Two different visual alignment errors can occur in putting: the alignment of the ball to the start
line and the face of the putter relative to this line at address (Dalton, 2013)
Shim et al (2012) found no effect of using a line on the ball, however this is still an approach regular used by Professional players.
Guadagnoli, M. & Lindquist, K. (2007) Challenge Point Framework and Efficient Learning of Golf. Int J of Sports Sci & Coaching 2(1) suppl, p185-197
Guadagnoli, M.A. & Lee T.D. (2010) Challenge Point: A Framework for Conceptualizing the Effects of Various Practice Conditions in Motor Learning, J Motor Behavior, 36(2), p212-224