Role of monkey midbrain near-response neurons in phoria adaptation

1. One striking characteristic of the way in which accommodation and convergence of the eyes are organized is that although the two responses are usually tightly coupled, a brief period of binocular viewing through prisms that require extra convergence brings about a persistent, adaptive alteration in the relationship between the two responses: the vergence during monocular viewing of a target at a given distance is biased in a convergent direction. We sought to discover the role of the near-response neurons we have previously described in the monkey midbrain in such phoria adaptation. 2. Phoria adaptation was produced in two monkeys by having them view binocularly stereoscopic targets under conditions that mimicked prism viewing, i.e., the mirrors of the stereoscope were set so as to require more convergence than that associated with a real target at the same distance as the images seen in the stereoscope. The activity of 57 near-response neurons located dorsally and dorsolaterally to the oculomotor nucleus was recorded before and after adaptation while the monkeys monocularly viewed targets at a range of distances. 3. Comparison of a neuron's response in normal binocular viewing with the response when the accommodation and vergence stimuli were in conflict allowed us to distinguish 24 neurons that behaved as though they were related exclusively to the vergence response. 5 neurons that behaved as though they were exclusively related to the accommodation response, and 12 neurons whose firing was not so simply related to either response. We were unable to classify the remaining 16 near-response cells by this method. 4. In accommodation-related neurons, the relationship between firing rate and accommodation did not alter, or only altered slightly, when the animal's phoria was adapted. 5. The relationship between firing rate and vergence was unaltered by phoria adaptation in only a small proportion of vergence-related neurons, showing that most neurons do not carry the entire signal responsible for phoria adaptation. On the other hand, in the majority of vergence-related neurons the relationship between firing rate and accommodation was altered by phoria adaptation, showing that most neurons do carry part of the phoria adaptation signal. 6. The implication is that the increase in vergence observed after adaptation is mediated at more than one site. A proportion of the phoria adaptation signal is present at the level of the midbrain vergence-related neurons, with the remainder of the signal being added later, presumably at the motoneurons.