Abstract
Listeners with normal hearing thresholds (NHTs) differ in their ability to steer attention to whatever sound source is important. This ability depends on top-down executive control, which modulates the sensory representation of sound in the cortex. Yet, this sensory representation also depends on the coding fidelity of the peripheral auditory system. Both of these factors may thus contribute to the individual differences in performance. We designed a selective auditory attention paradigm in which we could simultaneously measure envelope following responses (EFRs, reflecting peripheral coding), onset event-related potentials (ERPs) from the scalp (reflecting cortical responses to sound) and behavioral scores. We performed two experiments that varied stimulus conditions to alter the degree to which performance might be limited due to fine stimulus details vs. due to control of attentional focus. Consistent with past work, in both experiments we find that attention strongly modulates cortical ERPs. Importantly, in Experiment I, where coding fidelity limits the task, individual behavioral performance correlates with subcortical coding strength (derived by computing how the EFR is degraded for fully masked tones compared to partially masked tones); however, in this experiment, the effects of attention on cortical ERPs were unrelated to individual subject performance. In contrast, in Experiment II, where sensory cues for segregation are robust (and thus less of a limiting factor on task performance), inter-subject behavioral differences correlate with subcortical coding strength. In addition, after factoring out the influence of subcortical coding strength, behavioral differences are also correlated with the strength of attentional modulation of ERPs. These results support the hypothesis that behavioral abilities amongst listeners with NHTs can arise due to both subcortical coding differences and differences in attentional control, depending on stimulus characteristics and task demands.
Highlights
A number of recent studies suggest that listeners with normal hearing thresholds (NHTs) may suffer from auditory neuropathy, or a loss of ascending auditory nerve fibers (Schaette and McAlpine, 2011; Plack et al, 2014; Bharadwaj et al, 2015)
Despite having normal auditory thresholds, performance varied from below 40% to nearly 90%; almost all mistakes arose because listeners reported the content of one of the competing streams, rather than because they failed to understand the digits in the mixture (Ruggles and Shinn-Cunningham, 2011; Ruggles et al, 2011). These difficulties in focusing on target speech amidst competing speech were correlated with the strength of the subcortical response to periodic sound, known variously as the frequency-following response (FFR) or the envelope following response (EFR; see Ruggles and ShinnCunningham, 2011; Ruggles et al, 2012). These results suggest that poor subcortical encoding can lead to deficits in the ability to focus selective auditory attention on a source from a particular direction
The Attentional Modulation Index (AMI), which measures how strongly the neural representation is modulated by the focus of attention, was positive both when listeners attended to Stream A and when they attended to Stream B
Summary
A number of recent studies suggest that listeners with normal hearing thresholds (NHTs) may suffer from auditory neuropathy, or a loss of ascending auditory nerve fibers (Schaette and McAlpine, 2011; Plack et al, 2014; Bharadwaj et al, 2015). Auditory neuropathy, very like driven by noise exposure and aging (Schaette and McAlpine, 2011; Bharadwaj et al, 2014, 2015; Plack et al, 2014), is a likely contributor to individual differences in the encoding of subtle spectro-temporal features of supra-threshold sound. Such features are critical for segregating sound sources; if a listener cannot segregate sources, they will have trouble directing attention to whichever source is of interest. Auditory neuropathy may explain why some NHT listeners experience communication problems in noisy environments (Shinn-Cunningham et al, in press)
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