Kelly L. McDonald

Automatic and Controlled Processing of Melodic Contour and Interval Information Measured by Electrical Brain Activity

Most work on how pitch is encoded in the auditory cortex has focused on tonotopic (absolute) pitch maps. However, melodic information is thought to be encoded in the brain in two different relative pitch forms, a domain-general contour code (up/down pattern of pitch changes) and a music-specific interval code (exact pitch distances between notes). Event-related potentials were analyzed in nonmusicians from both passive and active oddball tasks where either the contour or the interval of melodyfinal notes was occasionally altered. The occasional deviant notes generated a right frontal positivity peaking around 350 msec and a central parietal P3b peaking around 580 msec that were present only when participants focused their attention on the auditory stimuli. Both types of melodic information were encoded automatically in the absence of absolute pitch cues, as indexed by a mismatch negativity wave recorded during the passive conditions. The results indicate that even in the absence of musical training, the brain is set up to automatically encode music-specific melodic information, even when absolute pitch information is not available.

Neural activity associated with distinguishing concurrent auditory objects

The neural processes underlying concurrent sound segregation were examined by using event-related brain potentials. Participants were presented with complex sounds comprised of multiple harmonics, one of which could be mistuned so that it was no longer an integer multiple of the fundamental. In separate blocks of trials, short-, middle-, and long-duration sounds were presented and participants indicated whether they heard one sound (i.e., buzz) or two sounds (i.e., buzz plus another sound with a pure-tone quality). The auditory stimuli were also presented while participants watched a silent movie in order to evaluate the extent to which the mistuned harmonic could be automatically detected. The perception of the mistuned harmonic as a separate sound was associated with a biphasic negative-positive potential that peaked at about 150 and 350 ms after sound onset, respectively. Long duration sounds also elicited a sustained potential that was greater in amplitude when the mistuned harmonic was perceptually segregated from the complex sound. The early negative wave, referred to as the object-related negativity (ORN), was present during both active and passive listening, whereas the positive wave and the mistuning-related changes in sustained potentials were present only when participants attended to the stimuli. These results are consistent with a two-stage model of auditory scene analysis in which the acoustic wave is automatically decomposed into perceptual groups that can be identified by higher executive functions. The ORN and the positive waves were little affected by sound duration, indicating that concurrent sound segregation depends on transient neural responses elicited by the discrepancy between the mistuned harmonic and the harmonic frequency expected based on the fundamental frequency of the incoming stimulus.

Aging and the processing of sound duration in human auditory cortex

Age-related declines in coding the fine temporal structure of acoustic signals is proposed to play a critical role in the speech perception difficulties commonly observed in older individuals. This hypothesis was tested by measuring auditory evoked potentials elicited by sounds of various durations in young, middle-aged and older adults. All stimuli generated N1 and P2 waves that peaked at about 104 and 200 ms post-stimulus onset. The N1 amplitude increased linearly with increases in the tonal duration in young, middle-aged, and older adults. The P2 amplitude also increased linearly with signal duration, but only in young and middle-aged adults. The results demonstrate that the N1 and P2 waves can resolve duration differences as short as 2-4 ms and that normal aging decreases the temporal resolving power for processing small differences in sound duration.

Left thalamo-cortical network implicated in successful speech separation and identification

The separation of concurrent sounds is paramount to human communication in everyday settings. The primary auditory cortex and the planum temporale are thought to be essential for both the separation of physical sound sources into perceptual objects and the comparison of those representations with previously learned acoustic events. To examine the role of these areas in speech separation, we measured brain activity using event-related functional Magnetic Resonance Imaging (fMRI) while participants were asked to identify two phonetically different vowels presented simultaneously. The processing of brief speech sounds (200 ms in duration) activated the thalamus and superior temporal gyrus bilaterally, left anterior temporal lobe, and left inferior temporal gyrus. A comparison of fMRI signals between trials in which participants successfully identified both vowels as opposed to when only one of the two vowels was recognized revealed enhanced activity in left thalamus, Heschls gyrus, superior temporal gyrus, and the planum temporale. Because participants successfully identified at least one of the two vowels on each trial, the difference in fMRI signal indexes the extra computational work needed to segregate and identify successfully the other concurrently presented vowel. The results support the view that auditory cortex in or near Heschls gyrus as well as in the planum temporale are involved in sound segregation and reveal a link between left thalamo-cortical activation and the successful separation and identification of simultaneous speech sounds.

Noise-induced increase in human auditory evoked neuromagnetic fields

Noise is usually detrimental to auditory perception. However, recent psychophysical studies have shown that low levels of broadband noise may improve signal detection. Here, we measured auditory evoked fields (AEFs) while participants listened passively to low‐pitched and high‐pitched tones (Experiment 1) or complex sounds that included a tuned or a mistuned component that yielded the perception of concurrent sound objects (Experiment 2). In both experiments, stimuli were embedded in low or intermediate levels of Gaussian noise or presented without background noise. For each participant, the AEFs were modeled with a pair of dipoles in the superior temporal plane, and the effects of noise were examined on the resulting source waveforms. In both experiments, the N1m was larger when the stimuli were embedded in low background noise than in the no‐noise control condition. Complex sounds with a mistuned component generated an object‐related negativity that was larger in the low‐noise condition. The results show that low‐level background noise facilitates AEFs associated with sound onset and can be beneficial for sorting out concurrent sound objects. We suggest that noise‐induced increases in transient evoked responses may be mediated via efferent feedback connections between the auditory cortex and lower auditory centers.

Neural encoding of sound duration persists in older adults.

Speech perception depends strongly on precise encoding of the temporal structure of sound. Although behavioural studies suggest that communication problems experienced by older adults may entail deficits in temporal acuity, much is unknown about the effects of age on the neural mechanisms underlying the encoding of sound duration. In this study, we measured neuromagnetic auditory evoked responses in young, middle-aged and older healthy participants listening to sounds of various durations. The time courses of cortical activity from bilateral sources in superior temporal planes showed specific differences related to the sound offsets indicating the neural representation of onset and offset markers as one dimension of the neural code for sound duration. Model free MEG source analysis identified brain areas specifically responding with an increase in activity to increases in sound duration in the left anterior insula, right inferior frontal, right middle temporal, and right post-central gyri in addition to bilateral supra-temporal gyri. Sound duration-related changes in cortical responses were comparable in all three age groups despite age-related changes in absolute response magnitudes. The results demonstrated that early cortical encoding of the temporal structure of sound presented in silence is little or not affected by normal aging.

Effect of tonal duration on auditory evoked potentials as a function of age

Behavioral research has shown that temporal resolution decreases with age as evidenced by higher‐gap‐detection thresholds in older versus young adults with normal hearing. In the present study, we examined the neural correlates of the temporal resolution function in young, middle aged, and older subjects, using a brief Gaussian‐shaped 2‐kHz tone that increased by 2 ms in separate trial blocks. All stimuli elicited a negative potential that peaked at about 100‐ms poststimulus (N1). The N1 amplitude increased linearly with increases in tonal duration. The growth function of the N1 amplitude was similar across the three age groups, although the slope of the function in older adults was shallower. Results suggest that the N1 response can resolve temporal information on the order of 2 ms. This electrophysiologic threshold parallels behavioral findings. The slower growth function of the N1 response in older adults may reflect a deficit in temporal processing. [Work supported by the Medical Research Council of C...