The effect of retroactive memory interference on the P300-based Complex Trial Protocol (CTP)

In a P300-based concealed information test (CIT), an increased response to a crime-related “probe” item of interest suggests concealed knowledge. Because the CIT's detection ability is based on knowledgeable parties recognizing the key item, weakening the crime memory might decrease probe identifiability and reduce diagnostic power. Research on retroactive memory interference (RI) has shown that acquiring new information after encoding a memory can degrade the original memory, which suggests that RI might pose a threat to CITs. To test this, Gronau et al. (2015) had participants complete a mock-crime, followed by either a control task or a RI manipulation task, intended to impair the crime memory. Both the simple guilty control and RI groups were subdivided into three time delay conditions: 1/3 of participants immediately completed the task and CIT, another 1/3 completed the task and returned a week later for the CIT, and the remaining participants completed both the task and CIT a week later. Results showed that RI reduced memory of crime details and skin conductance responses, while respiration line length was unaffected. Here, we extend Gronau et al. (2015), using the Complex Trial Protocol (CTP) version of the P300-based CIT, to investigate the influence of RI on recognition. The CIT effect was obvious in all six group × time delay subconditions, as evidenced by their significantly larger probe vs. irrelevant amplitudes, high percentage of bootstrapped iterations where probe > irrelevants, bootstrapped mean amplitude differences, and satisfactory hit rates. However, these indices of the CIT effect did not differ based on group or time delay, as was the case for target response error rates and P300 latencies. The only outcome of interest to vary by group or time delay was behavioral response times: both probe and combined irrelevant responses were delayed in the control (simple guilty) group. Thus, the evidence suggests that the RI manipulation used here does not threaten the P300-based CTP's accuracy. Results are considered as they compare to previous work, and limitations and possible explanations for our results are discussed.