Dopamine D2 receptor density in the limbic striatum is related to implicit but not explicit movement sequence learning.

A large body of literature suggests that motor sequence learning involves dopamine-modulated plastic processes in the basal ganglia. Sequence learning can occur both implicitly, without conscious awareness and intention to learn, and explicitly, i.e., under conscious control. Here, we investigated whether individual differences in implicit and explicit sequence learning of movement sequences in a group of 15 healthy participants are related to dopamine D2 receptor densities in functional subregions of the striatum. Sequence learning was assessed using the serial reaction time task, and measures of implicit and explicit knowledge were estimated using a process dissociation procedure. Correlation analyses were performed between these measures and D2 receptor densities, which had been measured previously with positron emission tomography. Striatal D2 densities were negatively related to measures of sequence learning. In the limbic subregion, D2 densities were specifically related to implicit but not explicit learning. These findings suggest that individual differences in striatal DA function underlie differences in sequence learning ability and support that implicit and explicit sequence learning depend on partly distinct neural circuitry. The findings are also in line with the general view that implicit learning systems are evolutionarily primitive and tend to rely more on phylogenetically old neural circuitry than does explicit learning and cognition.