INTRODUCTION
The cerebellum offers the perfect neuronal territory within which to achieve an understanding of simple forms of learning and memory that flow ‘‘continuously from molecules and cells through synapses and circuits to behavior without any grossly embarrassing gaps in the middle’’. Pavlovian eyeblink conditioning illustrates this beautifully for an elementary building block in learning: the capacity to make associations. Imagine receiving an air puff on your eye a quarter second after seeing a green light-emitting diode (LED) turn on, over and over again. The air puff is an unconditional stimulus (US) in that it yields an unconditional blink response (UR). From the reflex loop underlying this blink, a signal is transmitted to the inferior olive and subsequently through climbing fibers to a subset of Purkinje cells in mainly the simplex lobule (hemispheric lobule VI [HVI]) of the cerebellar cortex. This climbing fiber signal causes a complex spike in these Purkinje cells, whose activity moderates cerebellar nuclear cells that ultimately innervate the same eyelid musculature targeted by the reflex loop. The same Purkinje cells also receive massive sensory input through the mossy fiber-parallel fiber system, in large part originating in the pontine nuclei. Hidden within this sea of parallel fibers are signals encoding the green LED light, which by default is a neutral stimulus. Only on the condition that it is consistently paired with the air puff and only by virtue of the Purkinje cell’s capacity to forge associations between parallel and climbing fiber signals will one learn to perform a well-timed, conditional blink response (CR) upon exposure to the light, which is thusly called the conditional stimulus (CS).