memory) are unaffected. Other manipulations (e.g., semantic similarity of the words) affect only memory for earlier list words, but do not affect memory for the most recent few words. These results show that different factors affect short-term recall (disruption of rehearsal) and long-term recall (semantic similarity). Together, these findings show that long-term memory and short-term memory can vary independently of each other. Models Unitary model Not all researchers agree that short- and long-term memory are separate systems. The alternative Unitary Model proposes that short-term memory consists of temporary activations of long term representations (that there is one memory that behaves variously over all time scales, from milliseconds to years). It has been difficult to identify a sharp boundary between short-term and long-term memory. For instance, Tarnow reported that the recall probability vs. latency curve is a straight line from 6 to 600 seconds, with the probability of failure to recall only saturating after 600 seconds. If two different stores were operating in this time domain, it is reasonable to expect a discontinuity in this curve. Other research has shown that the detailed pattern of recall errors looks remarkably similar to recall of a list immediately after learning (it is presumed, from short-term memory) and recall after 24 hours (necessarily from long-term memory). Further evidence for a unified store comes from experiments involving continual distractor tasks. In 1974, Bjork and Whitten presented subjects with word pairs to remember; before and after each word pair, subjects performed a simple multiplication task for 12 seconds. After the final word-pair, subjects performed the multiplication distractor task for 20 seconds. They reported that the recency effect (the increased probability of recall of the last items studied) and the primacy effect (the increased probability of recall of the first few items) was sustained. These results are incompatible with a separate short-term memory as the distractor items should have displaced some of the word-pairs in the buffer, thereby weakening the associated strength of the items in long-term memory. Tzeng (1973) reported an instance where the recency effect in free recall did not seem to result from 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 as possible. After recall of the fourth list, participants were asked to recall items