A speech-motor-system perspective on nervous-system-control variables

The temporal organization of hand, eye, and head movements during reaching and pointing

Two experiments are reported that address the issue of coordination of the eyes, head, and hand during reaching and pointing. Movement initiation of the eyes, head, and hand were monitored in order to make inferences about the type of movement control used. In the first experiment, when subjects pointed with the finger to predictable or unpredictable locations marked by the appearance of a light, no differences between head and eye movement initiation were found. In the second experiment, when subjects pointed very fast with the finger, the head started to move before the eyes did. Conversely, when subjects pointed accurately, and thus more slowly, with the finger, the eyes started to move first, followed by the head and finger. When subjects were instructed to point to the same visual target only with their eyes and head, both fast and accurately, however, eye movement always started before head movement, regardless of speed-accuracy instructions. These results indicate that the behavior of the eye and head system can be altered by introducing arm movements. This, along with the variable movement initiation patterns, contradicts the idea that the eye, head, and hand system is controlled by a single motor program. The time of movement termination was also monitored, and across both experiments, the eyes always reached the target first, followed by the finger, and then the head. This finding suggests that movement termination patterns may be a fundamental control variable.

Control of Multimovement Coordination

The present paper provides some hypotheses concerning the role of sensorimotor mechanisms in the coordination and programming of multimovement behaviors. The primary database is from experiments on the control of speech, a motor behavior that inherently requires multimovement coordination. From these data, it appears that coordination may be implemented by calibrated, sensorimotor actions which couple multiple movements for the accomplishment of common functional goals. The data from speech and select observations in other motor systems also reveal that these sensorimotor linkages are task-dependent and may underlie the intermovement motor equivalence that characterizes many natural motor behaviors. In this context, it is hypothesized also that motor learning may involve the calibration of these intermovement sensorimotor actions. These observations in turn provide some alternative perspectives on the concept of a motor program, primarily suggesting that individual movements and muscle contractions are not wholly prespecified, but shaped by sensorimotor adjustments.