Emotional modulation of the postauricular reflex

Psychophysiology, 41 (2004), 426–432. Blackwell Publishing Inc. Printed in the USA.
Copyright r 2004 Society for Psychophysiological Research
DOI: 10.1111/j.1469-8986.00160.x
BRIEF REPORT
Emotional modulation of the post-auricular reflex
STEPHEN D. BENNING, a CHRISTOPHER J. PATRICK, a and ALAN R. LANG b
a Department of Psychology, University of Minnesota–Twin Cities, Minneapolis, Minnesota, USA
b Department of Psychology, Florida State University, Tallahassee, Florida 32306, USA
Abstract
A large literature now exists on emotional modulation of the startle blink reflex. The current study examined affective
modulation of the post-auricular reflex, which can be measured in relation to the same noise probe used to evoke the
startle reflex. We recorded the post-auricular reflex during viewing of pictures that varied systematically in emotional
valence, content, and intensity. A significant linear valence modulation effect was found, with pleasant pictures
potentiating and aversive pictures inhibiting the post-auricular reflex in comparison with neutral pictures. This
modulatory effect did not vary as a function of picture content, but it was most robust for highly intense emotional
pictures. Implications for the assessment of basic emotional action tendencies are discussed.
Descriptors: Post-auricular, Emotion, Appetitive, Startle, Reflex, Post-auricular reflex
It is well known that the startle blink reflex to an abrupt acoustic
probe is modulated by the emotional valence of foreground
stimulation. Startle magnitude is enhanced (potentiated) during
viewing of unpleasant pictures compared with neutral pictures,
and inhibited during viewing of pleasant pictures (cf. Lang,
Bradley, & Cuthbert, 1990; Vrana, Spence, & Lang, 1988). A
different reflex evoked by the same noise probe used to elicit
startle is the post-auricular reflex. Although several studies have
investigated attentional modulation of this reflex (e.g., Hackley,
Woldorff, & Hillyard, 1987; Patuzzi & O’Beirne, 1999; Sollers &
Hackley, 1997), the emotional modulation of the post-auricular
reflex has not been similarly investigated. The current study
examined post-auricular reflex modulation as a function of the
valence, thematic content, and affective intensity of picture
stimuli to determine whether this reflex might provide unique
information about on-line affective processing.
Emotion and Reflex Modulation
Reflexes are valuable to measure in studies of emotion because
they have the potential to tap the basic action dispositionsFappetitive-approach
and defensive-withdrawalFthat underlie posi-
This study was supported by grants MH17069, MH52384, and
MH65137 from the National Institute of Mental Health, Grant
AA12164 from NIAAA, and by funds from the Hathaway endowment
at the University of Minnesota. We are grateful to Daniel Blonigen and
Brian Hicks for their assistance with stimulus selection, counterbalancing,
and collection of data, and to Edward Bernat for his scholarly
comments on drafts of this manuscript.
Address reprint requests to: Christopher J. Patrick, Department of
Psychology, University of Minnesota, Elliott Hall, 75 East River Road,
Minneapolis, MN 55455, USA. E-mail: cpatrick@tc.umn.edu.
426
tive and negative emotional states. In this regard, Lang et al.
(1990) proposed a response-matching explanation for the linear
valence modulation effect observed for the startle reflex: A
defensive reflex, such as startle, is enhanced during aversive picture
viewing because the motivational state induced by the picture is
defensive, leading to a synergistic facilitation. Conversely, startle is
inhibited during pleasant pictures because here the ongoing
approach disposition opposes the defensive startle reaction. From
this perspective, a reverse pattern of modulation would be
expected for an appetitive reflexFthat is, potentiation during
pleasant pictures and inhibition during aversive pictures.
If this response-matching explanation of startle modulation in
humans is accurate, then intense emotional pictures that evoke
strong appetitive or defensive motivational states should produce
maximal modulatory effects on startle. Consistent with this,
Cuthbert, Bradley, and Lang (1996) reported that blink potentiation
was greater for high- versus low-arousal aversive pictures, and
that blink inhibition was greater for high- versus low-arousal
pleasant pictures. There is also evidence that modulatory effects
are stronger for picture contents that are tied to primary motives:
Blink inhibition is maximal for pleasant pictures that are sexual in
content, and potentiation is maximal for directly threatening
aversive pictures (Bradley, Codispoti, Cuthbert, & Lang, 2001;
Levenston, Patrick, Bradley, & Lang, 2000). The idea that startle
potentiation during aversive picture viewing reflects defensive
mobilization also fits with neuroscience research indicating that
fear-potentiated startle in rats is mediated by input from the
subcortical amygdala to the midbrain component of the startle
circuit, the nucleus reticularis pontis caudalis (Davis, Gendelman,
Tischler, & Gendelman, 1982; Hitchcock & Davis, 1986).
From this response-matching perspective, other defensive
reflexes should show modulatory patterns akin to startle.

Emotion and the post-auricular reflex 427
Moulder, Bradley, Requin, and Lang (1995) reported a parallel
linear valence modulation pattern (i.e., aversive4pleasant) for the
Hoffman reflex, a spinal flexion reflex elicited by aversive electric
shock. In this study, modulation effects were stronger for
emotional pictures of high intensity, and more pronounced for
participants who rated the shock as aversive. In contrast to this,
the magnitude of the spinal T reflex, elicited by nonaversive
tapping or vibratory stimulation of the Achilles tendon, is
comparably enhanced during both pleasant and unpleasant
pictures in relation to neutral (Bonnet, Bradley, Lang, & Requin,
1995)Fimplying facilitation by arousal (i.e., nonspecific activation;
cf. Lang et al., 1990). An opposing quadratic pattern,
involving inhibited probe response during both pleasant and
aversive pictures compared with neutral, has been reported for
other measures (e.g., noise-evoked P300 brain potential; Schupp,
Cuthbert, Bradley, Birbaumer, & Lang, 1997). This pattern
indicates a cross-modal attentional effect (cf. Anthony & Graham,
1985) arising from the fact that visual affective stimuli draw more
attention away from the acoustic modality (Lang et al., 1990).
The Post-Auricular Reflex
The post-auricular reflex is a vestigial muscle response in humans
that acts to pull the ear backward (Gray, 1901/1995). It is
recorded by positioning electrodes over the post-auricular muscle
behind the ear (O’Beirne & Patuzzi, 1999). Because this reflex is
evoked by an acoustic probe stimulus, it can be assessed
concurrently with the startle blink (cf. Hackley, 1993). The
post-auricular reflex has a much faster latency than the blink
reflex (i.e., 9–11 ms vs. 45–50 ms; Hackley et al., 1987), implying
a simpler brain stem circuitry. Following Cassella and Davis
(1986), Hackley (1993) theorized that the circuit for the postauricular
reflex parallels the startle circuit in terms of input (i.e.,
cochlear nucleus) and output (facial motor nucleus) components,
but that it does not include the nucleus reticularis pontis
caudalisFthe juncture of the startle circuit at which input is
received from the amygdala, accounting for the fear-potentiated
startle effect (Davis et al., 1982). If this model of the postauricular
circuit is correct, one might expect a different pattern of
modulation for the post-auricular reflex compared with the
startle reflex.
The post-auricular reflex has been assessed in a small number
of attention studies in humans. The magnitude of this reflex, like
that of startle, is attenuated if the reflex-eliciting noise is preceded
by the occurrence of a transient acoustic stimulus (prepulse)Fan
effect known as ‘‘prepulse inhibition’’ (Hackley, 1993; Hackley et
al., 1987). Prepulse inhibition is thought to reflect a low-level
sensory gating mechanism that affords ‘‘protection’’ at early
stages of perceptual processing (Braff, Geyer, & Swerdlow, 2001;
Graham, 1975). On the other hand, the post-auricular reflex
appears less subject to controlled attentional influences than
startle. Hackley et al. (1987) reported that prepulse inhibition
was enhanced for the startle reflex, but not the post-auricular
reflex, when participants were instructed to attend to the prepulse
stimulus. This dissociation in effects was interpreted in terms of
the faster, simpler circuitry of the post-auricular reflex (see also
Hackley, 1993).
However, when instructed to attend to one ear or the other in
an auditory task, participants showed generally enhanced postauricular
reflex magnitudes, and enhanced prepulse inhibition of
this reflex, on the side to which attention was directed (Hackley,
1993; Hackley et al., 1987). Hackley et al. (1987) attributed this
effect to motor priming (i.e., a tensing of muscles on the
instructed side) rather than attention per se. Consistent with this,
Patuzzi and O’Beirne (1999) reported a unilateral increase in
baseline activity of the post-auricular muscle, and an affiliated
increase in the magnitude of the post-auricular reflex to noise
probes, when participants were instructed to rotate their eyes
toward one side of the head or the other. On the other hand,
it has been shown that activity in the post-auricular muscle region
decreases during processing of near-threshold auditory
stimulation compared with more intense auditory stimulationFconsistent
with the idea that somatic activation decreases
to enhance perceptual sensitivity during orienting (Stekelenburg
& van Boxtel, 2001, 2002). However, the post-auricular reflex
was not assessed in these latter studies, so it is unclear what
impact these changes in baseline EMG activity would have on its
magnitude.
Current Study
The current study was conducted to examine modulatory effects
of emotion on the post-auricular reflex. To do this, we delivered
unwarned noise probes to participants during viewing of
pleasant, neutral, and unpleasant pictures and recorded
responses of the post-auricular muscle behind each ear. To gain
additional information about modulatory effects, we manipulated
both the intensity and thematic content of pleasant and
unpleasant pictures.
The status of the post-auricular reflex as a defensive or
appetitive reflex is unclear. However, the response-matching
hypothesis (Lang et al., 1990) makes clear predictions in either
case. If the post-auricular reflex is defensive in nature, it should
be potentiated during aversive pictures and inhibited during
pleasant pictures. If it is appetitive, it should be inhibited during
unpleasant pictures and potentiated during pleasant pictures.
Based on findings with the startle reflex, it was further predicted
that modulatory effects in either case would be maximal for
pleasant and aversive pictures of high intensity.
The literature on affective modulation of startle and other
reflexes provided a basis for alternative hypotheses as well. One is
that the post-auricular reflex might be facilitated by nonspecific
arousal rather than valence, with magnitude greater during both
pleasant and aversive pictures compared to neutral (cf. spinal T
reflex; Bonnet et al., 1995). Another is that the post-auricular
reflex would be inhibited as a function of cross-modal attention,
with reflex magnitude diminished during both pleasant and
unpleasant pictures relative to neutral (cf. probe-elicited P300;
Schupp et al., 1997).
Method
Participants
Participants were 24 male undergraduates (M age 5 19.2 years,
SD 5 1.25) recruited from psychology classes at the University of
Minnesota. The sample included only men because the current
data were collected as part of a pilot for a project involving male
prisoners. Individuals with visual or hearing impairments, as
assessed via a screening questionnaire, were excluded from the
study. Students received course credit for their participation.
Experimental Stimuli and Design
Participants viewed digitized color photographs (22 pleasant, 22
neutral, and 22 unpleasant) selected from the International
Affective Picture System (IAPS; Center for the Study of Emotion

428 S.D. Benning, C.J. Patrick, and A.R. Lang
and Attention, 1999). Fifty-four of the pictures were selected to
represent specific thematic contents, as follows: 1
Pleasant contents: erotic scenes (n 5 9; e.g., nude females,
intimate couples); adventure scenes (n 5 9; e.g., cliff diving,
motorcycle racing).
Neutral contents: inactive people or neutral human faces
(n 5 9); household objects or kitchen utensils (n 5 9).
Unpleasant contents: scenes of victimization of other people
(n 5 9; e.g., aggression, physical brutality, and combat);
threatening figures or weapons directed at the viewer (n 5 9;
e.g., pointed guns, menacing attackers).
Subsets of low and high intensity pictures, as defined by IAPS
normative ratings, were included in each emotional content
category (erotic, adventure, victim, threat). High-intensity pleasant
pictures were higher in rated valence and arousal, Ms(SDs) 5 7.33
(0.31) and 6.49 (0.53), respectively, than low-intensity pleasant
pictures, Ms (SDs) 5 5.92 (0.47) and 4.66 (0.63). High-intensity
unpleasant pictures were lower in normative valence and higher in
arousal, Ms(SDs) 5 2.79 (0.25) and 6.49 (0.52), respectively, than
low-intensity unpleasant pictures, Ms(SDs) 5 3.91 (0.36) and 4.64
(0.63). Low-intensity pleasant and unpleasant pictures were
equidistant from neutral in terms of rated valence and arousal,
as were high-intensity pleasant and unpleasant pictures.
Between 3 s and 5 s after the onset of each of these 54 pictures,
a brief binaural white noise probe (50 ms, 105 dB,o10 ms rise
time) was presented that elicited the post-auricular reflex. To
diminish the predictability of the noises, nine no-probe picture
trials were interspersed with the probe trials. Additionally, three
probed picture trials (IAPS numbers 4650, 7080, and 9252) were
included at the start of the series to familiarize participants with
the stimuli and to habituate large initial startle reactions.
Habituation trials and no-probe picture trials were excluded
from the analyses.
Noise probes were generated using a Coulbourn S81-02 white
noise module and S82-24 audio amplifier. The probe was
presented binaurally through Telephonics headphones (first half
of participant sample) or Etymotic insert earphones (second
half). Statistical tests revealed no significant effects of method of
noise delivery, so the data were collapsed across this factor in all
analyses reported. The noise probes occurred 3, 4, or 5 s after
picture onset. A total of nine probes were also delivered at
varying points during intertrial intervals (ITIs) to reduce the
predictability of the noise stimulus.
Six stimulus orders were used to balance the presentation of
pictures and startle probes across participants. As noted, each
order included 3 habituation trials followed by 54 probed picture
trials, interspersed with 9 no-probe picture trials. The 54 probed
picture trials were organized into three blocks, each consisting of
three pleasant, three neutral, and three unpleasant pictures.
Within and between orders, the positioning of pictures and
1 The 54 probed pictures, listed by their IAPS identification numbers,
were as follows: eroticF2381, 4000, 4233 (4617), 4274, 4230, 4653
(4750), 4690, 4687, 4290 (4651); adventureF4533 (8032), 8041, 8033,
5622 (8250), 5626, 5623, 8370 (8180), 8080, 8042; neutralF2190, 2210,
2214, 2372, 2480, 2495, 2850, 2890, 9700, 7002, 7030, 7034, 7040, 7050,
7150, 7205, 7705, 7710; victimF6010, 2520, 9594 (4621), 6571, 9400,
6530 (3550), 9250, 3400, 6350 (3500); threatF2100 (6241), 2682, 2130,
6242 (6244), 6370, 6243, 6510 (6250), 6260, 6230. The pictures in
parentheses are alternate exemplars from the same content category that
were substituted within some stimulus orders to achieve counterbalancing
of conditions (valence, content, intensity) across run orders.
startle probes was counterbalanced such that all valence, content,
and intensity conditions were represented equally across orders
at each serial position. Not more than two pictures of the same
valence occurred consecutively within any stimulus order, and
pictures of the same content never appeared consecutively.
During the experiment, participants sat in a padded recliner at
a distance of 120 cm from a 21-in. computer monitor positioned
directly in front of them. Physiological responses were recorded
using a PC computer running VPM data acquisition software
(version 11.2; Cook, Atkinson, & Lang, 1987). A second
computer controlled picture presentation. The post-auricular
reflex data for the current sample were collected as part of a
larger sample investigation of emotion and picture viewing in
which a variety of other response measures (i.e., heart rate, skin
conductance, facial muscle activity, startle blink, EEG, and
affective ratings) were recorded. 2 The current report is limited to
the post-auricular data because this response measure was
available for only a subset of participants and we wished to
highlight findings for this novel measure here.
Physiological Measures
The post-auricular reflex was measured as described by Sollers
and Hackley (1997). The pinna (outer ear) was pulled forward,
and behind each ear, a pair of Med Associates 0.25 cm Ag-AgCl
electrodes was positioned around the tendon of insertion for the
post-auricular muscleFreadily identifiable in most cases as a
fibrous strip connecting the pinna and the scalp midway up the
pinna. One electrode was placed directly adjacent to the tendon
on the surface of the pinna, and the other electrode was placed on
the scalp over the post-auricular muscle. Prior to placement, sites
were scrubbed with conductive gel to reduce impedances below 5
kO. Raw electromyographic (EMG) signals for each ear were
recorded for 50 ms before noise probe onset until 250 ms after
probe onset using a Coulbourn S75-01 high gain bioamplifier.
The sampling rate was 1,000 Hz.
The data were filtered on-line with a bandpass of 8–1,000 Hz,
and rectified off-line using Matlab software (MathWorks, 2000).
Post-auricular reflexes were scored after averaging the rectified
waveformsacrosstrialswithinconditions(Hackleyetal.,1987;
Sollers & Hackley, 1997). The magnitude of the post-auricular
reflex was scored from the aggregate waveform as a baseline-topeak
measure. The peak was calculated as the maximum EMG
activity within a window of 8–30 ms after noise probe onset, 3
whereas the baseline was calculated as the average rectified postauricular
EMG activity during the 50 ms before the onset of the
noise probe (Sollers & Hackley, 1997). Although all participants
showed substantial increases in post-auricular EMG activity
during the 8–30 ms post-probe-onset window, several participants
had reflex magnitudes that were difficult to differentiate from
background EMG activity. Therefore, the peak of each aggregate
waveform was manually scored to ensure that the algorithm
detected responses appropriately (cf. Sollers & Hackley, 1997); in
all but three aggregate waveforms, the manually assigned peak
was identical to the algorithmically picked peak.
2 Startle blink data for this subset of participants followed the usual
valence modulation pattern, with blink magnitudes during pleasant
pictures inhibited and those during unpleasant pictures potentiated
compared to those during neutral pictures (cf. Benning, Patrick, Hicks,
Blonigen, & Lang, 2001).
3 Analyses conducted with the peak calculated as the mean postauricular
activity within the 8–30 ms post-probe window and the baseline
calculated as above yielded identical patterns of results.

Emotion and the post-auricular reflex 429
Procedure
Participants provided informed written consent and then
completed a biographical form that screened for physical
ailments, medication use, and visual and auditory impairments.
Following electrode attachment, participants were advised they
would be viewing a series of pictures and rating their reactions to
each. They were instructed to watch each picture the entire time it
was on the screen and disregard occasional noises occurring
through the headphones or earphones. Before the main picture
series began, participants were given a demonstration of the Self-
Assessment Manikin (SAM; Lang, 1980) rating procedure, in
which they characterized their reactions to pictures on dimensions
of valence, arousal, and dominance (cf. Lang, 1980), as well
as interest (cf. Levenston et al., 2000). Each of the 66 pictures was
preceded by a blank screen for 3 s and was then presented for 6 s.
Seven seconds after picture offset, the ratings display appeared
and the participant completed the four ratings. The interval
between completion of the ratings and the onset of the next
picture ranged between 8 s and 14 s, averaging 11 s.
Data Processing and Analysis
Because the post-auricular reflex is a ‘‘microreflex’’ with a low
signal-to-noise ratio (cf. Hackley et al., 1987), averaging of trials
within condition was required to score the reflex. For one set of
analyses, we aggregated the rectified post-auricular waveforms
by thematic content within picture valence category. Because
there were equivalent numbers of human and object pictures
within the neutral category, we aggregated across trials within
each of these neutral contents, and we also aggregated across
trials for erotic and adventure contents within the pleasant
category, and victim and threat contents within the unpleasant
category. Each aggregate waveform thus incorporated data for
nine picture trials. A nine-trial aggregate waveform incorporating
data for all ITI probes was also formed. Post-auricular reflex
peak magnitude was scored from these aggregate waveforms,
along with mean post-auricular muscle activity during the preprobe
baseline. Effects of picture valence were examined by
collapsing magnitude scores across contents within valence
category. A subsidiary set of analyses was performed to examine
effects of affective stimulus intensity on post-auricular reflex
magnitude. For these analyses, we aggregated post-auricular
waveforms by picture intensity level (low, high) within pleasant
and unpleasant valence categories, again yielding aggregate
waveforms consisting of nine trials per condition, and extracted
peak magnitude scores from these aggregates.
Using the resultant scores, the following analyses were
performed:
1. To examine the effect of visual foreground engagement on
post-auricular reflex magnitude, we conducted a one-way
repeated measures ANOVA in which the neutral picture
condition was compared against the ITI (no-picture) condition.
A parallel one-way ANOVA was conducted to examine
the effect of foreground engagement on post-auricular
baseline activity.
2. To examine effects of picture valence (pleasant, neutral,
unpleasant) and ear recording site (left, right) on postauricular
magnitude, a 3 2 multivariate ANOVA was
performed with these variables included as within-subjects
factors. Significant main effects were subsequently examined
in terms of orthogonal linear (e.g., pleasant vs. unpleasant)
and quadratic (e.g., pleasant/unpleasant vs. neutral) contrasts
(cf. Bradley, Cuthbert, & Lang, 1993; Bonnet et al., 1995;
Moulder et al., 1995). A supplemental one-way MANOVA
was performed to examine effects of picture valence on
baseline post-auricular activity.
3. With regard to picture content and intensity, we were
interested in whether affect-modulation effects for the postauricular
reflex would be tied to specific contents or to
pictures of high intensity. We addressed these questions by
performing two sets of planned contrasts using one-way
ANOVAs. One involved comparisons of affective contents
within each valence against each other, and the other entailed
contrasts of pleasant and unpleasant pictures of each intensity
(low, high) against each other.
An a level of .05 was used as the criterion for significance in all
statistical tests.
Results
Picture Foreground versus ITI
Post-auricular reflex magnitudes were smaller during neutral
pictures compared with ITIs, Ms(SDs) 5 6.64 (6.18) mVand 7.98
(5.71) mV, respectively, F(1,23) 5 4.65, p 5 .04, Z 2 5 .17. There was
also a trend for baseline post-auricular EMG activity during neutral
pictures to be smaller than during ITIs, Ms(SDs) 5 0.74 (0.32) mV
and 0.84 (0.36) mV, respectively, F(1,23) 5 2.92, p 5 .10, Z 2 5 .11.
Picture Valence
Figure 1 displays grand average waveforms for the post-auricular
reflex by picture valence, with average reflex magnitudes depicted
in the inset bar graph. A significant main effect of picture valence
was found for post-auricular reflex magnitude, F(2,22) 5 4.16,
p 5 .03, Z 2 5 .27. 4 As shown in Figure 1, the post-auricular reflex
was larger during pleasant pictures than unpleasant pictures,
linear Valence F(1,23) 5 8.50, p 5 .01, Z 2 5 .27. Follow-up
paired t tests showed that post-auricular magnitudes tended to
be greater during pleasant than neutral pictures, t(23) 5 1.94,
p 5 .07, whereas post-auricular magnitudes during neutral and
unpleasant pictures did not differ, t(23) 5 1.11, p 5 .28. No main
effect of ear recording site (right vs. left) was found,
F(1,23) 5 1.00, p 5 .33, Z 2 5 .04, and the Ear Valence interaction
was not significant, F(2,22) 5 0.31, p 5 .74, Z 2 5 .01.
As further evidence of the consistency of the post-auricular
valence modulation effect, 75% (18/24; 95% confidence interval
5 55%–88%) of participants showed numerically greater
post-auricular reflex magnitudes during pleasant compared with
unpleasant pictures, and 71% (17/24; 95% confidence interval
5 51%–85%) showed numerically greater post-auricular
reflex magnitudes during pleasant versus neutral pictures. Both
proportions significantly exceeded chance, according to formulae
provided by Newcombe (1998).
The MANOVA with picture valence as the within-subjects
factor and mean pre-probe baseline post-auricular EMG activity
4 A supplementary analysis was performed in which picture trials were
subdivided into three sequential blocks, and post-auricular waveforms
were aggregated by valence (pleasant, neutral, unpleasant) within each
block. As in the primary MANOVA, this analysis yielded a significant
valence main effect, and also a significant main effect for block,
F(2,22) 5 6.77, p 5 .011, Z 2 5 .338, but no Block Valence interaction,
F(4,20) 5 0.95, p 5 .441, Z 2 5 .040. Thus, while the magnitude of the
post-auricular reflex declined over the session, the emotional modulation
of the post-auricular reflex remained constantFconsistent with the
findings of Bradley et al. (1993) for the startle blink reflex.

430 S.D. Benning, C.J. Patrick, and A.R. Lang
PA EMG Activity (µV)
6
5
4
3
2
1
0
-25 0 25 50 75 100 125
as the dependent variable revealed no effect of picture valence,
F(2,22) 5 0.30, p 5 .74, Z 2 5 .03. Post-auricular baseline Ms
(SDs) for pleasant, neutral, and unpleasant pictures were as
follows: 0.74 (0.29) mV, 0.75 (0.32) mV, and 0.72 (0.34) mV,
respectively.
Affective Content and Intensity
Within pleasant contents, the magnitude of post-auricular reflexes
during erotic pictures did not differ from those during adventure
pictures, Ms 5 7.23 and 7.21 mV (SDs 5 5.86 and 6.12), respectively,
F(1,23) 5 0.00, p 5 .96, Z 2 5 .00. Likewise, within unpleasant
contents, post-auricular reflex magnitudes did not differ
between victim and threat pictures, Ms 5 6.24 and 6.38 mV(SDs 5
5.20 and 5.58), respectively, F(1,23) 5 0.11, p 5 .75, Z 2 5 .01.
Post-auricular reflex magnitudes were significantly greater during
high-intensity pleasant pictures than during high-intensity
unpleasant pictures, Ms 5 8.11 and 6.81 mV (SDs 5 6.87 and
5.59), respectively, F(1,23) 5 7.78, p 5 .01, Z 2 5 .25, but postauricular
reflex magnitudes during low-intensity pleasant and
unpleasant pictures did not differ significantly from each other,
Ms 5 7.77 and 7.18 mV (SDs 5 6.48 and 6.18), respectively,
F(1,23) 5 2.31, p 5 .14, Z 2 5 .09.
Discussion
Pleasant
Neutral
Unpleasant
PA Peak Magnitude (µV)
8
7
6
5
Time Relative to Probe Onset (ms)
Pleasant Neutral Unpleasant
Picture Valence
Figure 1. Grand average post-auricular (PA) reflex waveforms (main
panel) and magnitudes SE (inset) by picture valence.
Our findings indicate that the magnitude of the post-auricular
reflex is reliably modulated by emotional valence. Specifically, we
found that the post-auricular reflex tended to be potentiated
during viewing of pleasant pictures compared with neutral, and
inhibited during viewing of aversive pictures, particularly during
high-intensity pictures. 5 This linear modulation pattern is
5 In a previous investigation of affect and attention during picture
viewing that employed a wider range of probe times (Bradley, Drobes, &
Lang, 1996), measurement of the post-auricular reflex was included along
with assessment of blink startle and tone probe responses (M. M.
Bradley, pers. comm., May 7, 2003). Although post-auricular reflex
magnitude in this study was somewhat higher during pleasant versus
unpleasant pictures, the difference was not significant. On the other hand,
more recently, a significant linear valence effect on post-auricular reflex
magnitude, mirroring the finding reported here, was obtained by investigators
at Indiana University (Ames, Merritt, Stout, & Hetrick, 2003).
opposite to that observed for the defensive startle blink reflex
(Lang et al., 1990). Analyses of post-auricular muscle activity
during the pre-probe baseline period indicated that this modulatory
effect was not attributable to differences in baseline EMG
(as would be expected if these results were due to head motion or
eye gaze; Patuzzi & O’Beirne, 1999; Stekelenburg & van Boxtel,
2001, 2002), because baseline post-auricular activity did not differ
across picture valence categories. On the other hand, baseline
post-auricular activity was reduced in connection with attenuated
post-auricular reflex response during viewing of neutral foregrounds
in relation to ITIs. This finding is consistent with the idea
that tonic activity in various somatic systems, including the postauricular
musculature, decreases to enhance perceptual sensitivity
during orienting (Stekelenburg & van Boxtel, 2001, 2002).
Lang et al. (1990) proposed a motivational priming hypothesis
to account for modulatory effects of emotional valence on
the startle reflex. Citing Konorski (1967), they postulated that
defensive reflexes like the startle blink are primed during negative
emotional states and inhibited during positive emotional states.
The potentiation effects observed for the neck (Cassella, Harty,
& Davis, 1986), blink (Lang et al., 1990), and whole-body (Davis
et al., 1982) reactions to noise probes under conditions of threat
are consistent with the idea that these are components of a
common defensive startle response. Although the post-auricular
reflex is likewise elicited by a sudden, loud noise, our data argue
against the post-auricular reflex being a short latency component
of the defensive startle reflex (Hackley, 1993). Indeed, there is
evidence that the post-auricular reflex may have a different
circuitry than the startle reflex (Cassella & Davis, 1986; Hackley,
1993), though more recent research with specific excitotoxic
lesions in the circuitry of the pinna reflex in rats (rather than with
general fiber-destroying electrolytic lesions; Cassella & Davis,
1986) suggests that the circuitry for the post-auricular reflex may
also include the nucleus reticularis pontis caudalis (Davis,
Walker, & Yee, 1999; Li & Frost, 1996).
Lang et al.’s (1990) motivational priming hypothesis postulates
that appetitive reflexes (salivation) would be facilitated
during positive emotional states and inhibited during negative
emotional states. In this regard, the post-auricular reflex behaved
as an appetitive reflex in the current study, as it was potentiated
during viewing of pleasant pictures and attenuated during
viewing of unpleasant pictures. The fact that the post-auricular
reflex increased during pleasant picture viewing indicates that
some other mechanism (appetitive motor priming) operated
against the influence of cross-modal sensory engagementFanalogous
to the impact of defense system activation on the startle
reflex during unpleasant pictures, which opposes the inhibitory
influence of cross-modal attention in picture viewing (Cuthbert et
al., 1996; Lang et al., 1997).
In the case of the startle reflex, a specific neural mechanism
has been identified to account for the facilitatory effect of
aversive foreground stimulationFnamely, an input from the
amygdala to the startle reflex circuit (Davis et al., 1982). A
corresponding input from the brain’s appetitive system to the
post-auricular circuit could conceivably account for potentiation
of the post-auricular reflex during pleasant foreground stimulation.
Research on the neural circuitry of the pinna reflex, the
animal analogue of the post-auricular reflex, indicates that the
motoneurons of the facial nerve that innervates the pinna receive
additional input from the retrorubral nucleus (Li & Frost, 1996),
a midbrain dopaminergic structure that has been associated with
sensitivity to reward stimuli in rats (Waraczynski & Perkins,

Emotion and the post-auricular reflex 431
2000) and with motor deficits arising from Parkinson’s disease in
humans (Heim et al., 2002).
An alternative possibility is that increased post-auricular
reactivity reflects enhanced orienting to the noise probe stimulus.
Indeed, there is evidence that unexpected noises can evoke
observable movement of the post-auricular (pinna) muscle in
primates in whom this muscle serves more than a vestigial
function. Naturalistic observations of Hanuman langurs, a lower
primate species, have revealed that in adult males, loud and
surprising sounds yield brief (i.e., 0.2–0.3 s) movements of both
the pinna and the head in the direction of the noise (Trivedi &
Mohnot, 2002), suggesting that the pinna reflex serves an
orienting function in this primate species. It may be that states of
attentional engagement, such as those associated with viewing of
pleasurable pictures, facilitate this phasic reflexive orienting
reaction. However, because pleasant and unpleasant pictures
engage more attention than neutral (Lang et al., 1990, 1997), the
predicted attentional pattern would have been one of postauricular
reflex inhibition for both affective categories compared
to neutral. Furthermore, juveniles and females of this species
exhibit greater short-duration pinna movements primarily
during play, feeding, and foraging for food (Trivedi & Mohnot,
2002), suggesting heightened responsiveness of this reflexive
system specifically during pleasurable activities.
Nevertheless, there is evidence that unpleasant pictures also
engage attention (Lang et al., 1997), so it is possible that some
types of aversive pictures might likewise facilitate the postauricular
reflex. The current study focused on threat and physical
attack pictures that tend to be perceived as fearful. Modulatory
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(Received March 4, 2003; Accepted October 13, 2003)