Age-Related Differences in Neuromagnetic Brain Activity Underlying Concurrent Sound Perception

Abstract

Deficits in parsing concurrent auditory events are believed to contribute to older adults' difficulties in understanding speech in adverse listening conditions (e.g., cocktail party). To explore the level at which aging impairs sound segregation, we measured auditory evoked fields (AEFs) using magnetoencephalography while young, middle-aged, and older adults were presented with complex sounds that either had all of their harmonics in tune or had the third harmonic mistuned by 4 or 16% of its original value. During the recording, participants were asked to ignore the stimuli and watch a muted subtitled movie of their choice. For each participant, the AEFs were modeled with a pair of dipoles in the superior temporal plane, and the effects of age and mistuning were examined on the amplitude and latency of the resulting source waveforms. Mistuned stimuli generated an early positivity (60 -100 ms), an object-related negativity (ORN) (140 -180 ms) that overlapped the N1 and P2 waves, and a positive displacement that peaked at ϳ230 ms (P230) after sound onset. The early mistuning-related enhancement was similar in all three age groups, whereas the subsequent modulations (ORN and P230) were reduced in older adults. These age differences in auditory cortical activity were associated with a reduced likelihood of hearing two sounds as a function of mistuning. The results reveal that inharmonicity is rapidly and automatically registered in all three age groups but that the perception of concurrent sounds declines with age.

Neural generators underlying concurrent sound segregation

“…ER-SAM analysis was applied to two previous MEG data sets ( [Alain et al, 2009] and [Alain and McDonald, 2007]). …”

“…In fact, when the evoked responses associated with a non-mistuned harmonic complex are subtracted from those associated with a mistuned one, a neural marker known as the object-related negativity (ORN) is revealed as a negative deflection 160-200 ms after sound onset (Alain et al, 2001). The ORN (and its magnetic counterpart, the ORNm) can be elicited whether listeners actively or passively listen to the sounds (Alain et al, 2001), is found in both young and old listeners (Alain and McDonald, 2007), and seems to be little affected by task difficulty or visual attention load (Dyson et al, 2005).…”

“…It is well known in the field of event-related potentials that the onset of a sound is associated with an obligatory -P1-N1-P2‖ long-latency response beginning around 50 ms after the sound has been perceived ( [Knight et al, 1980] and [Näätänen and Picton, 1987]). Accordingly, it is possible that the ORN component may itself be a manifestation of a temporally delayed P1, N1, and/or P2 in response to the detection of the mistuned harmonic ( [Alain et al, 2001], [Alain and McDonald, 2007] and [Alain et al, 2009]). The plausibility of this -delayed onset‖ hypothesis is bolstered by dipole source localization studies that place the neural generators of these P1, N1 and P2 responses in regions (Hari et al, 1987) proximal to where the ORN is believed to occur (i.e., the Sylvian fissure).…”

“…In the present study, we sought to localize generators of the ORN as well as the obligatory onset response by applying event-related synthetic aperture magnetometry (ER-SAM; and [Robinson and Vrba, 2004]) to two mistuned harmonic MEG data sets ( [Alain and McDonald, 2007] and [Alain et al, 2009]). Unlike ECD source modeling, the ER-SAM algorithm does not require a priori assumptions about the number or location of sources (e.g., basing the ORN model on an N1 source).…”

“…Selection of evoked field components and time points was guided by the auditory evoked field MEG data previously reported for these data sets ( [Alain and McDonald, 2007] and [Alain et al, 2009]). In those studies, the P1m, N1m and P2m evoked fields were found to peak around 66 ms, 115 ms, 180 ms (Experiment 1) and 53 ms, 109 ms, 197 ms (Experiment 2), respectively.…”