Although there exists disputation about the matching manner of the features in the Wheatstone-Panum limiting case

it is generally considered that the uniqueness constraint operates in the conventional stereopsis condition when there is an even number of two features within corresponding binocular regions of each eye.By using a simple two-to-two line-type conventional stereoscopic configuration

it was found that subjects could observe a third line under the condition when the left and the right line in one eye separately fuse with the left and the right line in the other eye

thus a phenomenon apparently violates the uniqueness constraint.By adopting two pairs of stereoscopic probes

we demonstrated that the extra line resulted from double fusion occurred between one line in one eye and two lines in the other eye.These results indicate that double fusion as well as single fusion may occur under the conventional stereopsis condition

and they imply that double fusion occurs in the Wheatstone-Panum limiting case.