Different Neural Codes for Spatial Frequency and Contrast

Abstract

S. S. Stevens (1957) argued that there are two different types of sensory dimensions, extensive and intensive, and that they have different types of neural codes. Stevens based his arguments on the results of psychophysical scaling experiments. However, if the two types of dimensions are coded in fundamentally different ways, the difference should be reflected in other tasks as well, including short-term memory tasks. In this paper, we present evidence that the short-term memory for spatial frequency, an extensive dimension, does not degrade over periods up to 10 sec, whereas the memory for contrast, an intensive dimension, does. In our experiments, the viewer must discriminate between two, successively presented stimuli. To respond correctly, the viewer must have an accurate memory of the first stimulus to compare with the second. If the memory degrades over time, discrimination performance must decline as the interval between the stimuli (ISI) lengthens. We find that spatial frequency discrimination, as measured by the difference threshold, is independent of ISI for intervals up to 10 sec, whereas the threshold for contrast discrimination progressively increases over this same period. This difference occurs at all contrast levels and has the same magnitude when the viewer judges both dimensions simultaneously as when the dimensions are judged separately.