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Abstract:
Humans are very good at detecting animals in briefly flashed photographs of natural scenes, both in central [Thorpe et al, 1996 Nature (London) 381 520 - 522)] and in parafoveal vision [Fabre-Thorpe et al, 1998, in Computational Neuroscience: Trends in Research Eds J Bower (New York: Plenum Press) pp 7 - 12]. To test how far this ability extends into the periphery, we tested, ten human subjects in a 180 deg panoramic viewing theatre. 1400 highly varied photographs (37.5 deg high by 25 deg wide) were flashed for 28 ms, and subjects were asked to release a button if the image included an animal. Image position varied randomly from trial to trial with nine possible positions covering the entire horizontal extent of the visual field. Performance was remarkably good and decreased linearly with eccentricity from 93.3 for central presentations, to 60.4 for images centred at 75 deg, a remarkable result given the very low ganglion cell densities so far in the periphery. Note that this level of performance was only possible if the subjects were made to guess--most subjects were totally unaware of what had been presented in the far periphery. The results imply that rapid, automatic, and largely unconscious processing may be far more sophisticated that has been thought in the past.
