The "Spinner" Illusion: More Dots, More Speed?

Hiroshi Ashida,Akiyoshi Kitaoka,Stuart Anstis,Alan Ho

doi:10.1177/2041669517707972

Abstract

The perceived speed of a ring of equally spaced dots moving around a circular path appears faster as the number of dots increases (Ho & Anstis, 2013, Best Illusion of the Year contest). We measured this “spinner” effect with radial sinusoidal gratings, using a 2AFC procedure where participants selected the faster one between two briefly presented gratings of different spatial frequencies (SFs) rotating at various angular speeds. Compared with the reference stimulus with 4 c/rev (0.64 c/rad), participants consistently overestimated the angular speed for test stimuli of higher radial SFs but underestimated that for a test stimulus of lower radial SFs. The spinner effect increased in magnitude but saturated rapidly as the test radial SF increased. Similar effects were observed with translating linear sinusoidal gratings of different SFs. Our results support the idea that human speed perception is biased by temporal frequency, which physically goes up as SF increases when the speed is held constant. Hence, the more dots or lines, the greater the perceived speed when they are moving coherently in a defined area.

Highlights

Visual motion provides us with vital information about the environment necessary for our daily survival
The results were collapsed across presentation sides of stimuli and their rotation directions
The speed of a higher radial spatial frequencies (RSFs) test stimulus was consistently overestimated by the participants, which agreed with the original spinner illusion observation

Summary

Introduction

Visual motion provides us with vital information about the environment necessary for our daily survival. One possible factor limiting our understanding of speed perception and vision in general is the inverse projection problem (Palmer, 1999). This problem arises because an infinite number of distal environmental objects with different shapes and sizes seen in the three-dimensional (3D) world can cast the same two-dimensional (2D) optical image onto the retina, introducing ambiguities difficult to solve. A moving distal environmental object further extends the inverse projection problem into the time domain and possibly introduces more uncertainty and inaccuracy for the visual system in processing visual motion information. The speed of a moving stimulus used in researches is expressed in terms of its spatial and temporal frequencies, and is susceptible to biases from other stimulus properties such as its luminance contrast (Thompson, Brooks, & Hammett, 2006; Thompson, 1982) and colour (Cavanagh, Tyler, & Favreau, 1984)

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Conclusion