Fixation disparities (FD) were measured as a function of forced vergence using binocular scleral search coils and simultaneously with nonius lines. The slope of the objective FD curve was significantly greater than the subjective FD curve for three of five subjects. This indicates an alteration in retinal correspondence of up to one degree, that shifts Panum's area to avoid the diplopia normally present with large disparities. This process allows for fusion in the presence of large objective fixation disparities which would normally cause diplopia. The shift in correspondence enhances the range of forced vergence, since the larger objective FDs serve as more effective stimuli to fusional vergence. The remaining subjects who lacked this effect had "flat" FD curves indicative of high vergence adaptation.
The spatial characteristics of the changes in retinal correspondence produced by forced convergence were studied. The vertical extent of lateral shifts in binocular correspondence were quantified by comparing the convergence of the eyes measured with binocular search coils to the convergence of the eyes as determined using nonius lines having vertical separations (gaps) between the nonius lines of 0.5-4.8 degrees. Lateral shifts in binocular correspondence only occurred for nonius gaps < 3-4 degrees. The effects of horizontal retinal eccentricity on lateral shifts in correspondence were determined by measuring the nonius horopter of the subject under forced convergence, using 11 nonius line eccentricities between 4.5 degrees left and right. The nonius horopter was shifted toward the fusion target maximally near the fixation point. There was no shift beyond 3 degrees of eccentricity. We conclude that the nonius horopter is 'dimpled' vertically and horizontally, facilitating local fusion by shifting the line horopter and the region of single binocular vision toward the point of regard over a region of 3 degrees around the fixation point.
Purpose
The purpose of the study was to measure and compare horizontal head
and eye tracking movements as baseball batters “took”
pitches and swung at baseball pitches.
Methods
Two former college baseball players were tested in two conditions. A
pitching machine was used to project tennis balls toward the subjects. In
the first condition, subjects acted as if they were taking (i.e. NOT
swinging) the pitches. In the second condition, subjects attempted to bat
the pitched balls. Head movements were measured with an inertial sensor; eye
movements were measured with a video eyetracker.
Results
For each condition, the relationship between the horizontal head and
eye rotations was similar for the two subjects as were the overall head-,
eye-, and gaze-tracking strategies. In the “take” condition,
head movements in the direction of the ball were larger than eye movements
for much of the pitch trajectory. Large eye movements only occurred late in
the pitch trajectory. Gaze was directed near the ball until about 150ms
before the ball arrived at the batter, at which time gaze was directed ahead
of the ball to a location near that occupied when the ball crosses the
plate. In the “swing” condition, head movements in the
direction of the ball were larger than eye movements throughout the pitch
trajectory. Gaze was directed near the ball until about 50ms-60ms prior to
pitch arrival at the batter.
Discussion
Horizontal head rotations were larger than horizontal eye rotations
in both the “take” and “swing” conditions.
Gaze was directed ahead of the ball late in the pitch trajectory in the
“take” condition, while gaze was directed near the ball
throughout much of the pitch trajectory in the “swing”
condition.
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