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2 Acta Neurobiologiae Experimentalis

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Dependence of the filled‑space illusion on the size and location of contextual distractors

100%

Marma V. , Bulatov A. , Bulatova N.

tom 80

nr 2

For most observers, the part of the stimulus that is filled with some visual elements (e.g., distractors) appears larger than the unfilled part of the same size. This illusion of interrupted spatial extent is also known as the ‘filled‑space’ or ‘Oppel‑Kundt’ illusion. Although the continuously filled‑space illusion has been systematically studied for over a century, there is still no generally accepted explanation of its origin. The present study aimed to further develop our computational model of the continuously filled‑space illusion and to examine whether the model predictions successfully account for illusory effects caused by distracting line‑segments of various lengths that are attached to different endpoints (i.e., terminators) of the reference spatial interval of the three‑dot stimulus. Our experiments confirm that the illusion manifests itself along a distracting segment located both inside and outside of the reference interval. In the case of two distractors arranged symmetrically with respect to the lateral terminator, we found that the magnitude of the illusion is approximately equal to the sum of the relevant values obtained with separate distractors. The results of experiments using vertical shifts of distractors supported the model’s assumption regarding the two‑dimensional Gaussian profile of hypothetical areas of weighted spatial summation of neural activity. A good correspondence between the experimental and theoretical results supports the suggestion that perceptual positional biases associated with the context‑evoked increase in neural excitation may be one of the main causes of the continuously filled‑space illusion.

The filled‑space illusion induced by a single‑dot distractor

100%

Bulatov A. , Marma V. , Bulatova N. , Mickiene L.

nr 1

In the present study, we tested the ability of our computational model of the filled-space illusion to account for data collected in experiments with stimuli comprising single-dot distractors. In three sets of experiments, we investigated this illusory effect as a function of distance between the distractor and lateral terminator of the reference spatial interval of the three-dot stimulus. We found that the model calculations properly predicted all of the observed changes in magnitude of the illusion for stimuli with a single distracting dot placed both within and outside the interval, as well as, for stimuli with two distractors arranged symmetrically relative to the lateral terminator. To additionally test the model, in a fourth set of experiments we performed psychophysical examination of the conventional Oppel-Kundt stimulus with a different number of equally spaced dots subdividing the filled part. Adequate correspondence between the computational and experimental data supports our assumptions concerning the origin of the filled-space illusion.