Undergraduate Honors Theses

Thesis Defended

Spring 2019

Document Type


Type of Thesis

Departmental Honors


Psychology & Neuroscience

First Advisor

Dr. Robert Spencer

Second Advisor

Dr. Heidi Day

Third Advisor

Dr. Janet Casagrand

Fourth Advisor

Dr. Nicole Speer


Post-traumatic stress disorder (PTSD) and other anxiety disorders are often characterized by impairment of the circadian system. Fear extinction learning is known to be modulated by the circadian system, but it is unclear if the time of extinction training or the time of extinction testing determines fluctuations in behavioral responses, or if these modulations are also present in female rats. Furthermore, it is unclear if fear relapse mechanisms show circadian rhythmicity. In experiment 1, rats were fear conditioned and extinction trained at zeitgeber time 4 (ZT4), ZT10, ZT16, or ZT22 and underwent five extinction testing sessions, separated by 6hr, beginning 24hrs after training. Rats trained at all times exhibited similar behavior at the time of extinction training. However, rats tested at ZT16 (active phase) demonstrated enhanced fear extinction recall memory and rats tested at ZT4 (inactive phase) demonstrated weaker fear extinction recall memory. In a follow-up experiment (experiment 3), results showed that male and female rats housed in constant darkness (DD) also exhibited time-of-day differences during fear extinction recall testing. These results suggest that fear extinction recall behavior may be regulated by the circadian system, and that molecular clocks in the prefrontal cortex may regulate molecular mechanisms associated with fear extinction memory retrieval. Follow-up experiments examined circadian modulation of fear relapse behavior. In experiment 2, rats trained at either ZT4 or Z16 underwent fear relapse testing 24, 36, or 48hrs later. No significant effects were observed; however, experiment 1, which tested rats for fear relapse 72hrs following extinction sessions, observed trending results that suggest that fear relapse behavior is enhanced at ZT16. For this reason, experiment 3 also examined fear relapse behavior 72hrs following extinction testing (as in experiment 1) and replicated the results of experiment 1 fear relapse. These data suggest that the circadian system may also modulate of fear relapse behavior, particularly spontaneous recovery of fear responses.