Looking at Visual
Objects:
The Role of the
Physiological studies of oculomotor
control have historically relied on individual spots of light as the visual
targets. However, in everyday life, we often orient towards extended objects
that may or may not be moving. For example, the above scene on the left
involves a large object that we can smoothly track by directing our gaze to its
center. At night (above right), the same scene has a sparser visual
representation of the object (only the strobe lights on the wing tips may be
visible), but we are still able to track the object`s invisible center. This
latter scenario is tractable for study in the laboratory, which allowed us to
investigate some of the neural mechanisms associated with it. We have described
these mechanisms using single-neuron
recording from and reversible
inactivation of the superior colliculus (SC), a structure crucial for
oculomotor control. The following are sample demonstrations of the basic
results we obtained.
Sample Behavior
The following movies show
sample stimulus configurations, along with a representative subject`s eye
position indicated by a green crosshair. During `extrafoveal tracking`, the
subject successfully inferred the invisible center of a stimulus consisting of
two peripheral features and tracked it for several seconds.
- Fixation (Quicktime format, running at half speed)
- Extrafoveal tracking (Quicktime format, running at half speed)
Behavior of Individual SC Neurons
We recorded from superior
colliculus (SC) neurons during the above behaviors. The SC contains a
topographic representation of visual space, which allowed us to record either
from peripheral neurons (representing one of the visual features of the
stimulus) or central ones (representing the foveal location of the invisible
center that was tracked). We found that the central neurons were the most
active during extrafoveal tracking, despite the lack of a visual stimulus.
These neurons were modulated based on the location of the goal that the subject
was trying to orient towards. During fixation, when the invisible center could
be ignored, the neurons were modulated to a lesser extent. The following movies
illustrate some of these findings.
- Sample SC neuron from the
central portion of the SC during fixation (the graph on the bottom is an
indication of the neuron`s activity) (Quicktime
format, running at half speed)
- The same neuron during
extrafoveal tracking (the gray curve in the bottom is the activity during
fixation shown in the above movie) (Quicktime
format, running at half speed)
Effects of Reversible Inactivation
of a Subset of SC Neurons
The following movie shows a
representation of extrafoveal tracking performance with and without SC
inactivation. The white crosshair shows the subject`s average eye position from
a baseline data set. The yellow crosshair shows this position after locations
in the SC encoding the lower left quadrant (in retinotopic coordinates) were
reversibly inactivated. The subject tracked as in the baseline condition, but
with a constant offset (relative to baseline) to the upper right quadrant,
indicating a biased estimate of goal location.
- Effects of reversible
inactivation on extrafoveal tracking (Quicktime
format, running at half speed)
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Created by Dr. Ziad M. Hafed on September 20,
2008