Learning in a Simple Motor System

Abstract

Motor learning is a very basic, essential form of learning that appears to share common mechanisms across different motor systems. We evaluate and compare a few conceptual models for learning in a relatively simple neural system, the vestibulo-ocular reflex (VOR) of vertebrates. We also compare the different animal models that have been used to study the VOR. In the VOR, a sensory signal from the semicircular canals is transformed into a motor signal that moves the eyes. The VOR can modify the transformation under the guidance of vision. The changes are persistent and share some characteristics with other types of associative learning. The cerebellar cortex is directly linked to the VOR reflex circuitry in a partnership that is present in all vertebrates, and which is necessary for motor learning. Early theories of Marr, Albus, and Ito, in which motor memories are stored solely in the cerebellar cortex, have not explained the bulk of the experimental data. Many studies appear to indicate a site of learning in the vestibular nuclei, and the most successful models have incorporated long-term memory storage in both the cerebellar cortex and the brainstem. Plausible cellular mechanisms for learning have been identified in both structures. We propose that short-term motor memory is initially stored in the cerebellar cortex, and that during consolidation of the motor memory the locus of storage shifts to include a brainstem site. We present experimental results that support our hypothesis.In many situations, inaccurate movements are worse than useless. If the movement is slow, errors can be corrected online by visual or other sensory feedback. However, rapid movements must be intrinsically accurate. This could be accomplished by hard-wiring an accurate movement, which has the obvious disadvantage that the movement cannot be readjusted for changes in downstream motor centers or (in the case of a reflex) for changes in sensory sensitivity. A better solution is to calibrate the movement by trial and error, that is, by motor learning. For our purposes, motor learning is defined as the process by which we acquire precise, coordinated movements, like those that are required in order to run, serve a tennis ball, and type accurately. Because accurate movements are necessary whenever predators must be avoided or prey captured, we can speculate that motor learning may have been one of the earliest types of associative learning to evolve. It has been demonstrated in the ventral nerve cord of insects (Horridge 1962) and is probably universal in freely-moving animals. Furthermore, because its substrate can be relatively simple and well understood, there is a real possibility of understanding motor learning both on the molecular level and on the level of brain circuitry.Motor learning is classified as procedural learning. Errors are monitored and used to guide adaptive changes without conscious awareness. Instead of learning a concept, the subject learns an accurate and appropriate motor response. Distinctions have also been made b...

Chronic Changes in Inputs to Dorsal Y Neurons Accompany VOR Motor Learning

“…However, the mechanism that maintains the new VOR gain after the flocculectomy is no longer identical to that in the intact animal because the system only works in the feed-forward mode. Our simulation results agree with those obtained experimentally (Broussard and Kassardjian 2004;Luebke and Robinson 1994;Partsalis et al 1995). Thus it appears that in the intact animal, changes in Purkinje cell sensitivities are necessary to compensate for the effects of the feedback loop.…”

“…Previous studies suggested that the flocculus plays an important role in the acquisition but not the retention of new VOR gains (Miles and Lisberger 1981). Broussard and Kassardjian (2004) showed that acute VOR motor memories are lost after flocculectomy, but chronic memories are largely retained. The authors suggest that the cerebellum has a major role in memory retention in the acutely adapted animal and that memories are shared between cerebellum and brain stem in the chronically adapted animal (cf.…”

The Site of a Motor Memory Shifts with Consolidation

“…Both of these structures are believed to participate in memory storage (Lisberger, 1994;du Lac et al, 1995) and may also participate in the consolidation of motor memory. Systematic investigations of consolidation in this simple system have begun only recently (Broussard and Kassardjian, 2004;Kuki et al, 2004). Here, we present evidence that VOR motor memory consolidates and that this occurs concurrently with a change in the location of memory storage.…”

“…Our results suggest that even in this simple system, motor memory may form initially in the cerebellar cortex and become distributed, during consolidation, between two or more sites. One possibility is that short-term learning takes place in the cortex, after which an appropriate error signal is generated by PCs to guide changes in the brainstem during consolidation (Broussard and Kassardjian, 2004). This does not rule out cortical processes such as synaptogenesis, which may also contribute to consolidation.…”

Análise da influência do nistagmo espontâneo e pré-calórico na vectoeletronistagmografia

“…O reflexo vestíbulo-ocular consiste na transformação de um sinal sensorial vindo dos canais semicirculares em uma resposta motora ocular, auxilia para estabilizar o olhar durante os movimentos cefálicos. Os movimentos oculares reflexos ocorrem na direção oposta à rotação cefálica e de velocidade equivalente a esta 7,8 . Há uma correlação entre os reflexos vestibulares e a faixa etária, ou seja, ocorre declínio do reflexo vestíbulo-ocular com o aumento da idade, em contrapartida, há um aumento do reflexo cérvico-ocular para compensar a redução da atividade vestibular 9 .…”