Academic journal article Psychonomic Bulletin & Review

Enhancement of Bistable Perception Associated with Visual Stimulus Rivalry

Academic journal article Psychonomic Bulletin & Review

Enhancement of Bistable Perception Associated with Visual Stimulus Rivalry

Article excerpt

Rapid, repetitive exchange of dissimilar, rival stimuli between the two eyes can produce slow alternations in perceptual dominance. This phenomenon, called stimulus rivalry, is potentially important for studying resolution of visual conflict associated with neural processing beyond the level of interocular competition. As previously implemented, however, stimulus rivalry can be difficult for some observers to experience, and it tends to occur within a relatively narrow range of contrasts and spatial frequencies. Here we show that it is possible to increase the incidence of stimulus rivalry by brief, periodic presentation of a composite configuration created by superimposition of the two rival stimuli. Possible reasons for the effectiveness of the composite in promotion of stimulus rivalry are discussed.

To identify neural correlates of conscious visual awareness (Crick & Koch, 1995), neuroscientists capitalize on bistable perception, a class of phenomena wherein visual experience fluctuates between alternative perceptual interpretations even though physical stimulation remains invariant (Andrews & Purves, 1997; Blake & Logothetis, 2002; Kim & Blake, 2005; Leopold & Logothetis, 1999). During bistable perception, there evidently exist modulations in neural activity coincident with the fluctuations in awareness, and identifying the sites of those neural modulations has become a challenge in singlecell recordings from awake, behaving animals (Leopold & Logothetis, 1996; Logothetis & Schall, 1989) and in human brain imaging (Haynes, Deichmann, & Rees, 2005; Lumer, Friston, & Rees, 1998; Polonsky, Blake, Braun, & Heeger, 2000; Tong & Engel, 2001; Wunderlich, Schneider, & Kastner, 2005).

One very intriguing form of bistability can be provoked by presentation of two dissimilar visual figures to the two eyes, with the rival figures rapidly turned "on" and "off" and repeatedly swapped between the two eyes several times a second (Figure 1, left). This stimulus presentation regime can produce alternations in perception in which the dominance of a given figure lasts several seconds and, therefore, transcends multiple eye swaps (Logothetis, Leopold, & Sheinberg, 1996). This form of bistability, unlike conventional binocular rivalry (Blake, 1989), cannot be explained by low-level interocular competition and, instead, implicates higher level neural competition between alternative stimulus representations (Wilson, 2003). However, this unique form of perceptual bistability, called stimulus rivalry, occurs within a rather narrow range of stimulus conditions (Bonneh, Sagi, & Kami, 2001 ; Lee & Blake, 1999), and, moreover, some observers experience difficulty in perceiving any bistability when viewing this eye-swapping display. Development of strategies for increasing the incidence of stimulus rivalry would promote its more widespread use as a tool for studying perceptual bistability, and that was the motive for the experiments described in this article.

The point of departure for our work was a concern about the possibly disruptive effect of the abrupt transients that occur as the gratings viewed by the two eyes change orientation every 333 msec. In their original article, Logothetis et al. (1996) were aware of this possibility, and they attempted "to minimize or eliminate the awareness of instances of stimulus reversals" (p. 623) by rapidly flickering (18 Hz) both gratings on and off throughout the period of stimulus presentation. Rapid flicker does indeed help perception of stimulus rivalry (Lee & Blake, 1999), but it does not eliminate onset transients created when the gratings are swapped between the eyes. To minimize those transients, we modified the eye-swapping procedure so that the off periods of rival stimulation associated with the flicker/swap procedure (Figure 1, left) were replaced by repetitive presentation of the composite of the two rival grating patterns (Figure 1, right). …

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