Note in Figure 14.2 that the relative efficiency of the various CS-US intervals diminishes sharply as exact simultaneity of stimuli is imposed, and that onset of the US prior to the CS also generates few CRs on test trials. Such results, in which “small amounts of conditioning” seem to be obtained from the simultaneous or the backward orders of presentation, are common when the US is electric shock or some other aversive stimulus, and the amount of “conditioning” obtained typically is on more than is generated by control procedures in which the CS and the US are not paired at all. For this reason, it is rather generally believed that backward conditioning does not occur, that the responses obtained on test trials do not represent “true conditioning, but that they are probably the results of pseudoconditioning or sensitization (see below). 11