Irrelevant auditory and tactile signals, but not visual signals, interact with the target onset and modulate saccade latencies

Multisensory temporal binding induces an illusory gap/overlap that reduces the expected audiovisual interactions on saccades but not manual responses

Throughout the day, humans react to multisensory events conveying both visual and auditory signals by rapidly reorienting their gaze. Several studies showed that sounds can impact the latency of visually guided saccades depending on when and where they are delivered. We found that unlocalized beeps delivered near the onset time of a visual target reduce latencies, more for early beeps and less for late beeps, however, this modulation is far weaker than for perceptual temporal judgments. Here we tested our previous assumption that beeps shift the perceived timing of target onset and result in two competing effects on saccade latencies: a multisensory modulation in line with the expected perceptual effect and an illusory gap/overlap effect, resulting from target appearance being perceived later/closer in time than fixation offset and shortening/lengthening saccade latencies. Gap/overlap effects involve an oculomotor component associated with neuronal activity in the superior colliculus (SC), a multisensory subcortical structure devoted to sensory-motor transformation. We therefore predicted that the interfering illusory gap/overlap effect would be weaker for manual responses, which involve distinct multisensory areas. In three experiments we manipulated the delay between target onset and an irrelevant auditory beep (stimulus onset asynchrony; SOA) and between target onset and fixation offset (real gap/overlap). Targets appeared left/right of fixation and participants were instructed to make quick saccades or button presses towards the targets. Adding a real overlap/gap (50% of SOA) compensated for the illusory gap/overlap by increasing the beep-related modulation of saccade latencies across the entire SOA range, whereas it barely affected manual responses. However, although auditory and gap/overlap effects modulated saccade latencies in similar ways, these were additive and could saturate, suggesting that they reflect independent mechanisms. Therefore, multisensory temporal binding affects perception and oculomotor control differently, likely due to the implication of the SC in saccade programming and multisensory integration.

Cross-modal perceptual enhancement of unisensory targets is uni-directional and does not affect temporal expectations

Temporal structures in the environment can shape temporal expectations (TE); and previous studies demonstrated that TEs interact with multisensory interplay (MSI) when multisensory stimuli are presented synchronously. Here, we tested whether other types of MSI - evoked by asynchronous yet temporally flanking irrelevant stimuli - result in similar performance patterns. To this end, we presented sequences of 12 stimuli (10 Hz) which consisted of auditory (A), visual (V) or alternating auditory-visual stimuli (e.g. A-V-A-V-…) with either auditory or visual targets (Exp. 1). Participants discriminated target frequencies (auditory pitch or visual spatial frequency) embedded in these sequences. To test effects of TE, the proportion of early and late temporal target positions was manipulated run-wise. Performance for unisensory targets was affected by temporally flanking distractors, with auditory temporal flankers selectively improving visual target perception (Exp. 1). However, no effect of temporal expectation was observed. Control experiments (Exp. 2-3) tested whether this lack of TE effect was due to the higher presentation frequency in Exp. 1 relative to previous experiments. Importantly, even at higher stimulation frequencies redundant multisensory targets (Exp. 2-3) reliably modulated TEs. Together, our results indicate that visual target detection was enhanced by MSI. However, this cross-modal enhancement - in contrast to the redundant target effect - was still insufficient to generate TEs. We posit that unisensory target representations were either instable or insufficient for the generation of TEs while less demanding MSI still occurred; highlighting the need for robust stimulus representations when generating temporal expectations.

Top-down control of saccades requires inhibition of suddenly appearing stimuli

Humans scan their visual environment using saccade eye movements. Where we look is influenced by bottom-up salience and top-down factors, like value. For reactive saccades in response to suddenly appearing stimuli, it has been shown that short-latency saccades are biased towards salience, and that top-down control increases with increasing latency. Here, we show, in a series of six experiments, that this transition towards top-down control is not determined by the time it takes to integrate value information into the saccade plan, but by the time it takes to inhibit suddenly appearing salient stimuli. Participants made consecutive saccades to three fixation crosses and a vertical bar consisting of a high-salient and a rewarded low-salient region. Endpoints on the bar were biased towards salience whenever it appeared or reappeared shortly before the last saccade was initiated. This was also true when the eye movement was already planned. When the location of the suddenly appearing salient region was predictable, saccades were aimed in the opposite direction to nullify this sudden onset effect. Successfully inhibiting salience, however, could only be achieved by previewing the target. These findings highlight the importance of inhibition for top-down eye-movement control.