Rotating grid method of measuring vergence limits: the effect of line frequency

When a grid consisting of equally spaced parallel lines is placed in the objective frontoparallel plane and is rotated from an initially vertical orientation about the primary sagittal line, the horizontal distance between the lines increases. When the two eyes fuse specific sections of the grid, this increase in horizontal separation can be used to vary the vergence stimulation. The vergence limits measured by this method were found to be influenced by spatial frequency. Using a selected spectrum of frequencies, it was found that vergence limits decrease nonlinearly with frequency. A range of line frequencies between 1.06 and 4.97 lines/deg is suggested for practical use. The vergence limit determined at one frequency was found to be different from that of the next line frequency for the same subject. The clinical implications of this are discussed. No optimum frequency for stimulating maximum vergence was found.