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A single case study of a patient with 'visual form agnosia' is presented. A severe visual recognition deficit was accompanied by impairments in discriminating shape, reflectance, and orientation, although visual acuity and colour vision, along with tactile recognition and intelligence, were largely preserved. Neuropsychological and behavioural investigations have indicated that the patient is able to utilize visual pattern information surprisingly well for the control of hand movements during reaching, and can even read many whole words, despite being unable to make simple discriminative judgements of shape or orientation. She seems to have no awareness of shape primitives through Gestalt grouping by similarity, continuity or symmetry. It is proposed that many of these perceptual disorders might be the combined result of (1) a selective loss of the cortical elaboration of the magnocellular visual processing stream, and (2) a selective output disconnection from a central processor of visual boundaries and shape primitives in the occipital cortex.

We have examined the dependence of rotational acuity on the orientation bandwidth of a stimulus using two-dimensional, band-pass filtered, spatial noise. Stimuli had a bandwidth of 0.5 octave of spatial frequency, centred at 5.0 cyc/deg, and an orientation bandwidth that covered the range from 0.0 to 25.0 deg. Thresholds were obtained on one principal (vertical), and one oblique axis (45 deg). It was found that acuity declined on both axes as bandwidth increased, in a manner that was compatible with simple statistical principles with virtually perfect sampling of the image. There was some evidence that the intrinsic noise is greater on the oblique axis than on the vertical, and that oblique axes are less densely sampled than the principal axes. These differences are small and are insufficient, either on their own or taken together to explain the oblique effect.

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