Abstract
Purpose::
To compare fMRI and VEP methods for assessing the cortical response lateralization that is characteristic of human albinism.
Methods::
We tested 18 participants with albinism, 17 controls and 6 participants with nystagmus. A full-field (±15°), high contrast (>96%) checkerboard target, that underwent pattern onset (33ms ON and 482ms OFF), was presented monocularly, to each eye, during VEP and fMRI recording sessions. For VEP sessions, 5 electrodes were used, one at Oz and the others placed at ±4cm and ±8cm lateral of Oz. A linked ears reference was used and a ground electrode was attached to the wrist. Artefactual responses were removed and the remaining responses (>150 for each eye) were averaged. The C1 was identified in the average trace obtained from each of the occipital electrodes and an interocular asymmetry index offered by Apkarian (1992) was calculated for each participant. For fMRI sessions, the stimuli were presented in an 18s block, which was followed by 18s of uniform grey. This cycle was repeated six times and each eye was tested separately. Activity in each hemisphere was quantified in terms of the number of voxels that modulated at the stimulus frequency (1/36s) with a correlation that exceeded 0.3 (p<0.01). The Michelson contrast between the number of activated voxels in each hemisphere was calculated and the difference between this measure for stimulation of the left and right eyes established an interocular asymmetry index.
Results::
VEP provided an excellent means by which those with albinism could be differentiated from normal, with only one patient and one control falling to the other grouping. Three individuals with nystagmus displayed responses characteristic of albninism. fMRI was marginally better than VEP at differentiating between those with albinism from controls with no individuals falling into the other grouping and only two individuals with nystagmus were classified as having characteristics of albinism with fMRI.
Conclusions::
VEP with a pattern onset stimulus regime provides a robust method to differentiate between those with albinism and controls, but can fail to differentiate between albinism and nystagmus. fMRI can achieve a better differentiation between those with albinism and normal controls than VEP and also offers an advantage in differentiating between albinism and nystagmus patients. fMRI may, therefore, offer clinical vision scientists a procedure that can assist in diagnosis of albinism in challenging cases.Apkarian, P. (1992) Opthalmic Paediatr. Genet. 13, 77-88.
Keywords: visual cortex • electrophysiology: clinical • imaging/image analysis: clinical