Bilateral Gamma/Delta Transcranial Alternating - IA de Investigación especializada en el MIT

Bilateral Gamma/Delta Transcranial Alternating
Current Stimulation Affects Interhemispheric
Speech Sound Integration

Basil C. Preisig1,2,3, Matthias J. Sjerps1,2, Alexis Hervais-Adelman3,
Anne Kösem4, Peter Hagoort1,2, and Lars Riecke5

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Abstracto

■ Perceiving speech requires the integration of different
speech cues, eso es, formants. When the speech signal is split
so that different cues are presented to the right and left ear
(dichotic listening), comprehension requires the integration
of binaural information. Based on prior electrophysiological
evidencia, we hypothesized that the integration of dichotically
presented speech cues is enabled by interhemispheric phase
synchronization between primary and secondary auditory cor-
tex in the gamma frequency band. We tested this hypothesis
by applying transcranial alternating current stimulation (TACS)
bilaterally above the superior temporal lobe to induce or

disrupt interhemispheric gamma-phase coupling. A diferencia de
to initial predictions, we found that gamma TACS applied in-
phase above the two hemispheres (interhemispheric lag 0° )
perturbs interhemispheric integration of speech cues, possibly
because the applied stimulation perturbs an inherent phase lag
between the left and right auditory cortex. We also observed
this disruptive effect when applying antiphasic delta TACS (en-
terhemispheric lag 180°). We conclude that interhemispheric
phase coupling plays a functional role in interhemispheric
speech integration. The direction of this effect may depend
on the stimulation frequency. ■

INTRODUCCIÓN

Our ability to discriminate and assign meaning to speech
sounds relies on the identification and integration of
spectrotemporal cues carried by the acoustic speech
señal. A common approach to study the mechanisms
underlying speech cue integration is to present distinct
cues to different ears (dichotic listening) and to investi-
gate the conditions under which they give rise to the sub-
jective experience of an integrated unified speech sound
(Preisig & Sjerps, 2019; Mathiak, Hertrich, Lutzenberger,
& Ackermann, 2001; Liberman & Mattingly, 1989; Rand,
1974). Although the auditory nerve projects from each
ear to both cerebral hemispheres, processing of acoustic
input is initially dominant in the neural pathway, incluir-
ing the auditory cortex that is “contralateral” to the ear of
presentación (Pollmann, Maertens, por cramon, Lepsien,
& Hugdahl, 2002; Sparks & Geschwind, 1968; Kimura,
1967); for reviews, ver (Hugdahl & Westerhausen, 2016;
Westerhausen & Hugdahl, 2008). Por lo tanto, the unifica-
tion of the binaurally presented speech cues requires in-
terhemispheric integration, eso es, the grouping and

1Donders Institute for Brain, Cognition and Behaviour, Nimega,
Los países bajos, 2Max Planck Institute for Psycholinguistics,
Nimega, Los países bajos, 3University of Zurich, 4Lyon
Neuroscience Research Center (CRNL), Lyon, Francia, 5Maastricht
Universidad

fusion of cues that are initially processed by different cere-
bral hemispheres. Además, processing of speech and
idioma, Por ejemplo, phoneme recognition, is dominant
in the left hemisphere (Mesgarani, Cheung, Johnson, &
Chang, 2014; Giraud & Poeppel, 2012; Chang et al., 2010;
Obleser, Zimmermann, Van Meter, & Rauschecker, 2007;
Jäncke, 2002; Zatorre & Belin, 2001). De este modo, the integration
of binaurally presented speech cues may require interhemi-
spheric transfer of information from the right to the left
auditory cortex via the corpus callosum, as described in
the so-called callosal relay model (Steinmann et al., 2014,
2018; Bayazıt, Oniz, Hahn, Güntürkün, & Ozgören, 2009;
Westerhausen, Grüner, Specht, & Hugdahl, 2009; Jäncke,
2002).

The interhemispheric transfer and integration of sen-
sory information has been suggested to be facilitated
through phase synchronization between neural oscilla-
tions in the two hemispheres (Fell & Axmacher, 2011;
Fries, 2005). Por eso, interhemispheric phase synchroni-
zation may play a crucial role for the integration of dichotic
speech cues. In support of this idea, Steinmann et al.
(2014) have shown modulation of interhemispheric gamma
(30–100 Hz) phase synchronization during dichotic speech
escuchando. Más precisamente, increased gamma functional con-
nectivity was observed in a condition requiring transfer of
speech cues for phoneme recognition, and this connectivity
was directed from the right to the left secondary auditory

Revista de neurociencia cognitiva 32:7, páginas. 1242–1250
https://doi.org/10.1162/jocn_a_01498

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corteza (Steinmann et al., 2018). De este modo, interhemispheric
phase synchronization in the gamma band in the posterior
superior temporal cortex plays a role in the interhemi-
spheric integration of speech.

It is still unclear whether this synchronization contrib-
utes functionally to speech integration or merely results
from it (Zaehle, Lenz, Ohl, & Herrmann, 2010). Más-
encima, it is unclear whether its role is limited to oscillations
in the gamma range. Besides their role in interhemi-
spheric integration of speech, cortical oscillations in the
lower gamma range (25–40 Hz) are also important
para
the processing of phonetic information such as formant
transitions or voicing (Rufener, Oechslin, Zaehle, &
Meyer, 2016; Giraud & Poeppel, 2012; Shamir, Ghitza,
Epstein, & Kopell, 2009; Poeppel, 2003). Slower oscilla-
tions in the delta and theta (∼1–8 Hz) band overlap with
intelligibility-relevant temporal fluctuations in the acous-
tic speech signal and may contribute functionally to the
processing of syllabic information during diotic speech
percepción (Riecke, Formisano, Sorger, Başkent, &
Gaudrain, 2018; Zoefel, Archer-Boyd, & davis, 2018;
Keitel, Ince, Bruto, & Kayser, 2017; Rimmele, Zion
Golumbic, Schröger, & Poeppel, 2015; Gross et al., 2013;
luo & Poeppel, 2007; for a comprehensive review, ver
Kösem & Wassenhove, 2017). De este modo, these slow oscillations
may contribute to diotic speech perception, but there
exists no evidence to suggest that they play a role in
interhemispheric integration.

en este estudio, we investigated the mechanisms under-
lying dichotic speech cue integration. We tested the hy-
pothesis that interhemispheric phase synchronization
plays a functional role in interhemispheric speech inte-
gration. We experimentally manipulated interhemispheric
phase synchronization by applying transcranial alternating
current stimulation (TACS) simultaneously above the audi-
tory cortex in the lateral superior temporal lobe of each
hemisferio. To functionally couple the two regions, nosotros
fixed the phase of TACS across the two stimulation sites
(in-phase condition). En cambio, to functionally decouple
the two regions, we reversed the phase of TACS at one
site (anti-phase condition; Preisig, Sjerps, Kösem, &
Riecke, 2019; Saturnino, Madsen, Siebner, & Thielscher,
2017). This approach has already been successfully
applied to modulate bistable perception in the visual do-
principal (Helfrich et al., 2014). To test for a specific role of
gamma oscillations for interhemispheric speech sound
integración, we applied TACS at 40 Hz (gamma condi-
ción). Además, we included 3.125-Hz TACS (delta
condición) and sham stimulation as control conditions
to enable establishing frequency specificity of the puta-
tive effect of gamma TACS on interhemispheric speech
integración.

Interhemispheric speech integration was assessed
using a dichotic listening task. An ambiguous speech
sound (“base,” perceptually intermediate between the
syllables /ga/ and /da/) was presented to the participants’
right ear and a disambiguating acoustic cue (“chirp,"

which was either a low or high third formant, F3) to their
left ear. Interhemispheric integration of the base and the
chirp is reflected by an increased number of /ga/ reports
in the low-F3 condition and an increased number of /da/
reports in the high-F3 condition (Preisig & Sjerps, 2019).
We predicted that interhemispheric phase synchroni-
zación (in-phase condition) would significantly increase
interhemispheric speech integration, as reflected by an
increased number of /ga/ reports in the low-F3 condition
and an increased number of /da/ reports in the high-F3
condición, compared with interhemispheric phase de-
synchronization (anti-phase condition). Además,
we predicted that functional coupling of bilateral auditory
cortices in the gamma, but not delta frequency band,
would strengthen interhemispheric speech integration,
compared with sham stimulation.

MÉTODOS

Participantes

Thirty-six right-handed volunteers (m = 22.56 años, DE =
2.93; 14 hombres) participated in the study. All participants
had normal or corrected-to-normal visual acuity. The par-
ticipants reported no history of neurological, psychiatric,
or hearing disorders, and all had normal hearing (hearing
thresholds of less than 25 dB HL at 250, 500, 750, 1000,
1500, 3000, y 4000 Hz, tested on both ears separately
using pure tone audiometry) and no threshold difference
between the left and the right ear larger than 5 dB for any
of the tested frequencies. All participants gave written in-
formed consent before the experiment. Ethical approval to
conduct this study was provided by the local ethics com-
mittee (CMO region Arnhem-Nijmegen). This study was
conducted in accordance with the principles of the latest
version of the Declaration of Helsinki.

Electric Stimulation

Electric currents were applied through two high-density elec-
trode configurations, each consisting of concentric rubber
electrodes: a central circular electrode (radius = 1.25 cm)
and a surrounding ring electrode (inner radius = 3.5 cm,
outer radius = 4.8 cm). Each electrode configuration was
connected to a separate battery-driven transcranial current
stimulator (Neuroconn, Ilmenau, Alemania), similar to pre-
vious two-channel approaches (Ten Oever et al., 2016;
Riecke, Formisano, Herrmann, & Sack, 2015). The elec-
trode configurations were centered according to the inter-
national 10–20 system over CP5 (above the left cerebral
hemisferio) and CP6 (above the right cerebral hemi-
sphere; ver figura 1). These scalp locations were chosen
to produce relatively strong currents in the target regions
over the auditory speech areas (es decir., left and right lateral
superior temporal lobe), as suggested by prior electric field
simulations on a standard head model using the simnibs
toolbox (Thielscher, Antunes, & Saturnino, 2015).

Preisig et al.

1243

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Cifra 1. Dual-site high-density
TACS setup. Left: The electrode
configurations were centered
according to the international
10–20 system over CP5 (arriba
the left cerebral hemisphere)
and CP6 (above the right
cerebral hemisphere). Right:
The interhemispheric phase
synchrony was manipulated
using in-phase TACS (0° phase
lag between stimulation sites,
dotted line) and anti-phase
(180° phase lag, dotted line)
TACS. The colors represent the
polarity (positive = red;
negative = blue) of the current
for the time stamp highlighted
by the dotted line. RH = right
hemisferio; LH = left
hemisferio.

TACS was applied at a frequency in the low gamma
banda (40 Hz) or in the delta band (3.125 Hz), the latter
matched the timescale of the syllabic envelope, eso es,
the duration of the syllable matched the length of a half
cycle of the delta TACS. Before starting the actual exper-
mento, we ensured that all participants tolerated the
TACS well. TACS intensity was adjusted individually to
the point for which the participant reported feeling com-
fortable or uncertain about the presence of the current
(1.4 ± 0.1 mA peak-to-peak, mean ± SD across par-
ticipants). Impedance was kept below 10 kΩ. el promedio
current density was 0.2 mA/cm2 at the center electrode
y 0.06 mA/cm2 at the concentric ring electrode. Stim-
ulation was ramped over the first and the last 10 sec of
each experimental block using raised cosine ramps.

The timing of the electric and auditory stimuli was con-
trolled using a multichannel D/A converter (National
Instrumentos, sampling rate: 11 kHz) and Datastreamer
software (Ten Oever et al., 2016). Visual stimulation and
response recording were controlled using Presentation
software ( Versión 18.0, Neurobehavioral Systems, Cª,
berkeley, California).

Behavioral Pretest

Interhemispheric speech sound integration was assessed
by simultaneously presenting an ambiguous base and a
disambiguating chirp (F3) to the right and left ear,
respectivamente. The chirp supported either a /da/ (high F3
∼ 2.9 kHz) or a /ga/ (low F3 ∼ 2.5 kHz) interpretation of
the ambiguous base. Because perceptual category bound-
aries may vary across individuals, a pretest was used to

define participant-specific ambiguous base stimuli for the
main experiment. The pretest included the presentation
of nine stimuli of the /da/–/ga/ continuum, cada 16 veces,
in random order. To make this pretest most similar to the
main experiment, syllables of the /da/–/ga/ continuum
were presented to the right ear, and a single F3 chirp
(identical to the F3 component in the ambiguous base
stimulus) to the left ear. Subjective category boundaries
were estimated by assessing individual psychometric
curves and identifying the point at which participants re-
ported perceiving the stimulus as /da/ or /ga/ in ∼50% of
the trials. The stimulus associated with this individual
category boundary was then used as the base stimulus
for the subsequent main experiment (Preisig & Sjerps,
2019). Further detail concerning stimulus creation is re-
ported in a previous publication using the same materials
(Preisig & Sjerps, 2019).

Experimental Design and Task

The experiment included four stimulation conditions and
sham stimulation. Electric stimulation was applied at one
of the two frequencies, 40 Hz and 3.125 Hz. Each of these
frequency conditions was presented in two interhemi-
spheric phase synchronization conditions: (A) “In-phase
stimulation” was applied with a phase lag of 0° between
the central electrodes placed over the left and the right
auditory speech areas (es decir., bilateral superior temporal
lobe) to induce interhemispheric synchronization. (B)
“Anti-phase stimulation” was applied with a relative phase
lag of 180° to induce interhemispheric desynchronization
(Preisig et al., 2019; Saturnino et al., 2017; ver figura 1).

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Revista de neurociencia cognitiva

Volumen 32, Número 7

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During “sham stimulation” (placebo), the onset ramp was
followed immediately by an offset ramp, eso es, no elec-
tric stimulation was applied during the actual experiment.
The ramp was repeated at the end of the block.

The experiment consisted of 10 experimental blocks.
Each block consisted of 48 trials containing the ambigu-
ous base stimulus (for which the F3 frequency was set at
the participant-specific subjective category boundary
value that was obtained in the pretest) y 12 trials con-
taining unambiguous base stimuli (which contained an
F3 component that supported a clear /da/ interpretation
[∼2.9 kHz] or a /ga/ interpretation [∼2.5 kHz]) presentado
to the right ear. The ambiguous base stimulus was paired
with a disambiguating F3 chirp presented to the left ear
(24 trials with the high F3 chirp and 24 trials with the low
F3 chirp). In the unambiguous stimuli, a chirp with the
same F3 frequency as the base was presented to the left
ear. Unambiguous stimuli did not require interhemispheric
integration for disambiguation because participants could
readily identify these stimuli based on monaural input
solo, eso es, the unambiguous base stimulus presented
to the right ear. The first half of the experiment included
five blocks of trials: gamma in-phase TACS, gamma anti-
phase TACS, delta in-phase TACS, delta anti-phase TACS,
and sham. The order of the first five experimental blocks
was reversed in the second half of the experiment. The or-
der of all blocks was pseudorandomized, such that blocks
of the same TACS frequency followed upon each other and
counterbalanced across participants. This pseudorandomi-
zation scheme was used to account for potential cross-
frequency carryover effects (zorros, Bruto, & Thut, 2015).
After each block, participants were asked to rate the
subjective strength of any sensations induced by the
stimulation on a visual analogue scale from 0 cm (No
subjective sensations) a 10 cm (strong subjective sen-
sations). Although sensation ratings were relatively low
in all conditions, TACS blocks (m = 2.64, DE = 1.38)

were rated significantly higher than sham blocks (m =
1.77, DE = 1.46), t(25) = 3.11, pag < .01. However, even though participants rated TACS and sham blocks dif- ferently (Turi et al., 2019; Zoefel, Allard, Anil, & Davis, 2020), this unlikely influenced our main results, as we found no association between sensation ratings and be- havioral performance, Pearson’s R(128) = −0.14, p = .10, across stimulation conditions. The auditory stimuli were presented with an ISI of on average 3.5 sec. The exact ISI was set so that the syllable onset occurred at one of six predefined, equidistant TACS phases (TACS/syllable onset lag: 30°, 90°, 150°, 210°, 270°, 330° ). This allowed compensating for indi- vidual differences in the optimal relative TACS syllable timing (Zoefel, Davis, Valente, & Riecke, 2019; Riecke et al., 2018; Riecke, Formisano, et al., 2015; Riecke, Sack, & Schroeder, 2015), with the aim to improve the detect- ability of putative stimulation effects in the group-level analysis. In this study, we did not observe any effect of TACS/syllable onset lag (Figure 2). Thus, the behavioral data were pooled across the six TACS/syllable onset lags for each stimulation condition. Every stimulus was pre- ceded by a fixation cross presented 600 msec before auditory stimulus onset. At 1450 msec after the fixation cross onset, the response options /ga/ and /da/ were pre- sented (one above and one below the fixation cross, falling within a visual angle of 9.43° ). The participants indicated their response by pressing the corresponding response button with their left index finger.1 Participants were instructed to perform as accurately and as fast as possible. The position (up vs. down) of the response options was counterbalanced across participants. Data Analysis In a first step, we assessed the reliability of the categorical judgments of individual participants on unambiguous D o w n l o a d e d l l / / / / j t t f / i t . : / / f r o m D o h w t t n p o : a / d / e m d i f r t o p m r c h . s p i l d v i r e e r c t c . m h a i e r d . u c o o m c n / j a o r t c i c n e / - a p r d t i 3 c 2 l 7 e 1 - 2 p 4 d 2 f 2 / 0 3 1 2 3 / 5 7 4 / 6 1 2 o 4 c 2 n _ / a 1 _ 8 0 6 1 1 4 6 9 1 8 8 p / d j o b c y n g _ u a e _ s 0 t 1 o 4 n 9 0 8 8 . p S d e f p e b m y b e g r u 2 e 0 s 2 t 3 / j / f . t o n 0 5 M a y 2 0 2 1 Figure 2. The phase angle histograms show the distribution of the participants’ best TACS/syllable onset lag in the stimulation condition labeled above each histogram (in-phase 40 Hz, anti-phase 40 Hz; sham, in-phase 3.125 Hz, anti-phase 3.125 Hz). The optimal timing of TACS and the syllable presentation, that is, average best lag was 87° ± 12° (mean ± SEM ) across participants. The distribution of the participants’ best lag pooled across TACS stimulation condition (in-phase 40 Hz, anti-phase 40 Hz; sham, in-phase 3.125 Hz, anti-phase 3.125 Hz) did not deviate significantly from uniformity (Rayleigh test for nonuniformity of circular data, z = 1.733, p = .177), suggesting that the best lag varied substantially across participants. Moreover, the best lags observed in the different stimulation conditions were observed to be uncorrelated ( ps >.08), Indicando que
participants’ best lag varied across stimulation conditions. To compensate for these individual differences in optimal relative TACS syllable timing,
the best lag was aligned across participants, and the remaining phase bins were phase wrapped, separately for each participant and stimulation
condición (for details, see Experimental Design and Task section). Initial analysis of the aligned data for an effect of TACS/syllable onset lag revealed
no significant result; por lo tanto, the six TACS/syllable onset lags were pooled in all subsequent analyses.

Preisig et al.

1245

endpoint trials (base and chirp stimuli with the same F3
endpoint, supporting the interpretation of /ga/ or /da/)
collected during the sham blocks. Por eso, for each partic-
ipant, we tested with a chi-square test whether the pro-
portion of /ga/ responses differed between /ga/ and /da/
endpoint stimuli. Based on this criterion, the data of four
participants were excluded from further analyses because
their classification accuracy did not significantly exceed
chance level. One additional participant was excluded
because of a technical error during the experiment.
De este modo, the final data set included data from 31 Participantes
(m = 22.63 años, DE = 3.20, 12 hombres).

Two dependent variables were analyzed: the categori-
cal response on each individual trial (0 = /da/; 1 = /ga/)
and the proportion of responses consistent with the pre-
sented F3 chirp (es decir., those in which interhemispheric in-
tegration occurred), per condition. These variables were
computed based on participants’ responses to the stimuli
requiring interhemispheric integration, eso es, the stim-
uli composed of an ambiguous base and a disambi-
guating F3 chirp. For each stimulation condition, el
proportion of integrated trials was calculated per TACS/
syllable onset lag, which were concatenated to build a
behavioral time series. To compensate for individual dif-
ferences (Cifra 2), el maximo (best lag) of the time
series was subsequently aligned across individuals.
Because we did not observe any effect of TACS/syllable
onset lag, the behavioral time series were pooled across
the six TACS/syllable onset lags for each stimulation con-
condición. Statistical analyses were conducted in R ( Versión
3.3.3) using parametric tests (normality assumption was
fulfilled, the dependent variable in each of our conditions
was normally distributed, Shapiro–Wilk test of normality,

ps > .19): Linear mixed-effect models were used to ana-
lyze categorical responses, and repeated-measures
ANOVAs were used to test for a stimulation effect, enterrar-
hemispheric phase effect, frequency effect, and interac-
ciones. Post hoc comparisons were conducted using
paired t tests and false discovery rate (FDR) corrections
for multiple comparisons were applied (Benjamini &
Hochberg, 1995).

RESULTADOS

The average classification accuracy (%) including unambig-
uous stimuli (extreme points from the /ga/–/da/ continuum)
during sham blocks was high (m = 90.59, DE = 8.74). Para
trials that required interhemispheric integration, par-
ticipants integrated the information from the F3 chirp on
average on 73.4 ± 9.7% (mean ± SEM) of the trials that
included an ambiguous base stimulus. Primero, probamos
whether participants’ responses to ambiguous base stimuli
were influenced by the frequency of the disambiguating F3
chirp presented to the contralateral ear. For this analysis,
we only included sham blocks. We observed that partici-
pants gave on average 34.12 ± 10.08% (mean ± SEM )
/ga/ responses to ambiguous bases combined with the high
(∼2.9 kHz) F3 chirp and 80.92 ± 9.14% (mean ± SEM) /ga/
responses to ambiguous bases combined with the low
(∼2.5 kHz) F3 chirp. To confirm that the chirp F3 frequency
influenced participants’ response (0 = /da/; 1 = /ga/ re-
respuesta), a logistic linear mixed-effect model with the fixed
factor “chirp type” (niveles: high F3 = −1; low F3 = 1), y
by-participant random intercepts and slopes were fitted to
los datos. The analysis revealed a main effect of chirp type
(B = 2.733, z = 13.052, pag < .001). This result indicates that D o w n l o a d e d l l / / / / j t t f / i t . : / / f r o m D o h w t t n p o : a / d / e m d i f r t o p m r c h . s p i l d v i r e e r c t c . m h a i e r d . u c o o m c n / j a o r t c i c n e / - a p r d t i 3 c 2 l 7 e 1 - 2 p 4 d 2 f 2 / 0 3 1 2 3 / 5 7 4 / 6 1 2 o 4 c 2 n _ / a 1 _ 8 0 6 1 1 4 6 9 1 8 8 p / d j o b c y n g _ u a e _ s 0 t 1 o 4 n 9 0 8 8 . p S d e f p e b m y b e g r u 2 e 0 s 2 t 3 / j . / f t o n 0 5 M a y 2 0 2 1 Figure 3. (A) The proportion of /ga/ responses (mean ± SEM ) as a function of chirp type (high F3, low F3) in the sham condition. (B) Participants’ average performance (mean ± SEM across participants) is shown for each stimulation condition relative to sham (gray reference line, shaded area represents SEM across participants). Dots represent the data points of single participants. 1246 Journal of Cognitive Neuroscience Volume 32, Number 7 interhemispheric speech integration occurred (i.e., the participants integrated the chirp and the contralateral ambiguous base) during the sham blocks as we expected (Figure 3A). Figure 3B shows participants’ average performance for each stimulation condition and the sham condition. In the stimulation conditions, overall performance ranged on average between 71.75% and 73.98%, whereas in the sham condition, it was significantly better (76.16%; average dif- ference: 3.46%), t(30) = 2.89, p = .007, d = 0.37. To test whether the strength of this general stimulation effect depended on the frequency or phase synchrony of the TACS, a two-way repeated-measures ANOVA, including the within-subject factors Stimulation Frequency (40 Hz, 3.125 Hz) and Interhemispheric Phase Synchronization (in-phase, anti-phase) was conducted for the dependent variable difference in interhemispheric integration of the chirp as compared with sham stimulation. Delta values be- tween the performance in each stimulation condition and the sham condition were included in the analysis. Contrary to our predictions, this analysis revealed no significant interaction Stimulation Frequency × Interhemispheric Phase Synchronization, F(1, 30) = 2.59, p = .12, ηp 2 = .01, or main effects of Stimulation Frequency, F(1, 30) = 0.12, p = .73, ηp 2 = .0004, or Interhemispheric Phase Synchronization, F(1, 30) = 0.20, p = .66, ηp 2 = .0008. The lack of a main effect of Interhemispheric Phase Synchronization implies no significant difference between in-phase versus anti-phase stimulation. To identify the specific TACS conditions under which the stimulation effect occurred, a one-way repeated-measures ANOVA, including the within-subject factors Stimulation Condition (sham, in-phase 40 Hz, anti-phase 40 Hz, in-phase 3.125 Hz, anti-phase 3.125 Hz) was conducted for the de- pendent variable the proportion of integrated trials. This analysis revealed a significant main effect of Stimulation Condition, F(4, 120) = 2.99, p = .02, ηp 2 = .02. Pairwise comparisons revealed significantly reduced performance in the in-phase 40-Hz condition, t(30) = −2.78, p = .049, FDR- corrected, d = −0.37, and anti-phase 3.125-Hz condition, t(30) = −2.76, p = .049, FDR-corrected, d = −0.40, com- pared with sham stimulation, but not in the anti-phase 40-Hz condition, t(30) = −1.44, p = .32, FDR-corrected, d = −0.22, or in-phase 3.125-Hz condition, t(30) = −2.16, p = .13, FDR-corrected, d = −0.29. These results indicate that the bihemispheric TACS modulated inter- hemispheric speech integration. DISCUSSION In this study, we tested the hypothesis that interhemi- spheric phase synchronization facilitates interhemispheric speech integration. To test this, we applied TACS simulta- neously above listeners’ left and right auditory speech areas (either in-phase or anti-phase) to synchronize or de- synchronize the two areas and measured the effect on interhemispheric speech integration. Based on previous evidence from electrophysiological studies (Steinmann et al., 2014, 2018), interhemispheric integration of speech might be causally related to phase synchronization of bilat- eral auditory speech areas in the gamma frequency band. No such effect has been reported for the delta frequency band. Thus, we predicted that functional coupling of bilat- eral auditory speech areas in the gamma, but probably not in delta frequency band, would strengthen interhemi- spheric speech integration, compared with functionally decoupling them. Our results show a reduction of interhemispheric in- tegration under gamma TACS compared with sham stim- ulation. This reduction was significant when gamma TACS was applied in-phase above the two cerebral hemispheres. We also observed a significant reduction when anti-phase delta TACS was applied. We found no significant difference between in-phase compared with anti-phase conditions for either gamma or delta TACS. Although we found a general reduction of performance during TACS versus sham stimu- lation, we observed no main effect or interaction in an over- all ANOVA comparing these reductions across the different TACS conditions. However, the observed pattern of signif- icant (in-phase gamma TACS, anti-phase delta TACS) and nonsignificant (anti-phase gamma TACS, in-phase delta TACS) changes in speech perception relative to sham stim- ulation strongly suggests that TACS modulated interhemi- spheric speech cue integration. Contrary to our prediction, in-phase, not anti-phase, gamma TACS perturbed interhemispheric speech cue in- tegration. This finding implies that full interhemispheric phase synchronization (0° difference) at 40 Hz is not ben- eficial for interhemispheric speech cue integration. This observation could be related to interindividual differences in interhemispheric auditory transfer times (Henshall et al., 2012). Strongest interhemispheric integration may occur when gamma phase in the two hemispheres differs in a manner commensurate with individual auditory trans- fer times. This notion is supported by findings showing that the auditory event-related N100 to dichotically pre- sented syllables occurs at a different latency over the right versus the left auditory cortex (Eichele, Nordby, Rimol, & Hugdahl, 2005). The reported lag is on average 15 msec, which closely matches with the half cycle duration of our gamma TACS (12.5 msec). In line with this, a recent study found that anti-phase TACS applied at 40 Hz does not affect response laterality during dichotic listening (Meier et al., 2019). Critically, the authors could show in a follow-up analysis that, only in participants with intrinsic gamma phase asymmetries closer to 0°, anti-phase gamma TACS led to a reduction of interhemispheric integration, that is, a shift in response laterality to right ear. These re- sults corresponds well with our finding that anti-phase gamma TACS, which imposes an interhemispheric lag of 12.5 msec, may not perturb speech cue integration. Our current experimental design does not allow further testing this idea; this may be done in future studies that Preisig et al. 1247 D o w n l o a d e d l l / / / / j t t f / i t . : / / f r o m D o h w t t n p o : a / d / e m d i f r t o p m r c h . s p i l d v i r e e r c t c . m h a i e r d . u c o o m c n / j a o r t c i c n e / - a p r d t i 3 c 2 l 7 e 1 - 2 p 4 d 2 f 2 / 0 3 1 2 3 / 5 7 4 / 6 1 2 o 4 c 2 n _ / a 1 _ 8 0 6 1 1 4 6 9 1 8 8 p / d j o b c y n g _ u a e _ s 0 t 1 o 4 n 9 0 8 8 . p S d e f p e b m y b e g r u 2 e 0 s 2 t 3 / j f t . / o n 0 5 M a y 2 0 2 1 parametrically manipulate interhemispheric phase asyn- chrony in multiple steps across the gamma cycle. Other studies have reported that bilateral 40-Hz TACS perturbs phonemic processing, decreasing discrimina- bility of syllables with different VOTs in young adults (Rufener, Zaehle, Oechslin, & Meyer, 2016) but increas- ing it in older adults (Rufener, Oechslin, et al., 2016) and in dyslexic individuals (Rufener, Krauel, Meyer, Heinze, & Zaehle, 2019). Therefore, we cannot rule out that our gamma TACS also affected local phoneme processing. We positioned our electrodes so as to stimulate espe- cially cortical speech areas in the lateral superior tempo- ral lobe; therefore, we believe that the observed effect originates from these areas. We cannot exclude that other regions were stimulated by spreading current and also contributed to the effect as our design did not in- clude control regions. In addition to that, gamma TACS might have affected deployment of attentional resources, considering that unilateral 40-Hz TACS may affect perfor- mance on dichotic working memory tasks ( Wöstmann, Vosskuhl, Obleser, & Herrmann, 2018). Surprisingly, our results suggest that not only gamma- phase coupling but also delta-phase coupling plays a role for interhemispheric speech cue integration. Our observa- tion that anti-phase delta TACS perturbed behavioral per- formance suggests that this type of stimulation disrupts cross-lateral transfer of speech cues as well. Previous stud- ies using dichotic stimulus presentation did not report phase coupling in this frequency band (Steinmann et al., 2014, 2018). Therefore, we speculate that anti-phase TACS may have caused a difference in neural excitability be- tween hemispheres during the processing of the binaural input: When the current was positive over one site, it was negative over the contralateral site, and vice versa. This may have been particularly relevant for the delta TACS condition, in which the applied current matched the syllabic envelope. Increased neural excitability in one hemisphere and decreased excitability in the other may have resulted in an interhemispheric difference in the ef- fectiveness with which the dichotic syllabic components (chirp or ambiguous base) were processed. Indeed, trans- cranial direct current stimulation has been shown to have polarity-specific effects on temporal and spectral pro- cessing of auditory input (Heimrath, Kuehne, Heinze, & Zaehle, 2014; Schaal, Williamson, & Banissy, 2013; Zaehle, Beretta, Jäncke, Herrmann, & Sandmann, 2011; Vines, Schnider, & Schlaug, 2006). An important additional consideration is that anti- phase delta TACS may disrupt interhemispheric cross- frequency dynamics between delta and gamma oscillations during speech perception (Giraud & Poeppel, 2012). Coupling of these frequency bands could be of particular relevance for interhemispheric integration, because re- gions in the left and right auditory cortex may be differ- ently tuned with respect to these frequency bands, with a relative leftward dominance of low-gamma neural oscil- lations and/or rightward dominance of slow frequency oscillations (Flinker, Doyle, Mehta, Devinsky, & Poeppel, 2019; Bouton et al., 2018; Giraud & Poeppel, 2012; Saoud et al., 2012; Poeppel, 2003). In addition to this, there is support that right hemispheric auditory process- ing may be tuned for spectral information (Preisig & Sjerps, 2019; Bouton et al., 2018) and left hemispheric auditory processing may be tuned for temporal informa- tion (Flinker et al., 2019; Saoud et al., 2012)—a theoretical framework originally formulated in the asymmetric sam- pling theory (Poeppel, 2003; for a similar framework, see Zatorre & Belin, 2001). In a previous study, we found that the laterality of initial chirp sound processing, that is, the ear of presentation, did not influence participants’ perceptual decisions (Preisig & Sjerps, 2019). However, stimulus laterality influenced the processing speed of integration. Thus, we cannot rule out that the ear of presentation contributes to the observed TACS effect. Our current experimental design does not allow further testing this idea; this may be done in future studies apply- ing interhemispheric cross-frequency delta–gamma TACS stimulation presenting the chirp to the left and the right ear, respectively. In summary, our results indicate that both gamma and delta TACS affect interhemispheric speech integration, but in different ways. The induced perturbations imply that interhemispheric phase coupling plays a functional role in interhemispheric speech integration. 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