An investigation into the contribution of receptive fields in the visual cortex to altered perimetric spatial summation in glaucoma

Classic literature on spatial summation suggests that the physiological basis for Ricco’s area lies in retinal ganglion cell [RGC] receptive fields. However, the finding of an enlarged Ricco’s area in glaucoma challenges this notion, as histological studies have found that RGC dendritic trees shrink before death in glaucoma, which should correspond to receptive field size shrinkage, rather than enlargement. Evidence has suggested a cortical contribution to determining the size of Ricco’s area, rather than a solely retinal basis. Pan & Swanson (2006) found that perimetric spatial summation could only be accounted for when considering cortical pooling by multiple spatial mechanisms. If an enlargement of cortical receptive fields is found in glaucoma, and if this is related to Ricco’s area measurements, this might partly explain the basis for the increase in spatial summation in the condition. The current thesis therefore aims to investigate how receptive field sizes at the retina and cortex contribute to Ricco’s area. Pattern Electroretinography and functional MRI population receptive field [pRF] mapping were utilised to test for differences in retinal and cortical receptive field size respectively, in a sample of glaucoma patients and age-similar controls. While there was no apparent enlargement of pRFs in glaucoma compared to controls, patients did demonstrate a significantly steeper relationship between pRF size and eccentricity in V1d. However, evidence for a cortical contribution to Ricco’s area was not found in these data, despite replicating an enlargement of Ricco’s area. In addition, though glaucoma patients demonstrated evidence for larger retinal receptive field sizes, this was not significantly associated with Ricco’s area. Overall, these results do not support the initial hypothesis of both retinal and cortical contributions to Ricco’s area. However, future work with more specific cortical modelling is outlined for fully characterising the relationship between cortical receptive field sizes and Ricco’s area