rTMS on left DLPFC affects emotional memory retrieval. The contribution of the stimulus valence

A reduced number of research has examined whether and where emotion and memory could be represented in the brain. A suggestive hypothesis proposed that the Lateral Prefrontal Cortex (LPFC) may be identified as the site of emotion-memory integration, since it was shown to be particularly sensitive to the activation of the emotional memories. Firstly, prefrontal cortex seems to be involved in memory mechanisms and, secondly, it is implicated in emotional regulation by managing the cognitive control over emotional behaviour. Nevertheless, there is little understanding around how prefrontal areas accomplish both emotional and memory functions. In the present research we explored the role of the dorsolateral prefrontal cortex (DLPFC) in memory retrieval process of positive vs. negative emotional stimuli. Secondly, the lateralization effect (left hemisphere) was explored in response to positive emotional material. These aspects were analyzed by using a repetitive transcranial magnetic stimulation (rTMS) paradigm that induced a cortical activation of the left DLPFC. Subjects were required to perform a task consisting in two experimental phases: an encodingphase, where some lists composed by positive and negative emotional words were presented to the subjects (9 lists, each composed of 20 words, 10 with a positive and 10 with a negative emotional content); a retrieval-phase, where the old and new emotional stimuli were presented for a recognition performance. For the retrieval-phase, stimulus material was composed by a total of 270 stimuli (9 lists, each composed by 30 words grouped in the old and new categories). An rTMS stimulation (5 Hz) was provided over the left DLPFC during the retrieval-phase. We found that the rTMS stimulation over this brain area affects the memory retrieval of emotional material, with higher memory efficiency (reduced RTs) for positive stimuli, whereas memory effectiveness (accuracy index) was not influenced by the stimulation. This result suggests that left DLPFC activation favours the memory retrieval of positive emotional information in comparison with negative information. The valence model of emotional cue processing may explain the decreasing of RT measure, by pointing out the distinct role the left (and right) hemisphere has in emotional cue processing. The model allowed to elucidate the relationship between emotional information retrieval and hemispheric lateralization effect, supposing withdrawal-related emotions are located to the right hemisphere, whereas approach-related emotions are biased to the left hemisphere. When one of the two cortical systems is hyper-activated, subjects had an unbalanced response in favour to one of the two emotional categories, respectively the positive or the negative one.