Detection of cortical vision loss usingfMRI and psychophysics
Abstract
The validity of functional magnetic resonance imaging (fMRI) as an indicator of human brain function is important for both theoretical and clinical purposes. One method to determine the validity fMRI is to compare visually evoked activation within retinotopic cortex with conventional behavior based visual field perimetry. To facilitate this goal, a Video Automated Perimeter (VAP) was developed to test the visual field behaviorally inside the MRI scanner. To confirm that the VAP was comparable with standard automatic perimetry, data were obtained using both the VAP and a conventional Humphrey Field Analyzer (HFA) for 9 patients having known, stable, cortical visual field defects. The median spatial cross-correlation of the VAP to HFA was 0.71, while the HFA to HFA correlation was 0.70. Thus the VAP system provides visual field maps equivalent to those obtained with HFA, but can be administered while the patient is in the MRI scanner, reducing test time and ensuring that both measures are obtained under matched conditions. VAP was then used to test the overall validity of fMRI visual field mapping. Forty-eight patients with cortical visual field loss participated in a total of 68 mapping studies to compare fMRI and behavioral visual field maps that extended out to 24 degrees eccentricity. The results of these studies were then analyzed using both automated and manual scoring techniques. Overall, the two measures resulted in 76% sensitivity, 65% specificity and 32% mismatches in the locations tested. A mismatch occurred when either the VAP or the fMRI visual field map had a loss of activation while the paired test did not. However, the locations of mismatches are potentially informative as diagnostic indicators of either neurovascular uncoupling or higher order lesions that cause cognitive deficits, such as neglect. Quantitative estimates of fMRI validity as an indicator of cortical function provide an important scale for weighing clinical decisions or theoretical interpretations. This is especially important when a misinterpretation could have costly consequences for the patient, such as surgically induced vision loss.
This paper has been withdrawn.