Transcranial Magnetic Stimulation Differentially Affects Speed and
Direction Judgments
Nestor Matthews, Bruce Luber, Ning Qian, and S Holly Lisanby, Exp. Brain Res.
2001, 140:397-406.
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Abstract
This study was conducted to determine whether humans' judgments about the
speed and direction of moving stimuli would be differentially affected by
transcranial magnetic stimulation (TMS). Subjects viewed two successively
presented moving stimuli that differed from each other both in the speed and
direction of motion. Single pulse TMS was applied either medially
(approximately 2 cm above the inion) or laterally (approximately 5 cm
lateral to and 4 cm above the inion), while subjects judged the speed and
direction differences. The physical stimulation (visual and TMS) was
identical on the two tasks, as was discriminability (d') when TMS was not
applied. We found significant criterion (b) shifts on the speed
discrimination task at both stimulation sites. Specifically, on TMS trials
the proportion of "slower" judgments increased significantly, consistent
with subjective reports that stimuli often appeared to slow when TMS was
applied. The subjective reports indicated no corresponding change in
perceived direction. We also found that speed discriminability was impaired
significantly more than direction discriminability, but only when TMS was
applied medially. Indeed, after controlling for TMS-related changes in
reaction time, speed discriminability was impaired significantly while
direction discriminability remained largely intact. This dissociation
suggests that the sensory response constraining speed discrimination is at
least partially independent from the sensory response constraining direction
discrimination. Combined with previous psychophysical data, the present data
suggest a double dissociation between speed and direction discrimination in
humans.
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