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PubMed1995

The relationship of choroidal blood flow and accommodation to the control of ocular growth.

We have carried out a number of different studies in chicks to examine the relationship between choroidal blood flow and myopic eye growth, and between accommodation and myopic eye growth. Our studies on choroidal blood flow show that myopic eye growth produced by form vision degradation leads to dramatic reductions in choroidal blood flow. These reductions appear directly attributable to the eye enlargement and the reduction in choroidal blood flow does not appear to be permissive for eye growth, since experimentally reduced choroidal blood flow hinders eye growth. Choroidal blood flow that is slightly above normal, however, may slightly enhance eye growth. Our studies on accommodation do not reveal any major necessary role of accommodation in regulating normal growth or in form vision degradation induced myopic eye growth. We found preliminary evidence, however, that chronically stimulating accommodation over a 2 week period, thereby producing excessive time in accommodation, may be sufficient for yielding a small but significant degree of myopic refractive error. Our studies suggest that neither fluctuations in choroidal blood flow nor an intact accommodative apparatus are essential for normal eye growth or myopic eye growth produced by form deprivation. Further studies are needed to confirm that excessive time in accommodation might be sufficient for producing myopia. Finally, our finding that choroidal blood flow is substantially reduced in myopic eyes may have implications for the etiology of the retinal problems suffered by humans with moderate to severe myopia.

Accommodation, Ocular↗
PubMed1992

Characteristics of near response cells projecting to the oculomotor nucleus.

1. Previous work has shown neurons just dorsal and lateral to the oculomotor nucleus that increase their firing rate with increases in the angle of ocular convergence. It has been suggested that the output of these midbrain near response cells might provide the vergence command needed by the medial rectus motoneurons. However, lens accommodation ordinarily accompanies convergence, and a subsequent study showed that only about one-half of these midbrain near response cells carried a signal related exclusively to vergence. One hypothesis suggested by this finding is that this subgroup of neurons might have a unique role in providing a "pure" vergence signal to the medial rectus motoneurons. 2. In the present study extracellular recordings were made from midbrain near response cells in monkeys while eye position and lens accommodation were measured. The monkeys viewed targets through an optical system that allowed the accommodative and ocular vergence demands to be manipulated independently. This approach was used to produce a partial dissociation of accommodative and vergence responses, so that an accommodative and vergence coefficient could be determined for each cell, by the use of the following equation FR = R0 + kda x AR + kdv x CR where FR is the firing rate of the near response cell, R0 is the predicted firing rate for a distant target, kda is the (dissociated) accommodation coefficient, AR is the accommodative response, kdv is the (dissociated) vergence coefficient, and CR is the convergence response. 3. The vergence and accommodation coefficients were determined for a large number of midbrain near response cells, including a subset that could be antidromically activated from the medial rectus subdivisions of the oculomotor nucleus. Some near response neurons were found with signals related exclusively to convergence (i.e., kdv greater than 0 and kda = 0), whereas several others had signals related exclusively to lens accommodation (i.e., kda greater than 0 and kdv = 0). The majority of the near response cells had signals related to both responses (i.e., kda not equal to 0 and kdv not equal to 0). Furthermore, the vergence and accommodation coefficients of near response cells appeared to be continuously distributed. Some cells had negative accommodation or vergence coefficients. 4. The 17 near response cells that could be antidromically activated from the oculomotor nucleus presumably provide vergence signals to the medial rectus motoneurons. Although all had positive vergence coefficients, only four of these cells carried signals that were related exclusively to vergence.(ABSTRACT TRUNCATED AT 400 WORDS)

Accommodation, Ocular↗