The ability to recall visual patterns from short-term memory often declines with the passage of time. There are two possible sources for this decline. One possibility is that internal variability or 'noise' introduced during he process of storage and retrieval grows over time. A second possibility is that internal noise remains constant, but the non-stochastic parts of the internal operations performed during storage and retrieval become lessoptimal over time. One way to distinguish between these possibilities is to measure changes in performance as varying amounts of externally added noise are introduced into a task. We applied this 'external noise masking' technique to a visual pattern discrimination task that involved the use of short-term visual memory. The task required observers to perform same/different discriminations with pairs of randomly generated noisy textures, separated by one of three time delays (100, 500, or 2000 ms). Increasing the delay between stimuli had little or no effect on internal noise, but reduced the efficiency of the non-stochastic parts of the internal operations by about 200%. In a subsequent experiment, externally-added noise was used to determine which spatial frequencies observers reliedupon to perform the pattern matching task at the long and short delays. The results showed that at least part of the reduction in efficiency with longer delays was due to observers' greater reliance on uninformative high spatial frequencies outside of the stimulus band. Possible explanations for the shift to higher frequencies will be discussed.