Two eyes: square root 2 better than one?

Classical data on the detection of simple patterns show that two eyes are more sensitive than one eye. The degree of binocular summation is important for inferences about the underlying combination mechanism. In a signal detection theory framework, sensitivity is limited by internal noise. If noise is added centrally after binocular combination, binocular sensitivity is expected to be twice as good as monocular. If the noise is added peripherally at each eye prior to combination, binocular sensitivity will be sqrt2 higher than monocular. In a large sample of observers (51), we measured contrast sensitivity for detection of gratings at several spatial frequencies using left, right, or both eyes.