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

ABSTRACT

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.

KEYWORDS:

• Mental time travel
• material-specific memory disorders
• temporal lobe epilepsy
• unilateral temporal lobectomy
• hemispheric specialization

Acknowledgments

This study was supported by the Italian Ministry of Health (Current Research).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Notes

1. According to this method, the performance of the normative sample is analyzed by means of a multiple regression model where demographic variables (age, education and gender) are entered as predictors and test scores as dependent variable. For each level of the demographic variables showing a statistically significant association with the dependent variable, adjustment factors can be derived from the corresponding parameter in the regression model. This method can lead sometimes to somehow opaque adjusted score values. Thus, for example, according to Giovagnoli et al. (1996), for older and less educated subjects up to 119 seconds need to be subtracted from the raw score on the Trail Making B. This means that a high performing, 75 years old individual with 3 years of education and a raw score of 120 sec would obtain an adjusted score of just 1 sec.

Additional information

Funding

This work was supported by the Italian Ministry of Health.