Physiological Startle Response to Frightening Stimuli in Light and Dark Environment

Abstract

Fear is an emotional response that humans experience in situations that are perceived as dangerous and is accompanied by physiological responses that would protect the body from potential harm. Different environments, such as darkness, can elicit altered responses in this physiological change because of the vulnerability and insecurity humans experience when in that situation. The purpose of our experiment was to determine if there is a significant difference in the physiological startle response in a dark environment compared to a light environment. We had 50 students from the UW-Madison Physiology 435 course participate in the study in which their heart rate (HR), electrodermal activity (EDA), and electromyography (EMG) were analyzed. They were shown a 2 minute video of ten cards being flipped over one-by-one, and were told they were performing a memory test. At the 1 minute mark, an emotionally provocative International Affective Picture System (IAPS) image appeared while paired with a high-pitched shriek. Participants were randomly selected to watch the video in the dark or in the light. Since it has been found that humans feel increasingly vulnerable around potentially threatening stimuli in dark environments, it was hypothesized that the participants who watched the video in the dark would have a greater startle response than those who watched in the light. Our results showed that there was no significant difference in heart rate, EDA, and EMG for participants startled in the dark and light conditions. However, we did find an unexpected significant difference in masseter EMG between female participants in the light and dark conditions. Future studies could investigate gender differences as well as a wider, more randomized population in order to explore different demographics and maintain research confidentiality. Lastly, our methods could be improved in the future by not allowing subjects to see the testing room before conducting the study and having the lights off when subjects tested in the dark enter the room.