What are the letters of speech? Testing the role of phonological specification and phonetic similarity in perceptual learning

Recent studies on perceptual learning have indicated that listeners use some form of pre-lexical abstraction (an intermediate unit) between the acoustic input and lexical representations of words. Patterns of generalization of learning that can be observed with the perceptual learning paradigm have also been effectively examined for exploring the nature of these intermediate pre-lexical units. We here test whether perceptual learning generalizes to other sounds that share an underlying or a phonetic representation with the sounds based on which learning has taken place. This was achieved by exposing listeners to phonologically altered (tensified) plain (lax) stops in Korean (i.e., underlyingly plain stops are produced as tense due to a phonological process in Korean) with which listeners learned to recalibrate place of articulation in tensified plain stops. After the recalibration with tensified plain stops, Korean listeners generalized perceptual learning (1) to phonetically similar but underlyingly (phonemically) different stops (i.e., from tensified plain stops to underlyingly tense stops) and (2) to phonetically dissimilar but underlyingly (phonemically) same stops (i.e., from tensified plain stops to non-tensified ones) while generalization failed to phonetically dissimilar and underlyingly different consonants (aspirated stops and nasals) even though they share the same [place] feature. The results imply that pre-lexical units can be better understood in terms of phonetically-definable segments of granular size rather than phonological features, although perceptual learning appears to make some reference to the underlying (phonemic) representation of speech sounds based on which learning takes place.