David C. Knight

Neural Substrates Mediating Human Delay and Trace Fear Conditioning

Previous functional magnetic resonance imaging (fMRI) studies with human subjects have explored the neural substrates involved in forming associations in Pavlovian fear conditioning. Most of these studies used delay procedures, in which the conditioned stimulus (CS) and unconditioned stimulus (UCS) coterminate. Less is known about brain regions that support trace conditioning, a procedure in which an interval of time (trace interval) elapses between CS termination and UCS onset. Previous work suggests significant overlap in the neural circuitry supporting delay and trace fear conditioning, although trace conditioning requires recruitment of additional brain regions. In the present event-related fMRI study, skin conductance and continuous measures of UCS expectancy were recorded concurrently with whole-brain blood oxygenation level-dependent (BOLD) imaging during direct comparison of delay and trace discrimination learning. Significant activation was observed within the visual cortex for all CSs. Anterior cingulate and medial thalamic activity reflected associative learning common to both delay and trace procedures. Activations within the supplementary motor area (SMA), frontal operculum, middle frontal gyri, and inferior parietal lobule were specifically associated with trace interval processing. The hippocampus displayed BOLD signal increases early in training during all conditions; however, differences were observed in hippocampal response magnitude related to the accuracy of predicting UCS presentations. These results demonstrate overlapping patterns of activation within the anterior cingulate, medial thalamus, and visual cortex during delay and trace procedures, with additional recruitment of the hippocampus, SMA, frontal operculum, middle frontal gyrus, and inferior parietal lobule during trace conditioning. These data suggest that the hippocampus codes temporal information during trace conditioning, whereas brain regions supporting working memory processes maintain the CS-UCS representation during the trace interval.

Amygdala and hippocampal activity during acquisition and extinction of human fear conditioning.

Previous functional magnetic resonance imaging (fMRI) studies have characterized brain systems involved in conditional response acquisition during Pavlovian fear conditioning. However, the functional neuroanatomy underlying the extinction of human conditional fear remains largely undetermined. The present study used fMRI to examine brain activity during acquisition and extinction of fear conditioning. During the acquisition phase, participants were either exposed to light (CS) presentations that signaled a brief electrical stimulation (paired group) or received light presentations that did not serve as a warning signal (control group). During the extinction phase, half of the paired group subjects continued to receive the same treatment, whereas the remainder received light alone. Control subjects also received light alone during the extinction phase. Changes in metabolic activity within the amygdala and hippocampus support the involvement of these regions in each of the procedural phases of fear conditioning. Hippocampal activity developed during acquisition of the fear response. Amygdala activity increased whenever experimental contingencies were altered, suggesting that this region is involved in processing changes in environmental relationships. The present data show learning-related amygdala and hippocampal activity during human Pavlovian fear conditioning and suggest that the amygdala is particularly important for forming new associations as relationships between stimuli change.

The role of the human amygdala in the production of conditioned fear responses

The amygdala plays a central role in the acquisition and expression of fear memories. Laboratory animal studies indicate that the amygdala both receives sensory information and produces learned behavioral and autonomic fear responses. However, prior functional imaging research in humans has largely focused on amygdala activity elicited by fearful stimuli, giving less attention to this regions role in the production of fear responses. In contrast, the present study used functional magnetic resonance imaging to investigate the amygdalas influence on the generation of conditional fear responses. Significant increases in amygdala activity were observed during the production of conditioned (learning-related), but not orienting, nonspecific, and unconditioned (nonlearning-related) skin conductance responses. Further, greater amygdala activity was demonstrated during conditioned response production than during conditioned stimulus presentation. These results suggest the amygdala not only responds to fearful stimuli, but also generates learning-related changes in human autonomic fear expression.

Functional MRI of Human Amygdala Activity During Pavlovian Fear Conditioning: Stimulus Processing Versus Response Expression

Although laboratory animal studies have shown that the amygdala plays multiple roles in conditional fear, less is known about the human amygdala. Human subjects were trained in a Pavlovian fear conditioning paradigm during functional magnetic resonance imaging (fMRI). Brain activity maps correlated with reference waveforms representing the temporal pattern of visual conditional stimuli (CSs) and subject-derived autonomic responses were compared. Subjects receiving paired CS-shock presentations showed greater amygdala activity than subjects receiving unpaired CS-shock presentations when their brain activity was correlated with a waveform generated from their behavioral responses. Stimulus-based waveforms revealed learning differences in the visual cortex, but not in the amygdala. These data support the view that the amygdala is important for the expression of learned behavioral responses during Pavlovian fear conditioning.

Human amygdala activity during the expression of fear responses.

The initial learning and subsequent behavioral expression of fear are often viewed as independent processes with potentially unique neural substrates. Laboratory animal studies of Pavlovian fear conditioning suggest that the amygdala is important for both forming stimulus associations and for subsequently expressing learned behavioral responses. In the present article, human amygdala activity was studied during the autonomic expression of conditional fear in two differential conditioning experiments with event-related functional magnetic resonance imaging and concurrent recording of skin conductance responses (SCRs). Trials were classified on the basis of individual participants SCRs. Significant amygdala responding was detected only during trials on which a signal both predicted shock and elicited significant conditional SCR. Conditional stimulus presentation or autonomic activity alone was not sufficient. These results indicate that amygdala activity may specifically reflect the expression of learned fear responses and support the position that this region plays a central role in the expression of emotional reactions.

Impact of continuous versus intermittent CS-UCS pairing on human brain activation during Pavlovian fear conditioning.

During Pavlovian fear conditioning a conditioned stimulus (CS) is repeatedly paired with an aversive unconditioned stimulus (UCS). In many studies the CS and UCS are paired on every trial, whereas in others the CS and UCS are paired intermittently. To better understand the influence of the CS-UCS pairing rate on brain activity, the experimenters presented continuously, intermittently, and non-paired CSs during fear conditioning. Amygdala, anterior cingulate, and fusiform gyrus activity increased linearly with the CS-UCS pairing rate. In contrast, insula and left dorsolateral prefrontal cortex responses were larger during intermittently paired CS presentations relative to continuously and non-paired CSs. These results demonstrate two distinct patterns of activity in disparate brain regions. Amygdala, anterior cingulate, and fusiform gyrus activity paralleled the CS-UCS pairing rate, whereas the insula and dorsolateral prefrontal cortex appeared to respond to the uncertainty inherent in intermittent CS-UCS pairing procedures. These findings may further clarify the role of these brain regions in Pavlovian fear conditioning.

Expression of conditional fear with and without awareness

Conditional responding during simple Pavlovian conditioning is often characterized as a form of implicit memory. The extent to which this type of associative learning is independent of awareness is an issue of continuing debate. Previous studies have demonstrated conditioning in the absence of awareness. However, their results have been questioned based on methodological concerns with postexperimental questionnaires. In the present study, skin conductance response (SCR) and unconditioned stimulus (UCS) expectancy were measured concurrently as participants were exposed to a differential delay fear conditioning procedure in which one tone (CS+) predicted a loud white noise, whereas a second tone (CS-) was presented alone. UCS predictability was varied on a trial-by-trial basis by presenting conditioned stimuli (CSs) at volumes just above or below the perceptual threshold. Differential UCS expectancy (awareness) was observed only on perceived trials, whereas differential SCR developed on both perceived and unperceived trials. Although perceived stimuli elicited larger SCRs, the magnitude of conditioning, indexed by differential conditioned response expression (conditioned SCR to CS+ minus the SCR to CS-), was not influenced by stimulus perception. These data indicate that conditional fear can be expressed when individuals are unaware of fear-eliciting stimuli and suggest that the degree of conditioning is independent of awareness during differential Pavlovian fear conditioning.

Neural correlates of unconditioned response diminution during Pavlovian conditioning

Pavlovian conditioning research has shown that unconditioned responses (UCR) to aversive unconditioned stimuli (UCS) are reduced when a UCS is predictable. This effect is known as UCR diminution. In the present study, we examined UCR diminution in the functional magnetic resonance imaging (fMRI) signal by varying the rate at which a neutral conditioned stimulus (CS) was paired with an aversive UCS. UCR diminution was observed within several brain regions associated with fear learning, including the amygdala, anterior cingulate, auditory cortex, and dorsolateral prefrontal cortex when a CS continuously relative to intermittently predicted the UCS. In addition, an inverse relationship between UCS expectancy and UCR magnitude was observed within the amygdala, anterior cingulate, and dorsolateral prefrontal cortex, such that as UCS expectancy increased the UCR decreased. These findings demonstrate UCR diminution within the fMRI signal, and suggest that UCS expectancies modulate UCR magnitude.

Functional MRI of human Pavlovian fear conditioning: patterns of activation as a function of learning.

fMRI was used to study human brain activity during Pavlovian fear conditioning. Subjects were exposed to lights that either signaled painful electrical stimulation (CS+), or that did not serve as a warning signal (CS-). Unique patterns of activation developed within anterior cingulate and visual cortices as learning progressed. Training with the CS+ increased active tissue volume and shifted the timing of peak fMRI signal toward CS onset within the anterior cingulate. Within the visual cortex, active tissue volume increased with repeated CS+ presentations, while cross-correlation between the functional time course and CS- presentations decreased. This study demonstrates plasticity of anterior cingulate and visual cortices as a function of learning, and implicates these regions as components of a functional circuit activated in human fear conditioning.

Neural substrates of explicit and implicit fear memory

Distinct aspects of our fearful experiences appear to be mediated by separate explicit and implicit memory processes. To identify brain regions that support these separate memory processes, we measured contingency awareness, conditional fear expression, and functional magnetic resonance imaging signal during a Pavlovian fear conditioning procedure in which tones that predicted an aversive event were presented at supra and sub-threshold volumes. Contingency awareness developed in conjunction with learning-related hippocampal and parahippocampal activity on perceived conditioning trials only. In contrast, conditional fear and differential amygdala activity developed on both perceived and unperceived trials, regardless of whether contingency awareness was expressed. These findings demonstrate the distinct roles of these brain regions in explicit and implicit fear memory processes.