Another important variable contributing to heterogeneity of neuroimaging results is the CS-US pairing or contingency rate.
Second, the choice of the contingency rate influences the definition of contrasts of interest between test and control conditions.
Specific experimental designs for fear conditioning in fMRI and PET were compared, focusing on the impact of critical experimental variables, such as timing parameters, the contingency rate, or characteristics of the stimuli, on neuroimaging results.
These studies were classified according to the type of study design (delay, trace, and extinction), the modality of the CS and US, the contingency rate, and the independent assessment of the conditioned response.
Results of the studies cited here indicate that activation of the amygdala seems to be independent of contingency rate: While thirteen studies employing continuous (100%) pairing, eight studies using 50% reinforcement and six studies with 0 %, 25 %, 33 %, 40 % and 80l reported amygdala activation, (e.g. [26], [29], [37], [49], [66]), others with the same pairing rates did not (e.g. [25], [60]).
Methodological differences were found a) in the conditioning protocol (delay, trace), b) in the contingency rate (100 %, 80 %, 50or less) and awareness, c) in the modality of CS and US (tactile, auditory, visual, olfactory), and d) with respect to the further assessment of the conditioned response (e.g. psycho-physiological measurements, verbal ratings).
Contingency rates in neuroimaging studies cited here are quite heterogeneous.
One study did not report any contingency rates [30].
Contingency rates determine how fast conditioned responses are acquired, and regulate extinction processes.
Finally, the choice of contingency rates is related to a problem specific to neuroimaging studies: the choice of contrasts between conditions.
Furthermore, we combined the contingencies "rate of unionization" and "knowledge intensity" with the control variable "size".
