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W. Payne, C. Harris; A Contingent CRT Display With Fixation Monitoring for Saccade Amplitude Adaptive Control Experiments . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2489.
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© ARVO (1962-2015); The Authors (2016-present)
Investigating the rapid adaptation of saccade amplitude requires an intra–saccadic stimulus change to be performed during the ongoing saccade. We have recently developed a computer controlled saccade contingent Cathode Ray Tube (CRT) display, integrating the Cambridge Research Systems ViSaGe visual stimulus generator and Skalar IRIS infra–red limbus tracker. This system enables us to exercise precise control over the stimulus with a well–characterised latency jitter.
The system is calibrated using both prior and in–trial calibration. In order to increase confidence in the fixations used for in–trial calibration, we use the saccade target in a simple 4–Alternative–Forced–Choice (4AFC) visual discrimination task; requiring the foveation of the target for a correct response. This enables us to monitor fixation accuracy and, in combination with auditory feedback and verbal encouragement, to control subject attention throughout the course of the experiment. The 4AFC task target consists of a 0.15 degree arrow–head, pointing in one of 4 directions. As this target is not visible unless foveated, it is drawn on a disk 0.3 degrees in diameter, allowing it to act as the target for reactive saccades. A simple pro–saccade task was used to elicit the eye movements which controlled the saccade contingent 167 Hz CRT display. A video–frame synchronized digital signal processing system was used to sample, filter and differentiate the eye movement data, which was then evaluated against a simple velocity threshold. Upon saccade detection, the stimulus display was modified according to the experimental protocol.
Of the 6 subjects tested, all showed some degree of amplitude adaptation.
This study shows that not only will this technology allow a visual error signal to be specified with a high degree of flexibility and control over it’s temporal, spatial, colour and luminance characteristics; It will also permit the use of sophisticated techniques which improve confidence in the results of experiments which use this equipment.
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