Is reduced vernier acuity in amblyopia due to position, contrast or fixation deficits?

Poor vernier acuity, exhibited by amblyopes, may reflect anomalies related to eccentric fixation, deficient position sensitivity, or reduced contour visibility. We have examined these factors by measuring contrast and vernier sensitivities with stimuli consisting of extended sinusoidal gratings of several spatial frequencies. Vernier thresholds were measured using both a classical single step position change and also a grating that was position-modulated sinusoidally along its entire length. For both types of target amblyopes exhibited deficient displacement sensitivity although accurate fixation was not strictly required. The vernier deficits were not critically dependent upon the type of displacement used, and the magnitudes of the vernier and contrast sensitivity deficits were closely related. Both were largest at high spatial frequencies, and those amblyopes with larger contrast sensitivity deficits also had larger vernier acuity deficiencies. Typically, contrast sensitivity and vernier acuity were normal, or nearly so, at very low spatial frequencies. Also, vernier acuities for the amblyopic and non-amblyopic eyes were approximately equal if grating contrast was set at some fixed multiple of detection threshold. We did not find a close relationship between the magnitude of the vernier deficit and reported perceptual distortions.