Physiology and Response Times to Pleasant and Unpleasant Smells


Karen Pratt


CU Boulder, Fall 2006


         The question I wanted to answer was, “Will people have different rates of response to unpleasant smells in comparison to pleasant smells, and if so, to which will they respond more quickly?” From past experience, I noticed that people had strong and immediate initial responses to unpleasant odors and seemingly less dramatic responses to pleasant odors.  From this observation, I hypothesized that individuals have faster rates of response to unpleasant smells in comparison to pleasant smells. 

         To test this hypothesis, I used the following items:  a reaction timer, a paper bag that was approximately 11 inches in length and was open at both ends, green tea oil, lavender oil, mint/basil oil, ammonium hydroxide solution, isopropyl alcohol, ethyl acetate, coffee grounds and strips of acid-free, absorbing paper.  I requested that each participant smell coffee grounds before and after exposure to each different smell.  During the test, I asked the participant to hold the paper bag over his/her nose, to close his/her eyes and to press the reaction timer as soon as he/she sensed the new smell.  I poured approximately 1 mL of the various scented products onto separate strips of acid-free, absorbing paper.  For each different smell, I wafted the corresponding paper strip at the open end of the paper bag opposite from the participant’s nose and simultaneously started the reaction timer.  For each participant, I ran tests of three pleasant smells and then three unpleasant smells.  The order of the smells was the same for each participant.  I ran the tests on 10 participants.

         Since unpleasant odors can often signal danger (such as the spray from a skunk) and pleasant odors can often signal good things (such as food), I predicted that individuals exposed to both pleasant and unpleasant smells will have quicker rates of reaction to the unpleasant smells than to the pleasant smells.

         The results of this experiment revealed that the average rates of response times for 10 participants were faster for the pleasant smells and slower for the unpleasant smells.  Mean response times (given in units of seconds) are as follows:  green tea oil = 3.9503, lavender oil = 3.9003, mint/basil oil = 3.3915, ammonium hydroxide solution = 7.1818, isopropyl alcohol = 5.218, ethyl acetate = 4.0265.  In addition, I thought that the response times were significantly different between the pleasant smells and unpleasant smells.  However, a t-Test reveals that the difference in the average response rates is statistically not significant as given by the P-value of 0.078095958 and t‑Stat = ‑2.22367619.  (In this experiment,

P > 0.05)

         My results are inconsistent with my predictions based on my hypothesis.  Some potential problems with this experiment include the following: 

o I tested each participant with the same order of the different smells and I should have done additional testing of each participant with a reversed order in the exposure to the various smells … going from unpleasant to pleasant.

o Also, I should have done additional testing of each participant inter-mixing the order of pleasant and unpleasant smells.

o It would have been useful to have a fresh paper bag for each individual being tested because the paper bag absorbed some of the odors.

o Since I had access to graduated cylinders, I attempted to pour approximately 1 mL of each product onto the strip of paper, however, using a small pipet would have given better accuracy in the amount of product exposed to the participant.

o Lastly, instructing participants on breathing techniques during each test would have given more consistency to the response times.

Because I was unable to locate any other experiments similar to this one on the CABLE web site, I could not make comparisons of the results of this experiment to others.

         From the results of my experiment, I propose a modified hypothesis:  individuals have different rates of response to pleasant smells in comparison to unpleasant smells, but the difference in response rates between the two types of smell is statistically insignificant.  This modified hypothesis would be consistent with the results of this experiment.