Visuo-verbal distinction revisited: new insights from a study on temporal lobe epilepsy patients in the debate over the lateralization of material-specific and process-specific aspects of memory.

Introduction: The automatic interaction between a cue and a memory trace can give rise to the vivid recollection of a purely sensory past experience. But are humans able to reach back intentionally to purely sensory experiences in the absence of any exogenous or endogenous cue? In the present study, we propose an alternative hypothesis, claiming that the retrieval of associated semantic memories, stored in the left hemisphere and acting as endogenous cues, is a prerequisite for intentionally recollecting sensory experience stored in the right hemisphere during mental time travels (MTT). Methods: To investigate this issue, we administered an MTT task to 26 epileptic patients (16 males and 10 females) who had undergone right or left temporal lobectomy and to 28 age and education matched controls. The task was devised so as to require the recollection of purely visual memories in the absence of external cues. Participants also performed two conventional recognition tasks with visual and verbal materials. The three between-subjects memory tasks were analyzed separately with the Kruskal-Wallis test and the Wilcoxon rank-sum test in order to investigate differences across groups. According to our hypothesis, we expected side asymmetries in the patients' performance on the two recognition tasks but not the MTT task. Results: While patients showed the well-known hemispheric asymmetry for visual and verbal material in the (external-cue dependent) recognition tasks, no side asymmetries emerged in the purely visual MTT task. Conclusions: In keeping with the view that visual memories cannot be targeted directly by a strategic search process, the lack of any side asymmetry in our MTT task can be interpreted as a trade-off between left-sided strategic search for associated semantic memories and right-sided storage of visual ones.