Acute laser lesion effects on acuity sweep VEPs.

Q-switched neodymium-YAG (infrared) laser lesions at energies up to and including retinal hemorrhages were placed under visual control in the parafovea and the fovea of anesthetized monkeys. Visual-evoked potential (VEP) data were obtained by parallel analog (vector voltmeter) techniques from scalp electrodes in response to high luminance counterphasing sine wave gratings. The gratings were swept downward in spatial frequency to determine an acuity estimate by recording of the VEP magnitude increase onset. Acuity estimates were determined immediately post-exposure and at 15 sec intervals up to 12 min. These were analyzed as a function of laser exposure site and retinal lesion produced. Significant delays in VEP lock-in were demonstrated in subjects that had parafoveal burns or parafoveal subretinal hemorrhages. Foveal burns caused severe short-term fluctuations before a sustained decrease in acuity. Contained foveal hemorrhages produced sustained acuity losses. Foveal exposures that did not produce an immediately visible lesion did not produce measurable changes in VEP response lock-in time. These results probably are independent of visible flash effects and indicate that there may be a transient neural shock effect from parafoveal lesions that can affect the fovea.