BOLD fMRI response of early visual areas to perceived contrast in human amblyopia

In this study, we used a temporal two-alternative forced choice psychophysi cal procedure to measure the observer's perception of a 22% physical contra st 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. Additiona lly, blood-oxygenation-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) was used to measure the neuronal response within early visu al cortical areas to monocular presentations of the same 22% physical contr ast gratings as a function of spatial frequency. For all six subjects with normal vision and for three subjects with amblyopia, the psychophysically m easured perception of 22% contrast as a function of spatial frequency was t he same for both eyes. Threshold contrast, however, was elevated for the am blyopic eye for all subjects, as expected. The magnitude of the fMRI respon se to 22% physical contrast within "activated" voxels was the same for each eye as a function of spatial frequency, regardless of the presence of ambl yopia. However, there were always fewer "activated" fMRI voxels during ambl yopic stimulation than during normal eye stimulation. These results are con sistent with the hypotheses that contrast thresholds are elevated in amblyo pia 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.