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Keywords:

  • hearing;
  • sound localization;
  • auditory space perception;
  • visual deprivation;
  • blindness;
  • auditory compensation;
  • cross-modal plasticity

Abstract

The ability to localize sounds in azimuth was tested in five cats that had been binocularly deprived of vision from birth for several months and in three normal age-matched controls. Brief tone bursts were presented in an eight-choice apparatus along 360° of the azimuthal plane at constant elevation. Using positive reinforcement techniques, the cats were trained to walk from the centre of the 3 m diameter circular enclosure to the hidden loudspeakers. The distribution of sound localization error from 55 trials per cat at each speaker position was measured, and its standard deviation was used to assess the precision of sound localization. All cats localized tones straight ahead of them most precisely; performance at lateral and rear positions was gradually less precise. When the sound localization ability of normal and binocularly deprived cats was compared across speakers, a significantly enhanced precision was found for binocularly deprived cats overall (P < 0.002; two-way analysis of variance). An improvement was found at each individual speaker position, but it was greatest at lateral and rear positions. In two sets of control experiments normal cats were retested (i) in the dark with the aid of an infrared camera and (ii) after 3 months of binocular lid suture. Normal cats in the dark did not show any differences in their sound localization behaviour. Late-deprived cats showed a tendency for better performance, which fell short of statistical significance. Our results in visually deprived cats agree well with some reports on the sound localization ability of blind humans, but disagree with others. Our data provide support for a hypothesis of compensatory plasticity, in which sensory functions get sharpened with the loss of another modality. They seem to rule out the necessity for vision to play a role in the postnatal calibration of auditory space.