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David P. Carey, Ceri T. Treventhan, Arash Sahraie; Revisiting manual localisation in the cortically blind field. Journal of Vision 2005;5(8):286. doi: 10.1167/5.8.286.
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© ARVO (1962-2015); The Authors (2016-present)
Background: Sensorimotor localisation in blindsight patients has been linked with residual activity in the occipito-parietal cortex. Despite the theoretical relevance of this claim, very few systematic group studies have been performed. Purpose: To investigate the incidence of and inter-relationships between manual localisation and other residual visual behaviours in a group of cortically-blind patients. Methods: Detailed perimetry was performed in the same testing session. Manual localisation was assessed using a touchscreen, and stimuli were generated using a SVGA graphics card (Cambridge Research Systems, UK) and presented on a 21″ CRT monitor at 37cm viewing distance. Appropriate adjustments were made to account for parallax errors. To minimise possible light scatter, the stimulus consisted of two adjacent semi-circular discs with opposite contrasts, and average luminance the same as the background. These were presented for 100ms. The fixation was monitored using a video eyetracker. Using a commentary-key paradigm, awareness of location judgment was recorded on a binary scale for each trial. Results: Although some residual localisation ability for some targets was suggested, considered analysis implies strategic behaviour on the patients' part. As some awareness for one or more of the stimuli was obtained, patients could reach to the restricted number of other target locations when no awareness was experienced. Conclusions: Data to date suggest caution should be applied when claiming residual manual localisation in the cortically blind. Nevertheless, the combination of eye-movement monitoring, controlled stimuli which minimise light scatter as a cue, simultaneous spatial awareness judgments and detailed analysis of the location and the extent of the brain damage will allow for detailed evaluation of the dorsal stream localisation hypothesis for the first time.
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