Not all researchers agree that short-term and long-term memory are separate systems. The Unitary Model proposes that Short term memory consists of temporary activations of long term representation. Some theorists propose that memory is unitary over all time scales, from milliseconds to years. Support for the unitary memory hypothesis comes from the fact that it has been difficult to demarcate a clear boundary between short-term and long-term memory. For instance, Tarnow shows that the recall probability vs. latency curve is a straight line from 6 to 600 seconds (ten minutes), with the probability of failure to recall only saturating after 600 seconds. If there were really two different memory stores operating in this time frame, one could expect a discontinuity in this curve. Other research has shown that the detailed pattern of recall errors looks remarkably similar for recall of a list immediately after learning (it is presumed, from short-term memory) and recall after 24 hours (necessarily from long-term memory).