The precise timing of the CS-US pairings has a great influence on whether the CS can be reliably conditioned to produce the CR. To begin with, a closer look at the development of the CR:

As can be seen, the dog initially only salivates when presented with the food US, but gradually the tone begins to elicit salivation, until, after a number of trials the dog salivates as soon as the tone is perceived.

As the figure above shows, the relative timing of the CS and US is crucial to the success of conditioning. Most of these results appear to have simple explanations. In delay conditioning the salivation elicited by the US gradually extends backwards as it becomes associated with the US. In the standard paradigm the same thing happens although it is a little less obvious because of the overlap between the CS and US. In trace conditioning one might assume that the very recent memory trace of the CS begins to be associated with the US and hence the UR gradually extends back, albeit weakly, to the actual occurrence of the CS. The strength of trace conditioning is generally inversely related to the delay between the CS and US. The same cannot be happening in backwards conditioning which is usually ineffective. Once the CS appears, the US has already elicited a response so the effectiveness of conditioning is not simply dependent on the temporal contiguity of the CS and US. Thus a crucial feature of successful CS-US pairing in time seems to be that the CS can be used to predict the occurrence of the US. This suggestion may also explain one of the most surprising results - if the CS and US are presented exactly simultaneously, so that both the onset and offset both occur together, then conditioning fails.