Abstract
Following superior parietal lobule and intraparietal sulcus (SPL-IPS) damage, optic ataxia patients underestimate the distance of objects in the ataxic visual field such that they produce hypometric pointing errors. The metrics of these pointing errors relative to visual target eccentricity fit the cortical magnification of central vision. The SPL-IPS would therefore implement an active “peripheral magnification” to match the real metrics of the environment for accurate action. We further hypothesized that this active compensation of the central magnification by the SPL-IPS contributes to actual object’ size perception in peripheral vision. Three optic ataxia patients and 10 age-matched controls were assessed in comparing the thickness of two rectangles flashed simultaneously, one in central and another in peripheral vision. The bilateral optic ataxia patient exhibited exaggerated underestimation bias and uncertainty compared to the control group in both visual fields. The two unilateral optic ataxia patients exhibited a pathological asymmetry between visual fields: size perception performance was affected in their contralesional peripheral visual field compared to their healthy side. These results demonstrate that the SPL-IPS contributes to accurate size perception in peripheral vision.
Highlights
The perceptual size of an object varies depending on where it is located in the field of view, in depth and depending on visual target eccentricity
Several other neural mechanisms come into play in the perception of size: the increase in size of the receptive field in peripheral vision (Nakahara et al, 2006) and the greater number of photoreceptors in the macula compared to the peripheral retina causing an overrepresentation of central vision in the visual cortex, called central magnification Daniel and Whitteridge, 1961)
Several studies have highlighted that areas located in the dorsal posterior parietal cortex in monkeys (Galletti et al, 1999) and humans (Fattori, 2009; Pitzalis et al, 2015; Rossit et al, 2013) are an exception: in this region, the representation of peripheral visual field is almost restored relative to the central visual field
Summary
The perceptual size of an object varies depending on where it is located in the field of view, in depth and depending on visual target eccentricity (horizontal distance relative to gaze). Several other neural mechanisms come into play in the perception of size: the increase in size of the receptive field in peripheral vision (Nakahara et al, 2006) and the greater number of photoreceptors in the macula compared to the peripheral retina causing an overrepresentation of central vision in the visual cortex, called central magnification Daniel and Whitteridge, 1961). This distortion of the representation of visual space is found in almost all structures of the visual cortex (Marino et al, 2008; Qiu et al, 2006). This is a region important for visuo-motor integration (Perenin et al, 1988)
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