Differential fear conditioning is associated with amplified processing of the CS+ relative to the CS− in sensory cortical networks. Amplified processing is observed for both visual and auditory CSs and using a range of US modalities. Generally speaking, the effects of fear conditioning are evident early (within the first 100 ms) and seem to occur more rapidly for auditory compared to visual CS+ cues. Amplified processing of fear-associated cues is likely driven by re-entrant bias signals emanating from multiple subcortical and cortical sources and acting on the sensory regions. The precise cortical mapping of the conditioned sensory responses is precluded by current evidence, although it appears that many response enhancements obtained using within-session conditioning designs with limited CS-US pairings, occur within secondary or associative sensory regions. When learning occurs over longer time periods (e.g., across two consecutive days) or after hundreds of CS-US pairings, then responses to the CS+ are altered in the lowest tiers of the sensory cortex (e.g., the visual C1). Evidence from steady-state studies suggests that CS+ sensory enhancement is related to the degree of fear system activation (i.e., heart rate acceleration in response to the CS+) and depends on CS-US associative strength rather than declarative expectancy of the US. Increased synchronized neuronal activity in the high frequency (> 20 Hz) EEG gamma band range is related to the formation of a cortical cell assembly representing CS+ features. Cross-modal forms of plasticity (e.g., a visual CS+ that elicits a response in regions of the auditory cortex) may also be observed during fear conditioning. This phenomenon appears to be more related to expectancy or preparation in the face of imminent US delivery.
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