Many studies have shown that blindness often results in enhanced auditory abilities (such as motion discrimination and pitch discrimination), thought to be supported by the positive effects of cross-modal neuroplasticity. However, a number of recent studies have demonstrated that blindness also results in degraded performance for certain auditory spatial tasks, such as vertical sound localization and spatial bisection. Although several hypotheses have been proposed to explain why some auditory abilities are enhanced while others are degraded, these are often limited to a specific subset of tasks, and a more comprehensive explanation is needed to account for why some auditory abilities are enhanced and others are degraded. This is illustrated by presenting the findings of two studies, that show that blindness enhances auditory distance discrimination (judging which of two sounds is closer), but worsens absolute distance perception (judging how far away a single sound is). Virtualization software was utilized to simulate sound sources presented at various distances within virtual rooms where the availability of level and direct-to-reverberant ratio (DRR) distance cues was experimentally manipulated. Participant groups were sighted controls, and fully blind individuals. All were audiometrically tested and had normal hearing. In the discrimination experiment, participants heard two 90-ms-duration broadband noises ranging from 1-8 m away, and reported which of the two sounds were closer. In the absolute distance experiment, participants heard static sound stimuli at virtual distances ranging from 1.2 to 13.8 m. Single speech, music, or noise sounds were presented, and participants estimated the distance of the sound source. The results showed that for all conditions, distance discrimination was significantly enhanced following blindness (p<0.05), suggesting that the ability to discriminate small changes in sounds is improved by blindness. However, absolute distance judgments were significantly worse in the blind group. Participants with visual losses were least accurate when estimating the distance of near sounds, increasing the likelihood of collision and falls due to misjudging the location of the sound source. This result suggests that auditory spatial representations of the world for blind listeners are compressed compared to those for normally sighted controls, and that visual information is needed to accurately calibrate internal representations of the auditory world. A framework is proposed comprising a set of nine principles that can be used to account for whether auditory abilities are enhanced or degraded.
Biography
Andrew Kolarik, School of Psychology, University of East Anglia, Norwich, United Kingdom.
Andrew Kolarik completed his BA and PhD in Cardiff University in 2006. He has held postdoctoral positions at several institutions, including the University of Cambridge and Anglia Ruskin University, and has recently taken up a Lectureship at the University of East Anglia. He specializes in spatial hearing, and the effects of sensory loss on auditory abilities.
