The dissociability of accommodation from vergence in the dark

A special case was set up in which accommodation and vergence were totally deprived of their stimuli, in order to determine whether their responses remained coordinated or whether they became dissociated. A dynamic, infrared optometer and infrared, eye-movement sensors were used simultaneously to monitor accommodation and vergence respectively in the dark. Responses were recorded continuously over an extended time course, and the two systems were found to behave essentially independently of each other in darkness. However, a statistical cross-correlation analysis revealed that, despite the readily apparent dissociation of the two responses in the time domain, there was a correlation between them in the frequency domain. The time-domain dissociation suggests that the tonic controllers of both motor systems are positioned downstream from the crosslinks that join them (accommodative vergence and vergence accommodation). The frequency-domain correlation suggests that the crosslinks are not completely dormant in the dark, but that they carry low-amplitude noise, at a more or less fixed temporal frequency, from one motor system to the other.