nd & Gallagher, 1993, p. 235). It is thus as a result of this decreased attention to B that blocking takes place.
In the other "equally successful" account of blocking it is held that differences in the processing of the unconditioned stimulus account for the phenomenon. According to this view the power of an unconditioned stimulus in conditioning "is a function of the difference between its reinforcing power and the conditioning strength of the conditioned stimuli that precede it in that trial" (Holland & Gallagher, 1993, p. 235). Therefore conditioning to B is blocked because the power of the US as a reinforcer is reduced in relation to B by the strength of its previous association with A. This position is seen to be confirmed by "unblocking" trials in which the strength of the reinforcer US is increased at the time the compound is paired with it. Frequently the blocking of conditioning to B is disrupted and "B acquires substantial conditioning" (Holland & Gallagher, 1993, p. 236).
But the question is far from being resolved since Holland and Gallagher (1993) also produced substantial evidence that the crux of blocking may be attentional. If the blocking effect relies on processing mechanisms that identify the reinforcing US as relatively weak in the case of B and as stronger when the US is increased in value, then "B should acquire conditioned inhibitory, rather than excitatory, tendencies when the value of the US" is decreased (Holland & Gallagher, 1993, p. 236). Yet in Holland's and Gallagher's (1993) experiment this did not occur. Instead the rats in their study "acquired substantial conditioning to B" whether the value of the US was increased or decreased -- leading them to believe that the attentional model was the more likely (Holland & Gallagher, 993, p. 235).
Since Kamin (1969) identified the blocking effect in associative learning, animal experiments have demonstrated that the effect is consistent and rep...