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Associative memory (AM) is the ability to remember and retrieve multiple items bound together. Previous studies aiming to modulate AM by various transcranial electric stimulation (tES) techniques were inconclusive, although overall suggestive that tES could be a tool for AM enhancement. However, evidence from a direct comparison between different tES techniques is lacking. Here, in a sham-controlled cross-over experiment, we comparatively assessed the effects of three types of tES—anodal tDCS, theta-band transcranial alternating current stimulation (tACS), and theta-oscillatory tDCS (otDCS), delivered over the left posterior parietal cortex, during a short-term digit-color AM task with cued-recall. The effects were tested in 40 healthy young participants while both oscillatory tES were delivered at a previously determined individual theta frequency (4–8 Hz). All three active stimulations facilitated the overall AM performance, and no differences could be detected between them on direct comparison. However, unlike tDCS, the effects of which appeared to stem mainly from the facilitation of low-memory demand trials, both theta-modulated tACS and otDCS primarily promoted AM in high memory demand trials. Comparable yet differential effects of tDCS, theta tACS, and otDCS could be attributed to differences in their presumed modes of action.

Associative memory (AM) is the ability to remember the relationship between previously unrelated items. AM is significantly affected by normal aging and neurodegenerative conditions, thus there is a growing interest in applying non-invasive brain stimulation (NIBS) techniques for AM enhancement. A growing body of studies identifies posterior parietal cortex (PPC) as the most promising cortical target for both transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (tES) to modulate a cortico-hippocampal network that underlines AM. In that sense, theta frequency oscillatory tES protocols, targeted towards the hallmark oscillatory activity within the cortico-hippocampal network, are increasingly coming to prominence. To increase precision and effectiveness, the need for EEG guided individualization of the tES protocols is proposed. Here, we present the study protocol in which two types of personalized oscillatory tES–transcranial alternating current stimulation (tACS) and oscillatory transcranial direct current stimulation (otDCS), both frequency-modulated to the individual theta-band frequency (ITF), are compared to the non-oscillatory transcranial direct current stimulation (tDCS) and to the sham stimulation. The study has cross-over design with four tES conditions (tACS, otDCS, tDCS, sham), and the comprehensive set of neurophysiological (resting state EEG and AM-evoked EEG) and behavioral outcomes, including AM tasks (short-term associative memory, face–word, face–object, object-location), as well as measures of other cognitive functions (cognitive control, verbal fluency, and working memory).

Spatial memory relies on efficient encoding, storage and retrieval of spatial information, which enables us to remember paths or locations of objects in everyday life. Moreover, this type of memory has been shown to decline with age and various neurodegenerative disorders. Parietal cortex has been shown to play an important role in the formation of short-term representations of spatial information. The aim of the current study was to test the possibility of immediate and long-term spatial memory enhancement, by increasing excitability of parietal posterior cortex. We used transcranial direct current stimulation (tDCS) over posterior parietal cortex in a placebo-controlled cross-over study. Participants received anodal (1.5 mA) or sham tDCS stimulation over P4 site (10-20 EEG system) for 20 minutes in two separate sessions. Immediately after stimulation, participants completed a spatial maze task, which consisted of learning block, 2D recall, and 3D recall. Spatial memory performance was tested 24 hours and 7 days after stimulation, to assess potential long-term effects. We found no significant effects of anodal stimulation on spatial memory performance either on immediate or delayed recall. This was the case with both, 2D and 3D spatial memory recall. Our results are in line with some studies that suggest that single brain stimulation sessions do not always produce effects on cognitive functions.

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