Comparison of changes in sensitivity and sensation: implications for the response-intensity function of the human photopic system.

Brightness magnitude estimations of foveally presented flashes deviate from a power function. Increment threshold functions for test lights presented upon these same flashes deviate drastically from a constant Weber fraction. These departures from the classic laws of sensation and sensitivity are shown to be in qualitative agreement and to be affected in similar ways by changes in steady adapting fields. Both sets of data can be fit over the lower range of flash intensities by models based on a saturating response function; both sets deviate from these models at high flash intensities in ways consistent with a nonsaturating response function.