Magnetic imaging in human classical conditioning

Acquired fears reflected in cortical sensory processing: a review of electrophysiological studies of human classical conditioning

The capacity to associate neutral stimuli with affective value is an important survival strategy that can be accomplished by cell assemblies obeying Hebbian learning principles. In the neuroscience laboratory, classical fear conditioning has been extensively used as a model to study learning-related changes in neural structure and function. Here, we review the effects of classical fear conditioning on electromagnetic brain activity in humans, focusing on how sensory systems adapt to changing fear-related contingencies. By considering spatiotemporal patterns of mass neuronal activity, we illustrate a range of cortical changes related to a retuning of neuronal sensitivity to amplify signals consistent with fear-associated stimuli at the cost of other sensory information. Putative mechanisms that may underlie fear-associated plasticity at the level of the sensory cortices are briefly considered, and several avenues for future work are outlined.

Learning related activation of somatosensory cortex by an auditory stimulus recorded with magnetoencephalography

Advances in non-invasive neuroimaging technology now provide a means of directly observing learning within the brain. Classical conditioning serves as an ideal starting point for examining the dynamic expression of learning within the human brain, since this paradigm is well characterized using multiple levels of analysis in a broad range of species. We used MEG to expand the characterization of conditioned responses (CR) recorded from the human brain with a simultaneous examination of their spatial, temporal and spectral properties. We paired an auditory conditioned stimulus (CS+) with a somatosensory unconditioned stimulus (US). We found that when the US was randomly omitted, presentations of CS+ alone, elicited greater desynchronization of beta-band activity in contralateral somatosensory cortex compared to presentations of an auditory stimulus that was never paired with the US (CS-), and compared the CS+ following a non-reinforced extinction session. This differentiation was largest between 150 and 350ms following US omission. We show that cross-modal CRs in the primary sensorimotor system are predominantly characterized by modulation of ongoing cortical oscillations.

Alteration in the response properties of primary somatosensory cortex related to differential aversive Pavlovian conditioning

The effects of differential aversive Pavlovian conditioning on the functional organization of primary somatosensory cortex (SI) were examined in 17 healthy participants. Neuroelectric source imaging from 60 electrodes was employed while nine subjects received an innocuous electric stimulus (conditioned stimulus, CS) to one finger (left or right) that was followed by painful electric shock to the lower back (unconditioned stimulus, US) and an innocuous stimulus to the other finger that was never followed by pain. Eight subjects received a presentation of the innocuous and painful stimuli with equal probability to both fingers (control group). The data included the electromyogram (EMG) from the left m. corrugator, and judgments of intensity, aversiveness, and CS-US contingency. Only the experimental group displayed EMG conditioning, differential contingency judgments, as well as a change of dipole orientation for the CS and an enhanced dipole moment for the US in the electroencephalogram. Intensity and unpleasantness ratings were altered in a more unspecific manner and did not differ between groups and stimulus conditions. The data suggest that SI contributes to memory processes in associative learning. Pavlovian conditioning of tactile responses might be important in the altered processing of painful stimuli in chronic pain patients where enhanced conditioning has been demonstrated.

The C50m response: conditioned magnetocerebral activity recorded from the human brain

Recent advances in neuroimaging technology now permit a precise determination of the dynamics of specific neural activity underlying human associative learning. We used magnetoencephalography (MEG) to characterize the dynamics of conditioned responses (CRs) within auditory cortex during habituation, delay and trace conditioning training, and delay conditioning extinction. Conditioned stimuli (CS) were visually presented geometric figures, and unconditioned stimuli (US) were aversive noise bursts. CS+ stimuli were paired with the US on 50% of presentations: CS- stimuli were never paired with the US. Auditory cortex was activated following the paired CS+ at an average of 49-62 ms following US onset. Our data support the presence of a differential conditioned response (C50m) in auditory cortex following the unpaired CS+ at an average of 30-61 ms after US omission. The current source strength of the auditory C50m was subsequently quantified for the unpaired CS+ and CS- during training, the unpaired CS+ during extinction, and habituation. During delay and trace training, the C50m was stronger for the unpaired CS+ than for the CS-, and was also stronger for the unpaired CS+ during training compared to both habituation and extinction. This is the first description of magnetocerebral conditioning in normal human auditory cortex. The C50m activity in auditory cortex elicited by visual stimuli constitutes a direct observation of associative neural plasticity within the human auditory cortex.

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.

Right-sided human prefrontal brain activation during acquisition of conditioned fear

This H2(15)O positron emission tomography (PET) study reports on relative regional cerebral blood flow (rCBF) alterations during fear conditioning in humans. In the PET scanner, subjects viewed a TV screen with either visual white noise or snake videotapes displayed alone, then with electric shocks, followed by final presentations of white noise and snakes. Autonomic nervous system responses confirmed fear conditioning only to snakes. To reveal neural activation during acquisition, while equating sensory stimulation, scans during snakes with shocks and white noise alone were contrasted against white noise with shocks and snakes alone. During acquisition, rCBF increased in the right medial frontal gyrus, supporting a role for the prefrontal cortex in fear conditioning to unmasked evolutionary fear-relevant stimuli.

Modulation of auditory neural responses by a visual context in human fear conditioning

Responses to a stimulus signaling danger depend not only on the nature of that stimulus, but also on the context in which it is presented. A large body of work has been conducted in experimental animals investigating the neural correlates of contextual modulation of fear responses. However, much less is known about this process in humans. In this study we used functional MRI in a fear conditioning paradigm to explore this phenomenon. Responses to acoustic conditioned stimuli in auditory cortex were modulated by the presence of a visual context which signaled the likelihood of receiving an aversive unconditioned stimulus. Furthermore, the presence of the aversive visual context was associated with enhanced activity in parietal cortex, which may reflect an increase in attention to the presence of environmental threat stimuli.

Fear conditioning and brain activity: a positron emission tomography study in humans

Regional cerebral blood flow (rCBF) was measured with H2 (15)O positron emission tomography in 8 healthy women before and after fear conditioning (i.e., paired shocks) and unpaired shocks to videotape cues. Conditioning was supported by enhanced peripheral nervous system recordings and subjective ratings. Fear conditioning increased rCBF in the central gray of the midbrain; bilaterally in the hypothalamus, the thalamus, and the left striatum; and in the right and left anterior cingulate and right prefrontal cortices. Regional CBF was attenuated bilaterally in the right and left prefrontal, temporal (including the amygdala), parietal, and occipital cortices, and in the left orbitofrontal cortex. When compared with unpaired shock presentations, fear conditioning resulted in elevated rCBF in the left cerebellum. Hence, in the present paradigm, only neural activity in the left cerebellum solely reflected processes associated with true Pavlovian conditioning.

A behavioral stages model of classical (Pavlovian) conditioning: application to cognitive aging

In the present article, it is argued that a five-stage sequential model of the behavioral and neurophysiological events that occur when organisms are exposed to signals predicting significant events suggests that classical conditioning produces multiple memory traces involving both excitatory and inhibitory processes. Further, these multiple brain structures and associated neurophysiological mechanisms are beginning to be understood; thus, using Pavlovian conditioning techniques to study aging and cognitive functions may provide insights into which brain structures or mechanisms are responsible for more general age-related declines in associative learning and memory. The evidence for this model is briefly reviewed and studies suggesting age-related effects on classical conditioning of various response systems are described within the context of the brain structures implicated by the model.

Subcortical correlates of differential classical conditioning of aversive emotional reactions in social phobia

BACKGROUND

Conditioning processes have been proposed to play a role in the development of anxiety disorders. As yet, the neurobiologic correlates of emotional learning have not been fully understood in these patients. Accordingly, brain activity was studied in subcortical and cortical regions involved in the processing of negative affect during differential aversive classical conditioning.

METHODS

Twelve patients with social phobia and 12 healthy control subjects were presented with paired conditioned (CS; neutral facial expressions) and unconditioned stimuli (US; negative odor vs unmanipulated air). Functional magnetic resonance imaging (fMRI) was utilized to examine regional cerebral activity during habituation, acquisition,a nd extinction trials. Activity was measured with echo-planar-imaging (EPI), and signal intensity in individually defined anatomic regions were analyzed.

RESULTS

Subjective ratings of emotional valence to the CS indicated that behavioral conditioning occurred in both groups. The presentation of CS associated with negative odor led to signal decreases in the amygdala and hippocampus of normal subjects, whereas an opposite increased activation in both regions was observed in patients. Regional differences were not found during habituation and extinction.

CONCLUSIONS

Results suggest that conditioned aversive stimuli are processed in subcortical regions, with phobic patients differing from control subjects.

Conditioning, awareness, and the hippocampus

For the past 50 years, psychologists have wrestled with questions regarding the relationship between conscious awareness and human conditioned behavior. A recent proposal that the hippocampus mediates awareness during trace conditioning (Clark, Squire, Science 1998;280:77-81) has extended the awareness-conditioning debate to the neuroscience arena. In the following commentary, we raise specific theoretical and methodological issues regarding the Clark and Squire study and place their finding into a broader context. Throughout our discussion, we consider the difficulties in assessing subjective awareness, the importance of establishing necessary and sufficient conditions for cognitive mediation effects, the influence of conditioned response modality, and the nature of hippocampal requirements across conditioning protocols. It is clear that trace eyeblink conditioning is a hippocampal-dependent task, but whether awareness is a necessary component of trace conditioning is not definitively proven. We propose that future functional neuroimaging studies and behavioral experiments using on-line measures of awareness may help clarify the relationship among classical conditioning, awareness, and the hippocampus.

Cerebral correlates of anticipated fear: a PET study of specific phobia

Cerebral correlates of anticipatory anxiety was studied in 14 women with specific snake or spider phobia, and in 6 nonphobic controls. Videofilms with neutral scenes were shown during positron emission tomographic measurements of regional cerebral blood flow (rCBF). The phobics, but not the controls, anticipated scenes with spiders or snakes to appear in the videofilms. This was reflected in higher anxiety ratings in the phobics. The primary visual cortex rCBF was lower in phobics than controls, while no rCBF differences were found in the other brain regions investigated. The results are suggested to indicate inhibition of corticogeniculate pathways, in turn representing a neurophysiological correlate of avoidant anticipatory coping.