Abstract
Freeman and Driver (2008) reported that brief sounds can bias the perceived direction of visual apparent-motion stimuli (see also Getzmann, 2007), an effect attributed to “temporal capture” of visual stimuli by the sounds (Morein-Zamir et al., 2003). Cortical area MT is a key substrate in visual motion perception (e.g. Britten et al., 1992; Salzman et al., 1990), but the spatial and temporal intervals (i.e. 14 deg and 300 ms, respectively) of Freeman and Driver's stimuli are much too large to engage area MT (Mikami et al., 1986 a, b; Newsome et al., 1986). Since such long-range motion stimuli are reportedly more sensitive to higher-order influences than are short-range motion (Horowitz & Treisman, 1989; Shiffrar & Freyd, 1993), we asked whether sound also impacts the perception of motion stimuli known to engage area MT. In experiment 1, subjects (N=7) judged the dominant motion direction of vertically-oriented bars that alternated between right and left of fixation. Spatial intervals ranged from 0.2 deg to 3.0 deg and temporal intervals varied from 60 ms to 240 ms. Without sound, perceived direction favors the smaller temporal interval (e.g. rightward motion is favored if the left-right interval is smaller than the right-left interval). We found that sounds systematically biased perceived direction in a manner consistent with temporal-capture. In experiment 2, subjects (N=7) judged the relative speeds of silent two-frame motion stimuli with those accompanied by two brief sounds (which either lagged or led the presentation of the individual bars). In further support of the temporal-capture hypothesis, perceived speed was determined by the timing of the sounds. Taken together, our findings suggest that brief stationary sounds may be able to shift the temporal tuning of area MT neurons for visual motion stimuli.
Supported by 2R01EY012872.