Primes and targets in rapid chases: tracing sequential waves of motor activation.

Single-cell recordings have indicated that visual stimuli elicit rapid waves of neuronal activation that propagate so fast that they might be free of intracortical feedback. Here, the time course of feedforward activation was traced by measuring pointing responses to color targets preceded by color primes initiating either the same or opposite response. The early time course of priming effects was strictly time locked to prime onset and depended only on properties of the primes, but was independent of the onset times of the actual targets as well as the perceptual effects of targets on primes. Results indicated that nonoverlapping feedforward signals by primes and targets traverse the visuomotor system in a rapid chase, controlling associated motor responses in strict sequence.