Agnosia And Action

Previously we discussed patient DF, who developed a severe form of apperceptive agnosia following carbon monoxide-induced anoxia. Although DF's visual system is unable to use shape information to make perceptual judgments and discriminations about an object, she is able to use the same information to accurately guide her prehensile movements to those same targets. For example, even though DF is unable to discriminate solid blocks of differing dimensions, she accurately scales her grasp when picking up the blocks, opening her hand wider for larger blocks than she does for smaller ones, just as people with normal vision do. DF also rotates her hand and wrist appropriately when reaching out to objects in different orientations, despite being unable to describe or distinguish the size, orientation, and shape of the different objects. It appears that although the perceptual mechanisms in DF's damaged ventral stream can no longer deliver any perceptual information about the size, orientation, and shape of objects she is viewing, the visuomotor mechanisms in her intact dorsal stream that control the programming and execution of visually guided movements remain sensitive to these same object features.

Although the discussion of the dorsal and ventral streams in this article has emphasized their separate roles, there are many connections between the two streams and thus the opportunity for "cross-talk." Recent investigations have shed light on the role that the communication between these two streams plays in object recognition. In some cases of associative agnosia, it has been reported that the ability to identify actions and to recall gestures appropriate to objects could play a significant role in preserving recognition of certain objects. Sirigu suggested that sensorimotor experiences may have a critical role in processing information about certain objects. It has been reported that the object categories that individuals with associative agnosia have difficulty reporting are those that they could not recall their action. The objects that they do not recognize would thus appear to be those that they do not associate with their sensorimotor experiences. The objects that they do recognize may be those whose action plays a critical part. This could help explain the "living" versus "nonliving" dissociation seen in CSVA. Action is certainly an important element for knowing tools, kitchen utensils, and clothes. In contrast, most animals do not evoke any gestures, and the only action linked with most fruits and vegetables is a simple gripping. It also appears that the recognition of action is well preserved in these individuals. The impairments in recognizing static objects perceived visually in associative agnosia sharply contrast with the relatively better ability to recognize objects from gestures illustrating their use and to recognize actions shown in line drawings.

It appears that the dorsal stream not only provides action-relevant information about the structural characteristics and orientation of objects but also is involved in the recognition of actions and in the recognition of objects when sensorimotor experience is evoked. This suggests that the dorsal pathway is involved in conscious visual perception and in the interpretation of goal-oriented action, even when shown in a static way. It is possible that when ventral stream damage in agnosia prevents direct access to representations of an object for perception, sensorimotor information from the dorsal stream may provide a limited mechanism for recognition. In other words, semantic information about objects may be accessed by the dorsal stream and passed onto the ventral stream for recognition. The preservation of how to manipulate an object may play a crucial part in assisting object recognition in patients with associative agnosia.

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