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===Noteworthy observations from Kamin’s studies of blocking===
Kamin (1968) reported that the light (L) and noise (N) CSs employed in the initial demonstrations of blocking were not equally salient, because suppression to L was greater than the corresponding suppression to N. Nevertheless, initial training to light CS (L) was just as effective in blocking conditioning to the noise CS (N) as the noise CS is in blocking conditioning to the light CS, when the latter is the added CS element to the LN compound. Salience refers to the rate of CR acquisition or development when the stimulus is paired with reinforcement. However, it is usually easier to block conditioning to a CS of weaker salience with a CS of stronger salience than the other way around (Marchant & Moore, 1973, with the notable exception of taste-potentiated odor aversion conditioning, Rusiniak, Hankins, Garcia, & Brett, 1979).

Kamin (1968) noted that the first presentation of a stimulus (typically L) in the CER procedure produced a transient suppression of ongoing bar pressing, which he referred to as a “mildly disruptive effect” akin to Pavlovian external inhibition. Transient disruptions of bar pressing are not conditioned responses, as they typically dissipate after the initial stimulus presentation. Conditioned suppression of bar pressing to a stimulus only dissipates after a series of non-reinforced extinction trials.

Kamin (1968, p 14) reported that prior conditioning to L did not attenuate conditioning to N-alone and vice versa. This finding validates one of the controls for proactive interference mentioned above.

Kamin (1968, p 16-18) reported that blocking in the two-stage paradigm was not overcome by extended compound-CS training in Stage-2.  By contrast, reducing the number of Stage-1 training trials to N attenuated the blocking effect observed on L test trials. Eight noise trials in Stage-1 was enough to produce complete blocking of L. Complete blocking was also observed with CSs of 1-minute duration instead of the usual 3-minute CS durations and with 3-milliampere foot shocks instead of the usual 1-milliampere foot shocks.  Furthermore, L was blocked just as effectively by a noise-off CS as a noise-on CS. In short, blocking in the CER procedure with rats is robust and easy to demonstrate, according to Kamin’s experiments. 

Kamin (1968, p 19) reported that blocking could be prevented by non-reinforced presentation of N (extinction training) before compound-CS training. This observation supports Kamin’s hypothesis that the amount of learning about the blocked CS (L) depends on to the depth of conditioned suppression to N at the start of Stage-2.
That is, learning about L depends on the discrepancy between the level of conditioned responding to N at the start of Stage-2 and the asymptotic level of conditioning achievable with the foot-shock reinforcement.