Properties of Saccadic Responses in Monkey When Multiple Competing Visual Stimuli Are Present

Abstract

Important insights into the neural organization of the saccadic system have been gained when the usually stereotyped movement trajectories of saccades have been altered by experimental manipulation. In the present study we produced trajectory variability in monkeys by using a visual search task in which both the location and color of an odd-colored target were changed randomly trial by trial, and the number of distractors was varied on each trial. We wished to determine whether increasing the number of distractors also increased the movement trajectory variation, i.e., the amount of initial directional deviation, endpoint deviation (averaging), and curvature of saccades. Overall, saccade latencies and the proportion of saccades directed to distractors decreased as the number of homogenous distractors increased. We also found that saccades have much more dispersion in their initial direction when distractors are present in comparison to the case when only a single target without distractors appears. However, initial dispersion decreases systematically as the number of distractors increases. The percentage of averaging saccades produced in the search task was not consistently dependent on the number of distractors. A significant fraction of averaging saccades still occurred for much wider spatial separations of stimuli than in previous studies using two visual stimuli with no specified target. The curvature of saccade trajectories increased dramatically when distractors were present, but the amount of curvature was not systematically affected by the number of distractors. Errors present in saccade trajectory in the search task were only poorly compensated. We conclude that these variable saccade trajectories result from incomplete or inaccurate specification of the target when competing stimuli are present and that a smaller number of more widely spread distractors facilitate saccade variability, perhaps due to the greater difficulty of target selection.

Pinna Cues Determine Orienting Response Modes to Synchronous Sounds in Elevation

“…The different response modes in our experiments are quite reminiscent of previous reports from visuomotor experiments, in which two visual targets evoked saccadic eye movements (Becker and Jürgens, 1979;Findlay, 1982;Ottes et al, 1984Ottes et al, , 1985Chou et al, 1999;Aitsebaomo and Bedell, 2000;Watanabe, 2001;Arai et al, 2004;Nelson and Hughes, 2007). Those studies revealed that the visuomotor system typically responds with averaging saccades when stimuli are presented in spatial-temporal proximity, and when the response reaction time is fast.…”

“…Second, in visuomotor experiments, visual stimuli were well separable, both at the retina, and in early sensory responses of neurons within the visuomotor pathways. This holds also for example for the midbrain superior colliculus (SC), a crucial sensorimotor interface for the programming and generation of eyehead orienting responses (Arai et al, 2004;Kim and Basso, 2008). The actual visual response selection leading to either averaging or bimodal responses is therefore attributable to neural processing, rather than to visual peripheral limitations.…”

“…Competition between different populations, combined with local-excitatory/global-inhibitory interactions, shapes the population that represents the saccade goal. Task constraints and stimulus saliency help favor neurons that represent the target to win this competition, yet also averaging may be the result of this competition (van Opstal and Van Gisbergen, 1990;Glimcher and Sparks, 1993;Arai et al, 2004;Kim and Basso, 2008).…”

“…Target selection has been addressed extensively in the visuomotor literature (Becker and Jü rgens, 1979;Findlay, 1982;Ottes et al, 1984Ottes et al, , 1985Chou et al, 1999;Aitsebaomo and Bedell, 2000;Watanabe, 2001;Arai et al, 2004;Nelson and Hughes, 2007). When two visual targets are presented in spatial-temporal proximity, saccadic eye movements often terminate between the two target locations (averaging).…”

A test of spatial temporal decoding mechanisms in the superior colliculus

“…The location of the visual target presented in relation to the evoked stimulation vector was loosely chosen to achieve distributions of distances in SC coordinates similar to those collected in experiment 1. Stimulation-evoked saccades that interacted with the visual target showed no obvious signs of curvature (saccades directed first toward one target and then toward the other in midflight; Arai et al 2004;Port and Wurtz 2003) and reflected a weighted combination of the visual and stimulation-evoked saccades; only this subset of movements was analyzed for the purpose of this study. Saccades observed on other trials clearly resembled pure stimulation movements (stimulation onset occurred well before the visually guided saccade), curved saccades (stimulation onset occurred during the visually guided saccade), or pure visual movements (stimulation onset occurred after the visually guided saccade) (McPeek et al 2003;Noto and Gnadt 2009) and were excluded from additional analyses.…”