Binocularity
The ability to focus upon an object with both eyes and create a single stereoscopic image.
Binocularity requires highly convergent orbits.
The evolutionary increases among primates in the degree of orbital convergence have been noted to correlate with the expansion of visual brain structures and, as a consequence, with the overall size of the brain.
It is therefore thought that the evolution of brain size in primates was associated with visual specialization.