The functional locus of the lateralized readiness potential HIROAKI MASAKI, a NELE WILD-WALL, b JO ¨ RG SANGALS, b and WERNER SOMMER b a Japan Society for the Promotion of Science, Tokyo, Japan b Humboldt-University, Institute for Psychology, Berlin, Germany Abstract The lateralized readiness potential (LRP) is considered to reflect motor activation and has been used extensively as a tool in elucidating cognitive processes. In the present study, we attempted to more precisely determine the origins of the LRP within the cognitive system. The response selection and motor programming stages were selectively manipulated by varying symbolic stimulus response compatibility and the time to peak force of an isometric finger extension response. Stimulus response compatibility and time to peak force affected response latency, as measured in the electromyogram, in a strictly additive fashion. The effects of the experimental manipulations on stimulus- and response-synchronized LRPs indicate that the LRP starts after the completion of response-hand selection and at the beginning of motor programming. These results allow a more rigorous interpretation of LRP findings in basic and applied research. Descriptors: Lateralized readiness potential, Response selection, Motor programming, Functional locus The lateralized readiness potential (LRP) is regarded as a useful and powerful tool in the study of human information processing. It is extracted from event-related brain potentials (ERP) and considered to reflect the activation of response-related processes following stimulus-related processing (Coles, 1989; De Jong, Wierda, Mulder, & Mulder, 1988; Gratton, Coles, Sirevaag, Eriksen, & Donchin, 1988). In previous research, the LRP has been successfully employed to investigate, for example, informa- tion transmission between perception- and response-related pro- cesses (e.g., Coles, Gratton, & Donchin, 1988; Miller & Hackley, 1992), motor programming (e.g., Leuthold, Sommer, & Ulrich, 1996), overlapping task processing (e.g., Osman & Moore, 1993), or dual route processes in spatial stimulus response compatibility (e.g., Stu¨ rmer, Leuthold, Soetens, Schro¨ ter, & Sommer, 2002). Although the somewhat global notion of LRP onset as marker of response activation is quite sufficient for many issues in cognitive psychology, there are other applications where a higher degree of precision as regards the functional significance of this component would be most welcome. For example, Sommer, Leuthold, and Schubert’s (2001) assessment of a motoric bottleneck after response initiation (De Jong, 1993) depended crucially on the locus of the LRP after or during response selection but prior to response initiation. The present study attempts to provide further information on how the onset of the LRP is related to the cognitive and motoric processes that have been suggested during choice reaction time tasks. The LRP is derived by recording ERPs from above the motor cortices in tasks that call for left- and right-hand or -foot responses. The ERP above the cortex ipsilateral to the effector required in a given trial is subtracted from the contralateral ERP. When these difference waves are averaged across hands, they yield the LRP, reflecting pure hand-related ERP asymmetry. In general terms, the LRP is considered as a measure of response activation or preparation (cf. Coles, 1989). The interval between a stimulus and the onset of the stimulus-synchronized LRP (stimulus-LRP interval) is a relative measure for the duration of premotoric processes, including perception and at least some aspects of response selection. In contrast, the interval between the onset of the response-synchronized LRP and the response (LRP-response interval) is a relative measure for the duration of the subsequent, that is, the motoric processes (Osman, Moore, & Ulrich, 1995). The LRP is considered to be generated at least partially in the primary motor cortex (e.g., Coles, 1989; Miller & Hackley, 1992). Note however, that even if LRP were exclusively generated in M1, it would, according to current neuroanatomical knowledge, not unambiguously specify its meaning, because the precise functions of M1 are themselves a matter of debate (e.g., Graziano, Taylor, & Moore, 2002). To determine the functional significance of ERP components, it has been suggested to assess the effects of experimental factors with known locus of action within the information processing system on the ERP component in question (e.g., Meyer, Osman, Portions of this article were presented at the 42nd Annual Meeting of the Society for Psychophysiological Research, Washington, DC, October 2–6, 2002. This study was supported by a fellowship of the Japan Society for the Promotion of Science to the first author. The authors would like to thank Allen Osman for valuable discussions, Kathrin Pusch for her assistance in collecting the data, and William Gehring, Toby Mordkoff, Birgit Stu¨ rmer, and one anonymous reviewer for their helpful comments on an earlier draft of this article. Address reprint requests to: Dr. Hiroaki Masaki, School of Sport Sciences, Waseda University, 2-579-15, Mikajima, Tokorozawa, Saita- ma, Japan, 359-1192. E-mail: masaki@waseda.jp. Psychophysiology, 41 (2004), 220–230. Blackwell Publishing Inc. Printed in the USA. Copyright r 2004 Society for Psychophysiological Research DOI: 10.1111/j.1469.8986.2004.00150.x 220