Can learning a frequency discrimination task occur without discrimination?

Following a recent report by Moore (Auditory learning: implications for speech perception. Proceedings of ISCA Workshop on Plasticity in Speech Perception [PSP2005], London, UK, 2005), showing in a frequency discrimination task that perceptual learning can occur in the absence of perceived stimuli, we further investigated the effects of training with identical stimuli (i.e. no perceptual difference between the stimuli) on frequency discrimination thresholds (DLF) and evaluated the influence of task demands on the degree of improvement following such training. A total of 20 adult normal-hearing participants were randomly assigned into three experimental groups: a "DLF-task" group, a "DLI-task" group, and a control group. For all groups, three DLF thresholds were obtained using an adaptive technique with feedback. For the two experimental groups, training was provided using two intervals of identical tone pairs. The participants in the "DLF-task" group were asked to select the tone with the higher pitch, whereas the participants in the "DLI-task" group, were instructed to select the louder of two tones. For the control group, DLF threshold-seeking tests were separated by two time intervals with no training. The results showed that: (1) training on a DLF task with identical stimuli can improve performance; (2) limited exposure to the stimuli and task in a group of naive listeners can already induce significant fast changes in performance; and (3) changing the task demands (i.e. to a DLI task) reduced improvement compared to the control group, which may suggest an interference with DLF learning. These results support the notion that perceptual learning can occur even in the absence of perceived stimuli. These findings shed light on the mechanisms that are involved in and influence auditory perceptual learning.