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We used psychometric responses of phenomena as criteria – to study perception distortion strength in peripheral areas, while looking towards vertically falling round spots. Spots were objects peripherally projected in the eye, having periodical grey-colored (or red, green, blue) grating moving horizontally within the spot area. Such excitation creates psychophysical perception distortion; falling projected in the brain has slant – Shapiro and Lu effect (2010). We studied the phenomenon by comparing perceived slants of two falling balls viewed in opposite peripheral areas: a) reference object – uniform and neutrally grey, falling at a distinct angle; and b) textured object: falling vertically, with vertical stripe structure that moved along horizontal direction within the object area. Psychometric curves were built using 2ATC paradigm. Observers in Part I were naïve participants, who during one hour responded to series of events where the reference objects had a randomly selected slant of fall trajectory. Subjective equivalence point was determined at the centers of the sigmoidal fit of psychometric curves where abscise was a reference real slant. Mean (22 participants) illusionary slant of falling textured objects was α = 28±5 deg (for viewing directionality ≈8 deg; falling vertical speed 0.18 deg/sec, texture Michelson contrast 0.82, texture spatial frequency 0.42 cycles/deg, “spinning” speed 2 deg/sec). In Part II two observers repeated the experiments in similar viewing conditions each day for two weeks. Distinct decay of α was determined with exponential time constant of 11±2 days. Most surprisingly, the decay curves converge to zero values for supposed infinitive length of demonstrations. Preliminary discussion of the latter with illusion authors revealed coincidence of our results with author’s suggestions. As such learning effect has been detected, the observers in Part III compared two equiluminant striped falling balls in opposite viewing fields, but balls contained different color content. Here we find differences of the slant in perception, however these differences were statistically insignificant.
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