Investigating Motor Encoding of Potential Reach Targets

Abstract

Neurophysiological and behavioural research has suggested that multiple potential actions are represented simultaneously. For example, neural activity associated with multiple potential actions has been observed in premotor structures of monkeys. Also, when humans are presented with two potential reach targets, they initially aim their reach toward the midpoint, or spatial average, of the targets. Within the context of the neurophysiological findings, it has been suggested that this ‘spatial averaging’ phenomena reflects competing neural activity associated with multiple motor plans being implemented simultaneously. However, it is also possible that the initial trajectory represents a perceptual average of spatial locations. The goal of this thesis was to examine these two accounts of spatial averaging. An initial experiment tested the idea that multiple motor plans specifying multiple motor parameters are prepared simultaneously. Participant's reached toward multiple potential targets, one of which was cued after movement onset as the movement target, that varied in location and orientation. Hand direction and orientation were found to be biased by both the spatial distribution and orientation; suggesting that fully specified motor plans are prepared simultaneously to potential targets. The second experiment tested whether spatial averaging behaviour reflects the use of a perceptual representation or a motor representation of potential targets. Participants preformed a rapid reaching task in which an obstacle blocked the movement to one of two potential targets. The initial movement direction was found to be a weighted average of the movement directions to each target; as opposed to being an average of the target locations. In chapter four, a visuomotor adaptation task in which individuals learned to produce identical movements to two separate visual targets allowed the perceptual representation and the motor representation of the targets to be dissociated and the predictions of each account to be tested separately. The initial movement direction was shifted in the direction of the adaptation; providing support for the idea that multiple motor plans are computed simultaneously. Taken together, the results presented herein provide support for the idea that the brain prepares multiple movement plans in advance of deciding between potential actions.