title: Rod increment thresholds in cone-rod dystrophy.
abstract: Rod system increment threshold functions (ITFs) were studied in patients with cone-rod dystrophy (CRD). Rod thresholds (I(t)) for a 104-min, 500-nm test stimulus (TS), superimposed on an 11 degrees long wavelength background (luminance I(b)), were measured in eleven CRD patients and fourteen normal subjects. Thresholds in normals were also measured using a 7-min TS. A modified version of Weber's law [logI(t) = logK + log(I(n)(b) + c)] was fitted to the data from each subject and test condition to yield a description of the ITF in terms of the free parameters K, c, and n. Four of the CRD patients exhibited an abnormally high absolute threshold; of these, two showed abnormalities in K and c consistent with a reduced efficiency of quantum capture by the rods. Abnormalities in the ITFs of CRD patients did not resemble the effect of reducing the diameter of the TS from 104 to 7 min in normals. This suggests that threshold abnormalities in the CRD patients did not result from altered spatial summation. The results illustrate use of a parametric representation of the ITF to evaluate the loss of sensitivity in visual disorders.
authors.forename: D W
authors.initials: DW
authors.lastname: Yates
authors.forename: D J
authors.initials: DJ
authors.lastname: Derlacki
authors.forename: D R
authors.initials: DR
authors.lastname: Pepperberg
authors.forename: K R
authors.initials: KR
authors.lastname: Alexander
authors.forename: G A
authors.initials: GA
authors.lastname: Fishman