Forming chunks in instrumental learning: The role of overshadowing

Lower order units combined into higher order functional units (e.g., letters into words) are called chunks. The hypothesis tested here suggests that chunks are formed when memories of goal events signal other goal events, a signal capacity that can be reduced when memories are seriously overshadowed by other more valid cues called grouping cues. Ln support of this hypothesis, it was found, in each of three experiments employing rats in runways, that the capacity of the memory of nonreward to signal reward depended on its validity relative to that of two other situational cues, runway brightness cues and a change in brightness cues, from one trial to the next. Specifically, the capacity of the memory of nonreward to signal reward was much more seriously reduced when it was in competition with more valid situational cues (.33 vs. 1.00, Experiment 1; .50 vs. 1.00, Experiment 2) than when it was in competition with situational cues having either the same validity (.33, Experiment 1) or a slightly higher validity (.33 vs .50, Experiment 3). It was also found that grouping each of two series was less effective in producing chunking than was grouping only one of the series.