Speech intelligibility reduces over distance from an attended location: evidence for an auditory spatial gradient of attention.

Speech reception thresholds (SRTs) were measured at a central focus of attention and at 20 degrees, 40 degrees, and 60 degrees locations distant in azimuth. Measurements were taken with one target collocated with two maskers, or with maskers flanking the target by +/-20 degrees. For 80% of trials, the target was played from the attended location ("expected"), and 20% came from another ("unexpected") location. For collocated stimuli, SRTs worsened with increasing distance from the expected location by 2.1 dB over the 60 degrees azimuth and by 5.1 dB for spatially separated target and maskers. In spatially separated conditions, a 2.9-dB change was still found when gaze was away from the attended location. Spatial attention appears to increase speech intelligibility against interferers, with gain decreasing with distance from the focus of attention. Spatial release from masking (RFM) was only found for the attended location. Further experiments suggest that target location uncertainty prevented RFM at unattended locations.