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John P. Wann, David T. Field, Mark A. Mon-Williams, David Milner; How would you catch a ball if you had visual form agnosia?. Journal of Vision 2005;5(8):290. doi: https://doi.org/10.1167/5.8.290.
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© ARVO (1962-2015); The Authors (2016-present)
Visual form agnosia offers a means for exploring the capabilities and, to some degree, the limitations of the human dorsal visual stream. Wann et al (2001) observed that a patient with visual form agnosia (DF) could modify appropriately her reach towards an object if there was a rapid change in its disparity-specified distance but in contrast to control participants a sudden change of optical size did not produce an equivalent modification in reach distance. This raises the question of how sensitive is DF to looming information that ordinarily signals object approach and time to collision (TTC)? We first established that DF has a reasonable ability to catch a ball that is thrown to her in a straightforward fashion. We next presented DF with a range of looming and changing size stimuli. DF was able to report verbally whether an object was approaching or receding when shown a simulation of a looming or contracting ball but was unable to make the same judgment when presented with an equivalent step-change in the size of static images (a task that appeared trivially easy to controls). When asked to hit a button when she gauged a looming ball would hit her, she graded her responses with changing TTC but seemed to rely upon optic size rather than the relative rate of dilation (Tau: Lee, 1976). We tested control participants (female, 48–56yrs) and found that they also failed to grade their responses in line with Tau. We will discuss how the skilled performer might extract a perceptual estimate equivalent to Tau from changing size, how this might degrade if high precision tasks are not practised and why this mechanism might fail in the case of visual form agnosia.
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