominance periods of coherent patterns to become perceptually entrained.

Figure 88.5..  

Rival display consisting of spatially distributed rival targets. Each eye's view consists of an array of blurred circles (Gaussian blobs) differing in contrast polarity between left- and right-eye views. Free-fuse these two targets and note how often all blobs of a given contrast polarity are simultaneously dominant. (This display is a version of one described by Logothetis, 1998.)

This tendency is even more pronounced in the color rival patterns shown in Figure 88.6, a variation of the display created by Diaz-Caneja (1928; see Alais et al., 2000). Here the addition of color further encourages grouping according to coherence, as you can confirm by comparing the rivalry associated with the gray-scale version and the color version.

Figure 88.6..  

Two versions of the well-known display of Diaz-Caneja (1928). Upon viewing the gray-scale version (upper pair of rival targets), observers often see a complete bulls-eye pattern or a complete pattern of vertical lines. This implies that portions of each eye's target are simultaneously dominant, presumably promoted by the spatial grouping that produces a coherent figure. This tendency toward grouping is even stronger in the color version of this figure shown at the bottom. (See color plate 76.)

In both Figures 88.5 and 88.6, periods of coherent dominance can be achieved only by very specific combinations of left-eye and right-eye components that are dominant simultaneously. This means, then, that one eye alone cannot be responsible entirely for dominance at any given moment. Instead, dominance may consist of a patchwork of visible features (Blake et al., 1992; Meenes, 1930), in this case collated from left- and right-eye views. Interocular grouping during rivalry has been nicely documented by Kovács et al. (1997), and it has been studied by others as well (Alais and Blake, 1999; Dörrenhaus, 1975).

One other intriguing characteristic of rivalry deserves mention, one having to do with the appearance of rival figures during transitions from suppression to dominance. Looking again at rivalry produced by the rival pair in the second row of Figure 88.1, pay particular attention to the emerging appearance of the rings as they assume dominance. You will probably notice that dominance originates within a restricted region of a ring and spreads from there to encompass the entire figure. These “waves” of dominance are typical of rivalry produced by all sorts of rival figures; they imply the existence of neural “cooperativity” wherein interconnections among neighboring ensembles of neurons promote the spread of activation over spatially extended regions of cortical tissue. Wilson et al. (2001) estimated the speed with which dominance waves travel around the circumference of circular rival targets like those pictured in the second row of Figure 88.1, coming up with an average wave speed of about 4 degrees of visual angle per second. This estimated wave speed was even faster when the contours forming the rival target were collinear (i.e., the target was a concentric grating), presumably because waves of dominance travel more effectively along collinear contours. Wave speed (expressed in units of visual degrees per second) was also faster for larger rival targets whose contours were imaged in more peripheral regions of the retina. Expressed in units of cortical tissue (not degrees of visual angle), however, wave speed remained constant with retinal eccentricity, implying that these waves are being generated in a visual area whose magnification factor matches that for visual area V1 (i.e., the exaggerated neural representation of the fovea in the visual cortex).

With these spatial and temporal characteristics of binocular rivalry in mind, we turn next to a consideration of the source of interest in rivalry. Why have vision scientists been fascinated with this phenomenon for almost two centuries, and why in recent years have neuroscientists become intrigued with rivalry?