Development of the primate visual cortex is reliant on a pulvinar input to area MT which bypasses V1

Early maturation of the dorsal stream-associated middle temporal (MT) area before any other visual association area suggests that the development of area MT may involve its direct connectivity with dorsal visual thalamic nuclei, rather than its connectivity with the primary visual cortex (V1). Despite this the thalamic nuclei that may be involved remained unclear. Possible candidates that fit the requirements of direct retinal input and project to area MT, include the medial portion of the inferior pulvinar (PIm) and the koniocellular layers of the lateral geniculate nucleus (LGN). With this in mind, this thesis investigated alternative disynaptic pathways from the retina to the visual cortex, bypassing V1, that are postulated to have a significant role in the early maturation of area MT and be responsible for the residual vision observed following a lesion of V1, especially during early postnatal life. Retinothalamocortical connectivity between PIm, LGN and area MT was investigated using a combination of intraocular anterograde tracers and intra-MT retrograde tracer injections in adult marmoset monkeys and at different time-points during normal postnatal development. In addition to neuroanatomical tracing, these pathways were further investigated using diffusion weighted imaging after longstanding partial V1 lesions in neonates and adults. In adult animals the presence of direct synapses of retinal afferents on area MT relay cells within the PIm, as well as the koniocellular layers of the LGN, was established. Thereby corroborating the existence of two disynaptic pathways from the retina to area MT that bypass V1. Whereas in the adult a small number of PIm cells were recipient of retinal projections, younger animals received increased ocular labelling in PIm. PIm was also found to have a greater contribution to area MT than either the direct projection from V1 or LGN soon after birth. Thereafter, the proportion of V1 input to area MT increased during development to become the dominant input while the proportion of LGN input remained small compared to that from either the PIm or V1. Following longstanding partial unilateral removal of V1, neonatal removal of V1 resulted in an increase in PIm to area MT connectivity and a sparing of retinal input to PIm compared with the nonablated adult control and adult V1 ablated animals. These results provide evidence that retinothalamocortical pathways to area MT exist in adult marmoset monkeys and are present early in life, prior to the maturation of all visual cortical areas and provide an alternate pathway that may be responsible for the early maturation of area MT. Lesions of primate V1 during infancy and in adulthood differentially alter the connectivity of area MT with visual thalamic nuclei such that the putative pathway involving the pulvinar and area MT may underpin the improved visual capacity observed following a lesion of V1 early in life (prior to the closing of the critical period) compared with adults.