Voluntary modification of automatic arm movements evoked by motion of a visual target.

We have investigated whether the processes underlying the visually evoked, automatic adjustments to a reach are: (1) modifiable by the subject's intention, and (2) available to initiate movement of a stationary arm. Unpredictable movement of a target (80 m/s through 10 cm, left or right in a third of trials) either evoked a mid-flight adjustment of a reaching movement or else acted as a trigger to start an arm movement. Subjects were instructed to respond as rapidly as possible by moving their finger either in the same or in the opposite direction to the target. The target shift evoked an early (125-160 ms) and/or a later (> 160 ms) class of response in the reaching arm. The early response was highly automatic in that it could not be reversed (move opposite) by the subjects' intention. However, the subjects' intention did influence the frequency of occurrence and the size of this early response. The later response was totally modifiable in that it changed direction according to the subjects' intention. Similar classes of response were observed in stationary limbs, but the early, more automatic response was substantially weaker than that elicited during a reach. Two possible mechanisms are proposed to explain these results. The first is a dual-pathway model, which assumes that the two response classes are each generated by separate visuo-motor processes with different properties. The second model assumes both responses are generated by a single visuo-motor mechanism that is under the control of a higher, attentional process.