Large group differences in binaural sensitivity are represented in preattentive responses from auditory cortex

Abstract

Correlated sounds presented to two ears are perceived as compact and centrally lateralized, whereas decorrelation between ears leads to intracranial image widening. Though most listeners have fine resolution for perceptual changes in interaural correlation (IAC), some investigators have reported large variability in IAC thresholds, and some normal-hearing listeners even exhibit seemingly debilitating IAC thresholds. It is unknown whether or not this variability across individuals and outlier manifestations are a product of task difficulty, poor training, or a neural deficit in the binaural auditory system. The purpose of this study was first to identify listeners with normal and abnormal IAC resolution, second to evaluate the neural responses elicited by IAC changes, and third to use a well-established model of binaural processing to determine a potential explanation for observed individual variability. Nineteen subjects were enrolled in the study, eight of whom were identified as poor performers in the IAC-threshold task. Global scalp responses (N1 and P2 amplitudes of an auditory change complex) in the individuals with poor IAC behavioral thresholds were significantly smaller than for listeners with better IAC resolution. Source-localized evoked responses confirmed this group effect in multiple subdivisions of the auditory cortex, including Heschl's gyrus, planum temporale, and the temporal sulcus. In combination with binaural modeling results, this study provides objective electrophysiological evidence of a binaural processing deficit linked to internal noise, that corresponds to very poor IAC thresholds in listeners that otherwise have normal audiometric profiles and lack spatial hearing complaints.NEW & NOTEWORTHY Group differences in the perception of interaural correlation (IAC) were observed in human adults with normal audiometric sensitivity. These differences were reflected in cortical-evoked activity measured via electroencephalography (EEG). For some participants, weak representation of the binaural cue at the cortical level in preattentive N1-P2 cortical responses may be indicative of a potential processing deficit. Such a deficit may be related to a poorly understood condition known as hidden hearing loss.

Keywords: auditory processing model; binaural hearing; electroencephalography; hemisphere differences; hidden hearing loss.

Graphical abstract

Figure 1.

Interaural correlation (IAC) thresholds across groups and a linear regression of IAC threshold and hearing threshold. A: the IAC-thresholds for the low and high IAC-threshold groups. Box and whisker plots represent minimum, first quartile, median, third quartile, and maximum IAC thresholds for the low- and high IAC-threshold groups. The scatter plot shows IAC thresholds for each individual. B: a scatter plot and a linear regression of IAC threshold and hearing threshold. For both panels, groups are differentiated by symbols, closed circles represent the low IAC-threshold group, open circles represent the high IAC-threshold group, and Xs are individual low IAC thresholds of the second phase that are not included in the electroencephalography (EEG) analyses. The horizontal lines represent the cut-off value at 0.15.

Figure 2.

Grand average event-related potential (ERP) waveforms based on global field power (GFP) following transition from reference (interaural correlation, IAC = 1) to one of five IAC levels are shown for each group. Data plotted for low (left, n = 11) and high (right, n = 8) IAC-threshold groups.

Figure 3.

Average global field power (GFP) amplitude of the N1-P2 range for the low (n = 11) and high (n = 8) interaural correlation (IAC)-threshold groups at each IAC-change level. Error bars represent SEM. Asterisks denote significant differences at the 0.05 level, Bonferroni corrected. The horizontal bars only apply to the low IAC-threshold group.

Figure 4.

Topographic organization of cortical source vertices sensitive to changes in interaural correlation (IAC) (red patches), group (blue patches), and the IAC × Group interaction (green patches) in left and right hemispheres. White ovals represent Heschl’s gyrus (HG).

Figure 5.

Scatter plot for Pearson correlation coefficient between interaural correlation (IAC)-threshold and N1-P2 source amplitude from the left planum temporale (PT), in response to an IAC-change of 0.25 (1 → 0.75). Closed circles represent the low IAC-threshold group, open circles the high IAC-threshold group.

Figure 6.

Threshold of the binaural model plotted as a function of interaural correlation (IAC)-change (relative to reference of IAC = 1) and internal noise (with respect to signal level). Behavioral IAC-threshold values and corresponding estimate of the models’ level of internal noise for that behavioral threshold are indicated by filled symbols for the low IAC-threshold individuals and open symbols for high IAC-threshold individuals.

Figure 7.

Discrimination performance of the binaural model for a comprehensive combination of reference and target interaural correlations (IACs) for two levels of internal noise representing the low (−9.5 dB) and high (−2.5 dB) IAC-threshold groups. A: poor discrimination observed for similar reference and target IAC level (dashed gray diagonal). Performance exceeds threshold as IAC difference between reference and target increases (threshold indicated by red and green contours). White arrows represent the amount of IAC change needed between reference and target to reach detection threshold. B: the range of IAC-change necessary for detection decreases as the reference IAC approaches 1 for both internal noise levels (red and green lines). The range of IAC change needed for detection is a fixed constant relative to the internal noise level (gray line).