BOLD fMRI Response of Early Visual Areas to Perceived Contrast in Human Amblyopia

1 Laboratory for Functional Magnetic Resonance Research, The John P. Robarts Research Institute, London, Ontario N6A 5K8; 2 Department of Medical Biophysics, The University of Western Ontario, London, Ontario N6A 5C1; 3 Department of Ophthalmology, London Health Sciences Centre, London, Ontario N6A 5A5; 4 Department of Psychology, The University of Western Ontario, London, Ontario N6A 5C2; and 5 Department of Diagnostic Radiology and Nuclear Medicine, The University of Western Ontario, London, Ontario N6A 5A5, Canada Goodyear, Bradley G., David A. Nicolle, G. Keith Humphrey, and Ravi S. Menon. BOLD fMRI Response of Early Visual Areas to Perceived Contrast in Human Amblyopia. J. Neurophysiol. 84: 1907-1913, 2000. In this study, we used a temporal two-alternative forced choice psychophysical procedure to measure the observer's perception of a 22% physical contrast grating for each eye as a function of spatial frequency in four subjects with unilateral amblyopia and in six subjects with normal vision. Contrast thresholds were also measured using a standard staircase method. Additionally, blood-oxygenation-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) was used to measure the neuronal response within early visual cortical areas to monocular presentations of the same 22% physical contrast gratings as a function of spatial frequency. For all six subjects with normal vision and for three subjects with amblyopia, the psychophysically measured perception of 22% contrast as a function of spatial frequency was the same for both eyes. Threshold contrast, however, was elevated for the amblyopic eye for all subjects, as expected. The magnitude of the fMRI response to 22% physical contrast within "activated" voxels was the same for each eye as a function of spatial frequency, regardless of the presence of amblyopia. However, there were always fewer "activated" fMRI voxels during amblyopic stimulation than during normal eye stimulation. These results are consistent with the hypotheses that contrast thresholds are elevated in amblyopia because fewer neurons are responsive during amblyopic stimulation, and that the average firing rate of the responsive neurons, which reflects the perception of contrast, is unaffected in amblyopia.