P188 Counteracting implicit conflicts by cathodal tDCS to the prefrontal cortex

The effects of transcranial direct current stimulation (tDCS) on cognitive functions are often conceived as not reliable, inconsistent, or even contradictory. Here, we describe a novel tDCS effect in the cognitive domain and characterize it in separate experiments with different participants and modulations of instruction and task. The effect is based on the prefrontal involvement in the mental representations of order: Small and first items are faster classified by left-hand responses in 2-AFC classification tasks whereas large and last items are faster responded to by right-hand responses, an automatic behavioral signature often described by the mental number line (MNL) metaphor. Online administration of 1 mA cathodal tDCS to the left prefrontal cortex alleviates the MNL effect in magnitude and parity judgment, thus substantiating a prefrontal involvement for linking numbers with space and producing implicit distractions in these tasks. In contrast, explicit conflicts based on visuo-spatial distraction remain unaffected. This dissociation is polarity-specific and unrelated to functional magnitude processing as classified by a regular numerical distance effect during the stimulation. Separate experiments replicate the neuromodulation effect and characterize the critical interaction of brain stimulation and neurocognitive functions. Our results corroborate current theoretical conceptualizations of order and spatial-numerical processing. Regarding tDCS, we demonstrate the replicability of polarity-specific cognitive neuromodulations with this novel effect and discuss its possible limitations. Further, it appears that tDCS effects are highly susceptible of task demands and emerge dependent on preceding (implicit) activations. Outlining the generalizations and limitations of neuromodulation effects in a systematic way can enhance the understanding of tDCS in the cognitive domain and facilitate targeted applications in respective populations. Finally, the neuromodulation of implicit MNL conflicts demonstrate how the emergence of dysfunctional cognition can be blocked by cathodal tDCS.