Rasmussen Anders, Hesslow Germund
Department of Experimental Medical Science, Lund University, Lund, Sweden.
Department of Experimental Medical Science, Lund University, Lund, Sweden.
Prog Brain Res. 2014;210:103-19. doi: 10.1016/B978-0-444-63356-9.00005-4.
The ability to anticipate future events and to modify erroneous anticipatory actions is crucial for the survival of any organism. Both theoretical and empirical lines of evidence implicate the cerebellum in this ability. It is often suggested that the cerebellum acquires "expectations" or "internal models." However, except in a metaphorical sense, the cerebellum, which consists of a set of interconnected nerve cells, cannot contain "internal models" or "have expectations." In this chapter, we try to untangle these metaphors by translating them back into neurophysiological cause and effect relationships. We approach this task from within the paradigm of classical conditioning, in which a subject, through repeated presentations of a conditional stimulus, followed by an unconditional stimulus, acquires a conditioned response. Importantly, the conditioned response is timed so that it anticipates the unconditioned response. Available neurophysiological evidence suggests that Purkinje cells, in the cerebellar cortex, generate the conditioned response. In addition, Purkinje cells provide negative feedback to the inferior olive, which is a relay for the unconditional stimulus, via the nucleo-olivary pathway. Purkinje cells can therefore regulate the intensity of the signal derived from the unconditional stimulus, which, in turn, decides subsequent plasticity. Hence, as learning progresses, the olivary signal will become weaker and weaker due to increasing negative feedback from Purkinje cells. Thus, in an important sense, learning-induced changes in Purkinje cell activity constitute an "expectation" or "anticipation" of a future event (the unconditional stimulus), and, consistent with theoretical models, future learning depends on the accuracy of this expectation.
预测未来事件并修正错误的预测行为的能力对任何生物体的生存都至关重要。理论和实证证据均表明小脑与这种能力有关。人们常认为小脑会形成“预期”或“内部模型”。然而，除了在隐喻意义上，由一组相互连接的神经细胞组成的小脑不可能包含“内部模型”或“有预期”。在本章中，我们试图通过将这些隐喻转化回神经生理因果关系来解开它们。我们从经典条件作用的范式来处理这个任务，在经典条件作用中，一个主体通过反复呈现条件刺激，随后呈现无条件刺激，从而获得条件反应。重要的是，条件反应的时机是经过调整的，以便它能预测无条件反应。现有的神经生理证据表明，小脑皮质中的浦肯野细胞产生条件反应。此外，浦肯野细胞通过核橄榄体通路向下橄榄核提供负反馈，下橄榄核是无条件刺激的中继站。因此，浦肯野细胞可以调节来自无条件刺激的信号强度，这反过来又决定了后续的可塑性。因此，随着学习的进行，由于浦肯野细胞不断增加的负反馈，橄榄体信号会变得越来越弱。所以，从一个重要意义上来说，学习引起的浦肯野细胞活动变化构成了对未来事件（无条件刺激）的“预期”或“预测”，并且与理论模型一致，未来的学习取决于这种预期的准确性。
