In a crowded scene we can effectively focus our attention on a specific speaker while largely ignoring sensory inputs from other speakers. How attended speech inputs are extracted from similar competing information has been primarily studied in the auditory domain. Here we examined the deployment of visuo-spatial attention in multiple speaker scenarios. Steady-state visual evoked potentials (SSVEP) were monitored as a real-time index of visual attention towards three competing speakers. Participants were instructed to detect a target syllable by the center speaker and ignore syllables from two flanking speakers. The study incorporated interference trials (syllables from three speakers), no-interference trials (syllable from center speaker only), and periods without speech stimulation in which static faces were presented. An enhancement of flanking speaker induced SSVEP was found 70-220 ms after sound onset over left temporal scalp during interference trials. This enhancement was negatively correlated with the behavioral performance of participants -- those who showed largest enhancements had the worst speech recognition performance. Additionally, poorly performing participants exhibited enhanced flanking speaker induced SSVEP over visual scalp during periods without speech stimulation. The present study provides neurophysiologic evidence that the deployment of visuo-spatial attention to flanking speakers interferes with the recognition of multisensory speech signals under noisy environmental conditions.
