Academic journal article Perception and Psychophysics

Illusory Temporal Order for Stimuli at Different Depth Positions

Academic journal article Perception and Psychophysics

Illusory Temporal Order for Stimuli at Different Depth Positions

Article excerpt

We used four experiments to examine how the perceived temporal order of two visual stimuli depends on the depth position of the stimuli specified by a binocular disparity cue. When two stimuli were presented simultaneously at different depth positions in front of or around a fixation point, the observer perceived the more distant stimulus before the nearer stimulus (Experiments 1 and 2). This illusory temporal order was found only for sudden stimulus presentation (Experiment 3). These results suggest that a common processing, which is triggered by sudden luminance change, underlies this illusion. The strength of the illusion increased with the disparity gradient and the disparity size (Experiment 4). We propose that this illusion has a basis in the processing of motion in depth, which would alert the observer to a potential collision with an object that suddenly emerges in front of the observer.

The perception of a temporal relationship for events in visual space depends on various factors related to the physical properties of stimuli and to the observer's conscious state. For example, visual latency for stimulus presentations depends on the luminance level of the stimuli (see, e.g., Rogers & Anstis, 1972). Apparent relative velocities for moving stimuli are affected by contrasts between the stimuli and the background (see, e.g., Anstis, 2001). The perception of visual simultaneity for two stimuli also depends on the eccentricity of the stimulus presentation (see, e.g., Ichikawa, 2002; Mitrani, Shekerdjiiski, & Yakimoff, 1986). Simultaneous changes in different submodalities of the visual system, such as color and motion (see, e.g., Arnold, Clifford, & Wenderoth, 2001; Aymoz & Viviani, 2004; Bedell, Chung, Ogmen, & Patel, 2003; Linares & López-Moliner, 2006; Moutoussis & Zeki, 1997a, 1997b; Nishida & Johnston, 2002; Viviani & Aymoz, 2001) and color and location (see, e.g., Pisella, Arzi, & Rossetti, 1998; Tanaka & Shimojo, 1996), are perceived as asynchronous events. Visual attention can also modulate processing around the attended area and, consequently, the temporal order and simultaneity for presentations of stimuli in attended and nonattended areas of the visual field (see, e.g., Hikosaka, Miyauchi, & Shimojo, 1993; Posner, Snyder, & Davidson, 1980; Shimojo, Miyauchi, & Hikosaka, 1997; Stelmach & Herdman, 1991). Studies examining the illusory flash-lag effect have shown that a continuous change in position (see, e.g., Bachmann, Luiga, Põder, & Kalev, 2003; Brenner & Smeets, 2000; Eagleman & Sejnowski, 2000; Krekelberg & Lappe, 2001; Nijhawan, 1994; Purushothaman, Patel, Bedell, & Ogmen, 1998; Whitney & Murakami, 1998) and in various other visual dimensions, such as luminance and color (see, e.g., Sheth, Nijhawan, & Shimojo, 2000), introduces temporal differences in the processing of the changing stimulus and flash. This illusory flash-lag effect can be enhanced by distracting attention from the stimulus (see, e.g., Baldo, Kihara, Namba, & Klein, 2002; Murakami, 2001; Shioiri, Yamamoto, & Yaguchi, 2002); it is reduced significantly by active observation (see, e.g., López-Moliner & Linares, 2006) or by a subjective mental set in which the observer controls the stimulus change (see, e.g., Ichikawa & Masakura, 2004, 2006). Current psychophysical studies are examining whether other factors can be added to the list of factors that affect the temporal relationship for events in visual space.

In the present study, we examined whether depth positions of the stimuli specified by a binocular disparity cue could be added to the list described above. Earlier studies using a single visual stimulus with a fixation point have demonstrated that processing of a stimulus nearer than the fixation plane is more rapid than that of a stimulus that is more distant than the fixation plane. For example, the latency of a converging eye movement is shorter than that of a diverging one if the angle of the convergence/ divergence is smaller than 20? …

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