Gaze anchoring to a pointing target is present during the entire pointing movement and is driven by a non-visual signal

A well-coordinated pattern of eye and hand movements can be observed during goal-directed arm movements. Typically, a saccadic eye movement precedes t he arm movement, and its occurrence is temporally correlated with the start of the arm movement. Furthermore, the coupling of gaze and aiming movement s is also observable after pointing initiation. It has recently been observ ed that saccades cannot be directed to new target stimuli, away from a poin ting target stimulus. Saccades directed to targets presented during the fin al phase of a pointing movement were delayed until after pointing movement offset ("gaze anchoring"). The present study investigated whether ocular ga ze is anchored to a pointing target during the entire pointing movement. In experiment 1, new targets were presented at various times during the durat ion of a pointing movement, triggered by the kinematics arm moment itself ( movement onset, peak acceleration/velocity/deceleration, and offset). Subje cts had to make a saccade to the new target as fast as possible while maint aining the pointing movement to the initial target. Saccadic latencies were increased by an amount of time that approximately equaled the remaining po inting time after saccadic target presentation, with the majority of saccad es executed after pointing movement offset. The nature of the signal drivin g gaze stabilization during pointing was investigated in experiment 2. In p revious experiments where ocular gaze was anchored to a pointing target, su bjects could always see their moving arm, thus it was unknown whether a vis ual image of the moving arm, an afferent (proprioceptive) signal or an effe rent (motor control related) signal produced gaze anchoring. In experiment 2 subjects had to point with or without vision of the moving arm to test wh ether a visual signal is used to anchor gaze to a pointing target. Results indicate that gaze anchoring was also observed without vision of the moving arm. The findings support the existence of a mechanism enforcing ocular ga ze anchoring during the entire duration of a pointing movement. Moreover, s uch a mechanism uses an internally generated, or proprioceptive, nonvisual signal. Possible neural substrates underlying these processes are discussed , as well as the role of selective attention.