Motor fusion, also known as fusional vergence or fusional reserves, is the maximum amount the eyes can converge or diverge while still maintaining binocular single vision. Positive fusional reserves, measured with base-out prisms, represent the eyes' maximum convergence ability. Negative fusional reserves, measured with base-in prisms, represent maximum divergence.
Sources
Arnoldi, K. 2009. Prisms. In: Rosenfield, M., Logan, N. eds. Optometry: Science, Techniques and Clinical Management. 2nd ed. Edinburgh: Butterworth-Heinemann, pp.305-317.
Ciuffreda, K.J., Ciuffreda, M.A. and Wang, B. 2006. Conceptual model of human blur perception. Vision Research, 46(9), pp.1426-1441.
Fray, K.J. 2013. Fusional amplitudes: exploring where fusion falters. American Orthoptic Journal, 63(1), pp.41-54.
Noorden, G.K. and Campos, E.C. 2002. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St. Louis: Mosby.
Rowe, F.J. 2010. Clinical Orthoptics. 2nd ed. Oxford: Wiley-Blackwell.
Rutstein, R. and Daum, K. 1997. Anomalies of Binocular Vision: Diagnosis & Management. St. Louis: Mosby.