Six Degrees of Auditory Spatial Separation

Abstract

The location of a sound is derived computationally from acoustical cues rather than being inherent in the topography of the input signal, as in vision. Since Lord Rayleigh, the descriptions of that representation have swung between Blabeled line^and Bopponent process^models. Employing a simple variant of a twopoint separation judgment using concurrent speech sounds, we found that spatial discrimination thresholds changed nonmonotonically as a function of the overall separation. Rather than increasing with separation, spatial discrimination thresholds first declined as two-point separation increased before reaching a turning point and increasing thereafter with further separation. This Bdipper^function, with a minimum at 6°of separation, was seen for regions around the midline as well as for more lateral regions (30 and 45°). The discrimination thresholds for the binaural localization cues were linear over the same range, so these cannot explain the shape of these functions. These data and a simple computational model indicate that the perception of auditory space involves a local code or multichannel mapping emerging subsequent to the binaural cue coding.

Multisensory stimuli improve relative localisation judgments compared to unisensory auditory or visual stimuli

“…Both auditory (Mills, 1958; Makous and Middlebrooks, 1990; Charbonneau et al , 2013; Wood and Bizley, 2015; Carlile et al , 2016) and visual localisation acuity declines with eccentricity (Mateeff and Gourevich, 1984; Perrott et al , 1993; Charbonneau et al , 2013). Few studies have attempted to directly compare spatial acuity for auditory and visual stimuli throughout the visual field and focus instead on the spatial capture observed when spatially separated auditory-visual signals are presented (Howard and Templeton, 1966; Bertelson and Radeau, 1981).…”

Neurons in primary auditory cortex represent sound source location in a cue-invariant manner

“…This was consistent with the idea that averaging heterogeneous spatial receptive fields leads to loss of information 36,37,48 . Increasing the number of channels past ~20 did not substantially improve population decoding performance, suggesting that there may be an upper limit on spatial resolution for absolute localisation of auditory stimuli in the cortex, as has been suggested in humans 51 .…”

“…Previous work has suggested that spatial representations in the auditory cortex may comprise three channels 49,50 , or many channels with 6° widths tiling space 51 . In order to investigate these ideas further, we modified our two-channel decoder to compare decoding performance of neural populations divided into N channels according to spatial tuning observed in response to BBN stimuli.…”

“…We tested a new model based on the two-channel decoder with N channels containing similarly tuned neurons. This design allowed us to address the possibility that there may be more than two channels 49–51 , and assess the impact of within-channel averaging on model performance. Thus, we made a version of the population decoder that compared channels of small populations of similarly tuned units that were summed together.…”

Multisensory stimuli improve relative localisation judgments compared to unisensory auditory or visual stimuli

“…Both auditory(Mills, 1958; Makous and Middlebrooks, 1990; Charbonneau et al, 2013; Wood and Bizley, 2015; Carlile et al, 2016) and visual localisation acuity declines with eccentricity (Mateeff and Gourevich, 1984; Perrott et al, 1993; Charbonneau et al, 2013; Carlile et al, 2016). Few previous studies have attempted to directly compare spatial acuity for auditory and visual stimuli throughout the visual field and focus instead on the spatial capture observed when spatially separated auditory-visual signals are presented (Howard and Templeton, 1966; Bertelson and Radeau, 1981).…”