We explored the usefulness of eye fixation durations as a dependent measure in a concealed knowledge test, drawing on Ryan, Hannula, and Cohen (2007), who found eye fixations on a familiar face to be longer than fixations on an unknown face. However, in their study, participants always had to select the known face out of three faces; thus, recognition and response intention could not be differentiated. In the experimental phase of our experiment, participants saw six faces per trial and had to select one of them. We had three conditions: In the first, one of the six faces was a known face, and the participants had to conceal that knowledge and select another face (concealed display); in another, one of the six faces was a known face, and the participants had to select that face (revealed display); or finally, all six faces were unknown, and participants had to select any of the six faces (neutral display). Using fixation durations as the dependent measure, we found a pure and early recognition effect; that is, fixations on the concealed faces (known but not selected) were longer than fixations on the nonselected unknown target faces in the neutral display. In addition, we found a response intention effect; that is, fixation durations on the selected known faces were longer than those on concealed faces (known but not selected).

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 Bto-be-concealedk nowledge 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.

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