| dc.description |
Due to their periodic nature, neural oscillations might
represent an optimal ‘‘tool’’ for the processing of
rhythmic stimulus input [1–3]. Indeed, the alignment
of neural oscillations to a rhythmic stimulus, often
termed phase entrainment, has been repeatedly
demonstrated [4–7]. Phase entrainment is central
to current theories of speech processing [8–10]
and has been associated with successful speech
comprehension [11–17]. However, typical manipulations
that reduce speech intelligibility (e.g., addition
of noise and time reversal [11, 12, 14, 16, 17]) could
destroy critical acoustic cues for entrainment (such
as ‘‘acoustic edges’’ [7]). Hence, the association between
phase entrainment and speech intelligibility
might only be ‘‘epiphenomenal’’; i.e., both decline
due to the same manipulation, without any causal
link between the two [18]. Here, we use transcranial
alternating current stimulation (tACS [19]) to manipulate
the phase lag between neural oscillations and
speech rhythm while measuring neural responses
to intelligible and unintelligible vocoded stimuli with
sparse fMRI. We found that this manipulation significantly
modulates the BOLD response to intelligible
speech in the superior temporal gyrus, and the
strength of BOLD modulation is correlated with a
phasic modulation of performance in a behavioral
task. Importantly, these findings are absent for unintelligible
speech and during sham stimulation; we
thus demonstrate that phase entrainment has a
specific, causal influence on neural responses to
intelligible speech. Our results not only provide an
important step toward understanding the neural
foundation of human abilities at speech comprehension
but also suggest new methods for enhancing
speech perception that can be explored in the future. |
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