Expansion of perceived size of components of visual stimulus

Abstract

Introduction. The present study focuses on the expansion effect of the relative size of isolated stimulus parts like angles, broken lines, and short stripes. Subjects (M, N, O, L, Ë, A, B, R, P, H, K, and Г) matched the test distance to the referential stimuli length by adjusting the position of the terminal spot. The testing distance was limited to the terminal spot and the vertex or sideline of a referential figure. The starting referential stimulus was the contour pentagon (facsimile in Fig. 1). Lines and their combinations were removed from the pentagon to show separate parts. The background's luminance and the stimuli lines were 0.01 cd/m2 and 23 cd/m2 , respectively. Results. A. All modified stimuli caused the effect of length expansion, which did not significantly differ in strength from the entire pentagon (13.8 arc min; Fig 1 medial section). The illusion grew slightly from 13.8 to 15.7 arc min P = 0.397 (Welch's t-test) in the stimulus without a lower horizontal line and from 13.8 to 15.2 arc min P = 0.561 in that without the bottom two lines. B. An angle, a pair of wings (Fig. 1 right section) was twice shorter than the pentagon, but the illusion, on the contrary, increased from 13.8 to 16.9 arc min P = 0.176. C. When the open rectangle was presented (left section in Fig.1), the illusion strength did not change: 13.8 → 13.1 arc min P = 0.716, regardless of the stimulus change in spatial structure. D. When the vertical segment was removed from the open rectangle, and two parallel horizontal lines remained, the illusion increased: 13.1 → 17.4 arc min P = 0.015. E. If two vertical segments were added to the two horizontal lines to make the stimulus a rectangle, the strength of the illusion slightly decreased: 17.4 → 15.5 arc min P = 0.278. Conclusions. Components of a visual stimulus presented as isolated should be considered full-fledged objects in the size comparison procedure. The individual shape of an object determines the strength of the size expansion. A sum of the expansions of the size of stimulus parts significantly differs from the expansion of the whole. The present experimental data favor the functional relationships of the contour segments forming the shape, not the integration of reactions to each segment during the relative size estimation.