|
TMS
-
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.
Slit view
-
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.
[1] 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). [2] 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). [3] 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.
MEG, EEG & pupilary response in Photosensitive epilepsy (Pokemon)
Electrophysiology
|
