Neuropsychologia 49 (2011) 1974–1980 Contents lists available at ScienceDirect Neuropsychologia journal homepage: www.elsevier.com/locate/neuropsychologia The enhancement of cortical excitability over the DLPFC before and during training impairs categorization in the prototype distortion task Géza Gergely Ambrus a,∗ , Márta Zimmer b , Zsigmond Tamás Kincses a,c , Irén Harza a , Gyula Kovács b,d , Walter Paulus a , Andrea Antal a a Department of Clinical Neurophysiology, Georg-August University of Göttingen, Robert Koch Straße 40, 37075 Göttingen, Germany b Department of Cognitive Science, Budapest University of Technology and Economics, Budapest, Hungary c Department of Neurology, Albert Szent-Györgyi Clinical Center, University of Szeged, Szeged, Hungary d Institute for Experimental Psychology, University of Regensburg, Regensburg, Germany article info Article history: Received 22 October 2010 Received in revised form 16 March 2011 Accepted 18 March 2011 Available online 31 March 2011 Keywords: Transcranial direct current stimulation Transcranial random noise stimulation Prototype distortion task Visual categorization Dorsolateral prefrontal cortex abstract The present study investigated the effects of transcranial weak electrical stimulation techniques applied to the right and left dorsolateral prefrontal cortex (DLPFC) on categorization learning measured using a variant of the prototype distortion task. During the training phase of this task subjects saw low- and high distortions of a prototype dot-pattern. 60 participants received 10 min of either anodal or cathodal transcranial direct current (tDCS), transcra- nial random noise (tRNS) or sham stimulation before and during the training. We have assessed the effects of the intervention during a test phase, where the subjects had to decide whether the consecutive high- and low-distortion versions of the prototype or random patterns that were presented belonged to the category established in the training phase. Our results show that the categorization of prototypes is significantly impaired by the application of anodal tDCS and tRNS to the DLPFC. The prototype-effect, observable in the case of the sham stimulation group, was severed in all active stimulation conditions. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Categorization plays an important role in guiding behavior and thus the survival of animals. Efficient categorization requires the economical allocation of cognitive resources and the adequate mapping of categories along relevant features of the perceived environment. Consensus leans toward a multi-system approach of human category learning, with a system for rule-based acquisi- tion and another one involving implicit learning (Ashby & Waldron, 1999; Poldrack & Foerde, 2008), although attempts are being made to construct single-system models to account for the experimental results (Zaki, 2004). One of the most widely applied methods used to study catego- rization in the human is the prototype distortion paradigm (Posner & Keele, 1968). The procedure usually consists of a training and a testing phase (see Fig. 1). In the case of the “A, not-A” version the subject is exposed to various distorted stimuli derived from a single prototype during the training phase. In the testing phase the participant is shown stimuli similar in appearance, and has to decide whether a presented stimuli belongs to the category estab- ∗ Corresponding author. Tel.: +49 551 398461; fax: +49 551 398126. E-mail address: g.ambrus@gmail.com (G.G. Ambrus). lished in the training phase. In the case of the “A vs. non-A” variant stimuli are generated using two distinct prototypes, and partic- ipants have to place the presented patterns in one of these two categories. Subjects can be unaware of the existence of the cat- egory/categories during the training phase, or they can be guided through feedback-aided learning. Several versions of this paradigm exist; the stimuli of the task in most cases consist of dot patterns, color grids or cartoon animals. It has been consequently shown that healthy participants are able to assign prototypes and derivatives of the prototype pattern to the category in the testing phase of the task. Despite the fact that the prototype patterns are not pre- sented during the training phase, the rate of correct categorization decisions is consequently higher for these stimuli than of the dis- torted versions of the prototypes. This phenomenon is known as the prototype effect, and, in addition to humans it has been reported in experiments involving birds (Jitsumori, 1996) and non-human primates (Smith, Redford, & Haas, 2008) as well (for reviews, see Jitsumori, 2006; Jitsumori & Delius, 2001), although the issue is still being debated (Vauclair, 2002). Data from amnestic patients with impaired declarative memory suggest a retained implicit ability to acquire category-level knowl- edge via exposure to multiple instances of the given category in the prototype distortion task (Knowlton & Squire, 1993; Squire & Knowlton, 1995). Alzheimer’s patients with affected prefrontal 0028-3932/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropsychologia.2011.03.026