<P> Ovid J.L. Tzeng (1973) also found an instance where the recency effect in free recall did not seem to result from the function of a short - term memory store . Subjects were presented with four study - test periods of 10 word lists, with a continual distractor task (20 - second period of counting - backward). At the end of each list, participants had to free recall as many words from the list as possible . After free - recall of the fourth list, participants were asked to free recall items from all four lists . Both the initial free recall and the final free recall showed a recency effect . These results went against the predictions of a short - term memory model, where no recency effect would be expected in either initial or final free recall . </P> <P> Koppenaal and Glanzer (1990) attempted to explain these phenomena as a result of the subjects' adaptation to the distractor task, which therefore allowed them to preserve at least some of the functions of the short - term memory store . As evidence, they provided the results of their experiment, in which the long - term recency effect disappeared when the distractor after the last item differed from the distractors that preceded and followed all the other items (e.g., arithmetic distractor task and word reading distractor task). Thapar and Greene challenged this theory . In one of their experiments, participants were given a different distractor task after every item to be studied . According to Koppenaal's and Glanzer's theory, there should be no recency effect as subjects would not have had time to adapt to the distractor; yet such a recency effect remained in place in the experiment . </P> <P> One proposed explanation of the existence of the recency effect in a continual distractor condition, and the disappearance of it in an end - only distractor task is the influence of contextual and distinctive processes . According to this model, recency is a result of the final items' processing context being similar to the processing context of the other items and the distinctive position of the final items versus items in the middle of the list . In the end distractor task, the processing context of the final items is no longer similar to the processing context of the other list items . At the same time, retrieval cues for these items are no longer as effective as without the distractor . Therefore, the recency effect recedes or vanishes . However, when distractor tasks are placed before and after each item, the recency effect returns, because all the list items once again have similar processing context . </P> <P> Various researchers have proposed that stimuli are coded in short - term memory using transmitter depletion . According to this hypothesis, a stimulus activates a spatial pattern of activity across neurons in a brain region . As these neurons fire, the available neurotransmitters in their store are depleted and this pattern of depletion is iconic, representing stimulus information and functions as a memory trace . The memory trace decays over time as a consequence of neurotransmitter reuptake mechanisms that restore neurotransmitters to the levels that existed prior to stimulus presentation . </P>

What is an adaptation of the short term memory model that involves the active manipulation