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When
tracking a visual target moving in the environment, we use a
combination of saccadic and pursuit eye movements to center and
stabilize the retinal images of objects of interest. Saccades are
discrete movements that quickly direct the eyes toward a visual target,
whereas pursuit is a continuous movement that smoothly rotates the eyes
to compensate for any motion of the target.
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Spatial scale of attention during pursuit
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With Rich
Krauzlis and Josh
Wallman (from City College of the City University of
New-York)
we try to understand the role of attention in oculomotor tracking by
manipulating the spatial scale of attention, and by measuring the eye
movements evoked by perturbations of a tracked stimulus (left). We
found that when the spatial scale of attention was small,
perturbation-evoked saccades occurred more frequently, and had
latencies that were shorter and less variable. Similarly, ramp
perturbations imposed when the spatial scale of attention was small
evoked larger changes in smooth pursuit eye velocity than when the
spatial scale of attention was large. To probe the selectivity of
attention, in additional experiments we perturbed only the central or
peripheral part of the stimulus - we found that subjects differed in
the extent to which they could suppress the response to the
perturbation of the non-attended ring. By fitting the latency
distributions with the Reddi and Carpenter LATER
model, we found that we could group the responses of our subjects into
several distinct strategies for changing latency as a function of the
spatial scale of attention. A poster
entitled "Attention
affects catch-up saccades during smooth pursuit" was
presented at the Society
For Neurosciences 31st annual meeting (Madelain, L., Krauzlis, R.J.,
Wallman, J., 2001). More data were presented at VSS
2003 and 2004. A paper appeared
in Vision Research (pp 2685-2703, 45, 20, 2005). |
| Demonstration
of the ring stimuli used to study the effects of the spatial scale of
attention on pursuit and saccades. The two rings are
spinning in opposite direction (at 40 rpm for the large one and at 60
rpm for the inner one). The mask stimulus (11 segments in both ring) is
displayed for two seconds. The probe stimulus appears for a short
period of time (here 4 frames i.e. about 160 ms) and then the mask
returns for the rest of the trial (here one second). The task of the
subject is to report the number of segments in either the large or
small inner ring during the brief period when the probe stimulus is
present. Here the probe stimulus has 5 segments in the large ring and 4
in the inner one. |
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Pursuit of perceived motion
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I
am currently working on the relationship between smooth pursuit eye
movements and perception of motion with Rich Krauzlis.
Monitoring eye movements during pursuit of apparent motion we have
shown that the perceived motion of an object can drive pursuit, even
when the motion is perceptually bistable and the object itself is
illusory. We found that smooth pursuit and the perception of motion
direction are in temporal register, indicating that pursuit can provide
a real-time readout for the state of motion perception. We presented a poster at the VisionScienceS Society in
May 2003 on these
results (poster + demos here).
A paper entitled
"Pursuit of the ineffable: perceptual and motor reversals during the
tracking of apparent motion"
appeared in Journal of Vision, 3(11), 2003. Below is an example of the
bi-directional stimulus we used (you can see a rightward or leftward
motion and switch perceived direction at will). |
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Journal of vision, 3(11), 2003
(full text), part of the Special issue on linking eye movements and
perception
