Preexposure of a Redundant Stimulus in a Different Context Attenuates Blocking

Using an extension of the blocking paradigm, the question of whether operations that reduce the associability of a stimulus (preexposure, redundancy) exert their effects independently of the context in which they are experienced was examined. Prior to conditioning two of the four groups received 20 presentations (4/day for 5 days) of a visual stimulus prior to its use as a redundant element in the blocking paradigm. Preexposure occurred in the same context as the training situation for one of the groups; the other group experienced the stimulus in an altered auditory, visual and olfactory environment. The other groups served as controls: one received the blocking regimen; the other served as an overshadowing control. An altered context during preexposure enhanced the associability of the redundant cue evidenced by an attenuation of blocking. This finding suggests that one indirect effect of context is the specification of a conditional relationship between stimulus and response such that operations which alter the associability of a stimulus are dependent upon the context in which they occur.

Suffix Effects and Probe Modality in Probed Recall: Implications for Readout from Sensory Memory

The effect of a redundant stimulus suffix was investigated in three experiments using a probed recall task. The probe modality was auditory in the first experiment and visual in the second. In the third experiment, both probe modalities were tested. The presentation rate was varied in all three experiments and was confounded with the delay of the suffix. The suffix was found to have a small and approximately equal effect on the last six serial positions. In contrast to the predictions from Crowder and Morton's (1969) PAS model, presentation rate and suffix delay did not interact with the suffix effect. The results indicate that at least six items can be simultaneously represented in PAS, that there is no appreciable decay of information from PAS during presentation of the last list items, and that readout of PAS information does not occur either during list presentation or between the end of list presentation and the beginning of overt report.

Locus of the Effect of a Surprising Reinforcer in the Attenuation of Blocking

Previous experiments on conditioned suppression in rats have shown that prior conditioning to one element of a compound conditioned stimulus paired with shock may block or prevent conditioning to the other element. Reliable conditioning may, however, occur to the added element (blocking may be attenuated), if a surprising second shock is added shortly after each compound trial. Experiment I confirmed this finding, and further showed that blocking was attenuated only when the second shock occurred 10 s after the compound trial, not when it occurred 100 s later. Experiment II showed that the surprising omission of an expected second shock 10 s after each compound trial would also attenuate blocking, thus implying that the surprising event does not itself act to reinforce conditioning to the added element, but rather permits the unconditioned stimulus (the first shock) to play its normal role as an effective reinforcer. This conclusion was confirmed by Experiment III, which showed that a surprising second shock does not produce any increase in conditioning to the added element on the trial on which it occurs; rather it serves to ensure adequate conditioning to that element on a subsequent compound trial. The implication is that the surprising event acts proactively to prevent subjects learning to ignore an otherwise redundant stimulus.

Serial Position Effects in the Recall of Picture Sequences

Sequences of 10 pictures of familiar objects were presented for immediate recall, and typically bow-shaped serial position curves were obtained. The effects of redundant stimulus suffixes, and of redundant stimulus prefixes, which could be either extra pictures, or written words (the names of similar familiar objects) were examined. In the prefix conditions, extra words produced a general decrement over positions 1–4, as compared with a control condition. Picture prefixes had no effect. In the suffix conditions, redundant words affected positions 9 and 10, whereas the main effect of redundant pictures was at positions 8 and 9. The differential serial position effects are taken to reflect the presence of two memory codes, a verbal one and a visual one, which vary in trace-strength or accessibility at different list positions.