The case of DF (visual form agnosia)

ResearchBlogging.org Thanks to Mind Hacks, I found out that my supervisor, Mel Goodale, was featured on ABC Radio National's "All in the Mind" series. In the interview (which you can listen to by clicking here), he talks about a patient named DF, whose unique brain damage (i.e, selective bilateral lesions in the lateral occipital complex due to an episode of hypoxia) resulted in the disruption of her ability to consciously identify objects on the basis of their shape or orientation. In other words, her "vision for perception" was compromised. The fascinating thing is that her "vision for action" was spared. In other words, she can't consciously "see" the shape of objects, but she can interact with objects on the basis of visual information about their shape.

* A view of DF's brain damage (taken from James et al, 2003).

A number of papers have explored the behavioral consequences of DF's pathology. In one of these papers, Goodale and Humphrey (1998) presented DF with a slot that could be rotated and set at various orientations. For the first task, they gave her a card and asked her to match the orientation of the slot by rotating the card in her hand. As illustrated in the figure below, DF was unable to match the orientation of the slot on the basis of her perception of it. For the second task, they simply asked DF to post the card into the slot. Her performance was virtually indistinguishable from that of a healthy control.

* Results are normalized to upright orientation to show deviation from a successful performance.

In the same paper, Goodale and Humphrey report the results of another task, in which DF was asked to pick up flat, non-symmetrical objects. Healthy controls typically accomplish this task by choosing stable grasp points (i.e. opposing vectors on parts of the object with high curvature) for the thumb and index finger, with the object's center of mass laying roughly between the two points. Despite the fact that DF is unable to distinguish between these objects, she is perfect at picking them up in an appropriate way (see figure below). Compare her performance with that of patient RV, who suffers from optic ataxia (caused by bilateral lesions of the occipitoparietal region). People with optic ataxia have preserved vision for perception, but their vision for action is compromised in some way. Thus, while RV is able to distinguish between the objects on the basis of vision, she cannot use that information to guide her grasping movements in an appropriate way. It is important to note that RV doesn't simply suffer from a motor impairment. With her eyes closed, she can successfully reach out and touch locations on her body or pick up objects at remembered locations in her peri-personal space. Her impairment is one of online control of visually guided movements.

* The lines connect the two opposing grasp points used by DF, RV, and a control subject.

The case of DF, when considered along with the case of RV, highlights a double dissociation between vision for perception and vision for action. While DF can accurately guide her hand to objects whose shapes aren't consciously available to her, RV cannot accurately guide her hand to objects on the basis of their shape, even though the shapes of these objects are consciously available to her. This dissociation can be demonstrated in healthy subjects by taking advantage of the fact that the ventral visual processing stream ("vision for perception") is fooled by certain optical illusions, while the dorsal visual processing stream ("vision for action") seems impervious to them. Right now, the best explanation for this difference is that the ventral stream uses allocentric coding (i.e. it deals with spatial relationships between objects in the visual field), while the dorsal stream uses egocentric coding (i.e. it deals with spatial relationships between the viewer's body and target objects in the visual field).

I think it's safe to say that my first exposure to the story of DF marks the beginning of my fascination with the brain.

Works Cited

James, T.W. (2003). Ventral occipital lesions impair object recognition but not object-directed grasping: an fMRI study. Brain, 126(11), 2463-2475. DOI: 10.1093/brain/awg248

Goodale, M.A., Humphrey, G.K. (1998). The objects of action and perception. Cognition, 67(1-2), 181-207. DOI: doi:10.1016/S0010-0277(98)00017-1

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