Motion processing

There is convincing evidence from cognitive neuroscience that area V5 is involved in motion processsing. For example, Anderson et al. (1996) used magneto-encephalography (MEG) and MRI (see Chapter 1) to assess brain activity in response to motion stimuli. They reported that "human V5 is located near the occipito-temporal border in a minor sulcus [groove] immediately below the superior temporal sulcus" (Anderson et al., 1996, p. 428). This finding was consistent with previous findings using other techniques. For example, the special involvement of V5 in motion processing has been found in PET studies (e.g., Zeki et al., 1991) and in studies using functional MRI (e.g., Tootell et al., 1995a).

There is additional evidence about the importance of area V5 in motion processing in studies on braindamaged patients suffering from akinetopsia. In this condition, stationary objects can generally be perceived fairly normally but objects in motion become invisible. Zihl, von Cramon, and Mai (1983) studied LM, a woman with akinetopsia who had suffered brain damage in both hemispheres. Shipp et al. (1994) used a high-resolution MRI scan to show that LM has bilateral damage to V5. She was good at locating stationary objects by sight, she had good colour discrimination, and her binocular visual functions (e.g., stereoscopic depth perception) were normal, but her motion perception was grossly deficient. According to Zihl et al. (1983):

She had difficulty .in pouring tea or coffee into a cup because the fluid appeared to be frozen, like a glacier. In addition, she could not stop pouring at the right time since she was unable to perceive the movement in the cup (or a pot) when the fluid rose.In a room where more than two people were walking she felt very insecure .because "people were suddenly here or there but I have not seen them moving".

LM's condition did not improve over time. However, she developed various ways of trying to cope with her lack of motion perception. For example, she stopped looking at people talking to her, because she found it disturbing that their lips did not seem to move (Zihl et al., 1991).

Striking evidence of the involvement of V5 in motion perception was reported by Beckers and Zeki (1995). They used transcranial magnetic stimulation to produce temporary inactivation of V5. The result was what appeared to be complete akinetopsia.

Van Essen and Gallant (1994) argued that V5 in primates seems to consist of two subdivisions. One subdivision is concerned with the motion of objects, and the other deals with the effects of our own movement through the environment. Neurons in the latter area are responsive to changes in the retinal size of objects. They are also responsive to the rotation of the retinal image of an object, as would occur when we tilt our head.

Zeki (1993) argued that area V3 is involved in processing dynamic form and in obtaining three-dimensional structure from motion. Evidence supporting this was obtained by de Jong et al. (1994). They presented moving dots that either simulated the forward motion of an observer over flat ground or moved in a random way. PET scans revealed that V3 was much more active in the former condition.

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