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Saccadic eye movements are traditionally cited as an especially successful combination of accuracy and velocity, such high level of performances being believed to be crucial for optimal vision. Although the structures subtending these properties are now well recognized, very little is known about the functional consequences on visually guided behaviors of reduced saccade performances, i.e., slowness and/or inaccuracy. We therefore investigated the impact of such impairments in patients with spino-cerebellar and Friedreich ataxia, i.e., diseases known to affect both saccade parameters. Subjects performed a classical eye movement task, in order to quantify saccade inaccuracy and/or slowness, a visually search task and a reading task and completed a questionnaire designed to evaluate their perceived visual discomfort in daily activities. The first main result was that saccade impairments did have an impact on visually guided behaviors, resulting in an increased time for target detection, especially when accurate foveation was needed, and in an increased reading time. The main responsible oculomotor factor was increased variability of saccade accuracy, and the least responsible factor was reduced saccade velocity. The second main result was that saccade disorders did not induce significant subjective discomfort, since no correlations were found between the results of the questionnaire and saccade parameters. These results emphasize the functional impact of increased variable error of saccade accuracy and question the rationale of high saccade velocities. The discrepancy between objective and subjective measures underlines the largely unconscious aspect of saccade control and leads us to consider the need for an adapted therapy.

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