DOI:10.1068/p5245 Perceptual alternation induced by visual transients

When our visual system is confronted with ambiguous stimuli, the perceptual interpretation spontaneously alternates between the competing incompatible interpretations. The timing of such perceptual alternations is highly stochastic and the underlying neural mechanisms are poorly understood. We show that perceptual alternations can be triggered by a transient stimulus presented nearby. The induction was tested for four types of bistable stimuli: structure-from-motion, binocular rivalry, Necker cube, and ambiguous apparent motion. While underlying mechanisms may vary among them, a transient flash induced time-locked perceptual alternations in all cases. The effect showed a dependence on the adaptation to the dominant percept prior to the presentation of a flash. These perceptual alternations show many similarities to perceptual disappearances induced by transient stimuli (Kanai and Kamitani, 2003 Journal of Cognitive Neuroscience 15 664 ^ 672; Moradi and Shimojo, 2004 Vision Research 44 449 ^ 460). Mechanisms linking these two transient-induced phenomena are discussed. DOI:10.1068/p5245 ô These authors contributed equally to this work. This effect, termed induced perceptual alternation (IPA), is demonstrated with a variety of ambiguous stimuli including structure-from-motion, binocular rivalry, Necker cube, and ambiguous apparent motion. The IPA manifests characteristics similar to another transient-induced phenomenon where visual transients induce perceptual disappearance of objects (Kanai and Kamitani 2003; Moradi and Shimojo 2004). In both cases, transient stimuli cause a drastic change of the percept to a constant visual stimulus. We attempt to offer a schematic model that explains how visual transients produce a drastic perceptual change both in perceptual rivalry and disappearance. 2 Experiment 1. Basic effect In our first experiment, we show that perceptual reversals can be induced by transients in a variety of bistable stimuli; structure-from-motion, binocular rivalry, Necker cube, and bistable apparent motion. 2.1 Methods 2.1. 1 Observers and apparatus. Four naive observers and one of the authors (RK) participated. All observers had normal or corrected-to-normal vision. The stimuli were generated on a Macintosh computer with Matlab PsychToolbox (Brainard 1997; Pelli 1997) and presented on a 22-inch CRT monitor (LaCie Blue Electron). The refresh rate of the display was 85 Hz; and the resolution, 128061024. The viewing distance was 57 cm. 2.1. 2 Stimuli. We displayed four types of bistable stimuli. These stimuli were structurefrom-motion (SFM), binocular rivalry (BR), Necker cube (NC), and bistable apparent motion (AM). The details of the stimulus parameters are described below. A fixation cross was placed in the geometric center of each stimulus. At random intervals, every 2 ^ 4 s, the background was flashed for 47 ms without occluding the bistable stimuli. The flash was white (56 cd mÿ2). SFM (figure 1a). The SFM consisted of 400 blue dots with a lifetime of 1 s presented against a black background. The shape of the structure was a cylinder rotating around the vertical axis centered at the fixation cross (red). The size of the cylinder was 3.9 deg in height and 3.12 deg in diameter. The rotation speed was 0.2 revolution sÿ1. For this stimulus, the transient was a flash (47 ms) that had a rectangular shape covering the background of the stimulus area of SFM. BR (figure 1b). Two orthogonal square-wave gratings, red ^ black versus green ^ black (458 and 1358) were presented in a circular area (diameter 7.8 deg). The gratings were presented dichoptically with the use of red and green color filters. The gratings had a spatial frequency of 0.8 cycle degÿ1 and the duty cycle was 20%. To support binocular fusion, a white ring (0.4 deg in width) was surrounding the stimulus. The combinations of color (red and green), eye (left and right), and orientation (458 and 1358) were randomly chosen for each trial. Transient flashes were presented by briefly changing the black part of the gratings (ie the background) into white for 47 ms. NC (figure 1c). The skeletal drawing of a cube was presented in blue against a black background. The length of each edge was 1.95 deg when viewed from a perpendicular angle. The flash (47 ms) was a solid white disk with a diameter of 7.81 deg centering at the fixation. AM (figure 1d). AM was created by presenting a pair of blue disks alternately against a black background. The diameter of each disk was 0.98 deg. In one frame, the disks were presented in the upper-right and lower-left quadrants, and in the other frame, in the upper-left and lower-right quadrants. These two frames were alternated every 200 ms, and there was no blank interval between the frames. The four disk positions were 1.17 deg away from the fixation point along both the horizontal and vertical axes. The flash was a white disk (4.69 deg in diameter) centering at the fixation cross. 804 R Kanai, F Moradi, S Shimojo, F A J Verstraten