READY FOR ACTION: FIXATIONAL LIMB MOVEMENTS REVEAL FORTHCOMING VOLUNTARY MOVEMENTS

Open Access
- Author:
- Cohen, Rajal G.
- Graduate Program:
- Psychology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 24, 2008
- Committee Members:
- David A. Rosenbaum, Committee Chair/Co-Chair
Dagmar Sternad, Committee Member
Paola Eulalia Dussias, Committee Member
Daniel J Weiss, Committee Member
Cathleen M Moore, Committee Member - Keywords:
- reflex tuning
fixational movements
cuing
microsaccades
kinematics
human
movement
motor
reciprocal
discrete
tremor
ideomotor
readiness
preparation
countermove
vision
drift - Abstract:
- Physiological indices of cognitive processes have been the focus of considerable interest. They can be found in brain activity, facial expressions, microsaccades, and neck muscle activation. Recent work suggests that they also occur in fixational limb movements, which may include drift, tremor, and small quick movements. In the studies reported here, limb movement was measured while participants pointed to a central location and anticipated a target-directed movement. During the pauses before voluntary movement, the amplitude of small “fixational” movement increased in the axis of forthcoming target-directed movement relative to the orthogonal axis, while frequency decreased in the axis of forthcoming target-directed movement relative to the orthogonal axis. These fixational movement biases may be online indicators of attention or some other cognitive process. Alternatively or additionally, they may reflect some functional aspect of motor preparation. This project investigates the phenomenon of fixational limb movement biases and seeks to elucidate whether, and in what manner, they reflect mental and physiological aspects of readiness. In Experiments 1-3, participants memorized and then performed simple sequences of pointing movements that included long pauses. Experiment 1 verified that the effects held across different limb segments, and Experiments 2 and 3 confirmed that they were robust to various exploratory manipulations, including changes in instruction mode, internal vs. external timing, and pause duration. The amplitude effect depended on the proportion of recent movements made in each axis, suggesting that fatigue and gravity might play a role. Analysis of results with and without the last submove indicated that the countermove was critical to the frequency effect. Furthermore, when the last submove was removed from the period of analysis, vision was critical to the amplitude effect. In Experiments 4-6, participants responded to cues. Cue validity had a large influence on the effects. Discreteness or reciprocity of the movement and size of the staging area had smaller influences. The influence of the size of the staging area provided support for the hypothesis that the effects are due at least in part to participants drifting toward the expected target and correcting. Additional analyses provided support for the idea that for rapid discrete movements with known start times, fixational movements in advance of movement onset promote synchronization of discrete movement onset with ongoing oscillation. Finally, correlations of effects with performance suggested that the countermove had a facilitatory effect on movement times. In sum, the dependency of fixational limb movements on the direction of intended or expected forthcoming movement most likely has several sources. Support was found for: (1) the countermove, which serves to build up elastic energy immediately before the voluntary movement; (2) engagement of attention by the direction of forthcoming movement, combined with effort to maintain position with respect to a visual cue; (3) synchronization of fixational movements with respect to the expected time and direction of discrete movement onset. Other plausible relations between fixational and voluntary movements are discussed.