Adaptive Gain Modification of the Blink Reflex

A reflex blink results from the interaction of only three forces: ( 1 ) an active upward force produced by the tonically active levator palpebrae muscle, which elevates the upper eyelid; (2) an active downward force created by the normally quiescent orbicularis oculi (00) muscle, which closes the eyelid; and (3) an intrinsic, passive downward force, which acts to pull the upper eyelid down. During a blink the levator palpebrae relaxes and the 00 transiently contracts, rapidly lowering the eyelid. As the 00 returns to its quiescent state, the levator palpebrae resumes its tonic activity, overcoming the passive downward tension and slowly raising the eyelid. (For a review see refs. 1-3.) Despite the mechanical and neuronal simplicity of reflex blinks, they manifest many characteristics of more complex motor systems? One such characteristic is the ability of the blink reflex to undergo adaptive gain modification. Adaptive gain changes occur when the nervous system alters the relation between the magnitude of a stimulus and the amplitude of the ensuing reflex to compensate for persistent perturbations that modify the normal response. These gain changes can either increase or decrease the amplitude of the movement, depending on the perturbation. Saccadic eye movements, a much more complex motor behavior than blinking, illustrate this form of motor learning.' Presentation of a target 10 degrees eccentric to the point of fixation normally elicits a 10-degree saccadic eye movement. However, if the investigator presents this stimulus and then shifts the target to a point 14 degrees eccentric during the saccadic eye movement, the eye will fall 4 degrees short of the target. If this target shift is done repeatedly, the saccadic system begins to compensate by producing a 14-degree saccade when presented with a target 10 degrees eccentric. Thus, the adaptive gain change recalibrates the relation between stimulus magnitude and the ensuing motor response so as to bring the movement-evoking input and the motor output back into register.