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P. Satgunam, N. Fogt; Orienting Head Movements During Head-Free Pursuit . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2132.
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Purpose: In studies of monkey head-free pursuit the head contributes to pursuit. Head movement may ensure that when a second (more eccentric) object appears it will be well within the ocular motor range so switching fixation from the pursuit target to the new object requires a rapid eye movement but not a rapid head movement. The advantage is the eye is presumably controlled more accurately than the head. This study was to determine whether human subjects would turn the head during pursuit of a target, reducing the need for a rapid head movement to view a more peripheral object that suddenly appeared at a predictable location. Methods: 4 subjects participated. Subjects pursued a laser target (3.4m distance, 20° amplitude) that began straight ahead and then moved randomly right or left at 20°/s. In trials 1-10, subjects simply followed the target (pursuit only (PO)). In trials 11-50, subjects followed the pursuit target until it reached 16° eccentricity at which time a 2cm number was presented for 1sec (eccentricity 34° right). The subject followed the pursuit target until the number appeared, at which time the subject read the number aloud (pursuit-fixation (PF)). To inform the subject when and where the number appeared, a buzzer directly over the number would sound when the number was exposed. Head movements were monitored using a magnetic search coil. Results: During the PO trials, 1 subject showed a consistent head movement (mean = 5.3°) while 3 did not. Subjects who did not move the head in the PO trials did not move the head during the pursuit portion of the PF trials. The subject who showed consistent head movements during the PO trials had similar head movements during the pursuit portion of the initial PF trials, but as the number of PF trials increased the head movement during pursuit declined to about 1°. During the first PF trial, all subjects showed a rapid rightward head movement of at least 5° when the number was presented. For all subjects, this rapid head movement declined by about 5° or more as the number of PF trials increased. Conclusions: Increasing head rotation during the pursuit portion of the rightward PF trials would reduce the need to make a rapid head movement to view the number. However, subjects did not increase their head movement during the pursuit portion of the PF trials, choosing instead to make a rapid head movement to fixate the number. Subjects may not be accustomed to head movement during pursuit or ocular pursuit may be disrupted by head movement. Once the subject had repeatedly localized the number, the rapid head movement declined demonstrating that when the eccentric target appears at a predictable location and time, the likelihood of a head movement when orienting toward the object is reduced.
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