Identifying human albinism: a comparison of VEP and fMRI.

PURPOSE To compare VEP and fMRI as a means of detecting the abnormal visual projections in albinism in different stimulation conditions. METHODS Cortical response to monocular full-field pattern-onset and hemifield pattern-onset and -reversal stimulation of 18 subjects with a known diagnosis of albinism, 17 control subjects, and 6 control subjects with infantile nystagmus syndrome (INS) was determined by VEP and fMRI. An asymmetry index was used to quantify the extent of response lateralization as measured by both VEP and fMRI. The extent to which each method and stimulus combination differentiated participant groups was summarized with a receiver operating characteristic (ROC) analysis, where A(A-C) and A(A-N) refer to areas under the ROC curve for albino versus control and albino versus nystagmus comparisons. RESULTS Cortical response to full-field monocular stimulation conditions offered robust detection of the abnormal response lateralization in albinism, with fMRI (A(A-C) = 1.00; A(A-N) = 0.91) being slightly more robust than the VEP under these conditions (A(A-C) = 0.91; A(A-N) = 0.79). Hemifield stimulation paradigms were somewhat poorer at differentiating between groups, particularly when VEP was used in combination with pattern-reversal stimulation (pattern-onset fMRI A(A-C) = 0.94, A(A-N) = 0.84, and VEP A(A-C) = 0.86, A(A-N) = 0.86; pattern-reversal fMRI A(A-C) = 0.90, A(A-N) = 0.88, and VEP A(A-C) = 0.69, A(A-N) = 0.64). However, when only the most posterior aspects of the occipital lobe were considered with hemifield stimulation, fMRI achieved the best differentiation between the subject groups, most notably with hemifield pattern-reversal stimulation (A(A-C) = 1.00; A(A-N) = 1.00). CONCLUSIONS An interocular comparison between the lateralization of cortical responses elicited by full-field stimulation reliably distinguished between those with albinism and control groups, when both fMRI and VEP were used to assess cortical responses. Hemifield stimulation of one eye offers an alternative method for assessing misrouting associated with albinism and is highly effective when cortical signals are assessed with fMRI, but less so when VEP is used.