UCLA

Fear is an adaptive response that allows animals to respond appropriately to danger, and Pavlovian fear learning provides a mechanism that allows animals to learn about, predict, and effectively respond to dangerous situations. In the laboratory, fear conditioning is a frequently used procedure used to study the learning, expression, and underlying neurobiology of fear and emotional behavior. In such an experiment, animals learn to associate some cue/conditional stimulus (CS) or contextual location with an aversive outcome/unconditional stimulus (US), such as a footshock. As a result of this learned association, the animal will now respond to the initially neutral CS with defensive behavior, a conditional response (CR) as a result of learning. This dissertation contains two chapters which investigate different aspects of cued and contextual fear learning in male and female rodents, first with a focus on the selection and associative nature of defensive responding, then with a focus on potential sex differences and any potential underlying mechanisms.Chapter #1 of this dissertation presents behavioral experiments in mice that assess defensive behavior after various auditory fear conditioning procedures. Particularly, we employed a serial compound conditional stimulus (SCS), consisting of a tone followed by a white noise, training procedure which allowed us to asses differential freezing and activity burst behaviors to each component of the SCS. A number of experiments were run with various control groups to assess the associative or non-associative nature of defensive responding following such a procedure. Contrary to some conclusions in the recent literature, while associative processes had some impact on the topography of flight behavior, cue-elicited activity bursts were primarily due to nonassociative learning processes. Such activity burst behavior was viewed as being similar to a fear-potentiated startle response. The results of the experiments led to the production of a rule for defensive behavior selection: “When afraid, freeze until there is a sudden novel change in stimulation, then burst into vigorous flight attempts,” (Trott et al., 2022, p. 2). Chapter #2 of this dissertation presents behavioral contextual fear learning experiments in rats and computational simulations of the same experiments using BACON, a conceptual and computational model of hippocampal function. Behaviorally, animals underwent a 3-day contextual fear conditioning procedure in which they received pre-exposure to a to-be-shocked context on Day 1, a shock after some placement-to-shock interval (PSI) on Day 2, and a fear test on Day 3. Experiments 1A and 1B results revealed that after shorter pre-exposure periods, male rats showed higher levels of contextual fear; however, after longer pre-exposure periods, female rats showed greater fear. In Experiment 2, when testing recently vs remotely acquired fear memories, males again showed more contextual fear. Additionally, while pre-exposure and PSI are both periods of time prior to shock, Experiment 3 revealed that they were not equivalent to each other such that there were overall sex differences and differences in the timing of fear expression based on whether animals received the majority of pre-shock context exposure as pre-exposure or PSI. Through the use of BACON, a computational model of hippocampal function, we simulated many of our key findings and suggest a potential mechanism for such findings, that of an increased contextual feature sampling rate in males. The experiments presented here asses both cued auditory and contextual fear learning in both rats and mice and add to a rich literature on fear conditioning and defensive behavior. The experiments and discussion within Chapter #1 introduce important controls and considerations for the conduction and interpretation of fear conditioning experiments, particularly for a set of increasingly used fear conditioning procedures which result in different freezing and flight behavior to different cues paired with a shock US. Results from Chapter #2 provide insight into sex differences in contextual learning and suggest a hippocampal mechanism for such differences, adding to a growing literature on sex differences in learning and hippocampal function.