Herrmann B, Henry MJ, Johnsrude IS, Obleser J (2016). Altered temporal dynamics of neural adaptation in the aging human auditory cortex. Neurobiology of Aging 45:10-22.
Neural adaptation is a phenomenon by which a neural response is reduced by the repetition of a stimulus. In audition, neural adaptation has been suggested to underlie a variety of important sensory functions such as detecting a rare sound among other sounds or segregating different acoustic streams. In our aging Western societies, difficulties in hearing become more and more a topic of public relevance and some of the hearing difficulties in aging might be related to changes in neural adaptation. The current electroencephalography (EEG) study investigated neural adaptation of responses in younger and older adults.
A pure tone was repeatedly presented in two different temporal sequence contexts. In one context (regular), the sequence continuously alternated between accelerating and decelerating, while the tone was repeated temporally irregular in the other context (irregular). The figure above shows example between-tone temporal intervals (top; sound onset-to-onset interval) and the temporal structure of events for each context (bottom).
Neural responses (time course) for tones preceded by different onset-to-onset intervals are displayed in the figure above (only the regular context is shown). Neural responses were particularly modulated by onset-to-onset intervals in the time window between 0.08 and 0.11 seconds after tone onset (N1 response amplitude). We extracted the N1 amplitude for tones preceded by different intervals separately for the two temporal contexts (regular, irregular).
N1 amplitudes were more strongly modulated by onset-to-onset intervals in the regular compared to the irregular context (i.e., a larger response range), showing that the extended temporal stimulation history affects the response magnitude to the current sound (bar graphs in the figure below show the response range, that is, the difference between maximum and minimum N1 amplitude). In addition, the overall N1 amplitude as well as the response range were larger in older compared to younger individuals.
In a further analysis, we only investigated N1 amplitudes in the regular temporal context (figure below), analyzing the responses in the accelerating versus decelerating sequence parts (see first figure). We observed that the N1 amplitudes were larger in the accelerating compared to the decelerating sequence parts and furthermore that this difference was decreased in older individuals. (The bar graph in the figure below shows the mean amplitude across all onset-to-onset intervals.)
Finally, the temporal context and aging effects were explained by a single-neuron computational model. Specifically, the data from the neuron model suggested that neurons in the aging auditory cortex recovered faster from adaptation over time, causing the variety of amplitude differences observed between age groups. In other words, the extended temporal stimulation history had less influence on the response magnitude elicited by a sound for older compared to younger adults. We interpreted the results to be due to reduced neural inhibition in auditory cortex accompanying aging as has been shown for animals.