The focus of attention at the virtual cocktail party--electrophysiological evidence

The width of the attentional focus during the selection of one of two concurrent normal human participants was investigated using event-related potentials. Two stories were presented from virtual locations located 15° to the left and right azimuth by convolving the speech message by the appropriate head-related transfer function determined for each individual participant. Task irrelevant probe stimuli (phoneme/da/uttered by the same speaker as the story) were presented in rapid sequence from the same virtual locations. Occasionally, probes were presented at locations 15 or 30° lateral of the standard probes. Probes coinciding with the attended message gave rise to a fronto-central negativity relative to the phoneme probes coinciding with the unattended speech message. This was similar to the typical ERP attention effect. On the attended side probes deviating from the standard location by 30° elicited a different type of negative response, tentatively identified as a reorienting negativity, whereas probes deviating by 15° did not. These results are taken to suggest that spatial information is used for message selection in a cocktail-party situation but that the focus of spatial attention is relatively wide.