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psychometric visual acuity

Psychometric visual acuity is visual acuity measured as a threshold with forced-choice psychophysical methods.1 Forced choice makes the observer pick one of several alternatives on every trial.1 A reply of "I cannot see it" is not accepted, and the observer's own criterion for seeing therefore plays a smaller role.1

On a Tumbling E or Landolt C chart, the patient reports which way the E or the gap in the C points.2 That task needs resolution only and no recognition of letters.2 The threshold is read from a psychometric function, the percentage of correct answers across stimuli of graded visibility, at the point midway between chance and perfect performance.1 With two alternatives that point is 75% correct, and with four it is 62.5%.1

Every patient should be encouraged to guess.3 Pushing patients to threshold makes the measurement more accurate, and many patients who stop at one line go on to read another line, or a line and a half, quite well.3

More detail

Single letters, or a finger pointing to one letter in a line, reduce contour interaction and make the task easier than reading a whole line.3 In amblyopia the difference can be startling, and a line acuity of 6/30 may correspond to 6/6 with symbols presented singly.4

Contour interaction around a Landolt C begins below a critical separation of 1.9 to 3.8 minutes of arc in normal eyes and 8.4 to 23.3 minutes of arc in amblyopic eyes.4 Scaled to each eye's minimum angle of resolution, the curves for normal and amblyopic eyes are essentially similar, so contour interaction follows resolution capacity.4 About two thirds of patients with amblyopia show crowding at the first examination.4

Sources (9)
  1. Schwartz, Visual Perception: A Clinical Orientation, ch. 11
  2. Schwartz, Visual Perception: A Clinical Orientation, ch. 7
  3. Eperjesi and Rundström, Practical Binocular Vision Assessment, ch. 2
  4. von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 14
  5. Arundale, K. 1978. An investigation into the variation of visual acuity test scores. Transactions of the Ophthalmological Societies of the United Kingdom, 98(2), pp.345-349.
  6. Flom, M.C., Weymouth, F.W. and Kahneman, D. 1963. Visual resolution and contour interaction. Journal of the Optical Society of America, 53(9), pp.1026-1032.
  7. Raasch, T.W., Rucker, F.J., Vogel, C.R. and Tan, K.K. 1998. Intelligent automated static perimetry. Ophthalmology, 105(5), pp.808-814.
  8. Rutstein, R. and Daum, K. 1997. Anomalies of Binocular Vision: Diagnosis & Management. St. Louis: Mosby.
  9. Westheimer, G. 1979. Scaling of visual acuity measurements. Archives of Ophthalmology, 97(2), pp.327-330.

Reviewed October 2026

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