Chronometric analysis supports fusion rather than suppression theory of binocular vision

Suppression theory attributes the singleness of vision during binocular viewing to inconspicuous binocular rivalry. In two experiments, reaction times (RTs) for detection of a small monocular probe were measured while the eyes viewed identical (“fusion”) or different (rivalry) stimuli. As expected, distributions of RTs obtained during binocular rivalry showed large positive skew, from trials in which detection was delayed because the probe was superimposed on the suppressed field. Opposite to the prediction of suppression theory, however, the RT distribution during fusion showed far less positive skew, implying that information is available from both eyes during binocular viewing, as held by fusion theory. These findings were confirmed and extended over a large range of probe luminances. During fusion, log mean RTs fell steeply as log probe luminance was increased up to a critical value, then less steeply for further increases. During rivalry, the same steep branch of the RT-luminance function appeared, but shifted as though the probe was about 0.25 log units dimmer. The second branch was also present, but steeper than for fusion, so that RTs for fusion and rivalry were the same at the highest values tested.