Classical fear conditioning in functional neuroimaging

Buchel, C. & Dolan, R.J. (2000). Classical fear conditioning in functional neuroimaging. Current Opinion in Neurobiology, 10, 219-223.

A brief overview of the brief history of the examination of classical conditioning with functional neuroimaging. Early PET studies showed a striking absence of expected amygdala activation, but later demonstrated the expected amygdala involvement. More recently, 'backward masking' designs indicate a hemispheric difference when the CS+ was presented out of awareness, with greater activation observed in the right amygdala. fMRI studies showed amygdala participation during initial acquisition and early phases of extinction, and also demonstrated the characteristic decreases in amygdala activation over time. Interestingly, blocked fMRI designs revealed that social phobics do not show the 'physiological' decrease of amygdala activation over time. Finally, the paper points out the controversy about the role of the amygdala. One camp regards the amygdala as a rapid subcortical information processing unit that is continuously involved in the processing of CSs in aversive classical conditioning, producing deliberately high "false alarm" rates and being kept under the supervision of cortical controllers. The other camp sees the amygdala as enabling or permitting associative plasticity that encodes acquired sensory contingencies which are later expressed at a cortical level; once the association has been learned, the systems mediating the modulation of plasticity disengage and hence we see the decline in amygdala activation. More on this debate here.