Abstract
Frequency tagging of sensory inputs (presenting stimuli that fluctuate periodically at rates to which the cortex can phase lock) has been used to study attentional modulation of neural responses to inputs in different sensory modalities. For visual inputs, the visual steady-state response (VSSR) at the frequency modulating an attended object is enhanced, while the VSSR to a distracting object is suppressed. In contrast, the effect of attention on the auditory steady-state response (ASSR) is inconsistent across studies. However, most auditory studies analyzed results at the sensor level or used only a small number of equivalent current dipoles to fit cortical responses. In addition, most studies of auditory spatial attention used dichotic stimuli (independent signals at the ears) rather than more natural, binaural stimuli. Here, we asked whether these methodological choices help explain discrepant results. Listeners attended to one of two competing speech streams, one simulated from the left and one from the right, that were modulated at different frequencies. Using distributed source modeling of magnetoencephalography results, we estimate how spatially directed attention modulates the ASSR in neural regions across the whole brain. Attention enhances the ASSR power at the frequency of the attended stream in contralateral auditory cortex. The attended-stream modulation frequency also drives phase-locked responses in the left (but not right) precentral sulcus (lPCS), a region implicated in control of eye gaze and visual spatial attention. Importantly, this region shows no phase locking to the distracting stream. Results suggest that the lPCS in engaged in an attention-specific manner. Modeling results that take account of the geometry and phases of the cortical sources phase locked to the two streams (including hemispheric asymmetry of lPCS activity) help to explain why past ASSR studies of auditory spatial attention yield seemingly contradictory results.
Highlights
The ability to focus attention on a sound of interest amidst irrelevant signals is vital for an animal’s survival
This study is the first, to our knowledge, that uses a wholebrain distributed source localization procedure to assess the effects of auditory attention on the auditory steady-state response (ASSR), that uses true binaural stimuli using head-related transfer functions (HRTFs) as stimuli when studying attention effects on the ASSR
Our results show that the ASSR from the cortical source contralateral to the attended hemifield is enhanced in a frequency-specific manner
Summary
The ability to focus attention on a sound of interest amidst irrelevant signals is vital for an animal’s survival. We took advantage of the ability of the cortex to phase lock to input acoustic oscillations around 40 Hz. By driving the auditory cortex with known frequencies, we explored what other cortical regions may be involved in attention. The ASSR to a particular stimulus is sensitive to additions of new sounds to the acoustic scene even when there is no spectral overlap between the different sources (Ross et al, 2005b). Together, these two properties suggest that the ASSR can be used to “frequency tag” neural responses. Cortical responses locked to the modulation frequency of a particular stimulus in a scene must be related to processing of that stimulus
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