Figure 3. Placement of facial EMG electrodes. muscle, and D=orbicularis muscle.

A=zygomaticus muscle, B=corrugator muscle, C=frontalis

Another muscle group that is measured using electromyographic equipment is the orbicularis muscle which contracts during the startle reflex (Figure 3D). This measure has been used frequently to assess exaggerated startle response, which is one of the most commonly reported symptoms of PTSD. While blinking is to some degree under voluntary control, the startle reflex has a short latency (30-120 ms) and thus occurs too quickly to be intentionally generated. However, the startle reflex itself can be modulated by different emotional states, such as fear. A large body of literature has examined the increase in the startle response during fearful situations, called fear-potentiated startle [5]. This is also a good laboratory measure of fear and has the potential to be used in assessment of PTSD.

2. Psychophysiology as a diagnostic tool for PTSD

2.1. Hyper-arousal to audiovisual combat stimuli

As mentioned above, one of the cardinal symptoms of PTSD, according to the DSM-IV, is physiological hyper-arousal, or exaggerated reactivity, to trauma reminders [1]. Early studies capitalized on this symptom and used combat-related stimuli to evoke arousal in Vietnam veterans with PTSD. The first studies to examine hyper-arousal using physiological measurements used standardized combat stimuli, such as combat sounds like mortar explosions or gunfire, and standardized pictures of combat. In these studies all participants would be exposed to the same sets of stimuli while their responses were measured. In a series of studies, Blanchard and collaborators have successfully discriminated veterans with or without PTSD on the basis of heart-rate responses to combat sounds (95% correct for veterans with PTSD and non-veteran controls [6]; 81% correct [7]; 86.4% correct for combat veterans with PTSD vs. without PTSD [8]). They measured heart-rate, systolic and diastolic blood pressure, skin conductance, and frontalis EMG in response to three conditions: (a) resting, (b) mental arithmetic, and (c) a combination of music, silence, and combat sounds and found that PTSD veterans had higher heart-rate responses to all sounds relative to non-PTSD veterans (Figure 4). Furthermore, Blanchard, Kolb, and Prins [9] found that a discriminant function based on measures of heart-rate alone correctly identified an initial sample of 84% of combat veterans with PTSD and 75% of all veterans. Studies using standard audiovisual combat imagery found that heart-rate was the most sensitive psychophysiological measure in distinguishing traumatized individuals with and without PTSD. While heart-rate alone showed a high degree of discrimination, there was still a relatively high rate of both false positive (i.e. non-PTSD patients classified as PTSD), and false negatives (PTSD patients classified as non-PTSD) according to interview diagnoses.


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