John Siderov

A particular focus of my research is to investigate and understand the problem of visual crowding, where the recognition of letter targets (as indeed other objects in visual space) is disrupted by the presence of nearby contours. Our work has attracted international interest. We have demonstrated that in central vision (i.e. foveal) for near threshold stimuli, crowding operates over a fixed spatial distance irrespective of target size, which contradicts a popular explanation of crowding (Siderov J., Waugh S.J. & Bedell H.E. Foveal contour interaction for low contrast acuity targets. Vision Research, 2013; 77: 10-13 and Siderov J., Waugh S.J. & Bedell H.E. Foveal contour interaction on the edge: Response to Drs Coates and Levi. Vision Research 2014; 96:145-148). We were the first to show that the intensity of foveal crowding reduces under low levels of luminance (mesopic) (Bedell H.E., Siderov J., Waugh S.J., Zemanova R., Pluhacek F., & Musilova L. Contour interaction for foveal acuity targets at different luminance. Vision Research, 2013; 89:90-95) but not under total darkness (scotopic luminance) (Musilova L., Pluhacek F., Marten-Ellis S.M., Bedell H.E., & Siderov J. Contour interaction under photopic and scotopic conditions. Journal of Vision, 2018; 18(5): 1-11). This finding has implications when assessing vision clinically under low luminances (Pluhacek F. & Siderov J. Mesopic visual acuity is less crowded. Graefe’s Archive for Clinical and Experimental Ophthalmology, 2018. /doi.org/10.1007/s00417-018-4017-6).

We have confirmed experimentally early suggestions that letter recognition in children and in patients with amblyopia is crowded through a combination of low level spatial interactions (contour interaction) in addition to higher level influences through putative attentive mechanisms or possibly factors relating to eye movements (Bedell H.E., Siderov J., Formankiewicz, M.A., Waugh S.J., & Aydin S. Evidence for an eye- movement contribution to normal foveal crowding. Optometry and Vision Science 2015; 92: 237-245). An additional finding suggests a different time course for the development of visual acuity in children based on how crowding develops, which has not previously been reported (Norgett Y. & Siderov J. Effect of test chart configuration on crowding in strabismic amblyopia. Journal of Vision. 2017; 17: 1-14; Norgett Y. & Siderov J. Foveal crowding differs in children and adults. Journal of Vision, 2014; 14: 1-10 and Norgett Y. & Siderov J. Crowding in children’s visual acuity tests – effect of test design and age. Optometry and Vision Science, 2011; 88(8): 920-927).