Emotional modulation of the postauricular reflex

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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.
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Emotional modulation of the postauricular reflex

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