Starting with a balanced stimulus of low, intermediate or high luminance (indicated with a grey dot and an arrow) the dot luminance of one of the two layers is manipulated while that of the other remains fixed.

<p>Mean percept durations are plotted for episodes when the sphere is perceived with the fixed dot luminance surface in the foreground (dotted lines) or with the variable dot luminance in the foreground (solid lines). For both the average group data (A) and the individual observers (B) manipulations of dot luminance mainly affect the mean dominance durations of the percept with the brightest dots in the foreground. Error bars represent the standard error of the mean.</p

Reversal rates as a function of dot luminance of the two motion-defined surfaces.

<p>For the average group data (n = 5) the reversal rates of individual observers were normalized to the mean reversal rate over all conditions. Logically following from the demonstrated effect of dot-luminance induced percept probability on the mean dominance durations (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003473#pone-0003473-g003" target="_blank">fig. 3</a> & <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003473#pone-0003473-g004" target="_blank">fig. 4</a>) these plots demonstrate that for both the group data and the individual observers' reversal rates decrease when the luminance difference between the two surfaces increases. Furthermore, reversal rates for balanced stimuli increase when the dot luminance, and thus stimulus strength, increases.</p

Percept dominance durations as a function of differential dot luminance.

<p>Dominance durations are plotted for percepts with defined luminance for the perceived ‘front’ and ‘back’ surface of the sphere. For both the group data (n = 5, data for each observer is normalized to the mean percept duration at intermediate contrast) and the individual observers, dominance durations are longest when observers perceive the brightest possible dots in the foreground and de dimmest possible dots in the background. A decrease of the dot luminance of one of the layers of a balanced high luminance stimulus does not decrease the durations of the episodes when this layer is perceived in the foreground. Instead, it increases the durations of the opposite perceptual interpretation. If however the dot luminance of a balanced low luminance stimulus is increased, this only influences the duration of the episodes when the varied dot luminance is perceived as the foreground (see also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003473#pone-0003473-g004" target="_blank">figure 4</a>).</p

Predominance as a function of individual surface dot luminance.

<p>For both the group data (n = 5) and individual observers a balanced increase of stimulus luminance (the diagonal) does not affect the predominance. If the luminance of only one layer of dots is manipulated the predominance shifts towards the perceptual interpretation with the brightest dots in the foreground.</p

Schematic representation of the stimulus.

<p>White dots on a black background moving with a sinusoidal speed profile create the impression of a three-dimensional sphere rotating in depth around the vertical axis. If all dots have the same luminance (A, Balanced) both rotation directions are equally likely and the sphere is perceived to switch rotation direction every few seconds. If the dots moving in opposite directions have a different luminance (B) the sphere is biased towards the perceptual interpretation with the brightest dots in the foreground. (C) In our experiment we used three different luminance levels (Low = L, Intermediate = I, High = H) for the two surfaces resulting in nine different sphere stimuli.</p

Mean accuracy (Acc) and response times (RT) for both the categorical and coordinate condition.

<p>Standard deviation in parentheses.</p

Mean regression coefficients related to all three stimulus types for A) V1, B) V2, and C) V3.

<p>Means are presented for both the categorical (cat) and coordinate instruction (coo) as well as the first stimulus (S1), the matching region of the second stimulus (S2 match), and the mismatching region of the second stimulus (S2 mismatch). Error bars represent the standard error of the mean (SEM).</p

The cross dot stimulus with all possible dot positions.

<p>The central cross was red in all first stimuli (S1) and green in all second stimuli (S2). Note that only one of these dots was present in a single stimulus.</p

Mean regression coefficients of the retention interval between the first and second stimulus for A) V1, B) V2, and C) V3.

<p>Means are presented for both the categorical (cat) and coordinate instruction (coo) as well as the regions that would match and mismatch in comparison to the first stimulus. Error bars represent the standard error of the mean (SEM).</p

All elements of the trialsequence: an interval (10000 ms), first stimulus presentation (300 ms), jittered interval (3000–8000 ms), second stimulus presentation (300 ms), and a fixation cross during which a response could be given (2000 ms).

<p>All elements of the trialsequence: an interval (10000 ms), first stimulus presentation (300 ms), jittered interval (3000–8000 ms), second stimulus presentation (300 ms), and a fixation cross during which a response could be given (2000 ms).</p