Redundant Visual Signals Boost Saccade Execution

Article excerpt

The redundant signal effect (RSE) refers to the fact that human beings react more quickly to a pair of stimuli than to only one stimulus. In previous studies of the RSE in the oculomotor system, bimodal signals have been used as the goal of the saccade. In consistency with studies using manual response times (RTs), saccadic RTs have been shown to be shorter for redundant multimodal stimuli than for single unimodal stimuli. In the present experiments, we extended these findings by demonstrating an RSE in the saccadic system elicited only by unimodal visual stimuli. In addition, we found that shorter saccadic RTs were accompanied by an increased saccadic peak velocity. The present results are of relevance for neurophysiological models of saccade execution, since the boost of saccades was elicited by two visual transients (acting as a "go" signal) that were presented not at the goal of the saccade but at various other locations.

When required to respond manually as quickly as possible to a stimulus onset, the human being delivers a faster response when two stimuli are presented than when only one is presented. This phenomenon is known as the redundant signal effect (RSE; Todd, 1912). The response time (RT) advantage observed in the double-signal over the single-signal condition is also called redundancy gain, and has been documented for stimuli delivered in the same mode and in different modalities (see, e.g., Miller, 1982, 1986; Mordkoff & Yantis, 1991). The redundancy gain can be explained according to either statistical facilitation (Raab, 1962) or neural coactivation (Miller, 1982), with the former postulating a horse race between stimuli for response activation and the latter hypothesizing that stimuli are pooled together before the response is emitted. To distinguish between these two possibilities, Miller (1982) devised a mathematical method known as the horse-race inequality. If the inequality is not violated, probability summation is sufficient to explain the redundancy gain; in contrast, if a violation is observed, some sort of neural coactivation should be assumed to have occurred.

Although most of the RSE studies have recorded manual RTs (Miller & Reynolds, 2003), a few have addressed the RSE using saccadic responses (Arndt & Colonius, 2003; Colonius & Arndt, 2001; Hughes, Reuter-Lorenz, Nozawa, & Fendrich, 1994). Notably, whereas manual RT studies have demonstrated the redundancy gain for both visual and multimodal stimuli, in those in which saccadic RTs were recorded only multimodal stimuli were used. Typically, in the latter studies the redundant signal condition consists of auditory and visual stimuli presented together at the same location (corresponding to the saccadic goal), whereas the single-signal condition consists of a unimodal stimulus. The results consistently showed shorter saccadic latencies on double- than on single-signal trials (see, e.g., Hughes et al., 1994).

Quite surprisingly, what has remained unexplored is whether or not redundant unimodal visual stimuli can produce an RSE in saccadic RTs. Hence, the aim of the present study was to address whether or not the execution of a visually guided endogenous saccade toward a prespecified target can be speeded up by a "go" signal consisting of two visual transients instead of only one, with the visual transients appearing at nontarget locations.

EXPERIMENT 1

The only evidence that unimodal visual stimuli can elicit an RSE comes from studies in which manual RTs were recorded (see, e.g., Miller, 2004; Miller & Reynolds, 2003). In these studies, the participants' task was to press a button as soon as they detected the occurrence of a visual transient ("go" signal). Note that the motor response in this task is not directed toward the "go" signal. Hence, to determine whether or not unimodal visual stimuli can elicit an RSE in the oculomotor system, we presented visual transients that did not coincide with the saccadic goal. …