Auditory Change Complex Responses to Spectrotemporally Modulated Stimuli

The non-language-dependent Audible Contrast Threshold (ACT) test is a clinically viable spectrotemporal modulation detection test and serves as an alternative to language-specific speech-in-noise tests. However, the ACT test requires active participation, which is naturally challenging for infants, young children, and individuals with developmental or intellectual differences. This article focuses on the specifications and design of an electrophysiological version of ACT (E-ACT). A test paradigm was developed based on auditory change complex (ACC) responses to spectrotemporally modulated stimuli. This study investigated the effects of two potential carriers for the test stimuli, differences in responses between brain hemispheres, represented by left and right mastoids, and the effect of the direction of ACC change to optimally design an E-ACT. Finally, several strategies for defining individual thresholds for the E-ACT were compared.

Two experiments were conducted with 18 and 47 adult participants, respectively, all with pure-tone hearing thresholds at or below 75 dB HL at frequencies up to and including 2 kHz. The stimulus, consisting of spectrotemporally modulated targets alternating with unmodulated references, each presented for approximately 1 sec, elicited ACC responses from the participants. In Experiment A, both noise and tonal-carrier stimuli were used, while in Experiment B, only tonal-carrier stimuli were included. Electroencephalogram data were analyzed using the objective Fmpi (individualized multi-point Fsp) detector to estimate whether a response was present.

The tonal-carrier stimuli elicited significantly more detected responses compared with the noise-carrier stimuli. Analysis of hemispheric dominance revealed a significantly higher detection rate for ACC responses from the right mastoid compared with the left. However, the highest detection rate was observed when averaging responses from both mastoids. When ACC responses were divided into subcategories based on the direction of auditory change, the reference-to-target change ("On") produced a significantly higher detection rate than the target-to-reference change ("Off"). Pooling the "On" and "Off" responses did not increase the detection rates. The most effective strategy for determining the E-ACT threshold was to select the direction of auditory change of the mastoid average that was individually strongest in the first recording at maximum modulation.

The present findings suggest that an electrophysiological version of ACT should be based on the tonal-carrier stimulus. To define individual thresholds for an E-ACT, the ACC should be determined as the average of left and right hemispheric responses, using only the direction of auditory change that is individually strongest during the first recording.