Direction-of-Motion Discrimination Is Facilitated by Visible Motion Smear

Article excerpt

Recent evidence indicates that motion smear can provide useful information for the detection and discrimination of motion. Further, it has been shown that the perception of motion smear depends critically on the density of dots in a random-dot (RD) stimulus. Therefore, in the present experiments, the contribution of perceived motion smear to direction-of-motion discrimination was evaluated using RD targets of different densities. Thresholds for direction-of-motion discrimination and the extent of perceived motion smear were determined for RD stimuli with densities of 1, 2, and 10 dots/deg^sup 2^, presented for 200 msec at a velocity of 4, 8, or 12 deg/sec. To evaluate the contribution of information about orientation from motion smear, thresholds for orientation discrimination were measured using parallel lines with the same length as the extent of perceived smear. Despite the opportunity for increased summation as RD density increases, our results indicate that direction-of-motion discrimination worsens. Because perception of motion smear is reduced with an increase in RD density, our results are consistent with a facilitation of direction-of-motion discrimination by visible motion smear.

One of the fundamental tasks of the visual system is to decode the direction information in the retinal-image motion that results from objects that move in space. Electrophysiological investigations have disclosed that nerve cells located in striate and extrastriate cortical areas have direction-selective characteristics (Cheng, Fujita, Kanno, Miura, & Tanaka, 1995; Cornette et al., 1998; Hubel & Wiesel, 1968; Singh, Smith, & Greenlee, 2000), and it is generally assumed that the brain constructs its percept of the direction of motion from the selective responses of such neurons (Azzopardi & Cowey, 2001; Blanke, Landis, Mermoud, Spinelli, & Safran, 2003; Britten, Shadlen, Newsome, & Movshon, 1992; Salzman, Britten, & Newsome, 1990). However, debate continues as to how these cortical neurons work together to generate the perception of motion (Adelson & Movshon, 1982; Marr & Ullman, 1981; Pack, Livingstone, Duffy, & Born, 2003; Peterson, Li, & Freeman, 2004; Purushothaman & Bradley, 2005; D. W. Williams & Sekuler, 1984; Zohary, Scase, & Braddick, 1996).

Geisler (1999) hypothesized that spatial orientation information from "motion streaks" may be used by the visual system to enhance the encoding of moving targets. The visual representation of an object that moves at a sufficient velocity should be smeared, because of the visual persistence that accompanies temporal integration (Bidwell, 1899; Bowen, PoIa, & Matin, 1974; Coltheart, 1980; McDougall, 1904). This motion smear produces a streak in the orientation parallel to the direction of motion. Neurons in the primary visual cortex tuned to orientations parallel to the motion trajectory should be activated by the streak, and the output from these orientation-selective detectors could combine with that from direction-selective detectors to determine the direction of motion. Geisler, Albrecht, Crane, and Stern (2001) presented neurophysiological evidence to indicate that orientation-tuned neurons in the primary visual cortex of cat and monkey do, in fact, respond to motion streaks. A related proposal by Barlow and Olshausen (2004) suggests that the visual system uses the anisotropies of local spatial frequency power spectra that result from motion blur to analyze the direction of motion streaks.

Support for a contribution of motion streaks to the processing of visual motion is available from psychophysical experiments. Geisler (1999) reported that adaptation to a tilted grating significantly shifts the perceived direction of a bright spot that moves vertically at 10 deg/sec, but does not do the same for a spot that moves at 2.5 deg/sec. In this experiment, the moving bright spot was presented on a dark background, which, when the velocity was 10 deg/sec, would be expected to produce a noticeable extent of visible motion smear. …