Selective Attention Increases Both Gain and Feature Selectivity of the Human Auditory Cortex

Laboratory of Computational Engineering, Helsinki University of Technology, Espoo, Finland.
PLoS ONE (Impact Factor: 3.53). 02/2007; 2(9):e909. DOI: 10.1371/journal.pone.0000909
Source: PubMed

ABSTRACT An experienced car mechanic can often deduce what's wrong with a car by carefully listening to the sound of the ailing engine, despite the presence of multiple sources of noise. Indeed, the ability to select task-relevant sounds for awareness, whilst ignoring irrelevant ones, constitutes one of the most fundamental of human faculties, but the underlying neural mechanisms have remained elusive. While most of the literature explains the neural basis of selective attention by means of an increase in neural gain, a number of papers propose enhancement in neural selectivity as an alternative or a complementary mechanism.
Here, to address the question whether pure gain increase alone can explain auditory selective attention in humans, we quantified the auditory cortex frequency selectivity in 20 healthy subjects by masking 1000-Hz tones by continuous noise masker with parametrically varying frequency notches around the tone frequency (i.e., a notched-noise masker). The task of the subjects was, in different conditions, to selectively attend to either occasionally occurring slight increments in tone frequency (1020 Hz), tones of slightly longer duration, or ignore the sounds. In line with previous studies, in the ignore condition, the global field power (GFP) of event-related brain responses at 100 ms from the stimulus onset to the 1000-Hz tones was suppressed as a function of the narrowing of the notch width. During the selective attention conditions, the suppressant effect of the noise notch width on GFP was decreased, but as a function significantly different from a multiplicative one expected on the basis of simple gain model of selective attention.
Our results suggest that auditory selective attention in humans cannot be explained by a gain model, where only the neural activity level is increased, but rather that selective attention additionally enhances auditory cortex frequency selectivity.

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    ABSTRACT: Cochlear outer hair cells (OHCs) are remarkable, mechanically-active receptors that determine the exquisite sensitivity and frequency selectivity characteristic of the mammalian auditory system. While there are three to four times as many OHCs compared with inner hair cells, OHCs lack a significant afferent innervation and, instead, receive a rich efferent innervation from medial olivocochlear (MOC) efferent neurons. Activation of the MOC has been shown to exert a considerable suppressive effect over OHC activity. The precise function of these efferent tracts in auditory behavior, however, is the matter of considerable debate. The most frequent functions assigned to the MOC tracts are to protect the cochlea from traumatic damage associated with intense sound and to aid the detection of signals in noise. While considerable evidence shows that interruption of MOC activity exacerbates damage due to high-level sound exposure, the well characterized MOC physiology and evolutionary studies do not support such a role. Instead, a MOC protective effect is well explained as being a byproduct of the suppressive nature of MOC action on OHC mechanical behavior. A role in the enhancement of signals in noise backgrounds, on the other hand, is well supported by (1) an extensive physiological literature (2) examination of naturally occurring environmental acoustic conditions (3) recent data from multiple laboratories showing that the MOC plays a significant role in auditory selective attention by suppressing the response to unattended or ignored stimuli. This presentation will argue that, based on the extant literature combining the suppression of background noise through MOC-mediated rapid adaptation (RA) with the suppression of non-attended signals, in concert with the corticofugal pathways descending from the auditory cortex, the MOC system has one evolved function-to increase the signal-to-noise ratio, aiding in the detection of target signals. By contrast, the MOC system role in reducing noise damage and the effects of aging in the cochlea may well represent an exaptation, or evolutionary "spandrel".
    Frontiers in Systems Neuroscience 02/2015; 9:12. DOI:10.3389/fnsys.2015.00012
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    ABSTRACT: Mounting evidence suggests that auditory attention tasks may modulate the sensitivity of the cochlea by way of the corticofugal and the medial olivocochlear (MOC) efferent pathways. Here, we studied the extent to which a separate efferent tract, the ‘uncrossed’ MOC, which functionally connects the two ears, mediates inter-aural selective attention. We compared distortion product otoacoustic emissions (DPOAEs) in one ear with binaurally presented primaries, using an intermodal target detection task in which participants were instructed to report the occurrence of brief target events (visual changes, tones). Three tasks were compared under identical physical stimulation: (i) report brief tones in the ear in which DPOAE responses were recorded; (ii) report brief tones presented to the contralateral, non-recorded ear; and (iii) report brief phase shifts of a visual grating at fixation. Effects of attention were observed as parallel shifts in overall DPOAE contour level, with DPOAEs relatively higher in overall level when subjects ignored the auditory stimuli and attended to the visual stimulus, compared with both of the auditory-attending conditions. Importantly, DPOAE levels were statistically lowest when attention was directed to the ipsilateral ear in which the DPOAE recordings were made. These data corroborate notions that top-down mechanisms, via the corticofugal and medial efferent pathways, mediate cochlear responses during intermodal attention. New findings show attending to one ear can significantly alter the physiological response of the contralateral, unattended ear, probably through the uncrossed-medial olivocochlear efferent fibers connecting the two ears.
    European Journal of Neuroscience 10/2014; 40(12). DOI:10.1111/ejn.12746 · 3.67 Impact Factor

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