Abstract
Purpose: During smooth pursuit of a compound stimulus (2 concentric rings, one 0.8 deg, the other 8 deg), the latency of saccades to a 1.5 deg perturbation is shorter when the spatial scale of attention is narrow (directed to small ring) than when it is broad (Madelain, Krauzlis & Wallman, Society for Neuroscience, 2001). A possible explanation for this attentional effect is that a narrow field of attention differentially increases the saliency of the small ring, thereby reducing its spatial uncertainty and leading to corrective saccades more quickly because the position error is detected more quickly. If this is so, increasing the contrast of the inner ring should have the same effect as attending to it.
Methods: On each trial the compound stimulus moved at 10 deg/sec until, at some point, its path was perturbed forward or backward by 1.5 deg, and, simultaneously, the number of radial segments in each ring briefly changed. Three subjects reported the number of segments in either the small or large ring, according to an auditory cue before each block of 24 trials. We measured the saccade latencies in response to the perturbation on 1344 trials for each subject.
Results: Catch-up saccades occurred much later when attending to the large ring than to the small, whether the contrast of the inner ring was 0.2, 0.4 or 0.8 (mean latency: large ring, 437, 427, 436 msec, respectively; small ring, 175,165, 161 msec).
Conclusions: Our results show that saccade latencies are shortened much more by attending to the small ring than by increasing its contrast. Therefore the shorter latencies when attention is narrowed are not due to attentional modulation of saliency. As an alternative explanation, we propose that short-latency corrective saccades are generated when the target leaves the spotlight of attention; with our perturbations this would have occurred only when the scale of attention was narrow.