- Interaction of TMS-induced phosphenes and visual stimuli
D.-A. Wu, Y. Kamitani, F. Maeda, S. Shimojo
Purpose: Whereas single-pulse transcranial magnetic stimulation (TMS) of the human visual cortex results in scotoma, dual-pulse TMS results in the transient visual percept of an area of brightness, termed a phosphene. In this study, we presented disk-shaped visual stimuli that overlapped the phosphene in the visual field. Subjects evaluated the brightness of the overlapping region when dim or bright disks were either flashed or steadily shown.
Procedure: Dual magnetic pulses were delivered by a Neotonus Neopulse stimulator. The coil position over the left occipital lobe, power, and inter-pulse interval were optimized for phosphene induction. Trials began with a dark background (< 0.1 cd/m^2), then a large homogeneous disk (15 deg in diameter) was either: flashed for 93 ms and followed by TMS (40 ms after flash offset), or shown steadily for 6 s, during which (at the 4th second) TMS was triggered. The disks were set either dimmer or brighter than phosphenes perceived on a dark background. The four conditions (flashed/steady x dim/bright) were randomly repeated five times each. After each trial the disk was redisplayed and subjects adjusted the luminance of the region that overlapped with the phosphene to match their percept.
Results: 1) For both dim and bright flashed disks, the region overlapping the phosphene was reported to be brighter than the rest of the disk (p < .01). However, in steady displays the region was darkened in bright disks, but brightened in dim disks (p < .01). 2) With the 40 ms delay between the flashed disk and TMS, which maximizes the brightening effect, the disappearance of the disk was perceived first, followed by the simultaneous perception of the phosphene and its brighter spatially overlapping region.
Discussion: Phosphenes clearly cannot be considered a mere brightening of the visual field. Phosphenes show non-linear brightness interactions with visual stimuli that are both physically and perceptually non-concurrent.
Funding: NIH/ERS, Anonymous donation to Caltech.
- Perceptual Priming in Slit Viewing
C. Yin, S. Shimojo, & S. A. Engel
Abstract: When your office door is slightly ajar, and something moves down the hall, you are able to perceive that object although only a small segment of its visual image is projected upon the retinae at any time. Prior studies of this ability, known as "slit viewing", have suggested that a necessary condition for successful shape perception is the correct perception of the moving contours’ orientations. In cases with a narrow slit that is one or two pixels wide, the contours’ orientations are not perceivable and slit viewing usually fails.
We tested whether prior successful slit viewing experiences with the same shape may improve performance with narrower slits. Five observers were asked to determine in a two alternative forced choice task which of two views of a line-drawn object were more rotated in the counterclockwise direction, with an angular disparity of 10 deg. Slit width was initially 1 pixel wide, was ramped up to 8 pixels and then ramped back down to 1 pixel (specifically in these steps: 1, 2, 4, 8, 4, 2, 1). Observers saw the same pair of views at the same rotations, but with different presentation order, at each slit width. At the widest slit, all observers were able to see the shape easily.
Preliminary results suggest that perceptual priming did occur as both accuracy and confidence ratings increased on the downward ramp. In a second study, object rotation was changed from trial to trial to see if the priming was object-based. Preliminary results suggest that the perceptual priming in slit viewing is view-based. In a third study, slit width was only ramped up to 2 pixels (in these steps: 1, 2, 2, 2, 2, 2, 1). Preliminary results suggest that practice does not increase performance in the smaller slit views, suggesting that the benefit stems mainly from top-down factors.
- Position capture by object motion through a slit
Katsumi Watanabe, Romi Nijhawan, Shinsuke Shimojo
Moving items cause the position of flashed items to appear shifted in the direction of motion (Whitney & Cavanagh, 2000). Is this motion-induced position capture due to retinal or perceived motion? In order to examine this issue, we employed a slit-view motion display where retinal motion is minimized but motion perception remains vivid.
 Two white diamonds (3 deg) translated horizontally in opposite directions on a gray background at 7.2 deg/s, one above and one below the fixation. When the diamonds were in vertical alignment, two vertical lines (1 deg) were flashed for 13 ms, one inside each diamond (full-view condition). Observers (N=3) saw the lines shifted in the direction of the moving diamonds; when the top line appeared shifted to the left, the bottom line appeared shifted to the right, and vice versa (p < .05).
 In the slit- view condition, observers saw exactly the same display except the diamonds were visible only through a narrow (5.8 min) vertical slit (within which the lines were flashed) so that only small elements of the diamonds were visible at any one time. Even though horizontal retinal motion was greatly reduced in this condition, observers perceived the diamonds moving behind the occluders and the motion-induced position capture occurred undiminished (p < .05).
 Finally, we allowed observers to see the diamonds through only a 1-pixel slit (1.4 min), eliminating any horizontal motion component. Surprisingly, the moving diamonds were still perceived. Although the motion direction was ambiguous, observers tended to see the diamonds moving in the same direction. Despite that this object motion perception was all in the observer’s mind, the motion-induced position capture was still significant (p < .05; both lines appeared shifted in the same direction), though somewhat reduced. Thus the motion-induced position capture must be attributed to high-level motion processing responsible for dynamically integrating object motion and shape.