motor fusion (fusional reserves)

Motor fusion is the ability to align the eyes so that sensory fusion can be maintained.¹ Retinal disparity outside Panum’s area drives these fusional movements, and the two eyes move in opposite directions as vergences.¹ Motor fusion depends on the extrafoveal retinal periphery, so no stimulus for it exists once the images of the fixated object fall on both foveae.¹

The fusional reserves are the amount of vergence that can be induced before fusion fails and the patient reports blur or diplopia.² Base-out prism measures the convergent reserve, base-in prism measures the divergent reserve, and vertical prism measures the vertical reserves, each at distance and at near.² The examiner records the prism power at three points: blur, break and recovery.² The blur point marks the relative convergence or divergence.²

Positive fusional vergence is fusional convergence measured from the phoria position to the base-out limit of clear, single binocular vision.³ Negative fusional vergence, also called fusional divergence, is measured from the phoria position to the base-in limit.³