Psychiatry Research 125 (2004) 147–160 0165-1781/04/$ - see front matter  2003 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.psychres.2003.12.008 Exploration of somatosensory P50 gating in schizophrenia spectrum patients: reduced P50 amplitude correlates to social anhedonia Sidse M. Arnfred *, Andrew C.N. Chen a, b Department of Psychiatry, Hvidovre Hospital, University Hospital of Copenhagen, Brøndbyøstervej 160, DK-2605 Brøndby, a Denmark Human Brain Mapping and Cortical Imaging Laboratory, Aalborg University, DK-9220 Aalborg, Denmark b Received 27 May 2003; received in revised form 30 September 2003; accepted 16 December 2003 Abstract Originally, the hypothesis of a sensory gating defect in schizophrenia evolved from studies of somatosensory evoked potentials (SEP), although the idea has primarily been pursued in the auditory modality. Gating is the relative attenuation of amplitude following the second stimulus in a stimulus pair. Recently, SEP P50 gating was seen when recording the SEP P50 in a paradigm similar to the one used for auditory P50 gating. Hypothetically, abnormality of somatosensory information processing could be related to anhedonia, which is considered a core feature of schizophrenia. Twelve unmedicated, male, schizophrenia spectrum patients (seven schizophrenic and five schizotypal personality disorder patients) and 14 age-matched healthy men participated in recordings of pair-wise presented auditory and median nerve stimuli. The patients had smaller amplitudes of the SEP P50 at the first stimulus, but no gating defect. The reduced amplitude was particularly evident in subjects with high scores on the Revised Social Anhedonia Scale. Early somatosensory information processing seems abnormal in schizophrenia spectrum patients. This could be in agreement with the theory of loss of the benefit of regularity in schizophrenia, while the results are in-conclusive regarding sensory gating theory.  2003 Elsevier Ireland Ltd. All rights reserved. Keywords: Information processing; Auditory P50 gating; Evoked potentials; Social anhedonia; Schizophrenia 1. Introduction In studies of somatosensory evoked potentials (SEP) in schizophrenia, findings of increased mean amplitudes in the early SEP and decreased *Corresponding author. Present address: Bispebjerg Bakke 23, Copenhagen NV, Denmark. Tel.: q45-35-31-26-61yq45- 24-62-75-12; fax: q45-35-31-39-53. E-mail address: s.arnfred@tdcadsl.dk (S.M. Arnfred). amplitude in the late SEP (Shagass, 1976; Buchs- baum, 1977; Shagass, 1977; Shagass et al., 1977, 1978, 1979) led Shagass to formulate the theory of an impaired sensory filtering mechanism leading to inefficient functioning of later stages of proc- essing (Shagass, 1977). Based on this and other findings within early information processing and the description of disturbed perception of young schizophrenic patients (McGhie and Chapman,