Auditory perceptual learning.

Abstract

An interesting phenomenon in science is how the discovery of new techniques, findings, or theoretical understanding can produce cyclical waves of interest in a research area. Such is the case for perceptual learning, which is currently enjoying a renaissance following previous periods of excitement, some 50 and 100 years ago (Meyer 1899; Seashore et al. 1908; Gibson 1953). In this issue, Karmarkar and Buonomano (2003) energize this resurgence with exciting work on auditory perceptual learning. There seem to be two converging reasons for the revival of interest in this field. The first derives from discoveries in neuroscience of plasticity in the mature brain (for review, see Calford 2002). Initially prompted by findings that lesions in the peripheral somatosensory system led to remapping of the lesioned limb or digit in the cortex, this field evolved in several directions, including studies of how intensive, specific training could also lead to remapping in the cortex. The second reason for the current interest in perceptual learning derives from an increased understanding of the nature of learning difficulties, particularly in children. In the mid-1990s, neuroscientists latched on to a stream of research in developmental psychology showing that some children with language-based learning impairments (LLIs), such as dyslexia, had poor visual and auditory temporal processing abilities. Since then, there has been a huge surge of activity (for review, see Ramus 2001) showing that children with LLI have a wider range of difficulties processing sensory stimuli. These difficulties typically involve multiple sensory, cognitive, and motor systems. But most controversial, and potentially most significant from an applied perspective, has been the finding that training based on the principles of perceptual learning can effectively treat these processing problems and the LLI that they are associated with (Merzenich et al. 1996; Tallal et al. 1996; Kujala et al. 2001). Against this background, Karmarkar and Buonomano (2003) present data on the learning of an auditory interval discrimination task. During training, adult listeners had to decide whether a test pair of tones were separated by a shorter or longer interval than a target pair presented at the onset of a trial block. The frequency of the tones and the target interval were varied between listeners. All listeners were tested on a battery of similar stimuli at both trained and untrained frequencies and target durations before and after 10 d of training. For listeners who learned, the results showed generalization of training across tone frequency but not across target interval. Generalization of training to a tone-duration discrimination task was also observed. This suggested that training for auditory temporal information occurs in centralized brain circuits that are accessed across frequency channels. One of the goals of Karmarkar and Buonomano’s work was to ensure that enhanced performance resulted from improved timing per se, rather than from an enhanced ability to store and/or compare a standard and a comparison stimulus. To achieve this, they allowed the participant to hear the standard several times only at the beginning of a block, and on each trial, participants were presented only with the comparison stimulus. As it seems unlikely that the relatively few presentations of the standard were sufficient to develop a concept of the standard, the authors interpreted the improved performance as indicating that the participants formed a dynamic representation of the time frame, and adjusted this timing based on the feedback from their decisions. However, their results can be interpreted as showing an enhanced ability to store and/or compare stimuli from trial to trial. A strategy of comparing the current interval with the previous one (for which the correct response was known) would have worked well, as trials that were long relative to the standard were also longer than an immediately preceding short trial, and vice versa. Using this strategy, the participant’s ability to discriminate successive intervals would be improved by training, but the learning would be confined to intervals around the trained interval, since generalization to other intervals was not observed. This hypothesis could be tested by examining generalization to a standard closer to the trained standard (e.g., one threshold away). Alternatively, trial by trial comparisons could be controlled as an independent variable by use of different presentation methods such as a conventional, two-interval task, or the method of constant stimuli. This fascinating study addresses several other research themes that are central to an understanding of perceptual learning and its application. A major one is generalization. The authors focused on the implications of the pattern of learning generalization that they observed for the locus of the learning. Thus, learning across frequency suggested that the learning occurred outside of the core auditory pathway, Corresponding author. E-MAIL David.Moore@ihr.mrc.ac.uk; FAX 44 115 9518503. Article and publication are at http://www.learnmem.org/cgi/doi/ 10.1101/lm.59703.