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B.T. Garee, N. Fogt; The Effect of Voluntarily Increasing Head Movement During Eye-Head Pursuit . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2133.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Recently head movements of a total congenital ophthalmoplegia patient were examined. Head movements in rapid orienting tasks were qualitatively similar to saccadic eye movements in normals. The eye and head may share a common neural signal that specifies the overall orienting behavior. The relative contributions of eye and head to the overall orienting behavior can be modified by changing the common signal, so the head can be moved to a greater extent when ocular saccades are restricted. These studies examined saccadic behaviors, but little has been done on head-free pursuit. Limited studies of head-free pursuit have concluded that there is a common eye-head pursuit signal. This experiment examined whether normal subjects could pursue a target with the head. This requires that the common eye-head pursuit signal be voluntarily modified. Methods: Two authors participated. Subjects tracked a horizontal step-ramp target (ramp velocity 20°/s, amplitude < 15°, random direction) with the head. The ramp would cross the straight ahead around the time the head moved. In Experiment 1a the target was tracked several times. In Experiment 1b subjects tracked the target 30 times. In Experiment 2 subjects tracked the target 30 times and tried to point a laser attached to the head at the target. Eye movements (Exp. 1a only) and head movements (all experiments) were monitored using magnetic search coils. Results: In Exp. 1a, head tracking overshoots occurred in all trials while eye tracking was accurate. In Exp. 1b head tracking overshoots occurred in all trials. The mean head tracking errors in Exp. 1b (positive values indicate overshoot) when the ramp reached 10° eccentricity were 5.5° (subject 1) and 15.4° (subject 2). In Exp. 2, these errors were -1.6° and -0.4° respectively. Conclusions: When the contribution of head movement to the pursuit response is voluntarily increased, the head does not pursue a target as efficiently as the eye (or eye and head) normally does. This may indicate that the common signal for eye-head pursuit is less adaptable than that for saccades. Alternatively, it could be that since the eye was close to the target, head tracking was not attended to as closely as it would be with restricted eye movements. It may also be that many years are required to adapt or modify the common pursuit signal to pursue a target accurately with the head. Head tracking was much improved in Exp. 1b. Thus the head overshoot in Exp. 1 resulted from the inability to calibrate the initial velocity of the head to that of the target and not because of the inertia of the head. Providing ophthalmoplegia patients with a head position cue for a short time may allow them to rapidly learn to pursue with the head.
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