JM Gold, R Sekuler, A Baerveldt, RF Murray, AB Sekuler, PJ Bennett
Department of Psychology, University of Toronto, andPurpose. Memory for visual patterns decays with the passage of time. This decay could be due to a reduction in internal signal strength (e.g., degradation of an internal filter or template), an increase in internal variability, or both. To distinguish among these possibilities, we estimated the effect of time delay on internal additive noise, multiplicative noise, and signal strength by measuring observers' equivalent input noise, response consistency, and calculation efficiency in a visual pattern matching task.
Method. Three observers performed a same/different discrimination task with pairs of randomly generated 2D textures (2-4 c/image band-pass filtered noise fields) separated by either a short (100 msec) or long (2000 msec) time delay. The textures were unique on every trial, and were embedded in one of several different levels of Gaussian white noise. To see representative textures, click.
The contrast of the textures was varied across trials to obtain same/different discrimination thresholds. These data were used to estimate additive internal noise and calculation efficiency for each time delay. To obtain a measure of internal multiplicative noise, the same observers performed the matching task in high external noise with two passes through the same stimulus set.
Results. Contrast energy thresholds at each time delay increased linearly with external noise spectral density. Comparisons of human and ideal performance showed that increasing the time delay between stimuli had no effect on additive internal noise but reduced calculation efficiency by about 200%. The consistency of responses during the two passes through the stimulus set did not change with time delay, demonstrating that the passage of time had no effect on multiplicative internal noise.
Conclusions . These results (1) imply that visual memory's decay over time is due only to a reduction in internal signal strength, and (2) 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. To see that paper's abstract click here; to download a .pdf file click here.