P1253297275VJdkr

1
Transcranial Magnetic Stimulation Differentially
Affects Speed Direction Judgments 4674
Nestor Matthews1, Bruce Luber 2,3, Ning Qian1,
Sarah H. Lisanby2,3 Center for Neurobiology
Behavior, Columbia University1 Department of
Psychiatry, Columbia University2 Department of
Biological Psychiatry, New York State Psychiatric
Institute3.
Results
Conclusions
Purpose To determine the extent to which humans
speed and direction judgments could be
differentially affected by Transcranial Magnetic
Stimulation (TMS).
Spontaneous Reports - Each of the five subjects
spontaneously reported that the motion appeared
to slow when TMS was applied. There was no
corresponding change in perceived
direction. Data Analysis - The data were
analyzed with respect to four dependent
variables Criterion (b) discriminability (d)
reaction time efficiency ( d / reaction time
). For each subject and task, the data from the
TMS conditions were normalized to the data from
the no-TMS condition (dotted, horizontal line on
each graph).
1. TMS significantly reduced perceived speed
without affecting perceived direction. This was
true at both stimulation sites. 2. Speed
discrimination was significantly more impaired
than direction discrimination, but only when
TMS was applied medially. This was also true for
our efficiency measure, which controls for
tradeoffs between the speed and accuracy
of responding. 3. Averaging across subjects,
stimulation sites, and SOAs, TMS significantly
reduced the efficiency of speed discrimination
but did not alter the efficiency of direction
discrimination. This suggests a single
dissociation between the mechanisms responsible
for these two tasks.
Methods Subjects viewed two successively presente
d moving stimuli that differed from each other
both in the speed and direction of motion, as
schematically shown here.
Discriminability (d)
Reaction Time
Efficiency
Criterion (b)
1st Stimulus 200 msec
2nd Stimulus 200 msec
In separate trial blocks, subjects judged the
speed (faster / slower) or the direction
(clockwise / anti-clockwise) of the 2nd
stimulus relative to the first.
The Bottom Line
TMS significantly affected speed judgments, but
not direction judgments. This dissociation is
the converse of the earlier finding that
axis-of-motion affects direction discrimination
but not speed discrimination (Matthews Qian,
Vision Res, 39, 2205-11). Taken together, the
two studies constitute evidence for a double
dissociation between the mechanisms responsible
for speed and direction discrimination.
Single pulse TMS was applied with a round coil
either medially (2 cm above the inion)
or laterally (5 cm lateral to and 4 cm above the
inion). The pulse occurred either 0, 40, 80,
120, 160, or 200 msec after the onset of the
2nd stimulus, or not at all (control condition).
The physical stimulation (visual and TMS) was
identical on the two tasks, as was
discriminability (d) when TMS was not applied.
This poster can be viewed at http//brahms.cpmc.co
lumbia.edu/nestor/arvo.htm