counterbalanced order. The anode was placed over F3 and the cathode on the right supraorbital scalp. Active stimulation con- sisted of 2.0 mA tDCS for 20 min. Traditional sham consisted of 2.0 mA tDCS for the first minute and 0 mA thereafter. Novel sham consisted of 2.0 mA tDCS for three 1-minute periods (at 0, 6:40, and 13:20) interspersed between 17 min of 0 mA stimulation. After the end of each session, participants were asked if they thought the stimulation was real. They also rated cutaneous sensations and experiences before and after each session. Results: Traditional sham was perceived as “real” by fewer (9/24) participants than novel sham (16/24; McNemar’s Test, p¼.016) and active stimulation (18/24, p¼.022). More participants reported tingling (15 vs. 7, p¼.008) and discomfort (13 vs. 7, p¼.031) after novel than traditional sham. Reports of tingling and discomfort did not differ significantly (p>.5) after active vs. novel sham stimula- tion. Reports of tingling (Phi¼.358, p¼.002) and discomfort (Phi¼.323, p¼.006) correlated with whether participants perceived stimulation as “real.” Conclusions: Blinding with traditional sham is ineffective at 2.0 mA. A novel sham with 3 discrete bursts of stimulation produced better blinding, likely due to increased cutaneous sensations. 297 Baseline cue-reactivity to an alcohol-related cue-exposure may predict relapse after HF-rTMS treatment S.C. Herremans * a , R. De Raedt b , P. Van Schuerbeek c , F. Matthys a , J. De Mey c , C. Baeken a,d a Department of Psychiatry, Universitair Ziekenhuis Brussel, Vrije Universiteit Brussel (VUB), Laarbeeklaan 101, 1090 Brussels, Belgium, b Department of Experimental Clinical and Health Psychology, Universiteit Gent, Henri Dunantlaan 2, 9000 Ghent, Belgium, c Department of Radiology, Universitair Ziekenhuis Brussel, Vrije Universiteit Brussel (VUB), Brussels, Belgium, d Department of Psychiatry and Medical Psychology, Universitair Ziekenhuis Gent, Universiteit Gent, De Pintelaan 185, 9000 Ghent, Belgium Background: Alcohol addiction is a chronic relapsing disorder. The underlying neurobiology of relapse is complex; previous research implicates the ventromedial prefrontal cortex (vmPFC), anterior cingulate cortex (ACC) and ventral striatum (VS). The application of high-frequency (HF) repetitive transcranial magnetic stimulation (rTMS) could possibly serve as a new treatment option for alcohol addiction. To day, it is unknown which patients might benefit from this intervention. Objective: Consequently, we explored if baseline differences in brain activity - during an alcohol-related cue-exposure - between relapsers and abstainers treated with HF-rTMS could possibly serve as a biomarker predicting future relapse. Methods: Before the start of the HF-rTMS treatment, which was administered as an intensive protocol (consisting of 15 active sessions spread over one week), patients were confronted with an alcohol-related cue-exposure. Because it is unclear whether a block or event-related paradigm is more suitable to identify im- aging biomarkers, patients were confronted with both of them in a consequent manner. Relapse, defined as the consumption of any amount of alcohol, was assessed four weeks after the stimulation. We performed whole brain analysis and subsequent regions of interest (ROI) analyses of vmPFC, ACC and VS. Results: Fourteen against six patients had relapsed. Only the block paradigm could demonstrate brain activity differences between relapsers and abstainers at baseline. Whole brain analysis showed the implication of the reward system, while additional ROI analysis demonstrated ACC activity differences between relapsers and ab- stainers during the exposure to the alcohol cues. Conclusions: ACC activity at baseline could possibly serve as a biomarker to identify alcohol-dependent patients at risk for relapse after HF-rTMS treatment. Block paradigms are more sen- sitive than event paradigms in identifying imaging biomarkers for relapse. 298 Optimization in transcranial electrical neuromodulation based on the reciprocity principle Mariano Fernández-Corazza a,b , Sergei Turovets a,c , Phan Luu c,d , Don Tucker c,d a Neuroinformatics Center, University of Oregon, Eugene, OR, US b Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina c Electrical Geodesics Inc., Eugene, OR, US d Department of Psychology, University of Oregon, Eugene, OR, US One of the key challenges in dense array transcranial electrical neuromodulation is to find a current injection pattern that max- imizes the current density at a cortical region of interest and minimizes it in the rest of the head, which is mathematically modelled as an optimization problem. These optimal patterns can be found with help of the Least Squares (LS) or Linearly Con- strained Minimum Variance (LCMV) algorithms. However, these methods return currents for all available electrodes in a dense array head harness. Therefore, each electrode requires to be driven by an independent current source, which is not always practical and at least expensive. The aim of this study is to analyze several optimization methods that comply with hardware and safety constraints. The reciprocity principle is instrumental in finding optimal injection patterns guided by the solution of the forward problem in Electroencephalography and taking into ac- count the mentioned constrains. We have compared the perfor- mance of the reciprocity family methods in terms of bias, focality, intensity, and directionality using the LS and LCMV solutions as the baseline. Four targets on the cortex differing by depth and orientation were defined in a detailed seven-tissue MRI/CT based Finite Element head model. The optimal current injection pat- terns were computed using the LS, LCMV and three reciprocity derivative methods assuming 128 and 256 electrode nets, one current injection source and no fewer than 10 electrodes engaged as sources or as sinks from the safety considerations. All reci- procity algorithms showed good performance comparable to the LCMV and LS solutions. The best “ad hoc” reciprocity version has demonstrated the focality similar to the LCMV and LS solutions, and higher intensity on the target in the desired direction. Comparing the 128 and 256 electrode cases we found that use of the larger electrode number improves the focality and intensity parameters. Abstracts / Brain Stimulation 8 (2015) 395e411 403