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To answer the question 'what is suppressed during binocular rivalry?" a series of three experiments was performed. In the first experiment observers viewed binocular rivalry between orthogonally oriented patterns. When the dominant and suppressed patterns were interchanged between the eyes observers continued seeing with the dominant eye, indicating that an eye, not a pattern, is suppressed during rivalry. In a second experiment it was found that a suppressed eye was able to contribute to stereopsis. A third experiment demonstrated that the predominance of an eye could be influenced by prior adaptation of the other eye, indicating that binocular mechanisms participate in the rivalry process.

and ROBERTFOX Vanderbilt Uniuersity; Nashville, TennesseeSuccessive durations ofbinocular rivalry are sequentially independent, random variables. To explore the underlying control process, we perturbed the cycle during a 3D-sec viewing period by immediately forcing an eye to return to dominance whenever it became suppressed. During this period offorced dominance, that eye's individual dominance durations were unusually brief, but immediately following the period offorced dominance that eye's suppression durations were unusually long. However, no long-term change in the sequential pattern ofrivalry 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.Among the fascinating features of binocular rivalry are the abrupt, seemingly unpredictable shifts in dominance and suppression that occur over time, Casual observation suggests that these shifts are unrelated in any compelling way to volitional attempts to alter the temporal course of rivalry alternations, and this suggestion is borne out by results from experiments in which observers tried but largely failed to maintain dominance of one eye's view indefinitely (Lack, 1978). Moreover, this phenomenal impression of unpredictability receives quantitative support from stochastic analyses, which reveal that successive durations of phases of dominance and suppression are sequentially independent random variables (Fox & Herrmann, 1967;Wade, 1975;Walker, 1975). The duration of any given phase, in other words, is unrelated to the duration of prior phases.These conclusions are based on data gathered by voluntary reports of practiced observers who have viewed rivalry for intervals on the order of 1 min, with interspersed periods of rest. The stimulus conditions were designed to promote abrupt, clear-cut changes in phenomenal state, with minimum incidence of intermediate, or mixed, dominance. It is unlikely, however, that these conclusions are peculiar either to these stimulus conditions or to the response tendencies of observers, for the same pattern of sequential independence is found when

Grating acuity, the ability to resolve high-contrast square-wave gratings, was measured in a falcon and in humans under comparable conditions. This behavioral test of falcon acuity supports the common belief that Falconiformès have superb vision-the faclon's threshold was 160 cycles per degree, while the human thresholds were 60 cycles per degree. Falcon acuity, however, was much more dependent on lumanance, declining sharply with decreases in luminance.

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