Two distinct origins of long-term learning effects in verbal short-term memory

Abstract

Verbal short-term memory (STM) is highly sensitive to learning effects: digit sequences or
nonword sequences which have been rendered more familiar via repeated exposure are
recalled more accurately. In this study we show that sublist-level, incidental learning of
item co-occurrence regularities affects immediate serial recall of words and nonwords,
but not digits. In contrast, list-level chunk learning affects serial recall of digits. In a first
series of experiments, participants heard a continuous sequence of digits in which the
co-occurrence of digits was governed by an artificial grammar. In a subsequent STM test
participants recalled lists that were legal or illegal according to the rules of the artificial
grammar. No advantage for legal lists over illegal lists was observed. A second series of
experiments used the same incidental learning procedure with nonwords or non-digit
words. An advantage for legal versus illegal list recall was observed. A final experiment
used an incidental learning task repeating whole lists of digits; this led to a substantial
recall advantage for legal versus illegal digit lists. These data show that serial recall of
non-digit words is supported by sublist-level probabilistic knowledge, whereas serial recall
of digits is only supported by incidental learning of whole lists.