To investigate the effects of dopamine on the dynamics of semantic
activation, 39 healthy volunteers were randomly assigned to ingest
either a placebo (n = 24) or a levodopa (n = 16)
capsule. Participants then performed a lexical decision task that
implemented a masked priming paradigm. Direct and indirect semantic
priming was measured across stimulus onset asynchronies (SOAs) of 250,
500 and 1200 ms. The results revealed significant direct and indirect
semantic priming effects for the placebo group at SOAs of 250 ms and
500 ms, but no significant direct or indirect priming effects at the
1200 ms SOA. In contrast, the levodopa group showed significant direct
and indirect semantic priming effects at the 250 ms SOA, while no
significant direct or indirect priming effects were evident at the SOAs
of 500 ms or 1200 ms. These results suggest that dopamine has a role in
modulating both automatic and attentional aspects of semantic
activation according to a specific time course. The implications of
these results for current theories of dopaminergic modulation of
semantic activation are discussed. (JINS, 2004, 10,
15–25.)