Memory indexing: a novel method for tracing memory processes in complex cognitive tasks.

We validate an eye-tracking method applicable for studying memory processes in complex cognitive tasks. The method is tested with a task on probabilistic inferences from memory. It provides valuable data on the time course of processing, thus clarifying previous results on heuristic probabilistic inference. Participants learned cue values of decision alternatives that were arranged within spatial frames. Later, they were told about the validities of cue dimensions and performed memory-based binary choice tasks: first, according to spontaneously adopted decision strategies and, subsequently, according to instructed decision strategies (a noncompensatory lexicographic strategy and a compensatory equal weighting strategy). During decision making, participants saw only the empty spatial frames without cue values. The spontaneously adopted and instructed decision strategies were reflected in discriminable gaze patterns on the empty spatial frames. When retrieving information no longer visible, participants tended to fixate on locations at which information was visible during the learning phase (the looking-at-nothing phenomenon). Gaze patterns were consistent with cue-wise and alternative-wise patterns of information search predicted for the instructed decision strategies as well as for the spontaneously adopted strategies identified based on decision outcomes. These findings extend previous results on the connection between memory and gaze. Furthermore, the successful application of memory indexing suggests its wider applicability in studying memory-based tasks.