Abstract
Purpose: :
When an image containing spatial information is tilted, it induces more compensatory ocular torsion than an image without spatial information (Pansell, Sverkersten & Ygge, 2006). The aim of the present study was to investigate the effect of amplitude (of image rotation) and interval (image rotation frequency) on visually induced ocular torsion.
Methods: :
Ocular torsion was recorded in five healthy individuals using a head mounted video oculography system (Chronos vision, Germany). The stimulus consisted of an image showing a city scene rich in spatial orientation cues. Centered on the image was a fixation point which the subject was instructed to look at. The stimulus was displayed on a computer screen (1600x1200 @ 60Hz) at a distance of 50 cm. After ten seconds, the image started to rotate (clockwise) around the fixation point in steps of 3.75°, 7.5° or 15° at intervals of 1.5, 3 or 9 seconds. The image continued to rotate in steps for 60 seconds. The rotations had a smooth acceleration and deceleration phase and lasted one second. All subjects viewed all combinations of amplitudes and intervals in a randomized order. Between each of the nine conditions there was a washout period when nothing was displayed on the screen for 30 seconds.
Results: :
An immediate transitory torsional response was found for each stimulus rotation. The 15 degree amplitude induced a larger transient response (2.40± 0.16 deg) compared to the 7.5-degree (1.8±0.19 deg) and the 3.75-degree (1.38±0.12 deg) rotations. In the tests with short time intervals, the torsional position was accumulated over time, shifting the torsional position in the same direction as the stimuli (maximum 5 deg). In the tests with longer time intervals the torsional position returned towards the initial reference position before the next stimuli took appeared.
Conclusions: :
A larger stepwise rotation induced a larger torsional response. The time interval between rotations had little influence on the transient torsional response. However, a short time interval induced a shift of the torsional position, something which was not seen at longer time intervals.
Keywords: eye movements • space and scene perception