Abstract
Purpose: To study a new illusion of apparent rotation. By alternatively increasing and decreasing the amount of transparency of a grey annulus superimposed to a radial arrangement of black and white circular sectors, the annulus appears to rapidly rotate. We call this phenomenon “windmill illusion”. Phenomenology of the illusion: (i) The rotation does not belong to the boundaries of the annulus but only to its grain and matter, that appear to flow within and along the annulus area ambiguously in either clockwise and anticlockwise directions; (ii) the granular matter of the annulus emerges clearly only under transparency modulation and only if the illusory rotation is perceived; (iii) the intentional motion of the gaze in one direction (e.g. clockwise) disambiguates the illusory rotation that follows the gaze direction; ( iv) the loss of transparency annuls the apparent rotation; (v) by rotating the whole stimulus while the transparency of the annulus is increased and decreased, the annulus appears to rotate in the opposite direction to the real rotation; (vi) by replacing the annulus with another shape (disk, square, vertical or horizontal rectangle) the apparent motion follows the main directions of the boundaries of the specific shape, i.e. the matter appears to flow along the horizontal rectangle and in either left and right directions; (vii) by replacing the annulus with a disk of different sizes, if the disk is smaller than the sectors, the apparent rotation belongs to the disk, if it is larger than the sectors, the rotation belongs to them, if it has the same size as the sectors both rotate but only in their matter or grain; (viii) the apparent rotation is clearly perceived under neon color spreading conditions. Experiments: The previous phenomenal conditions were psychophysically measured. Results confirmed previous phenomenal observations and are theoretically discussed and related to other phenomena reported in the motion perception literature.
Supported by Fondazione Banco di Sardegna, ERSU and Fondo d'Ateneo ex 60% (to BP). We thank Marina Oggiano, Deborah Cisternino and Loredana Ticca for assistance in testing the subjects.