A PET investigation of implicit and explicit sequence learning

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 greater than or equal to 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. (C) 1996 Wiley-Liss, Inc.