The centrifugal visual system (CVS) is a feedback pathway of predominantly visual information from the brain to both eyes, but principally to the contralateral retina. The CVS is often considered to be something of a peculiarity, regarded as being specific to birds (Aves). Indeed, so-called ‘higher’ vertebrate species are assumed not to even possess such a centrifugal pathway when, in fact, an efferent projection to the retina has been conclusively demonstrated in all vertebrate groups (including humans). Perhaps this point of view reflects the lack of progress made in the elucidation of function in the bird, the dominant model for CVS research in the 120 years since being first described. In the series of experiments presented here, I have begun to investigate the role of the CVS in the modulation of eye growth. In addition, I have addressed a number of unknowns that exist regarding the midbrain connectivity of the CVS. In a series of four parallel lesion experiments, the centrifugal efferent pathway to the retina was unilaterally disrupted in post hatch chicks, raised under different developmental conditions. Under normal visual conditions but in the absence of centrifugal efferents, eyes contralateral to the lesion developed shorter eyes and moderate, relative hyperopia (long-sightedness). In contrast, under constant light conditions, ipsilateral eyes became significantly shorter than fellow (i.e. contralateral) eyes. Compensation for, and recovery from, plus and minus lens-imposed defocus in contralateral eyes was largely unaffected. Centrifugal efferents emanate from two distinct midbrain populations: the isthmo-optic nucleus (ION) and the surrounding scattered cells within the ectopic area (EA). From experiments using pathway tracing techniques, I have demonstrated that, unlike cells of the ION, EA cells do not receive input from primary visual areas. In addition I present evidence for a possible ‘cross-talk’ pathway between centrifugal cells on either side of the midbrain, and discuss its potential involvement in the normally symmetrical eye growth.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:567443 |
| Date | January 2012 |
| Creators | Dillingham, Christopher Mark |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/41964/ |
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