Underlying psychometric function for detecting gratings and identifying spatial frequency. 
Observers detected and/or identified the spatial frequencies of grating stimuli.  Spatial frequency varied from 3.8 to 5.5 cycles per degree, and contrast varied from 0.001 to 0.33.  In nearly all cases, the psychometric functions that relate performance on the different tasks to contrast are multiples of one underlying function, provided that the functions are expressed in standard normal deviates.  The underlying function is positively accelerated at contrasts less than 0.01 and levels off at contrasts greater than 0.05.  A vector model interprets the results and relates them to the responses of individual spatially tuned mechanisms.