Specificity of binaural perceptual learning for amplitude modulated tones: a comparison of two training methods

The specificity of auditory perceptual learning has been taken as an indicator of the likely locus within the brain at which underlying neuronal changes occur. This study examined interaural level difference (ILD) discrimination learning with sinusoidally amplitude modulated (SAM) tones and whether training-induced threshold improvements generalize from one side of auditory space to the other and to an untrained carrier frequency. A novel, dual-staircase adaptive method was adopted that was designed to prevent participants from identifying the nature of the adaptive track. ILD thresholds obtained with this method were compared with a constant-stimulus technique using otherwise identical stimuli. Adaptive thresholds derived from psychometric functions were found to be biased compared to those obtained from reversals. Although adaptive and constant-stimulus procedures appeared to yield different temporal patterns of learning, no global differences were found between them in terms of training outcomes. These data show that ILD discrimination learning with SAM tones does generalize to an untrained carrier frequency but does not generalize across the midline. This implies that the neural substrate for binaural plasticity is found at a relatively high level of the auditory pathway where information is combined across frequency and where each side of auditory space is represented separately.