Memory for visual patterns decays with the passage of time. This decay could be due to a reduction in internal signal strength, an increase in internal variability, or both. To distinguish among these possibilities, we estimated the effect of time delay on internal noise and signal strength by measuring observers' equivalent input noise, response consistency, and calculation efficiency in a same/different visual pattern matching task.

Increasing the time delay between stimuli from 100 to 2000 ms had no effect on additive internal noise but reduced calculation efficiency by about 200%. The consistency of responses between two passes through the stimulus set did not change with time delay, demonstrating that the passage of time had no effect on signal-dependent internal noise. These results imply that visual memory's decay over time is due only to a reduction in internal signal strength, and strongly constrain theories of visual memory.

Note. Details of the analysis of calculational efficiency, additive and multiplicative internal noise are given in J Gold, PJ Bennett & AB Sekuler(1999) "Signal but not noise changes with perceptual learning" Nature 402 176-178.