2 Visual Systems Revisited

David Ingle

Abstract:
In 1967 4 of us co-authored the “2 visual system” hypothesis: identifying vs. localizing systems. I stressed that both visual modes must be subcortical in lower vertebrates. Held’s contrasting functions were both cortical, without anatomical underpinning. Ungerleider & Mishkin (1982) added anatomical data to crystalize established clinical concepts of temporal vs. parietal lobe ‘perceptual’ functions. Goodale & Milner (1995) advanced the 2VS concept by showing that parietal lobe functions include a primitive kind of “object vision” used for grasping of objects. I take issue with their attempt to strip parietal cortex of “perceptual” functions, using both clinical & scan evidence.

My 1967 paper focussed on “size-dependent” coding of shape discriminations by fish. I soon presented other evidence for spatial mechanisms underlying object discrimination in fish and then showed that humans show some “shape invariance” during hand-shaping. This discovery anticipated the Goodale-Milner formulation by 10 years.

In monkey, temporal lobe is not necessary for discrimination of mirror-image shapes (a spatial task in my view) so that one must ask whether parietal cortex plays a critical role features spatial location. Data from both lesion studies & brain scans in humans show that processes related to perceptual rotation of shapes are linked to parietal areas.

I add a new source of information which distinguishes spatial & feature-representation in man. My own “visual persistences” are vivid afterimages which remain fixed in body-centered coordinates (following eye-closure) as I can translate these images throughout the field or exchange them from hand to hand. However, I find that larger configurations rotate with the hand, but smaller features do not. This form/space dissociation parallels observations of both Richard Held & Ivo Kohler: prismatic “adaptation” applies to the localizing mode of vision but does not alter percepts based on local features.

The ability to move detailed visual images up to 180 deg. within the frontal field implies tight collaboration of identification & spatial systems. While I’m not aware of evidence for direct coordination between T & P regions, both areas project to prefrontal cortex, where a higher level synthesis of the 2 VS’s could occur. A top-down influence of PFC or premotor regions on parietal cortex may account for my motor-to-visual phenomena.

Furthermore the developmental construction of space may depend on the role of PFC & premotor region in guiding eye & hand movements.

The lecture will take place:

in Room 203, 44 Cummington St.
on Thursday, April 18, 2002
at 12:15 pm