Although it is well-known that aging often results in hearing loss in older adults, audiologists are challenged when the influence of changes in the peripheral (cochlear) hearing functions are difficult to separate from concurrent changes in higher-order auditory processing in older listeners. Recent work in our laboratory shows that P300 auditory event-related potentials (P3AERPs) may provide valuable insights into differences in higher-order auditory processing between younger and older listeners and a potential clinical tool to evaluate central auditory changes in older listeners.
Research in electrophysiologic testing can provide clinical tools needed to investigate the higher-order auditory processing abilities of children with speech-language disorders and/or auditory processing disorders and older adults with early Alzheimer's disease. Electrophysiological tasks can be simplified for use in individuals with cognitive limitations, such as those with advanced Alzheimer's disease. Cortical event-related potentials recorded from brief stimuli such as clicks or synthesized speech stimuli (e.g., /ta/) can be used to reflect neural encoding at the level of the auditory cortex, particularly in listeners who are either not capable of responding or unwilling to respond to auditory stimuli (Martin, Tremblay, & Korczak, 2008).
P300 event-related potentials provide a valuable tool to evaluate higher-order auditory processing by recording endogenous late auditory-evoked potentials that can be elicited by simple auditory discrimination tasks. In accordance with the International 10-20 system, neuroelectrical activity is recorded from surface electrodes placed on the midline at central (Cz) scalp locations, referenced to linked electrodes on earlobes and a forehead ground (Fpz) electrode. Central (Cz) scalp locations provide good topography for amplitude/latency correlations that reflect neurocognitive operations underlying fundamental discrimination processes required in the P300 (Polich et al., 1997).
The procedure used in eliciting the P300 is called the "oddball paradigm" because two different stimuli are presented in a random series with one stimuli occurring less frequently than the other. During the P300 assessment, the subject is asked to distinguish between these stimuli by counting the infrequent stimuli (e.g., 2 kHz tone bursts) and ignoring the frequent stimuli (e.g., 1 kHz tone bursts). The result is a P300 waveform characterized by a positive deflection in the 250–400 milliseconds range, which is seen in response to the infrequent stimuli.
Much like other evoked potentials, P300 responses are measured and studied in terms of amplitude (magnitude of response) and latency (speed of response). P300 latency and amplitude are marked on the waveform of the infrequent stimulus and eyeblinks are recorded on a separate electro-oculography (EOG) channel. The latency of the P300 is a sensitive measure of the stimulus classification speed and the amplitude of the P300 has been used to measure attentional resource allocation (Polich et al., 1997). P300 latencies have been found to be significantly longer in young adults with auditory processing disorders than in control adults without auditory processing disorders in the presence of competing noise (Krishnamurti, 2001). In another study, older adults with age-associated memory impairment (AAMI) showed smaller P300 amplitudes and longer P300 latencies than control older adults without AAMI (Anderer et al., 2003).
In the current study, the P300 latency and amplitude measures were obtained in listening conditions with and without auditory distraction in younger and older listeners. The eight younger listeners in this study included three males and five females ages 22–26.5 years (mean=23.6 years). The group of four older listeners included two males and two females ages 70–84 years (mean=79 years). The younger listeners all had normal hearing, and the older listeners had normal pure tone thresholds in the speech frequencies and sensorineural hearing loss that did not exceed 40 dB in the higher frequencies.
P300 responses were recorded under two listening conditions: with and without distraction in competing noise. An oddball paradigm was used and subjects were asked to count the number of infrequent stimuli (2 kHz tone bursts) while ignoring frequent stimuli (1 kHz tone bursts). In the first condition without auditory distraction, the subjects attended to infrequent auditory stimuli without any competing noise. In the second condition with auditory distraction, the subjects attended to auditory stimuli while ignoring white noise presented to the opposite ear.
Younger listeners showed shorter P300 latencies than older listeners under both no-distraction and distraction conditions. These results indicate that aging slows down information-processing during listening conditions with and without distraction. P300 amplitudes were greater for younger listeners than older listeners and a greater change was seen from the no-distraction to distraction condition for the older group. This difference may be related to reduction in the amount of neurological substrate for auditory processing in the aging auditory system.
As the national and international population of elders continues to increase, audiologists are challenged to evaluate and improve central auditory functioning in people who are older. P300 event-related potentials appear to have tremendous promise in evaluating the functional status of the central auditory nervous system, but this assessment is limited in clinical utility because of factors such as high cost, limited clinician training, and need for specialized software. Results of the current study indicate that P300 testing can provide physiological measures of auditory processing under distraction conditions (such as competing noise) in younger and older adults. More research is needed, however, before the use of P300 is included in test batteries for aging adults.