The Yin and Yang of Visual Streams – Part 2

Many thanks to Dr. Curt Baxstrom, who commented on Part 1.  I had no intent of doing a Part 2 but a PDF he sent  of a recent review article on the subject was too good not to share and comment on.  The paper comes out of the Unit on Learning and Plasticity in the Laboratory of Brain and Cognition at the National Institutes of Mental Health (NIMH).

The stated goal of the Unit on Learning and Plasticity is to elucidate how the structure, function and selectivity of the cortex change with experience or impairment, even in adulthood. We use functional brain imaging (fMRI and MEG) and behavioral (eye tracking, psychophysics) techniques to investigate two main areas of interest.

You can access the article, A New Neural Framework for Visuospatial Processing, through the NIMH website of one of its authors, Dr. Chris BakerHere is the first of several key figures.

Figure  “a” above shows the original formulation of the dorsal and ventral visual streams by Ungerleider & Mishkin (their work was done on macaque monkeys). The ventral stream is a multisynaptic pathway projecting from the striate cortex (area OC) to area TE in the inferior temporal cortex, with a further projection from area TE to ventral prefrontal region FDv. The dorsal stream is a multisynaptic pathway projecting from striate cortex to area PG in the inferior parietal lobule, with a further projection from area PG to dorsolateral prefrontal region FDD. On the basis of the effects of lesions in monkeys, the ventral stream was termed a ‘What’ pathway supporting object vision, whereas the dorsal stream was labelled a ‘Where’ pathway supporting spatial vision.

As I pointed out previously in our blog here, Dr.  Bob Sanet and I drew comparisons between Skeffington’s Identification and Centering (I & C) concept with the What and Where pathways.  In this sense Skeffington was well ahead of his classical vision contemporaries who were trying to relate I & C to Accommodation and Vergence, when clearly he was linking these terms to what ultimately would become the What and Where Pathways.  Regrettably there was a disconnect between Skeffingtonian terms and what was being taught in most Colleges of Optometry rooted in classical vision science research, but he hit on two crucial points:

1) The multimodal nature of visual processing (which is why he inserted other circles in his model on anti-gravity & speech-language, onto which Merrill Bowan has grafted other multimodal factors) and

2) The notion of vision as an “emergent” process, now in vogue again as a term that denotes the complexity of self-organizing systems.  This emergence flies in the face of reductionism that, to this day, has led to a false sense of security that “vision is fine” when doctors inappropriately equate patency of structure with normalcy of function.

Pardon the digression …. looks like we’ll have to create a Part 3 to finish!

– Leonard J. Press, O.D., FCOVD, FAAO

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