Influence of direction and eccentricity on pro- and anti-saccade metrics
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The ability to process and respond to environmental cues requires the transformation of a sensory stimulus into an appropriate motor response, a process known as a sensorimotor transformation. The anti-saccade task can be used to investigate the ability of a subject to suppress a reflexive saccade towards a visual stimulus (pro-saccade) and generate a voluntary saccade 180° away from it. Additional steps are involved in the anti-saccade sensorimotor transformation that do not occur in the pro-saccade, which may produce performance differences between pro- and anti-saccade metrics. We were interested in exploring these differences to gain insight on the mechanism of the sensorimotor transformation of the anti-saccade and to uncover any directional biases in saccadic performance. Two experiments were performed, one in which stimuli were presented at 20 angular positions with a constant eccentricity of 12°, and another using 18 possible eccentricities along the horizontal. Pro-saccades had faster SRTs and velocities, larger amplitudes, higher accuracy and less variation in their trajectories than anti-saccades. Pro- and anti-saccade performance was shown to exhibit a similar dependence on both saccade goal direction and eccentricity. Differences manifested as a generalized reduction in anti-saccade performance that can be described as a scalar multiple of pro-saccade performance at all locations. Possible causes of this reduced performance were speculated to be i) the involvement of higher cortical structures, ii) errors in the internal representation of the stimulus, iii) sensorimotor coordinate transformation inaccuracy, and iv) online updating of the motor plan and the speed accuracy trade off inherent to saccades. The results of this study are comparable to previous monkey and human studies however certain differences were found that require further investigation. Further investigation is also required to determine the validity of the possible causes of performance reduction in the anti-saccade task and their specific contributions.