Analysis of Contrast Processing Segregated Into Magnocellular and Parvocellular Systems in Asymptomatic Carriers of 11778 LHON

Abstract

Purpose: : To detect evidence of specific impairment of parvocellular (PC) and magnocellular (MC) functions in asymptomatic carriers of Leber`s Hereditary Optic Neuropathy (LHON) from a large Brazilian pedigree.

Methods: : Contrast detection thresholds were measured in 22 carriers of 11778 LHON (mean age= 35 ± 13.4 yrs) and 22 control subjects (mean age= 40 ± 10.3 yrs). Assessment was with self–developed software running the steady–/pulsed–pedestal paradigm (Pokorny & Smith, 1997). Stimuli consisted of a luminance pedestal (four square array, 1 degree each) and a constant background presented in a high–resolution display monitor. The luminance level of one of the squares in the pedestal (test stimulus) had a randomized temporal modulation (flashes with 17ms and 133ms) under different adaptation levels provided by the pedestal (presented with 7, 12 and 19 cdm²) and the background (12 cdm²). The six resulting combinations were presented in the pulsed and steady paradigms, in a total of 12 subsets. In the steady– paradigm, the test stimulus was briefly presented on a continuously present pedestal and in the pulsed– paradigm, both test stimulus and pedestal were flashed simultaneously. Psychophysical correlates from MC and PC functions are revealed by the steady– and pulsed– paradigms, respectively.

Results: : Approximately 28% of the carriers had higher contrast thresholds than the 90% normal limits based on the control group. This was true for both paradigms. For the steady– paradigm, the thresholds that exceeded normal limits were associated with high temporal frequencies (33%) whereas in the pulsed– paradigm, they were more frequent at the lower temporal frequencies (29%). There was no correlation between the contrast thresholds and age, visual acuity or color vision scores.

Conclusions: : We demonstrated an impairment of temporal contrast detection in the LHON asymptomatic carriers. However, there was no evidence of selective damage to either the MC or PC streams, indicating that both are impaired in 11778 LHON carriers. This corroborates our previous results showing uniform losses along the luminance spatial contrast sensitivity function (Ventura et al., IOVS, 2005).

Keywords: contrast sensitivity • visual impairment: neuro-ophthalmological disease • neuro-ophthalmology: optic nerve