Marjorie E. Anderson, PhD
University of Washington
The simplest way to produce movement might be for a central command generator to activate the effector (or an intermediate) with signals appropriate to produce the movement. However, neither the nervous system nor the engineer usually works that way. At multiple levels, and along many parallel pathways, information from both peripheral and central sources is fed back to influence the motor command or the efferent signals at multiple points. These feedback signals, which will incorporate current information from proprioceptors and exteroceptors, can be used for on-line control and adjustment. The cerebellum is but one of the neural sites at which abundant sensory information is available to modify input to motor efferent generators, such as the primary motor cortex.
Longer-term changes in the gain of the response to different sensory modalities, including so-called long-term potentiation (LTP) or long-term depression (LTD) also have been demonstrated at multiple sites. Several mechanisms may contribute to changes in sensory gain, including activity-dependent changes in transmitter release, sprouting, up- or down regulation of receptors, or postsynaptic changes in gene expression.
Motor output also can be influenced by what could be called "state" (alert vs. sleeping; agitated vs. calm; hungry vs. satiated), as well as by the cognitive knowledge of past results. (Did my dart hit the bull's eye, or didn't it? Could I successfully button my button without looking at it, or is vision of the button necessary?)
Dr. Randolph Nudo will examine the evidence for functional plasticity in one efferent signal generator, the motor cortex. A variety of experimental manipulations have been shown to produce changes in cortical motor maps, and recent studies have begun to examine structural mechanisms that might underlie these changes, which should have profound implications for recovery after injury.
Dr. Jonathan Wolpaw will address the needs and status of brain-computer interfaces designed to produce interactions with the external world when the brain cannot access appropriate effector output. Even when appropriate central signals can be acquired, constant interaction and adaptation of both the user and the brain-computer interface are required to successfully translate the user's intentions into an appropriate effect.
Dr. Fay Horak will specifically address the postural adaptations that occur when information from individual sensory modalities is lost. Sensory substitution, changes in the gain of the responses to remaining senses, increased use of prediction based on past behavior, and voluntary changes in strategies all make important contributions to stabilize posture. Rehabilitation must tap into these mechanisms if maximum postural stability is to be regained after disease or injury.