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Eye movements and Motion
Perception
We worked out the preliminary engineering and software
development for interfacing the Eye Movement Monitor (Permobil Meditech, Ober-2),
used to analyze eye movements, with the Macintosh computers on which the stimuli
are displayed. We implemented basic tests for measuring saccades and a step ramp
stimulus for measuring smooth pursuits and obtained data from 10 normal subjects
and three motion impaired patients.
Contents
back to Research
 Neurology and Behavior. The focus of the eye movement research
will be the contribution of the posterior parietal cortex to eye movements as it
relates to motion perception. The role of these areas has been studied to a high
degree in monkey (Segraves et. al., 1987; Newsome and Paré, 1988; Movshon
et. al., 1990; Wurtz et. al., 1990a, 1990b; Leigh and Zee, 1991, pp. 152-156; Andersen
et. al., 1997) and neurological patients their physiology provides a good model to
understand how the human brain functions The human homologues of these areas are
thought to be involved with the processing of motion, attention, making planned movements,
and generating eye movements (Andersen et. al., 1997).
Previous physiological research with eye movement generation
and control in monkey show the involvement of cortical areas that are also involved
with processing visual motion. The striate cortex propagates the stimulus velocity
and position information necessary for the generation of smooth pursuit and saccades
(Segraves, et. al., 1987). The middle temporal area (MT) contributes information
on the object motion, not position, and lesions to this area cause a decrease in
saccade and pursuit accuracy to moving targets. Unilateral lesions to the medial
superior temporal cortex (MST), which receives input from both MT and from extraretinal
sources, cause defective eye movements to targets moving towards the side of the
lesion (ipsilateral directional pursuit deficits) (Wurtz, et. al., 1990a, 1990b).
It has been suggested that visual, rather than motor, processes may determine many
of the characteristics of smooth pursuit (Movshon et. al., 1990).
In humans, these ipsidirectional deficits have been
seen in patients with focal brain lesions in the lateral occipital-parietal cortex
(Barton, 1996; Morrow & Sharpe, 1990; Thurston, et. al., 1988).
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Eye Movements and Motion Perception. Single, moving targets have
been the traditional stimulus for testing eye movements. The ipsilateral directional
deficits have been shown by measuring pursuit of sinusoidal horizontally moving points.
Pursuit gains for targets moving toward the lesion are lower, with the presence of
‘catch-up’ saccades as a signal of this pursuit asymmetry. Another type of target
used extensively in the mentioned research is the step/ramp paradigm, which is capable
of measuring both pursuit and saccade response to a moving target (Rashblass, 1961).
There are many kinds of step-ramp stimuli, but generally it requires that a subject
fixate on a stationary target and at some point in time, the target changes position
to another location on the screen (the step) and proceeds to move at a constant velocity
either towards the center of the screen (centripetal motion) or away from the center
(centrifugal motion).
Random dot kinematograms (RDKs) have been used as a
stimulus for stimulating the motion processing centers of human and monkey brain
(Newsome and Paré, 1988; Watamanuik and Sekuler, 1992; Vaina, 1994; Barton,
1994; Barton, 1995). Recently, RDKs have been used as stimuli to examine how well
smooth pursuit distinguishes different directions of motion (Watamanuik & Heinen,
1994, 1998). From this work with normal human subjects, the human eye movement system
is capable of tracking the global motion of an RDK, suggesting a processed motion
signal may be an input to the eye movement system. The initiation of pursuit is similar
to pursuit to single targets, but with fewer saccades and higher gain. In order to
make a comparison between oculomotor and psychophysical performance, Watamanuik and
Heinen utilized a strategy previously used to measure smooth pursuit precision. This
strategy is an adaptation of signal detection theory to measure the precision of
smooth pursuit eye movements. Signal detection theory states that the discrimination
of two stimuli, one composed of noise and one containing a signal and noise, can
be conceptually represented as Gaussian-shaped distributions (see below).
One can determine, based on the means and standard deviations
of each distribution, the ease by which an observer can distinguish signal from noise.
This theory, a fundamental basis behind psychophysical studies, has only recently
been used to analyze the oculomotor system (Kowler & McKee, 1987). Watamanuik
and Heinen found that the directional precision of smooth pursuit is similar to performance
of psychophysical direction discrimination (Watamanuik & Heinen, 1994, 1998).
This similarity suggests that smooth pursuit eye movements and psychophysical responses
share common stages of motion processing.
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Eye Movements Monitoring System. There are a variety of methods
used to record eye movements (Stolk, 1995; Wooding, 1995). The Permobil Meditech Ober2 Eye Tracking goggles measure horizontal and vertical positions of the eyes using
infra-red oculography. This method works by using infra-red light sources, detectors,
and the different reflective properties of the iris compared with the sclera (the
white of the eye) to generate voltage information that can be converted to eye position.
Infra-red light signals are shown on the boundary of the iris and the sclera. When
the eye moves in one direction (up or down; left or right), less infra-red light
is reflected back to the detector on one side of the eye than the other. The voltage
signals (0-5V range for the Ober2) from the detectors can then be calibrated and
used to calculate eye position. The Ober2 uses pulsed infra-red light emitting diodes
as sources and solid state silicon diodes as detectors (Permobil Meditech, Inc.,
1993). These are mounted on a circuit board inside the goggles close to the eye.
The horizontal (X-axis) diodes measure horizontal eye position first, followed by
a vertical (Y-axis) measurement by the vertical positioned diodes a few milliseconds
later.
System Setup: Ober2 Interface. A hardware
and software interface was set up between a Macintosh Centris 650 and an Acer Pentium
PC to:
• control the start, stop, and interrupt of the Ober2
recording from within the software of the Macintosh test stimuli program
• mark the Ober2 recorded data during the stimulus display
• monitor the recording status of the goggles from within the test
The figure below is a block diagram of the eye movements
test system. Eye position is recorded on the PC side (2.A-F); the stimulus is presented
on the Macintosh side (2.1-3). Once the operator has set up the Ober2 system and
started testing the horizontal and vertical eye position is recorded as a voltage
from the infrared light detectors within the goggles (2.A). The goggle signals run
through an isolation box (2.B) before being sent to the Ober2 recording card in the
PC (2.D). Here the voltage signals (range: 0-5 V) are digitized and saved in binary
format in a recording file (2.F). An interface connection (2.4) between the Ober2
I/O card (2.E) and the Macintosh Centris 650 (2.1) allows signals sent from the Macintosh
to start and stop recording, interrupt recording, and to mark the data as it is being
recorded. Mark signals appear as letters ‘X’ or ‘W’ in the data file.
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Eye Movements Tests. All of the following
tests are generated by a Macintosh Centris 650 onto a 21” monochrome monitor 60 cm
away from the subject. The Ober2 goggles sampled vertical and horizontal eye position
at a rate of 240Hz.
Saccade Test
This test allows the measurement of reflexive saccades to stationary targets in the
subject’s periphery. The subject fixates on a 0.5ºx0.5º square located
in the center of the screen. After 2 seconds, the point disappears and relocates
to a new position a set distance away in one of twelve positions corresponding to
points around an analog clock. The subject saccades to the new position. After 0.5
seconds, the square is replaced by a 'T', at which point the subject remains fixated
and presses a button. The 'T' disappears and the center point reappears. The test
continues until all 12 positions have been shown. The points can be presented in
a predictable order (clockwise from 1 to 12) or in a random order. For normal subjects,
5º, 10º, and 15º amplitude displacements are used. For patients, varying
the eccentricity of the target points will determine what areas of the visual field
are still intact. This allows the tester to ensure that later test stimuli are displayed
within the patient’s intact visual field.
Antisaccade Test
The aim of the antisaccade test is to examine a subject’s ability to suppress saccades
to suddenly appearing targets and initiate saccades in the opposite direction. For
the antisaccade test, the subject fixates on a center fixation. After a set amount
of time, the center fixation disappears and a target appears in the subject’s periphery,
as with the Saccade Test. The direction of the saccade, however, depends what fixation
point appears. If the fixation point was an ‘O’, the subject is to saccade in the
direction of target displacement. If the fixation point was an ‘X’, the subject must
make an antisaccade opposite the direction of target displacement.
Step-Ramp Test
The step-ramp stimulus can be used to test a subject’s smooth pursuit and saccade
to a moving target. The subject fixates on a 0.5ºx0.5º square in the center
of the screen. After a random delay between 45 and 2000 ms, the target goes off and
is stepped horizontally to the right or to the left of the original target position.
Two paradigms of target motion are used: (1)Centrifugal: a step of 3 degrees followed
by a ramp of 15 or 30 degrees per second away from the center of the screen; (2)Centripetal:
a step of 9 degrees followed by a ramp of 15 or 30 degrees per second toward the
center of the screen.
RDK Stimuli
The stimuli consisted of sparse RDKs as illustrated in panel i) of the Core Motion Tests. The Motion Coherence Test has been modified to control the
Ober2 system that records eye movements and to present the appropriate stimuli for
this research. Random-dot cinematograms (RDKs) are used to test the subject’s ability
to perceive coherent direction of motion and the subject’s ability to generate pursuit
eye movements to the direction of motion of RDKs at varying coherence levels.
Average eye velocity and direction of eye motion to
RDKs of varying coherence levels will be collected. The oculomotor difference threshold
(Kowler and McKee, 1987) will be calculated for the precision of eye velocity magnitude
and direction response to several levels of motion direction coherence.
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last update: 12/11/98
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