Objectives: Cochlear implant listeners are able to at least partially adapt to the spectral mismatch associated with the implant device and speech processor via daily exposure and/or explicit training. The overall goal of this study was to investigate interactions between short-term unsupervised learning (i.e., passive adaptation) and the degree of spectral mismatch in normal-hearing listeners' adaptation to spectrally shifted vowels. Design: Normal-hearing subjects were tested while listening to acoustic cochlear implant simulations. Unsupervised learning was measured by testing vowel recognition repeatedly over a 5 day period; no feedback or explicit training was provided. In experiment 1, subjects listened to 8-channel, sine-wave vocoded speech. The spectral envelope was compressed to simulate a 16 mm cochlear implant electrode array. The analysis bands were fixed and the compressed spectral envelope was linearly shifted toward the base by 3.6, 6, or 8.3 mm to simulate different insertion depths of the electrode array, resulting in a slight, moderate, or severe spectral shift. In experiment 2, half the subjects were exclusively exposed to a severe shift with 8 or 16 channels (exclusive groups), and half the subjects were exposed to 8-channel severely shifted speech, 16-channel severely shifted speech, and 8-channel moderately shifted speech, alternately presented within each test session (mixed group). The region of stimulation in the cochlea was fixed (16 mm in extent and 15 mm from the apex) and the analysis bands were manipulated to create the spectral shift conditions. To determine whether increased spectral resolution would improve adaptation, subjects were exposed to 8- or 16-channel severely shifted speech. Results: In experiment 1, at the end of the adaptation period, there was no significant difference between 8-channel speech that was spectrally matched and that shifted by 3.6 mm. There was a significant, but less-complete, adaptation to the 6 mm shift and no adaptation to the 8.3 mm shift. In experiment 2, for the mixed exposure group, there was significant adaptation to severely shifted speech with 8 channels and even greater adaptation with 16 channels. For the exclusive exposure group, there was no significant adaptation to severely shifted speech with either 8 or 16 channels. Conclusions: These findings suggest that listeners are able to passively adapt to spectral shifts up to 6 mm. For spectral shifts beyond 6 mm some passive adaptation was observed with mixed exposure to a smaller spectral shift, even at the expense of some low frequency information. Mixed exposure to the smaller shift may have enhanced listeners' access to spectral envelope details that were not accessible when listening exclusively to severely shifted speech. The results suggest that the range of spectral mismatch that can support passive adaptation may be larger than previously reported. Some amount of passive adaptation may be possible with severely shifted speech by exposing listeners to a relatively small mismatch in conjunction with the severe mismatch.
