A PET investigation of implicit and explicit sequence learning

Abstract The purpose of this study was to determine the mediating neuroanatomy of implicit and explicit sequence learning using a modified version of the serial reaction time (SRT) paradigm. Subjects were seven healthy, right‐handed adults (three male, four female, mean age 26.7, range 18–43 yr). PET data were acquired via the oxygen‐15‐labeled‐carbon dioxide inhalation method while subjects performed the SRT. Subjects were scanned during two blocks each of (1) no sequence (Random), (2) single‐blind, 12‐item sequence (Implicit), and (3) unblinded, same sequence (Explicit). Whole‐brain‐normalized images reflecting relative regional cerebral blood flow (rCBF) were transformed to Talairach space, and statistical parametric maps (SPMs) of z‐scores were generated for comparisons of interest. The threshold for significant activation was defined as z‐score ≥ 3.00. Behavioral data demonstrated significant learning ( P < .05) for Implicit and Explicit conditions. Tests of explicit knowledge reflected non‐significant explicit contamination during the Implicit condition. Foci of significant activation in the Implicit condition were found in right ventral premotor cortex, right ventral caudate/nucleus accumbens, right thalamus, and bilateral area 19; activation in the Explicit condition included primary visual cortex, peri‐sylvian cortex, and cerebellar vermis. Activations in visual and language areas during the Explicit condition may reflect conscious learning strategies including covert verbal rehearsal and visual imagery. Right‐sided premotor, striatal, and thalamic activations support the notion that implicit sequence learning is mediated by cortico‐striatal pathways, preferentially within the right hemisphere. © 1996 Wiley‐Liss, Inc.