Abstract
purpose. To determine if multiple states for the initiation of pursuit, as
assessed by acceleration in the “open-loop” period, can be learned
and gated by context.
methods. Four normal subjects were studied. A modified step-ramp paradigm for
horizontal pursuit was used to induce adaptation. In an increasing paradigm, target velocity doubled 230 msec
after onset; in a decreasing paradigm, it was halved. In
the first experiment, vertical eye position (±5°) was used as the
context cue, and the training paradigm (increasing or decreasing)
changed with vertical eye position. In the second experiment, with
vertical position constant, when the target was red, training was
decreasing, and when green, increasing. The average eye acceleration in
the first 100 msec of tracking was the index of open-loop pursuit
performance.
results. With vertical position as the cue, pursuit adaptation differed between
up and down gaze. In some cases, the direction of adaptation was in
exact accord with the training stimuli. In others, acceleration
increased or decreased for both up and down gaze but always in correct
relative proportion to the training stimuli. In contrast, multiple
adaptive states were not induced with color as the cue.
conclusions. Multiple values for the relationship between the average eye
acceleration during the initiation of pursuit and target velocity could
be learned and gated by context. Vertical position was an effective
contextual cue but not target color, implying that useful contextual
cues must be similar to those occurring naturally, for example, orbital
position with eye muscle weakness.
When primates track a small object moving across their field of
view, they use pursuit eye movements to stabilize the image of the
object upon the fovea. Because slippage of images across the retina is
an important stimulus that drives the pursuit system, which, in turn,
generates eye movements to reduce the magnitude of this slip, pursuit
is regarded as a visual feedback (“closed-loop”) control system.
However, there are inherent delays in the processing of visual
information necessary to produce a pursuit command, so that whenever a
target of interest changes its speed or direction unpredictably, the
pursuit system must operate in an “open-loop” mode for roughly 130
msec,
1 2 without the benefit of immediate feedback.
Consequently, as is the case for other open-loop ocular motor
subsystems such as saccades
3 4 and the vestibulo-ocular
reflex,
5 the open-loop period of pursuit must be
accessible to long-term calibration so that the speed of the eyes is
brought to that of the target as quickly as possible and kept there
without any motor instability or oscillations that would interfere with
visual acuity. These considerations predict an adaptive capability for
calibrating the open-loop, initial portion of the pursuit tracking
response, and indeed such has been demonstrated in both monkeys and
humans.
6 7 8 9 10 11 12 Typically, the average acceleration of the
eye in the first 100 to 130 msec of pursuit tracking was used as a
measure of the open-loop response of the pursuit system and was shown
to be modifiable in various learning paradigms. Recently, for the
vestibular, saccadic, and vergence systems it has been shown that more
than one adaptive eye movement response can be learned and gated in or
out depending on context.
13 14 15 16 17 18 19 20 21 Here, we investigated
context-specific adaptation of pursuit, looking at the influence of a
nonvisual cue, vertical eye position, and a visual cue, target color.