Through the eyes to memory: Fixation durations as an early indirect index of concealed knowledge.

In three experiments, we investigated an early memory effect in eye fixations, namely increased durations of the second fixations to known relative to unfamiliar stimuli. This effect occurs even if knowledge of the stimulus is deliberately concealed. In Experiment 1, we found the early memory effect using object materials and a gaze-contingent stimulus presentation that controlled for parafoveal stimulus processing. In Experiment 2a, we looked for the effect under conditions commonly used in the concealed information test (CIT). To this end, participants encoded the "to-be-concealed" knowledge incidentally while doing a mock crime task, which was followed by a CIT. Beyond the control of parafoveal stimulus processing in Experiment 1, this procedure allowed minimization of influences of carry-over processes associated with the preceding stimulus. Experiment 2b replicated Experiment 2a but applied a 1-week retention interval between the encoding of the to-be-concealed knowledge and the CIT. We observed an early memory effect in all experiments, suggesting that the effect is robust, irrespective of the paradigm, stimulus materials, and retention interval used.