The human startle response is a sensitive, noninvasive measure of central nervous system activity that is currently used in a wide variety of research and clinical settings. In this article, we raise methodological issues and present recommendations for optimal methods of startle blink electromyographic (EMG) response elicitation, recording, quantification, and reporting. It is hoped that this report will foster more methodological validity and reliability in research using the startle response, as well as increase the detail with which relevant methodology is reported in publications using this measure. Descriptors: Startle, Blink, Electromyographic (EMG), HumanDue to the dramatic increase in the use of the startle blink response in research and clinical settings, Gregory Miller, then Editor of Psychophysiology (2001), appointed a committee to consider guidelines for startle blink research in humans. The result is this article, the aim of which is to propose a series of suggestions that might guide researchers in the collection and reporting of data based on the blink component of the startle response. Due to space limitations, this article will not deal with several areas of interest to startle researchers, such as affect, attention, psychopathology, and prepulse inhibition, but will instead focus on the fundamental methodology applied when startle blink electromyographic (EMG) data are used to investigate any research question. One goal of this article is to bring a higher degree of both reliability and validity to this research area by summarizing recent research in which alternative methods have been compared and by providing criteria for choosing among them. Another goal is to encourage the reporting of relevant methodological details in publications in this area of research. We hope that this article will serve as a guide for researchers new to the area of startle, showing them the potential ramifications of deciding to do things one way rather than another. Moreover, experienced researchers may benefit from a review of the methodological advances that have been made in this area over the past few years, and may even reconsider some of their current practices.

This study examined empathy-related responding in male adolescents with disruptive behavior disorder (DBD), high or low on callous-unemotional (CU) traits. Facial electromyographic (EMG) and heart rate (HR) responses were monitored during exposure to empathy-inducing film clips portraying sadness, anger or happiness. Self-reports were assessed afterward. In agreement with expectations, DBD adolescents with high CU traits showed significantly lower levels of empathic sadness than healthy controls across all response systems. Between DBD subgroups significant differences emerged at the level of autonomic (not verbal or facial) reactions to sadness, with high CU respondents showing less HR change from baseline than low CU respondents. The study also examined basal patterns of autonomic function. Resting HR was not different between groups, but resting respiratory sinus arrhythmia (RSA) was significantly lower in DBD adolescents with high CU traits compared to controls. Results support the notion that CU traits designate a distinct subgroup of DBD individuals.

Spontaneous pericranial electromyographic (EMG) activity is generally small and is contaminated by strong low-frequency artifacts. High-pass filtering should suppress artifacts but affect EMG signal power only minimally. In 24 subjects who performed a warned simple reaction time task, the optimal high-pass cut-off frequency was examined for nine different pericranial muscles. From four experimental conditions (visual and auditory reaction signals combined with hand and foot responses), 1-min EMG recordings were selected (bandwidth: 0.4-512 Hz) and divided into 60 1-s data segments. These segments were high-pass filtered, the -3-dB cut-off frequency varying from 5 to 90 Hz, and subjected to power spectral analysis. Optimal high-pass filter frequencies were determined for the mean power spectra based on visual estimation or comparison with a theoretical spectrum of the artifact-free EMG signal. The optimal frequencies for the different muscles varied between 15 and 25 Hz and were not influenced by stimulus or response modality. For all muscles, a low-pass filter frequency between 400 and 500 Hz was appropriate.

In earlier studies, positive but inconsistent relationships have been reported between mental effort and electromyogram (EMG) amplitude in task-irrelevant limb muscles. In this study, we explored whether facial EMG activity would provide more consistent results. Tonic EMG activity of six different facial and jaw-elevator muscles was bilaterally recorded during a two-choice serial reaction task with paced presentation of auditory or visual signals. In Experiment 1, task load (signal presentation rate) was kept constant for 20 min at the level of the subject's maximal capacity. In Experiment 2, task load was increased in a stepwise fashion over six successive 2-min periods from sub- to supramaximal capacity levels. EMG amplitude and coherency between momentary bilateral amplitude fluctuations were measured. In Experiment 1, EMG amplitude of frontalis, corrugator supercilii, and orbicularis oris inferior showed a strong gradual increase throughout the task period, whereas taks performance remained fairly stable. Orbicularis oculi, zygomaticus major, and temporalis EMG showed a much smaller increase or no increase. In Experiment 2, the first three muscles showed a fairly consistent increase in EMG amplitude with increasing task load. Orbicularis oculi and zygomaticus major were not active until task load became supramaximal. Effects of stimulus modality or laterality were not found in any experiment. These results are consistent with the notion that EMG amplitude of frontalis, corrugator, and orbicularis oris provides a sensitive index of the degree of exerted mental effort.

Based on the assumption that facial mimicry is a key factor in emotional empathy, and clinical observations that children with disruptive behavior disorders (DBD) are weak empathizers, the present study explored whether DBD boys are less facially responsive to facial expressions of emotions than normal controls. Facial electromyographic (EMG) activity in the zygomaticus major and corrugator supercilii muscle regions, and heart rate activity were studied in 22 clinically referred 8-12-year-old DBD boys and 22 age-matched normal controls during exposure to dynamic happy and angry expressions. Dispositional emotional empathy was assessed by a self-report questionnaire for children. The happy and angry facial expressions evoked distinct facial EMG response patterns, with increased zygomaticus muscle activity to happy expressions and increased corrugator muscle activity to angry expressions. The corrugator (but not the zygomaticus) muscle response pattern was less pronounced for DBD boys than the normal controls. Attending to the emotional expressions was associated with equivalent cardiac deceleration in both groups, reflecting a similar orienting/attention response. Lower empathy scores were obtained for DBD boys than for normal controls. In conclusion, facial mimicry responses to angry facial expressions were subnormal in DBD boys, which may be a sign of a deficient early component in the process of emotional empathy, and thus play a role in impaired empathic responding.