Fast, transient cardiac accelerations and decelerations during fear conditioning in rats.

The current study reports on a number of heart rate responses observed in rats subjected to a discriminatory Pavlovian fear conditioning procedure. Rats learned that a series of six auditory pips was followed by a footshock when presented alone, but not when the pip series was preceded by a visual safety signal. Each auditory pip in the series evoked a fast transient (<1s) cardiac deceleration. This was the case on both trials followed by shock and on trials not followed by shock. The onset of the safety light evoked a similar fast deceleration. We propose that these transient decelerations are similar to the human Evoked Cardiac Response 1 (ECR1), a brief modest deceleration evoked by simple sensory stimuli that is thought to reflect an early process of stimulus registration. Immediately following these pip-evoked decelerations, modest fast accelerations were observed. These accelerations were larger when the pip series was followed by shock than when it was not followed by shock. We propose a potential linkage between these accelerations and the human acceleratory ECR2 component, which is associated with more elaborate processing following stimulus registration; something likely to take place when the pip series predicts an aversive event. Both the ECR1- and ECR2-like responses were embedded within a slow, gradual heart rate increase across the entire pip series. This tonic increase was significantly larger on trials with footshock and is therefore probably associated with anticipatory fear of the upcoming shock. An additional special type of cardiac response was found to the first pip in the series not preceded by the safety signal; here, a much larger and more sustained deceleration was apparent. This response appears relatable to the prolonged deceleration reported in humans in response to aversive picture content. We discuss the cardiac responses found in rats in the current study in the context of heart rate responses known in the human literature.

N150 in amygdalar ERPs in the rat: is there modulation by anticipatory fear?

The hypothesis was tested whether the amygdalar N150 of rats, a slow, negative component in the event-related potential from the lateral amygdala, is sensitive to a state of anxious anticipation. A conditioning procedure was applied in which a series of six auditory stimuli was followed by a shock when presented alone, but not when the auditory stimuli were preceded by a visual stimulus. Heart rate recordings confirmed that the auditory stimulus train induced a state of increasing anticipatory fear and that this condition was modulated by the visual stimulus. During behavioral training, a N150 appeared in the amygdalar event-related potential evoked by the auditory stimuli, replicating previous findings. However, the amplitude of the N150 was not affected by whether or not the visual stimulus had been presented before. These results failed to support the idea that the N150 is related to the expectancy of an aversive event. An alternative interpretation, emphasizing the increase in arousal and attention that is inherent to aversive learning, is discussed.

Slow late component in conditioned stimulus-evoked potentials from the amygdala after fear conditioning in the rat.

Male Wistar rats were subjected to a differential Pavlovian fear conditioning procedure in which one of two tones (6 or 10 kHz) was followed by an electric shock (CS+) and the other was not (CS-). Before and after fear conditioning, we recorded the evoked potentials elicited by CS+ and CS- from electrodes aimed at the lateral nucleus of the amygdala. Before conditioning, a slow, negative component with peak amplitude around 150 ms was present in the evoked potentials. This component was sensitive to habituation. After fear conditioning, both CS+ and CS- elicited the same late component, albeit with a larger amplitude. This enhancement was temporary: decreasing amplitude was observed in the course of CS test presentations under extinction. Prior research revealed a comparable slow component in the amygdala of the cat under similar experimental conditions. The collective results indicate that the large late component in the amygdala is enhanced by fear conditioning, suggesting that such enhancement reflects the anticipation of a biologically significant event.