Abstract
Levelt’s four propositions (L1–L4), which characterize the relation between changes in “stimulus strength” in the two eyes and percept alternations, are considered benchmark for binocular rivalry models. It was recently demonstrated that adaptation mutual-inhibition models of binocular rivalry capture L4 only in a limited range of input strengths, predicting an increase rather than a decrease in dominance durations with increasing stimulus strength for weak stimuli. This observation challenges the validity of those models, but possibly L4 itself is invalid. So far, L1–L4 have been tested mainly by varying the contrast of static stimuli, but since binocular rivalry breaks down at low contrasts, it has been difficult to study L4. To circumvent this problem, and to test if the recent revision of L2 has more general validity, we studied changes in binocular rivalry evoked by manipulating coherence of oppositely-moving random-dot stimuli in the two eyes, and compared them against the effects of stimulus contrast. Thirteen human observers participated. Both contrast and coherence manipulations in one eye produced robust changes in both eyes; dominance durations of the eye receiving the stronger stimulus increased while those of the other eye decreased, albeit less steeply. This is inconsistent with L2 but supports its revision. When coherence was augmented in both eyes simultaneously, dominance durations first increased at low coherence, and then decreased for further increases in coherence. The same held true for the alternation periods. The initial increase in dominance durations was absent in the contrast experiments, but with coherence manipulations, rivalry could be tested at much lower stimulus strengths. Thus, we found that L4, like L2, is only valid in a limited range of stimulus strengths. Outside that range, the opposite is true. Apparent discrepancies between contrast and coherence experiments could be fully reconciled with adaptation mutual-inhibition models using a simple input transfer-function.
Highlights
Binocular rivalry is a phenomenon which occurs when our eyes receive stereo-incompatible inputs at the same retinal location
Several studies have challenged the role of adaptation [but see e.g. 6,7,8], there is substantial evidence that adaptation plays a significant role in binocular rivalry alternations [9,10,11,12,13,14]
Where changes in dominance duration of the weaker stimulus were quite small in the contrast experiments, those changes were much larger in the coherence experiment, at least in some of our subjects
Summary
Binocular rivalry is a phenomenon which occurs when our eyes receive stereo-incompatible inputs at the same retinal location. Levelt’s second proposition (L2) entails (together with L1 and L3) that if the stimulus strength is changed in one eye, it affects exclusively the dominance durations of the contralateral eye, while having no effect at all on the dominance durations of the eye in which the stimulus strength is manipulated [16] This result may seem counterintuitive at first glance, but is explained within the framework of reciprocal inhibition where a given stimulus generates not an isolated response but one linked to the response generated by another, competing stimulus. L2 was reconfirmed in the way it was stated by Levelt [17,18], and later in a more attenuated form, which states that there are changes (albeit much smaller) in dominance durations of the ipsilateral eye [19,20] This view on L2 was challenged by Brascamp et al [21] who, by testing a wider range of stimulus contrasts, found that dominance durations mainly changed for stimuli in the eye which received the higher-contrast image [see 22,23]. The fourth proposition (L4) posits that increasing the stimulus strength in both eyes shortens the suppression phases of stimuli in both eyes, leading to increases in the rivalry alternation rate, i.e., decreases in dominance duration of the two competing percepts
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