Startle Reflex – A Pure Measure of Valence

The startle reflex is a reflexive response in the physical body that occurs in response to an unexcepted, intense stimulus. Types of startle reflexes include knee jerks, neck jerks, and eye blinks. The purpose of the startle reflex is to alert us to danger and to protect our body from harm. The startle reflex is managed by the amygdala. In the laboratory, the startle reflex is measured with the eye blink. The amplitude of the eye blink is positively correlated with the magnitude of the startle reflex. An electromyogram (EMG) is the tool that measures the magnitude of the eyeblink. An electrode is played underneath the lower eyelid on the orbicularis oculi muscle. This electrode measures how much muscle activity occurs after people experience a startle probe. A startle probe occurs when participants experience 50 milliseconds of loud, white noise that was not expected.

The startle reflex is a pure measure of the valence of an emotion that occurs in high-arousal situations. When viewing IAPS photos of negative events (e.g., mutilations, snakes, spiders) participants showed a greater eyeblink magnitude compared to when viewing IAPS photos of positive events (e.g., opposite-sex erotica, food, and children; Bradley et al., 1990; Vrana et al., 1988). Remember, the IAPS photos all elicit high-arousal emotions – either positive or negative. So, these findings tell us that high arousal negative and positive emotions both cause the eyeblink, it’s just negative emotions cause LARGER eyeblink magnitude and positive emotions cause a SMALLER change in the eyeblink.  So, the startle reflex is a pure measure of whether someone experiences a high-arousal negative emotion or a high-arousal positive emotion.