Rapid integration of face detection and task set in visually guided reaching

The superior colliculus (SC) has been increasingly implicated in the rapid processing of evolutionarily relevant visual stimuli like faces, but the behavioural relevance of such processing is not clear. The SC has also been implicated in the generation of upper-limb Express Visuomotor Responses (EVRs) on upper limb muscles, which are very short-latency (within [~]80 ms) bursts of muscle activity time-locked to visual target presentation. This reasoning led us to investigate the influence of faces on EVRs. We recorded upper limb muscle activity from young healthy participants as they reached toward left or right targets in the presence of a distractor stimulus presented on the opposite side. Across blocks of trials, we varied the instruction as to which stimulus served as the target or distractor. Doing so allowed us to assess the impact of instruction on muscle recruitment by examining trials when the exact same stimuli required a reach to either the left or right. We found that EVRs were uniquely modulated in tasks involving face selection, promoting reaches toward or away from faces depending on instruction. Follow-up experiments confirmed that this phenomenon required highly salient repeated faces, and was not observed to non-facial salient stimuli nor to faces expressing different affect. We conclude that our results attest to an integration of top-down task set and bottom-up feature detection to promote rapid motor responses to faces at latencies that match or precede the arrival of face information in human cortex. STATEMENT OF SIGNIFICANCEThe tecto-reticulo-spinal pathway is hypothesized to mediate the express visuomotor response (EVR). This study extends this hypothesis by demonstrating that face detection in the subcortex impacts low-latency movement via the EVR at latencies preceding cortical activity for face perception. To date, this constitutes the most direct evidence for direct behavioural relevance of rapid face detection in the brainstem. Further, we find that this response can be modulated by task context, allowing for different instruction-based responses given the exact same visual stimulus and implicating top-down cortical control of the EVR.