Single versus multiple probe blocks of P300-based concealed information tests for self-referring versus incidentally...

The aim of the present study was to compare two protocols and two information types on detection of concealed knowledge. Four independent groups of subjects were run. Two were tested on probe stimuli of a self-referring (AUTO) nature and two were tested on incidentally acquired details of a mock crime (MOCK). Each pair of groups was run either in a one-probe (1PB) or multiple probe (6PB) block. In the single probe block, which was repeated three times with a different probe on each block, one probe item was randomly repeated multiple times in a Bernoulli series with frequently presented, meaningless or irrelevant items. There was also a rare target item designed to force attention to the stimulus screen. All stimulus types were of the same category within each block. In contrast, in the multiple probe (and category) block, rare probes, rare targets and frequent irrelevant items were repeatedly presented in a Bernoulli series within one block.

There was a difference in task demand as measured by reaction time between the two protocols (the multiple probe protocol was more demanding), and a difference trend in P300 detection sensitivity between protocols for both information types combined in favor of the 1PB (p<.07). With both protocols combined, there was a trend (p<.07) favoring detection of familiar versus incidentally learned information. In terms of P300 amplitudes, both protocols showed the usual result that P300 to probes was greater than that to irrelevants. Also, as with detection rates, self-referring information was better detected in terms of Probe-Irrelevant P300 amplitude differences than mock crime information, regardless of protocol. There was no effect of time passage across the repeated blocks of the one-probe protocol. Methodological problems with the multiple probe protocol as utilized in most recent publications are discussed.