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Biological Psychology 78 (2008) 216–219
www.elsevier.com/locate/biopsycho
Brief report
Proneness to hypomania predicts EEG coherence between
left motor cortex and left prefrontal cortex
Carly K. Peterson *, Eddie Harmon-Jones
Texas A&M University, Department of Psychology, 4235 TAMU, College Station, TX 77843, United States
Received 17 August 2007; accepted 24 January 2008
Available online 2 February 2008
Abstract
Previous research has demonstrated that hypomania is associated with approach motivation and activity in the left prefrontal cortex (PFC).
Other research has linked left motor cortex excitability to approach motivation, suggesting the existence of connections between the motor cortex
and PFC. The present research extends this work using unilateral hand contractions to manipulate contralateral cortical activity, and examining the
relationship between motor cortex and PFC inter-electrode EEG coherence and hypomania. Within the right-hand contraction condition,
hypomania related to greater connectivity between the left motor cortex and left PFC, relative to connectivity between the left motor cortex and
right PFC. No relationships were found within the left-hand condition. The present research provides additional support for the role of the left PFC
in bipolar disorder, as well as an important extension of research linking motor cortex excitability to emotion and approach motivation.
Published by Elsevier B.V.
Keywords: Bipolar disorder; EEG coherence; Hypomania; Prefrontal cortex (PFC)
Asymmetrical prefrontal cortical activity is associated with
1998; Schiff and Lamon, 1994). Muscle contractions on one
motivational direction, with greater relative left prefrontal
side of the body affected emotive outcomes, presumably as a
cortical (PFC) activity relating to approach motivation and
result of activation of the contralateral hemisphere (Schiff et al.,
greater relative right PFC activity relating to withdrawal
1998). In addition, right-hand contractions, compared to left-
motivation (Harmon-Jones, 2004). Activity in the left PFC also
hand contractions, caused increased self-reported approach
relates to bipolar disorder, speci?cally in approach motivational
affect to a mildly positive approach-oriented stimulus
situations. For example, bipolar spectrum individuals in a
(Harmon-Jones, 2006), greater behavioral aggression in
manic state exhibit greater relative left PFC activation to
response to an anger-inducing event (Peterson et al., 2008),
challenging and rewarding goal-striving tasks compared to
and contralateral activations in the central region that spread to
control participants (Harmon-Jones et al., in press). Also,
the PFC (Harmon-Jones, 2006; Peterson et al., 2008).
individual differences in hypomania relate to greater relative
The present research utilized the method of unilateral hand
left frontal cortical activity in response to anger-inducing
contractions to manipulate activations over the central cortical
events (Harmon-Jones et al., 2002).
regions. EEG coherence between central regions and PFC was
Research using transcranial magnetic stimulation (TMS) has
examined and related to hypomania. Coherence measures the
linked motor cortex excitability to emotion and to approach
degree EEG signals (within a given frequency band) measured
motivation, suggesting the existence of connections between
at two distinct scalp locations are linearly related to one
the motor cortex and frontal cortical regions involved in
another. High EEG coherence occurs between regions
emotive processes (Hajcak et al., 2007; Schutter et al., in press).
connected by known white matter tracts (Thatcher et al., 1986).
One method of manipulating activation over the motor cortex
Previous research examining the effect of unilateral hand
without TMS uses unilateral hand contractions (Schiff et al.,
contractions on coherence found that while right-hand
contractions caused greater motor cortex/anterior site coher-
ence over the left hemisphere, this effect was not found during
left-hand contractions (Peterson et al., 2008). Rather, left-hand
* Corresponding author. Tel.: +1 979 458 4056; fax: +1 979 845 4727.
contractions appeared to cause greater motor cortex/posterior
E-mail addresses: carlypeterson@gmail.com (C.K. Peterson),
eddiehj@gmail.com (E. Harmon-Jones).
site coherence over the left hemisphere. No effects of hand
0301-0511/$ – see front matter. Published by Elsevier B.V.
doi:10.1016/j.biopsycho.2008.01.011

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C.K. Peterson, E. Harmon-Jones / Biological Psychology 78 (2008) 216–219
217
contractions on coherence were found over the right hemi-
signals were visually scored and portions of the data that contained artifacts
sphere.
were removed. Then, a regression-based eye movement correction was applied
(Semlitsch et al., 1986), after which the data were again visually inspected. All
Based on research demonstrating that right-hand contrac-
epochs 1.024 s in duration were extracted through a hamming window.
tions cause increased approach tendencies as well as greater
Coherence, the magnitude of squared coherency, was computed for the
coherence between the motor cortex and left PFC, we predicted
alpha band (8–13 Hz; no normalization; mean excluded) using Neuroscan
that greater motor cortex/PFC coherence over the left hemi-
software version 4.3 (El Paso, TX) for all four 45 s periods of hand contraction.
sphere during right-hand contractions will be associated with
Alpha power was used because it has been found to relate inversely with cortical
activity and it is the frequency band used in past frontal asymmetry and emotive
proneness to hypomania. No relationships are predicted
research. Coherence estimates then were square-root transformed, to more
between hypomania and right motor cortex/PFC coherence,
closely resemble the absolute value of Pearson correlation coef?cients, and
or during left-hand contractions. Additionally, hypomania and
natural log transformed to normalize the distribution. Asymmetry indexes for
frontal alpha power asymmetry may not be correlated, because
frontal regions (mid-frontal (F3/4), lateral frontal (F7/8), frontal-temporal (Ft7/
the hand contractions do not constitute a strong approach
8), and frontal-central (Fc3/4)) and parietal region (P3/4; for comparison
purposes) were then computed for each motor cortex hemisphere (C3/C4)
motivational situation, as used in past work. That is, although
by subtracting coherence between the motor cortex and right electrode from
mania was found to relate to increased relative left frontal
coherence between the same motor cortex and left electrode.1 For example, the
activity at resting baseline in one study (Kano et al., 1992), this
variable C3F3 coherence minus C3F4 coherence taps left motor – left PFC
relationship has not been replicated in two other studies
coherence relative to left motor – right PFC coherence. For all variables, higher
(Harmon-Jones et al., 2002, in press). However, these latter two
scores indicate greater coherence between the motor cortex and the left
electrode. Indexes were computed in this manner in order to control for the
studies revealed that mania was associated with increased
dynamic relationship between the left and right frontal cortices. That is,
relative left frontal activity during approach motivational
previous research using lesion patients (e.g. Robinson and Downhill, 1995)
situations. These results are consistent with the capability
and TMS (Schutter et al., 2001) has revealed that the inactivation of one frontal
model of frontal EEG asymmetry and personality, which posits
hemisphere causes an over-activation of the opposite frontal hemisphere. These
that there are meaningful individual differences in approach
results suggest that each frontal hemisphere may regulate the other. Addition-
ally, controlling for coherence between each motor cortex and the opposite
and withdrawal tendencies, but these differences are best
hemisphere frontal electrode may tighten the speci?city to coherence between
elicited in speci?c situations (Coan et al., 2006).
electrodes of the same hemisphere. This process is similar to estimating partial
coherence, which controls for volume conduction that may cause erroneous
high coherence (Nunez et al., 1997). Partial coherence estimates are most useful
1. Method
when applied to speci?c hypotheses, such as those made in the present study
(Nunez et al., 1997).
1.1. Participants and design
Asymmetry indexes were also created to examine relationships with EEG
alpha power by subtracting the natural log of the right site from the natural log
Thirty-eight right-handed female introductory psychology students at Texas
of the left site. Because alpha power is inversely related to cortical activity,
A&M University participated in exchange for course credit. A review of
higher scores indicate greater relative left than right cortical activity (Davidson
unobtrusive video recorded during the hand contractions indicated that two
et al., 2000). Because all a priori comparisons were directional and were
participants did not follow instructions and thus were removed from analyses, so
derived from theory, which was based on past research, they were evaluated
that n = 36 (right hand: n = 17; left hand: n = 19).
using a one-tailed criterion of signi?cance (Hayes, 1988; Rosenthal et al., 2000)
1.2. Procedure
2. Results
Participants were instructed to squeeze a ball as hard as they could with their
right or left hand while their opposite hand remained ?at with the palm facing
Zero-order correlations were computed within each condi-
down; hand contraction assignment was determined randomly and experimen-
tion to test our prediction of a relationship between proneness to
ters were blind to condition. Four 45 s contraction trials occurred with a 15 s
relaxation period between each trial. The same procedure was used in Harmon-
hypomania and left motor cortex/relative left PFC coherence
Jones (2006), Peterson et al. (2008), and Schiff et al. (1998). EEG was recorded
during right-hand contractions. Although correlations between
during contractions. Then, participants completed a 15-question version of the
hypomania and motor cortex/relative left frontal coherence
hypomanic personality scale (HYP; Eckblad and Chapman, 1986; Klein et al.,
were predicted based on prior research, the correlations
1996). High scores on this HYP are associated with elevated rates of manic
involving the parietal region were not and thus a Bonferroni
symptoms (Klein et al., 1996). Sample items include ‘‘In unfamiliar surround-
ings, I am often so assertive and sociable that I surprise myself’’ (true), and ‘‘I
correction ( p = .0125) was used.
have often been so excited about an involving project that I did not care about
For those who made right-hand contractions, proneness to
eating or sleeping’’ (true).
hypomania related signi?cantly to left motor cortex/relative left
mid-frontal
and
frontal-central
site
alpha
coherence
1.3. Data collection and reduction
(r(15) = .57, p < .05 and r(15) = .54, p < .05, respectively)
(see Fig. 1). Relationships between hypomania and left motor
EEG was recorded from 27 tin electrodes mounted in a stretch-lycra
electrode cap (Electro-Cap, Eaton, OH). The reference electrode was placed
cortex coherence with other frontal sites were positive but not
on the left ear (A1), and data were also acquired from an electrode on the right
ear (A2), so that an off-line, averaged ears’ reference could be computed.
Vertical and horizontal eye movements (EOG) were also recorded to facilitate
1 An alternative method of computation could have been subtracting right
artifact correction of the EEG. All electrode impedances were under 5000 V,
motor cortex/right electrode coherence from left motor cortex/left electrode
and homologous sites were within 1000 V of each other.
coherence, although this method would not control for volume conduction. No
EEG and EOG were ampli?ed (60 Hz notch ?lter) with Neuroscan Synamps
signi?cant relationships with proneness to hypomania were found using this
(El Paso, TX), bandpass ?ltered (.1–100 Hz) and digitized at 500 Hz. The
method (r’s < .16).

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218
C.K. Peterson, E. Harmon-Jones / Biological Psychology 78 (2008) 216–219
p > .86) were smaller than the same relationships found within
the right-hand contraction condition (mid-frontal: z = 1.54,
p = .06; frontal-central: z = 1.71, p < .05).
Zero-order correlations were also computed within-condi-
tion to examine the relationship between proneness to
hypomania and EEG alpha power asymmetry. No signi?cant
relationships emerged in either condition ( ps > .20).
3. Discussion
As predicted, greater connectivity between the left motor
cortex and left PFC relative to connectivity between the left
motor cortex and right PFC was associated with hypomania
when the left motor cortex was activated by contralateral hand
contractions. Motor cortex/parietal site coherence did not relate
to hypomania, and no relationships were found between EEG
Fig. 1. Proneness to hypomania predicts left motor cortex/relative left mid-
coherence and proneness to hypomania during left-hand
frontal site coherence during right-hand contractions.
contractions. Additionally, no relationships were found
between proneness to hypomania and EEG alpha power
signi?cant ( ps > .10). The relationship between hypomania
asymmetry. We would not necessarily expect the latter
and left motor cortex coherence with the parietal region was not
relationship, as the contraction of the hand muscles does not
signi?cant ( p = .85).
constitute a strong approach motivational situation as used in
Right motor cortex/relative left frontal site coherence did not
past work (Harmon-Jones et al., 2002, in press). That is, a right-
relate to hypomania ( ps > .10), and these coherence values
hand contraction may ‘‘prime’’ approach motivation but
were signi?cantly smaller than the left motor—left PFC
evidence of this prime would only be seen given an approach
coherence (mid-frontal: t(15) = 3.4, p < .01; frontal-central:
motivation situation (Peterson et al., 2008).
t(15) = 2.2, p < .05). Right motor cortex/relative left parietal
These results suggest that individuals with hypomania have
site coherence did not signi?cantly relate to hypomania
strong connectivity between the left motor cortex and left PFC.
(r(15) = À.48, p = .05).
That this effect is only evident during right-hand contractions
Additionally, the correlation between hypomania and left
suggests that the activation in the left motor cortex associated
motor cortex/relative left mid-frontal site coherence differed
with the muscle contraction is an important part of this
signi?cantly from the same relationships over the right motor
relationship. Perhaps the appetitive behaviors associated with
cortex. See Table 1 for all correlations and t-scores for right-
hypomania and activation of the left PFC, such as goal-striving
hand contractions.
and reward responsiveness (Harmon-Jones et al., in press),
Within the left-hand condition, no signi?cant relationships
requires close connectivity with the motor cortex. Greater
between proneness to hypomania and motor cortex/PFC
coherence between these regions may facilitate activation of
coherence occurred ( ps > .50), and none of the relationships
approach ‘‘motor’’-vational behaviors, a notion consistent with
differed between hemispheres ( ps > .55). In addition, the
research suggesting that individuals with hypomanic/manic
correlations between proneness to hypomania and left motor
tendencies show greater approach motivation sensitivity (Har-
cortex/relative left mid-frontal site coherence (r(17) = .07,
mon-Jones et al., 2002, in press; Meyer et al., 1999; Nusslock
p > .76) and between proneness to hypomania and left motor
et al., 2007). While the results are preliminary given that they are
cortex/relative left frontal-central site coherence (r(17) = À.04,
based on a small sample of only female participants, the present
research extends recent work suggesting close connections
Table 1
between the motor cortex and frontal cortical regions (Hajcak
Zero-order correlations between proneness to hypomania and motor cortex/
et al., 2007; Schutter et al., in press) to an enhanced understanding
relative left PFC coherence during right-hand contractions
of the neural circuitry underlying hypomania.
Index
Motor cortex
t-Score
Acknowledgement
Left
Right
Mid-frontal (F3–F4)
.57**
À.41
2.32*
The research described within this article was supported by a
Lateral frontal (F7–F8)
.41
À.10
1.16
grant from the National Science Foundation (BCS 0350435).
Frontal-temporal (Ft7–Ft8)
.38
À.17
1.22
Frontal-central (Fc3–Fc4)
.54*
À.41
2.10*
Parietal (P3–P4)
.05
À.48
1.22
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