The mismatch negativity as an index of temporal processing in audition
Introduction
The temporal aspects of sounds carry important information for various auditory functions such as speech and music perception. It has been proposed that the perception of speech signal is primarily based on temporal cues (Rosen, 1992, Shannon et al., 1995). Studies evaluating speech–sound intelligibility on the basis of temporal vs. spectral cues have indicated that when the temporal cues are preserved, almost all spectral cues can be removed from the speech signal and, yet, the speech content can be recognized (Rosen, 1992, Shannon et al., 1995). Moreover, single-channel cochlear-implant users are able to understand speech although their hearing aid principally transmits temporal sound information (Tyler, 1993). A deficiency in discriminating temporal sound features could cause severe disturbances in the linguistic domain. Numerous studies showed that deficiency in speech perception, caused by developmental factors or brain damage at some stage of life, is associated with a difficulty in discriminating temporal sound features (see, for example, Tallal and Piercy, 1973, Tallal, 1980, Robin et al., 1990, Farmer and Klein, 1995, Merzenich et al., 1996, Tallal et al., 1996, Kujala et al., 2000).
The present work addresses the applicability of the mismatch negativity (MMN; Näätänen et al., 1978) in measuring the temporal-discrimination accuracy of the central auditory system. The MMN reflects sensory-memory functioning and can be used to study discrimination accuracy in the auditory system (Näätänen, 1992, Näätänen, 1995, Tiitinen et al., 1994, Amenedo and Escera, 2000). The MMN is obtained by presenting a repetitive auditory stimulus (‘standard’) which is occasionally replaced with a deviant stimulus differing from the standard stimulus in some respect. The MMN elicitation is based on a memory trace formed by the auditory system for the standard stimulus to which each incoming stimulus is compared, with a deviant stimulus eliciting an MMN (Näätänen, 1990).
The MMN has been obtained for various auditory-stimulus differences, such as those in pitch, intensity, and duration (Näätänen, 1992). It has also been obtained for more complex stimulus features, such as phonetic (Aaltonen et al., 1987, Sams et al., 1990, Näätänen et al., 1997) or sound-pattern changes or even changes in abstract features of auditory stimuli (Näätänen et al., 1993, Tervaniemi et al., 1994, Paavilainen et al., 1998, Paavilainen et al., 1999; for a review, see Näätänen et al., 2001). The MMN reflects sound-discrimination accuracy since it is elicited even by just-perceptible differences between sounds (Kraus et al., 1995, Tremblay et al., 1998). The MMN can be obtained without the subject's attention; therefore, it is a suitable tool for studying perceptual functions in individuals who cannot concentrate on task performance (Näätänen, 1995). The MMN has already been successfully used in studying auditory perception in unconscious patients (Kane et al., 1993) and in those having difficulties in understanding or following instructions, like patients with receptive aphasia (Aaltonen et al., 1993, Alain et al., 1998, Ilvonen et al., 2001) or children and infants (Alho et al., 1990, Kraus et al., 1996, Cheour et al., 1998).
Currently, there is an urgent need to improve the reliability of the MMN measurement paradigms, since the main problem in using the MMN at the single-subject level is its fairly poor signal-to-noise ratio. This problem is of course pronounced when the discrimination of fine auditory differences is studied. The reliability of the MMN as an index of discrimination has been studied for simple sound features, such as pitch, intensity, and duration (Pekkonen et al., 1995, Lang et al., 1995, Escera and Grau, 1996, Frodl-Bauch et al., 1997, Joutsiniemi et al., 1998, Schröger, 1998, Tervaniemi et al., 1999, Kathmann et al., 1999). In these studies, MMN test–retest reliability varied between 0.3 and 0.78.
Section snippets
Subjects
Ten healthy subjects (5 males) of 21–27 years (mean 24 years) were included in the present study. Nine of them were right-handed.
Stimuli and procedure
The stimuli were tone pairs with different silent inter-stimulus intervals (ISIs). The tones were 500 Hz in frequency and 30 ms in duration (including 5 ms rise and fall times). Their intensity was 50 dB above the individual hearing level. In the standard-tone pair, the offset-to-onset ISI between the two tones was 120 ms, and in the deviant pairs 20, 60, and 100 ms.
ERP to the auditory stimuli
After artifact rejection, there were on the average 529–536 sweeps in the single-subject averages for the different deviant pairs when presented among the standard pairs (standard error of mean, SEM, 19–24). The standard tone pair as well as the 20, 60, and 100 ms ISI equiprobable tone pairs elicited positive and negative responses that were largely similar in amplitude (Fig. 1). However, as Fig. 1 shows, the onset of the negative deflection was temporally displaced for the different types of
Discussion
Since there is an urgent demand for methods to objectively evaluate temporal-discrimination abilities applicable at the individual level, the present study aimed at determining how reliably the MMN could reflect the discrimination accuracy of sound intervals. In both measurement sessions, a significant MMN was elicited by the 20 and 60 ms ISI deviant tone pairs, whereas the 100 ms ISI deviant tone pair elicited no MMN in either of the sessions. In addition, the MMN amplitude was larger for the
Acknowledgements
This study was supported by the Academy of Finland (grant number 73038).
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