Top-down attention modulates auditory sustained responses but not the neural processing advantage for vowels

Abstract

Temporal and spectral regularities characteristic of vowels are preferentially encoded by the auditory system, giving rise to a prolonged enhancement of a negative electromagnetic response in the auditory cortex - sustained negativity (SN). However, it remains unclear how sustained attention to the auditory stream interacts with this intrinsically enhanced processing. Here, we used MEG to examine SN and its differential response to vowel-like acoustic patterns versus spectrally complex noise, while modulating subjects attention to the auditory stimulation. Four stimulus types (600 ms duration; vowel-like sounds with and without pitch, periodic non-vowel sounds, and aperiodic noise) were presented to 30 adults during passive listening and an active gap-detection task. In 16% of randomly selected trials, a 50-ms silent gap was inserted into the stimuli, which served as a target. The number of non-target trials between successive targets ranged from 1 to 10, with the highest probability corresponding to an interval of six trials. Only non-target trials were analyzed. Analysis of reaction times and omission errors indicated that target expectancy increased with the number of trials since the previous target. Task-related attention enhanced an early orienting-related negativity ([~]50-150 ms) across all stimulus types, irrespective of target expectancy. It also accelerated the processing of periodicity in non-vocal sounds but did not affect the processing of vowels. In contrast, the later segment of the SN (150-600 ms) was strongly modulated by proximity to the previous target, decreasing immediately after target events and increasing as the likelihood of target occurrence rose. Despite these attentional and expectancy-related modulations, the enhanced neural processing of periodic vowels relative to noise remained stable. These findings indicate that sustained attention modulates both early and late SN responses and accelerates the extraction of periodicity in non-vocal sounds, yet does not alter the neural advantage conferred by vowel-related regularities.