Visual Perception and Psychophysics
This domain tests how the visual system encodes luminance, contrast, spatial detail, color, motion, and depth, plus the psychophysical methods used to measure these functions.
Core rules
- Snellen acuity compares a patient's minimum angle of resolution to a standard. A 20/20 letter subtends 5 arcmin overall; the critical detail is 1 arcmin. A 20/40 letter has a minimum angle of resolution of 2 arcmin.
- logMAR is the logarithm of the minimum angle of resolution; lower values are better. 20/20 equals logMAR 0.0; 20/40 equals 0.3.
- Contrast is (Lmax - Lmin)/(Lmax + Lmin). Contrast sensitivity peaks at intermediate spatial frequencies, roughly 3-6 cycles per degree, and declines at high and low frequencies.
- Weber law states that discrimination depends on the ratio of increment to background; Fechner law relates sensation logarithmically to stimulus intensity.
- Color vision is trichromatic at the receptor level and opponent-process at postreceptor levels; protan and deutan defects are common, tritan defects are rare.
Worked example
A patient reads the 20/40 line at 20 feet. Reasoning: the 20/40 letter subtends 10 arcmin at that distance, and its critical detail subtends 2 arcmin. The minimum angle of resolution is therefore 2 arcmin, and logMAR is log10(2) = 0.30. If the patient could read 20/20, logMAR would be 0.0, which is better.
Traps that cost marks
- Reversing logMAR direction: higher logMAR means worse acuity.
- Using Ishihara plates to diagnose tritanopia; they mainly screen red-green defects.
- Confusing visual acuity with contrast sensitivity; a patient can have 20/20 acuity and abnormal contrast sensitivity.