Enhanced long-term memory encoding after parietal neurostimulation

Abstract

Neurostimulation, e.g. transcranial direct current stimulation (tDCS), shows promise as an effective cognitive intervention. In spite of low spatial resolution, limited penetration, and temporary influence, evidence highlights tDCS-linked cognitive benefits in a range of cognitive domains. The left posterior parietal cortex (PPC) is an accessible node in frontoparietal networks engaged during long-term memory (LTM). Here, we tested the hypothesis that tDCS can facilitate LTM by pairing LTM encoding and retrieval with PPC stimulation. Healthy young adults performed a verbal LTM task (California Verbal Learning Task: CVLT) with four different stimulation parameters. In Experiment 1, we applied tDCS to left PPC during LTM encoding. In Experiment 2, we applied tDCS just prior to retrieval to test the temporal specificity of tDCS during a LTM task. In later Experiments, we tested hemispheric specificity by replicating Experiment 1 while stimulating the right PPC. Experiment 1 showed that tDCS applied during LTM encoding improved the pace of list learning and enhanced retrieval after a short delay. Experiment 2 indicated anodal left PPC tDCS only improved LTM when applied during encoding, and not during maintenance. Experiments 3 and 4 confirmed that tDCS effects were hemisphere specific and that no effects were found after right PPC stimulation during encoding. These findings indicate that anodal tDCS to the PPC helps verbal LTM in healthy young adults under certain conditions. First, when it is applied to the left, not the right, PPC and second, when it is applied during encoding.

Effect of prefrontal and parietal tDCS on learning and recognition of verbal and non-verbal material

“…Our first finding confirms the role of the posterior parietal region in episodic memory, as indicated by neuroimaging studies (Wagner et al, 2005;Skinner and Fernandes, 2007;Simons et al, 2008). Previous tDCS and TMS studies yielded conflicting results: Stimulation of the left PPC either enhanced (Jacobson et al, 2012;Jones et al, 2014) or disrupted verbal memory (Manenti et al, 2010). Using pronounceable but meaningless words (non-words), we obtained no effect of PPC stimulation.…”

“…tDCS studies have demonstrated enhanced retrieval of non-verbal material following anodal stimulation of the right PFC (Chi et al, 2010) and improved verbal memory following anodal tDCS over the left PFC (Javadi and Cheng, 2013). Anodal tDCS over the left parietal cortex has also been shown to enhance verbal memory (Jacobson et al, 2012;Jones et al, 2014).…”

Focalised stimulation using high definition transcranial direct current stimulation (HD-tDCS) to investigate declarative verbal learning and memory functioning

“…Firstly, the PT itself might not be important for verbal memory performance; rather, diffuse stimulation of nearby or more distal regions may be responsible for the previously observed improvements in cognitive functioning. Anodal tDCS to the left posterior parietal cortex, a region further posterior to the PT, has been demonstrated to improve the rate of list learning and delayed recall using a similar declarative verbal memory list learning task (Jones et al, 2014). Past studies using standard tDCS electrodes placed at the location of the PT (Cp5) could have activated similar posterior regions, whereas with HDtDCS, due to its high spatial resolution, stimulation is far more specific.…”

A network approach for modulating memory processes via direct and indirect brain stimulation: Toward a causal approach for the neural basis of memory

“…The findings from a couple of tDCS studies suggest that cortical excitability of parietal/ temporoparietal regions during encoding may relate to the probability of decay of encoded memory (Flöel et al, 2012, Jones, Gözenman, & Berryhill, 2014. Two studies using parietal tDCS stimulation examined the effects of anodal tDCS on memory decay (i.e., immediate vs. delayed recall), but targeted quite disparate parietal regions (right temporo-parietal cortex and posterior parietal cortex, respectively).…”

“…In contrast, stimulation of posterior parietal cortex (PPC) resulted in an opposite pattern. Anodal stimulation of left PPC during verbal encoding improved immediate free recall without affecting delayed recall (Jones et al, 2014). An important caveat in directly comparing the results from these two studies is that the two studies evaluated 'delayed recall' with a very different delay from the learning session (1 week vs. ~20 minutes), but it is interesting that the neural excitability of two disparate parietal regions during encoding seemed to nonetheless affect the durability of the encoded memory at differential temporal delays.…”