Many forms of learning require temporally ordered stimuli. In Pavlovian eyeblink conditioning, a conditioned stimulus (for example a tone), must precede the unconditioned stimulus (for example an airpuff), by at least about 100 ms, for learning to occur. Prior research have shown that the conditioned response, that is learned during eyeblink conditioning, is likely caused by a pause response in Purkinje cells, in the cerebellum. The predominant view is that synapses between Purkinje cells and their afferents undergo long term depression, which results in a pause response. This raises a serious conceptual challenge because long term depression is most effectively induced at short intervals between the conditional and unconditional stimulus (which do not support acquisition of conditioned responses). To resolve this discrepancy, we recorded from Purkinje cells during conditioning with short or long intervals between the conditional and unconditional stimulus. The results, published in Journal of Neuroscience, show that while a 150 ms interval did result in a pause response, shorter intervals did not. These results can explain why conditioning is not possible with short intervals. In addition, the results question whether long term depression can be the mechanism behind the acquisition of conditioned responses.
Link to full paper: http://www.jneurosci.org/content/34/5/1731.short