Active head movements contribute to spatial updating across gaze shifts

Abstract

Keeping visual space constant across movements of the eye and head is a not yet fully understood feature of perception. To understand the mechanisms that update the internal coordinates of space, research has mostly focused on eye movements. However, in natural vision, head movements are an integral part of gaze shifts that enlarge the field of vision. Here, we directly compared spatial updating for eye and head movements. In a virtual reality environment, participants had to localize the position of a stimulus across the execution of a gaze shift. We found that performing head movements increased the accuracy of spatial localization. By manipulating the speed of the visual scene displacement that a head movement produced, we found that spatial updating takes into account the sensorimotor contingencies of vision. When we presented gaze-contingent visual motion, subjects overestimated the position of stimuli presented across gaze shifts. The overestimation decreased if subjects were allowed to perform eye movements during the head movement. We conclude that head movements contribute to stabilizing visual space across gaze shifts and that contingencies of head movements, rather than being cancelled, facilitate the updating.

Keywords: eye movement; head movement; self-motion; space constancy.

Figure 1.

Illustration of the experimental setup. Participants performed either eye-only or eye-head gaze shifts from the fixation cross (FC) to the gaze shift target (GT). Participants had to judge the position of the comparison stimulus (C), presented after the gaze shift, against the probe stimulus (P), presented prior to the gaze shift. Please note, the displayed grating was only visible during the gaze shift and not during the presentation of the stimuli.

Figure 2.

(a) Average head movement main sequence, illustrating the linear relationship (y = 13.22 + 1.27x) between head movement amplitude and peak velocity. Data were collapsed across trials and participants in the first experiment. (b) Gaze shift traces for the head movement (shown in red) and the eye movement component (shown in blue) from an example trial in which the participant performed an unrestricted (left) and sequential gaze shift (right). The dashed lines represent the position of the fixation cross and gaze shift target on the horizontal axis. The eye moves to the target first, followed by the rotation of the head. (c) Psychometric functions of two representative participants for the stationary target - grating condition split by the eye movement restriction during the head movement.

Figure 3.

(a) Duration of the fixation of the gaze shift target during the execution of the head movement component of the gaze shift for the stationary target - grating (SG), stationary target - grey background (SN), flashed - grating (FG) and flashed - grey background condition (FN) split by the eye movement restriction during the head movement. (b) Saccade and head movement amplitudes for the individual conditions. The black dashed line represents the position of the gaze shift target (GT). Triangles connected by colored dashed lines represent the amplitudes of the eye movements, whereas circles with solid lines represent the amplitudes of head movements. Same conventions as in (a). (c) PHAs for the individual conditions. The dashed line represents the position of the probe (PP). Same conventions as in (a).

Figure 4.

(a) Saccade amplitudes for the stationary target - grating (SG), stationary target - grey background (SN), flashed - grating (FG) and flashed - grey background condition (FN) of the first and second experiment split by color. The dashed line represents the position of the gaze shift target (GT). (b) PHAs for the individual conditions of the first and second experiment. The dashed line represents the position of the probe (PP). Same conventions as in (a).

Figure 5.

(a) Duration of the fixation of the gaze shift target during the execution of the head movement component of the gaze shift for the individual visual velocity gains split by the eye movement restriction during the head movement. (b) Saccade and head movement amplitudes for the individual visual velocity gains. The dashed line represents the position of the gaze shift target (GT). Same conventions as in (a). (c) PHAs for the individual visual velocity gains. The dashed line represents the position of the probe (PP). Same conventions as in (a).