Subcortical Plasticity Following Perceptual Learni

Authors: Samuele Carcagno Christopher J Plack

Publish Date: 2010/09/28

Volume: 12, Issue: 1, Pages: 89-100

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Abstract

Practice can lead to dramatic improvements in the discrimination of auditory stimuli In this study we investigated changes of the frequencyfollowing response FFR a subcortical component of the auditory evoked potentials after a period of pitch discrimination training Twentyseven adult listeners were trained for 10 h on a pitch discrimination task using one of three different complex tone stimuli One had a static pitch contour one had a rising pitch contour and one had a falling pitch contour Behavioral measures of pitch discrimination and FFRs for all the stimuli were measured before and after the training phase for these participants as well as for an untrained control group n = 12 Trained participants showed significant improvements in pitch discrimination compared to the control group for all three trained stimuli These improvements were partly specific for stimuli with the same pitch modulation dynamic vs static and with the same pitch trajectory rising vs falling as the trained stimulus Also the robustness of FFR neural phase locking to the sound envelope increased significantly more in trained participants compared to the control group for the static and rising contour but not for the falling contour Changes in FFR strength were partly specific for stimuli with the same pitch modulation dynamic vs static of the trained stimulus Changes in FFR strength however were not specific for stimuli with the same pitch trajectory rising vs falling as the trained stimulus These findings indicate that even relatively lowlevel processes in the mature auditory system are subject to experiencerelated changeSounds whose waveforms repeat periodically elicit a sensation of pitch which plays a fundamental role in the perception of speech and music as well as in the segregation of concurrent sound sources Plack and Oxenham 2005a Auditory nerve fibers “phase lock” to the waveform of pure tones that is neural firing tends to occur at the same time during each cycle of the sound wave Several models of pitch assume that this temporal information which is preserved in upper brainstem structures for fundamental frequencies F0s up to several hundred hertz Liu et al 2006 is used to encode pitch eg Meddis and OMard 2006 McLachlan 2009Musicians Wong et al 2007 Bidelman et al 2009 and speakers of a tone language Krishnan et al 2005 2009a b Swaminathan et al 2008 show more robust subcortical “phase locking” in response to pitchevoking sounds compared to English speakers without musical experience This has been demonstrated using scalp recordings of the frequency following response FFR an evoked potential which reflects neural phase locking of brainstem nuclei inferior colliculus and lateral lemniscus Smith et al 1975 Gardi et al 1979 to the envelope of a sound The enhancement of the FFR in musicians and speakers of a tone language has been explained as a result of subcortical plasticity driven by the extensive practice these populations of listeners have in identifying and discriminating sounds on the basis of pitch However FFR differences between expert and naive pitch listeners may be caused by factors other than neural plasticity Monaghan et al 1998 such as genetic predispositions Moreover assuming that FFR enhancements in expert pitch listeners reflect subcortical plasticity it remains unclear whether this plasticity is limited to a critical developmental period or is retained also in adulthood A more direct approach to the study of neural plasticity consists in measuring neural activity before and after a period of training The only study using this approach in adults found more robust FFR phase locking to the waveform of one out of three tones in a group of English speakers following a period of training in an auditory identification task with pseudowords with the same pitch contour as Mandarin tones Song et al 2008 The lack of a control group in this study however does not allow the unequivocal conclusion that FFR changes between the pre and posttraining recordings were due to auditory training per se One purpose of the present study was to provide a more rigorous test of the experiencedependent plasticity of the FFR To this end we compared FFR changes between a group of adult listeners following an auditory training protocol of ten 1h sessions and a control group that did not receive any training FFR enhancements in Mandarin speakers have been found to be specific for stimuli with the same pitch contour as Mandarin tones Xu et al 2006 Krishnan et al 2009a To assess the specificity of FFR training effects with respect to pitch contour we trained participants with one of three stimuli respectively with a rising falling or static pitch contour and assessed FFR changes after training for all three stimuli Changes in performance in the behavioral discrimination task were also assessed for all stimuliThirtynine participants 16 females 35 right handed two ambidextrous completed the experiment The participants ranged in age between 19 and 35 years mean = 23 SD = 2 They all had normal hearing for both ears with absolute pure tone thresholds below 20 dB HL at octave frequencies from 250 to 8000 Hz None had prior experience in psychoacoustic tasks or musical training All participants gave written informed consent and were paid an hourly wage for their participation in the experiment All procedures of the study were approved by the Department of Psychology Ethics Committee Lancaster UniversityIllustration of the psychophysical tasks A For the static stimulus participants were required to discriminate between a standard stimulus with a fixed 140 Hz F0 and a comparison stimulus with a lower F0 B F0 contour of the rising stimulus SUp For the dynamic stimuli participants were required to discriminate between a standard stimulus with a 400 ms halfcycle FM and a comparison stimulus with a shorter halfcycle FM The start and end F0 points were the same for the standard and comparison stimuliFor the behavioral sessions the stimuli were generated digitally with 32bit resolution and a 48kHz sampling rate on a Macintosh workstation The stimuli were played through an MAUDIO Firewire 410 DAC and presented binaurally via Sennheiser HD580 headphones For the FFR sessions all stimuli were generated digitally with 16bit resolution and a 40kHz sampling rate The stimulus files were played through a DAC included in the evoked potentials data acquisition system Intelligent Hearing Systems–Smart EP and presented binaurally through mumetal shielded ER2 insert earphones Binaural FFRs have greater amplitude than FFRs recorded monaurally Clark et al 1997 The signaltonoise ratio for binaural FFRs should therefore be greater than for monaural FFRs and allow for more accurate FFR measurementParticipants were randomly assigned to one of three experimental groups GUp n = 9 GDown n = 9 GStatic n = 9 or to a control group GControl n = 12 All participants took part in a preliminary session during which they were familiarized with the stimuli and procedures of the experiment by running two blocks of the discrimination task for each stimulus An additional familiarization block for each stimulus was run during the first behavioral discrimination thresholds session Pretraining FFRs and behavioral discrimination thresholds were then measured in separate successive sessions During the training phase participants of the experimental groups ran ten sessions of the auditory discrimination task During each training session participants of the GUp and GDown groups completed 20 blocks of the discrimination task with the SUp and SDown stimuli respectively while participants of the GStatic group completed 18 blocks of the discrimination task with the SStatic stimulus The mean duration of the training phase was 27 days Participants of the control group waited for a similar amount of time mean 32 days without receiving any training After the training phase FFRs and behavioral thresholds were measured again in three separate sessions

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