The Effect of Non-stressful and Stressful Visual Stimuli on Heart Rate and Blood Pressure

Ian Krohn, Laura Hagen, and Summer Maxey

CU Boulder, Fall 2006

We tested the effects that stressful images have on the heart rate (HR) and blood pressure (BP) of individuals.  The Sympathetic Nervous System (SNS) is responsible for the “flight or fight” mechanism that resides within animals.  The SNS increases the amount of epinephrine (adrenaline) in a mammal’s bloodstream during stress.  The increase in epinephrine causes an increase in the animal’s HR, BP, and other physiological mechanisms.  These physiological mechanisms allow an animal to determine whether it should run (flight) or stand its guard (fight).  Since epinephrine is released into the bloodstream in response to stress, we hypothesized that HR and BP will increase after viewing stressful images.

To test this hypothesis we made two slide shows: one slide show depicting non-stressful images and another slide show depicting stressful images.  The experiment consisted of 10 participants: five male and five female.  The participant sat in a quiet room in front of a computer while each slide show played.  We obtained a mercury sphygmomanometer to test BP after each slide show.  We tested the systolic BP because it measures the peak pressure in the arteries during the cardiac cycle. Also, HR was determined by counting beats per minute.  Both the HR and BP were obtained by the same experimenter in all trials.  Since there tends to be an increase in epinephrine in the blood stream due to stress, we predicted that there will be an increase in HR and BP after observation of the stressful images in comparison to the non-stressful images.
Our results indicated that HR was significantly greater for the stressful images (mean = 83 beats/minute) rather than the non-stressful images (mean = 68 beats/minute; t = 2.262, P < 0.05).  The systolic BP was also significantly greater for stressful images (mean = 118.8 mmHg) than non- stressful images (mean = 111.5 mmHg; t = 2.262, P < 0.05).
The results of our experiment are consistent with predictions based on our hypothesis. However, even with significant results, it is possible we may have made a Type I error, meaning we detected significance when, in actuality, there was none. Other confounding variables, such as caffeine intake, phobias to certain images, and apprehension to medical supplies could have caused an abnormal increase in HR and BP. To ensure more accurate results it would be appropriate to run the test again, this time with a larger sample size, and controlling the confounding variables, such as monitoring caffeine intake for the day, and surveying fears of the population beforehand.  Other previous student studies have shown similar trends in the increase of BP and HR due to stressful stimuli, specifically nicotine and dramatic music sequences (Mead et al. 2003, Joung et al. 2002). To further test this experiment, an alternative hypothesis could compare not only the effects of stressful and non-stressful stimuli, but the effects of these stimuli on males versus females.  This alternative experiment could yield support for the possibility that males are more conditioned to stressful stimuli than females.