Abstract
When participants reach for a target, their hand can adjust to a change in target position that occurs while their eyes are in motion (the hand’s automatic pilot) even though they are not aware of the target’s displacement (saccadic suppression of perceptual experience). However, previous studies of this effect have displayed the target without interruption, such that the new target position remains visible during the fixation that follows the saccade. Here we test whether a change in target position that begins and ends during the saccade can be used to update aiming movements. We also ask whether such information can be acquired from two targets at a time. The results showed that participants responded to single and double target jumps even when these targets were extinguished prior to saccade termination. The results imply that the hand’s automatic pilot is updated with new visual information even when the eye is in motion.
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Notes
Even when jumps are perceptually accessible and participants are instructed to stop their movement (Pisella et al. 2000; Cressman et al. 2006) or to point in the opposite direction (Day and Lyon 2000) if the target moves, people fail to inhibit a deviation toward the displaced target. Recent evidence from our laboratory does suggest, however, that automaticity can be reduced by instructions that downplay the relevance of the target jump to the task, such as when people are told to ignore the jump (Cameron et al. 2009).
We note that this is a higher percentage of jump trials (50%) than was employed by Pisella et al. (2000) (20%) and closer to the percentage employed by Prablanc and Martin (1992) (66%). Unlike Pisella et al.’s experiment, the target perturbation in our experiment was paired with the saccade, which removes awareness of the jump. In this way, our study is more similar to Prablanc and Martin’s (1992). We would expect this lack of awareness to prevent any influence of the proportion of jump trials on a participant’s explicit strategy (though there is always the possibility that the nature and frequency of target perturbations might have an implicit effect on a participant’s performance).
In Experiment 1, average saccade MT was 72.9 ms (SD: 8.16 ms). Average time of the jump was 35.4 ms (SD: 7.25 ms) after the start of the saccade. Therefore, on average, the target disappeared 55.4 ms into the saccade, or 17.5 ms prior to the completion of the saccade.
We note that EOG was sufficient for Prablanc and Martin (1992) to detect an influence of their larger target jump on saccade amplitude, suggesting that this methodology does not preclude detection of an online response when such a response is present, though it may only detect the effect when the jump is large.
Two kinds of forward jump were included in an attempt to match the backward jump in both the initial target position (position 4) and the region traversed by the jump (between positions 3 and 4).
Given the high velocity of the eyes during a saccade, however, one would certainly expect post-saccadic information to be more reliable.
These are (1) suppression of a flashed stimulus, (2) suppression of continuous motion, and (3) suppression of a discrete displacement (i.e., a jump) (MacAskill et al. 2003).
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Acknowledgments
This research was supported by a scholarship from the Natural Sciences and Engineering Research Council of Canada (NSERC) awarded to B.C. and by a NSERC Discovery Grant awarded to R.C. We thank Dr. James Danckert and an anonymous reviewer for helpful comments on the manuscript.
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Cameron, B.D., Enns, J.T., Franks, I.M. et al. The hand’s automatic pilot can update visual information while the eye is in motion. Exp Brain Res 195, 445–454 (2009). https://doi.org/10.1007/s00221-009-1812-7
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DOI: https://doi.org/10.1007/s00221-009-1812-7