But: Psychophysical and electrophysiological studies have shown that the cortical response related to binocular summation increases with low contrast stimuli. In this study, we investigated this effect using functional MRI (fMRI). Since this technique records signals related to blood flow, it is important to optimise the parameters of stimulation in order to maximise the measurement of the cortical response. Presentation time, and spatial and temporal frequency were varied in order to explore the phenomenon in depth.

Objectives and Methods: Three normal healthy volunteers participated in this study. Their binocular function was defined by >60 seconds of stereopsis using the TNO test.

The data were acquired with a 3T clinical scanner using 2mm³ isovoxel for the functional images and 1mm³.for the high-resolution anatomical images.

In order to weight the magno or parvocellular layer response, two different checkerboard stimuli were used. The high temporal frequency checkerboard flickered at 14Hz and had a spatial frequency of 50min of arc. The low temporal frequency checkerboard flickered at 4Hz and had a spatial frequency of 5min of arc. The contrast was 20% for all checkerboard stimuli. Each cycle was of 24 seconds and consisted of one stimulation and a rest condition. The duration of the stimulation was 6, 9, or 12 seconds. Binocular and monocular conditions were repeated every 2 cycles. The data were analyzed with the Brain Voyager software.

Results: The fMRI signal increased in all subjects with the increase in presentation time. For all stimulations, the signal was greater in the binocular condition compared to the monocular condition. Moreover, the binocular/monocular ratio was different depending on the location of the cortical response.

Discussion: These results suggest the existence of different cortical processings underlying the phenomenon of binocular summation.

Conclusion: Efforts are being done to make this method capable to evaluate abnormal binocular function.