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Abstract:
GOAL: This study was conducted to determine whether humans'
judgments about the speed and direction of moving stimuli could be
differentially affected by transiently altering the electrical
state of the visual system. METHOD: Subjects viewed two
successively presented moving stimuli that differed from each other
both in the speed and direction of motion. Single pulse
transcranial magnetic stimulation (TMS) was applied with a 90 mm
round coil 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.
RESULTS: 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 the stimulus
often appeared to slow or stop when TMS was applied. There was no
corresponding criterion shift on the direction-discrimination task.
Additionally, after controlling for TMS-related changes in reaction
time, speed discriminability was impaired significantly while
direction discriminability remained largely intact. CONCLUSION: We
believe this dissociation suggests that the sensory response
constraining speed discrimination is at least partially independent
from the sensory response constraining direction
discrimination.
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