(2001) trained animals for only 4 days at an isi that is outside of a range known to be optimal for learning . neural recording studies electrophysiological recording studies of cerebellar cortex have helped to better understand the role that pcs play in the eyeblink cr learning process. mccormick and thompson (1984b) recorded pc activity during eyeblink training and found cell populations that discharged in a pattern apparently related the behavioral cr, while other pc populations discharged in patterns that coincided with either presentation of the cs or us. similar results were found by berthier and moore (1986) with single unit recording of pcs in lobule hvi. they found that populations of neurons fire in relation to various aspects of eyeblink training, including cs and us presentation and cr execution. (berthier and moore, 1986; gould and steinmetz, 1996). recently, similar stimulus- and response-related pc activity has been found in ant (green and steinmetz, 2005). finally, electrophysiological recordings of pcs in hvi and ant have revealed a difference in the overall population responses of pcs. the majority of pcs show excitatory patterns of activity during eyeblink conditioning in hvi (berthier and moore, 1986; gould and steinmetz, 1996; katz and steinmetz, 1997), and inhibitory patterns of activity in ant (green and steinmetz, 2005). in a single unit recording study where the individual purkinje cells were shown to be located in the area controlling blinks and to receive climbing fibre input on us presentations, only inhibitory responses were found. in a recent study of similarly characterized purkinje cells which were followed for up to more than fifteen hours, it was found that repeated presentations of the cs and us caused the gradual development of a pause in purkinje cell firing. this pause response, called a purkinje cell cr, was also obtained when direct mossy fibre stimulation was used as the cs and direct climbing fibre stimulation as the us. unpaired presentations of the cs and us caused extinction of the purkinje cell cr. when paired presentations were reintroduced after extinction, purkinje cell crs reappeared rapidly, mirroring the "savings" phenomenon demonstrated at the behavioral level. purkinje cell crs were also adaptively timed. feedback control of learning the deep cerebellar nuclei have been shown to inhibit the inferior olive and this inhibition probably serves a negative feedback control function. as learning proceeds, the olive becomes inhibited and it has been shown that this inhibition has temporal properties that makes ideally suited to serve as a feedback signal for controlling learning. stimulating this pathway during paired cs - us presentations causes extinction. recordings from purkinje cells show that the olive is depressed during the purkinje cell cr. summary taken together, results from lesion, inactivation, and neural recording studies seem to demonstrate that the cerebellar cortex is not essential for basic eyeblink cr learning or retention, but that significant contributions from cortex underlie normal learning. synaptic mechanisms underlying ebc the parallel fiber – purkinje cell synapse long term depression (ltd) at the pf-pc synapse is hypothesized to have significant functional consequences for learning the behavioral cr in ebc (ito, 1984). for example, as a result of training, inp cells discharge prior to cr execution and fire in a pattern of increased frequency of response that predicts the temporal form of the behavioral cr (mccormick & thompson, 1984). this pattern of activity clearly indicates that the inp is capable a generating a conditioned response. purkinje cells of the cerebellar cortex tonically inhibit deep nuclear cells. therefore, an ltd-mediated decrease in pc activity at the appropriate time during a cs-us interval could release the inp from tonic inhibition and allow for execution of a cr. an increase in pc activity could have the opposite effect, prohibiting or limiting cr execution. it has been hypothesized that crs are generated by the inp as a result of release from pc inhibition (i.e. perrett et al., 1993).