Abstract
Repeated visual processing of an unfamiliar face suppresses neural activity in face-specific areas of the occipito-temporal cortex. This "repetition suppression" (RS) is a primitive mechanism involved in learning of unfamiliar faces, which can be detected through amplitude reduction of the N170 event-related potential (ERP). The dorsolateral prefrontal cortex (DLPFC) exerts top-down influence on early visual processing. However, its contribution to N170 RS and learning of unfamiliar faces remains unclear. Transcranial direct current stimulation (tDCS) transiently increases or decreases cortical excitability, as a function of polarity. We hypothesized that DLPFC excitability modulation by tDCS would cause polarity-dependent modulations of N170 RS during encoding of unfamiliar faces. tDCS-induced N170 RS enhancement would improve long-term recognition reaction time (RT) and/or accuracy rates, whereas N170 RS impairment would compromise recognition ability. Participants underwent three tDCS conditions in random order at ∼72 hour intervals: right anodal/left cathodal, right cathodal/left anodal and sham. Immediately following tDCS conditions, an EEG was recorded during encoding of unfamiliar faces for assessment of P100 and N170 visual ERPs. The P3a component was analyzed to detect prefrontal function modulation. Recognition tasks were administered ∼72 hours following encoding. Results indicate the right anodal/left cathodal condition facilitated N170 RS and induced larger P3a amplitudes, leading to faster recognition RT. Conversely, the right cathodal/left anodal condition caused N170 amplitude and RTs to increase, and a delay in P3a latency. These data demonstrate that DLPFC excitability modulation can influence early visual encoding of unfamiliar faces, highlighting the importance of DLPFC in basic learning mechanisms.
Highlights
Repetition of a visually presented stimulus leads to suppression of neural activity in the cortical areas responsible for processing the stimulus [1,2]
The Transcranial direct current stimulation (tDCS) condition effect remains when grand-averaged amplitude values of only the last four repetitions of Av1 are included (F(2,20) = 7.49, P,0.01, gp2 = 0.43). These results demonstrate that the tDCS condition effect on N170 amplitude reflects differential repetition suppression (RS) between the active tDCS conditions during the first five repetitions of stimuli
This trial average effect was significant at both F3 (F(2,9) = 4.54, P,0.05, gp2 = 0.5) and F4 (F(2,9) = 5.96, P,0.05, gp2 = 0.57), where an additional interaction of tDCS condition x trial average was found (F(4,7) = 1.91, P,0.05, gp2 = 0.52). This interaction further revealed that a significant RS effect was present only in the right anodal/left cathodal condition at this site (F(2,20) = 4.1, P,0.05, gp2 = 0.3) between Av1 and Av2, as well as Av1 and Av3 (P,0.05, Figure 4C). These results suggest that while having the same neurophysiological influence (i.e.: cortical excitability augmentation for the anode), tDCS polarity did not have the same induced effects on RS of the P3a depending on the hemisphere over which it was applied
Summary
Repetition of a visually presented stimulus leads to suppression of neural activity in the cortical areas responsible for processing the stimulus [1,2]. RS has been associated with perceptual priming, a basic form of memory formation and learning whereby accuracy and reaction time (RT) are implicitly improved in recognizing a previously presented stimulus [1,5,6]. In elderly patients with Alzheimer’s disease, poor performance in recognition tests assessing episodic memory for faces and corresponding names is associated with reduced suppression of activity at encoding in medial temporal lobe structures. This contrasts with cognitively unimpaired young and elderly subjects showing stronger RS effects and normal retrieval performance [10]. Learning of novel faces is severely impaired in the absence of RS to unfamiliar faces in the fusiform face area (FFA) as characterized in developmental prosopamnesia [12]
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