Transcranial direct current stimulation modulates spatial memory in cognitively intact adults.

Two forms of spatial processing are involved in object location memory. Coordinate processing uses a fine-grained code to provide exact knowledge for the location and is believed dependent on the right posterior parietal cortex (PPC). Categorical processing relies on spatial relationships between objects and is believed dependent on the left PPC. We used transcranial direct current stimulation (tDCS) to test these brain-behavior relationships during the encoding and subsequent recall of object location associations. Twelve right-handed, healthy young participants received 20 min of tDCS (2mA) during three separate sessions. Stimulation delivery was counterbalanced across participants and sessions and included anodal ("excitatory") stimulation of right PPC with concurrent left PPC cathodal ("inhibitory") stimulation (R+L-), the reverse montage (R-L+), and sham stimulation. Participants completed different versions of the Object Location Touchscreen Task (OLTT) during each session, which assesses coordinate (recall of the location without the environment) and categorical processing (recall of the location with the environment). Encoding occurred during the last 5 min of stimulation, while the delay phase occurred 15 min after stimulation. Participants performed more accurately during the coordinate phase following R-L+ stimulation when compared to R+L- performance. Categorical performance was not significantly affected by stimulation. Findings suggest two possibilities that will be examined in future studies with larger sample sizes: (1) The R-L+ facilitates left-hemisphere dominant categorical processing, the benefits of which persists even when environmental details are absent, possibly due to increased mental imagery; (2) Cathodal stimulation decreased spurious neuronal noise thereby allowing for more efficient processing by the "critical" neuronal populations in the right PPC.