Modulatory Effects of Attention on Lateral Inhibition in the Human Auditory Cortex

<div><p>Reduced neural processing of a tone is observed when it is presented after a sound whose spectral range closely frames the frequency of the tone. This observation might be explained by the mechanism of lateral inhibition (LI) due to inhibitory interneurons in the auditory system. So far, several characteristics of bottom up influences on LI have been identified, while the influence of top-down processes such as directed attention on LI has not been investigated. Hence, the study at hand aims at investigating the modulatory effects of focused attention on LI in the human auditory cortex. In the magnetoencephalograph, we present two types of masking sounds (white noise vs. withe noise passing through a notch filter centered at a specific frequency), followed by a test tone with a frequency corresponding to the center-frequency of the notch filter. Simultaneously, subjects were presented with visual input on a screen. To modulate the focus of attention, subjects were instructed to concentrate either on the auditory input or the visual stimuli. More specific, on one half of the trials, subjects were instructed to detect small deviations in loudness in the masking sounds while on the other half of the trials subjects were asked to detect target stimuli on the screen. The results revealed a reduction in neural activation due to LI, which was larger during auditory compared to visual focused attention. Attentional modulations of LI were observed in two post-N1m time intervals. These findings underline the robustness of reduced neural activation due to LI in the auditory cortex and point towards the important role of attention on the modulation of this mechanism in more evaluative processing stages.</p></div

Comparison of baseline data.

<p>Comparison of baseline data.</p

TMNMT Neurophysiological Mechanism and Music Modification.

<p>The individual tinnitus frequency is removed from the frequency spectrum of the music. This leads to increased lateral inhibition onto the neurons inside of the notch. Blue circles indicate excitatory pyramidal cells in auditory cortex. Red circles indicate inhibitory interneurons. Redistribution of energy from low frequencies to high frequencies is labeled as equalizing of the energy spectrum of the music.</p

Behavioral Results.

<p>Mean change of THQ score in the three notch width groups. Error bars indicate 95%-CI. A negative change indicates an improvement of tinnitus distress.</p

Depiction of the dipoles chosen as the auditory region of interest (ROI).

<p>Mean neural activation for the TT over all conditions was averaged between 70 and 130 ms and plotted on a cortical surface. Grey cylinders indicate the locations of selected dipoles.</p

Grand averaged source waveforms for the N1m time window.

<p>Pre CF = pre measurement control tone. Pre TF = pre measurement tinnitus tone. Post CF = post measurement control tone. Post TF = post measurement tinnitus tone. The onset of the stimulus is at 0 milliseconds and indicated by the dotted line. N1m source strength is lower for the TF, because the TF has a higher carrier frequency than the CF.</p

Flowchart of the Study Design.

<p>Participants were recruited and their tinnitus frequency matched before trial start. Following recruitment and tinnitus pitch-matching, participants received the pre measurement (behavioral and neurophysiological). After three month of TMNMT with 1-, ½- or ¼-octave notch width this was followed by the post measurement (same measures as pre).</p

Results for the main effect of <i>noise type</i> in the bilateral temporal region of interest (ROI) for the two different intervals of interests (IOI).

<p>Only regions surviving cluster-based permutation are colorized. <i>F</i>-values corresponding to a corrected <i>p</i> < .05(A) or <i>p</i> < .01. (B) Black cylinders indicate the locations of dipoles selected as the temporal ROI.** <i>p</i> < .01. (A) Left: Statistical parametric map of <i>F</i>-values for the main effect <i>noise type</i> averaged between 70 and 105 ms. Right: Corresponding mean ROI activity for the test tone (TT) after white noise (WN, black) and after notched white noise (NWN, grey). Bars denote within-subject 95%-confidence intervals. (B) Left: Statistical parametric map of <i>F</i>-values for the main effect <i>noise type</i> averaged between 180 and 268 ms. Right: Corresponding mean ROI activity evoked by the TT after WN (back) and after NWN (grey). Bars denote within-subject 95%-confidence intervals.</p

Depiction of target and distractor stimuli for the visual and auditory tasks.

<p>(A) Auditory distractor and target: Small changes of loudness (increase of 2 dB) were implemented as the target masking sound. This loudness change could occur on four different time points during the presentation of the masking stimulus (0.5 s, 1.1 s, 1.7 s, 2.6 s) and lasted until the end of the masking stimulus. The depiction shows a loudness change at 1.7 s. (B) Visual distractor and target stimuli: When the pattern of small crosses displayed exactly one small square of crosses, the stimuli were considered as targets.</p

Neurophysiological Results.

<p>Mean relative change of N1m (blue) and ASSR (red) source strength evoked by the tinnitus tone relative to baseline and a 500 Hz control tone. Error bars indicate 95%-CI.</p