Mask-triggered thrust reversal in the negative compatibility effect.

Rapid motor responses to visual stimuli can involve both the activation and inhibition of motor responses. Here, we trace the early processing dynamics of response generation, examining whether activation and inhibition events form a strict sequence when elicited by sequential stimuli, as we would expect if motor events are driven by fast, stimulus-triggered feedforward sweeps. We employed identical stimuli in two complementary paradigms. In response priming, responses to a target stimulus are speeded or slowed by a masked prime triggering the same or an alternative response, respectively. By prolonging the prime-target interval, the response-priming effect can reverse to form the negative compatibility effect (NCE), especially when the mask contains response-relevant features. We report two experiments in which primed pointing movements going in ten possible directions were measured with response-relevant, response-irrelevant, or no masks interleaved between the primes and targets, while selective visual attention was varied. We showed that in response priming, initial responses are controlled exclusively by the prime. In the NCE, however, even the earliest movement phase is controlled jointly by the prime, mask, and target information, and a massive force in counterdirection to the primed response reverses the priming effects specifically for slow responses. We conclude that response priming reflects a strict sequence of feedforward response activations, whereas the activation/inhibition events in the NCE are not strictly serial, but integrate information from the different stimuli over time. Even though the mask features and visual attention modulate the NCE, its major source is a mask-induced, direction-specific thrust reversal of the initial response.