[Abstract Title]. - Society for Neuroscience

Program#/Poster#: 263.12/DD12
Topic: D.06.c. Eye and head control
Support: MRC
Title: Evidence for sampling and storage of visual motion information in the predictive control
of head and eye during head-free pursuit
Authors: *R. ACKERLEY, S. COLLINS, G. R. BARNES;
Fac. of Life Sci., Univ. of Manchester, Manchester, United Kingdom
Abstract: During head-free pursuit of a target, both the eyes and head move together using
visual feedback and internal (extra-retinal) mechanisms. This necessitates that the vestibuloocular
response is suppressed so that gaze (head + eye) and head move in the same direction. If a
moving target is extinguished, the continuation of any movement must be controlled by internal
mechanisms alone. Experiments were conducted to investigate the use of internal representations
in reproducing target motion. After a period of fixation, the target moved with a randomized
step-ramp horizontal motion (10-40 deg/sec, left or right) and subjects were instructed to follow
the target with both eyes and head. In control trials the target was illuminated continuously. In a
second set of trials (mid-ramp extinction), the target was visible in the first part of the ramp and
then extinguished for 400-600ms before reappearing for the second part of the ramp. Some midramp
extinction catch trials (10%) were embedded in the control trials. In a third set of trials
step-ramp stimuli were presented in pairs of identical velocity. In the first of each pair (shortramp
extinction), target motion was equivalent to the first part of the mid-ramp extinction but the
second part (reappearance) was absent. Subjects tracked the target when visible and attempted to
continue following it when extinguished, predicting its future trajectory. In the second of each
pair (initial extinction), target motion was again equivalent to the mid-ramp extinction but the
target was visible in the second part only. Subjects were encouraged to initiate eye and head
movement so as to track the unseen target in anticipation of its appearance, using the extinction
of fixation as a „go‟ cue. In all tasks, subjects were able to continue smooth eye and head
movement even in the absence of visual input, a manifestation of the action of internal feedback.
Moreover, they were able to grade their gaze and head velocity on the basis of expected target
velocity. In both the mid-ramp extinction and catch trials, gaze and head velocity were sustained
throughout target extinction. In these conditions motion information was derived from the initial
part of the same trial and depended on expectation of target reappearance. For the initial
extinction, eye and head movements were initiated ~200ms after extinction of fixation and prior
to target appearance. In this case, grading of gaze and head velocity must have been derived from
the prior short-ramp. In both cases, results indicate that sampled velocity information can be used
to both generate and sustain eye and head responses when the target is not visible but when the
future target trajectory can be predicted.
Disclosures: R. Ackerley , None; S. Collins, None; G.R. Barnes, None.