This slide shows quantitative psychometric functions illustrating how performance declines as spatial frequency increases for the resolution task of discriminating a vertical from a horizontal grating, and for the detection task of discriminating a grating from a uniform field of the same mean luminance. Psychometric functions are usually considered the gold standard in psychophysics and these data clearly show that observers are able to discriminate vertical from horizontal gratings in the periphery only for very low frequencies - below 5.5 cyc/deg in this example, before they begin to make mistakes and performance falls to chance levels.
Nevertheless, high-frequency patterns remain visible over a broad range of frequencies beyond the classical resolution limit. In fact, in my experience peripheral gratings are so visible that they are actually easier to see if I don't look straight at them, much like the situation at night when the best way to see a dim star is to fixate a few degrees to one side or the other. Notice that with this rigorous psychophysical method that detection acuity can exceed resolution acuity by an order of magnitude in peripheral vision.
The transition from veridical to non-veridical perception is so striking in these experiments, and occurs over such a narrow range of spatial frequencies, that it is possible for subjects (with a bit of practice) to become very skilled at finding the onset of aliasing by a simple method-of-adjustment procedure. We have found that the resolution limit found this way corresponds to the corner of the psychometric function where performance first dips below 100% correct. Because it is so much faster than the psychometric function method, and gives highly repeatable results, we often use the onset-of-aliasing criterion for when surveying the visual field.
WWWaveTM 1996
World Wide Web automated virtual environment TM 1996
Kevin Haggerty, Indiana University.
This slide show was automatically converted to web pages by the WWWave.