Neurobehavioral mechanisms of human fear generalization

Abstract

While much research has elucidated the neurobiology of fear learning, the neural systems supporting the generalization of learned fear are unknown. Using functional magnetic resonance imaging (fMRI), we show that regions involved in the acquisition of fear support the generalization of fear to stimuli that are similar to a learned threat, but vary in fear intensity value. Behaviorally, subjects retrospectively misidentified a learned threat as a more intense stimulus and expressed greater skin conductance responses (SCR) to generalized stimuli of high intensity. Brain activity related to intensity-based fear generalization was observed in the striatum, insula, thalamus/periacqueductal gray, and subgenual cingulate cortex. The psychophysiological expression of generalized fear correlated with amygdala activity, and connectivity between the amygdala and extrastriate visual cortex was correlated with individual differences in trait anxiety. These findings reveal the brain regions and functional networks involved in flexibly responding to stimuli that resemble a learned threat. These regions may comprise an intensity-based fear generalization circuit that underlies retrospective biases in threat value estimation and overgeneralization of fear in anxiety disorders.

Figure 1. Stimulus set and task design

(a) The stimulus dimension consisted of 5 images, of the same identity, morphed between neutral and fearful endpoints. (b) The task involved rating whether each face was or was not expressing fear by pressing one of two buttons. In the first phase (preconditioning), subjects saw each morph increment in the absence of the US. Fear learning involved repeated pairings of the S3 (CS+) with an electrical shock US, and the S1 (CS−) unreinforced. The generalization test followed fear learning and involved presentation of each of the morph increments. The CS+ was intermittently reinforced during the generalization test. Images are not to scale.

Figure 2. Behavioral results

(a) Mean normalized SCRs from preconditioning and the generalization test show that response output was undifferentiated along the neutral-to-fearful continuum during preconditioning (white bars) and shifted towards stimuli of high emotional intensity during the generalization test (black bars). (b) Difference scores, reflecting the change in response output from preconditioning to generalization test, show an average decrease in psychophysiological responses to the S1 and S2 and an average increase to the S3, S4, and S5. (c) Mean reaction times show that subjects were fastest to categorize the S4 and S5 as fearful. (d) A majority of subjects (71%) mistakenly identified the S4 as the CS+ indicating a strong illusory correlation. Error bars reflect standard error of the mean (SEM), (*) denote significant differences (p < 0.05) and (**) at p < 0.01.

Figure 3. Brain regions involved in differential fear learning show generalized patterns of activity

(a) Regions of interest were identified by contrasts of CS+ versus CS−. Contrasts of each non-conditioned stimulus (S2, S4, and S5) versus the CS− revealed significant differential activity for the S4 and S5 versus the CS− (one-sample t tests) in the right caudate (x = 10, y = 4, z= 2) and right insula (x = 34, y = 12, z= 6) during the generalization test. Activity was significantly enhanced for the S5 contrast from preconditioning to the generalization test (paired-samples t test) in the right caudate. (b) Regions of interest were identified by contrasts of CS− versus CS+, and revealed activity in the subgenual (x = −2, y = 32, z= −6) and rostral (x = −6, y = 44, z= 10) ACC. Contrasts comparing non-conditioned stimuli versus the CS+ showed significant differential activity between the S2 and CS+ during the generalization test. All regions were small volume corrected with a family-wise-error p < 0.05, shown here at p < 0.001 (uncorrected) for visualization purposes. Error bars reflect SEM. For one-sample t tests, (*) denote significant differences (p < 0.05) and (**) at p < 0.01. For paired-samples t test, (/) denotes significant differences (p < 0.05).

Figure 4. Brain-behavior interactions in fear generalization

(a) Normalized SCR scores for the S4 illustrate the variability in response across subjects and across runs. (b) Correlations between SCR scores and brain activity were revealed in the insula and amygdala, such that increases in arousal from pre-to-post fear learning were associated with increases in brain activity in these regions.

Figure 5. Learning-related and generalization-related increases in amygdala-visual cortex activity its relation to trait anxiety

(a) Single trial connectivity analysis, using the amgydala as a seed region, showed learning-related changes in amygdala connectivity with a face-selective region within the fusiform gyrus that was identified from an independent functional localizer task. Connectivity between these regions was undifferentiated prior to fear learning, and showed a linear increase in connectivity during the generalization test. ANOVA revealed a phase (preconditioning, generalization test) by stimulus interaction for the CS+ versus the CS−, as well as for the S4 versus the S2. (b) Increases in functional connectivity on S4 trials, from preconditioning to the generalization test, were positively related with trait anxiety scores (r ₍₁₃₎ = 0.62, p = 0.018). Increases in amygdala-FFG connectivity were not related with anxiety scores for the CS+ (r ₍₁₃₎ = −0.12, p = 0.69).