Background: Transcranial direct current stimulation (tDCS) is a neuromodulation method that has been used to modulate learning. We tested whether anodal tDCS targeted at the left DLPFC could enhance learning in a semantic variant of discrimination eyeblink conditioning, i.e., whether the stimulation would have a specific effect on the discrimination ability, rate of acquisition, amplitude of the conditioned response (CR), or all of these.
Methods: Immediately prior to the eyeblink conditioning, the participants received either active stimulation of 1 mA for 10 min or sham stimulation. The anode was placed over F3 and the cathode over the right supraorbital area. The conditioned stimuli (CSs) were common Finnish male and female names that were presented as text. Male names were reinforced with an unconditioned stimulus.
Results: Stimulation had no effect on the learning rate or discrimination ratio, but the stimulated participants showed steeper CR acquisition in the initial phase of the experiment. Nevertheless, the participants in the stimulation group showed greater eyeblink CRs to the non-reinforced CS.
Discussion: Contrary to our initial hypothesis, the magnitude and rate of CRs to non-reinforced CS was higher in the active stimulation group than in the sham stimulation group, which may suggest deterioration of discrimination and contingency awareness in the used task. Our observations may suggest a lack of effect on the participants' ability to discriminate between two different types of CS. Furthermore, cathodal modulation of the right prefrontal cortex may explain the change in magnitude and rate of CRs to non-reinforced CS.
Keywords: Conditioning; Eyeblink conditioning; tDCS.
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