Picking up where we left off in Part 2, the new neural framework for visuospatial processing realizes that the idea of splitting visual streams into two primary “When” and “Where” pathways, corresponding to Dorsal and Ventral streams, is too simplistic. More likely the visual system derives questions and answers like any good journalist, processing information about Who, What, When, Where, Why, and How.
Let’s go back to the figure we introduced in Part 2. Researchers had bandied about the idea of whether the dorsal stream is better depicted as a motoric “How” pathway for visually guided action rather or a perceptual “Where” pathway.
Part “c” of the figure shows that the upper or dorsal stream is not a single stream, but stems from an occipito-parietal circuit comprising at least three distinct pathways:
1) the parieto–prefrontal pathway, targeting the prefrontal cortex (shown by a dashed green arrow) supporting spatial working memory.
2) the parieto–premotor pathway, targeting the premotor cortex (shown by a dashed red arrow) supporting visually-guided actions.
3) the parieto–medial temporal pathway, targeting the medial temporal lobe, both directly and through the posterior cingulate and retrosplenial areas (shown by a dashed blue arrow), supporting navigation.
Now let’s talk structure and function for a moment. Structure first. The common origin of these three pathways is the occipito-parietal circuit which is typically hooked up like this. Portions of V1 (primary visual cortex) representing central as well as peripheral field project to area V6 (part of parieto-occipital area PO). V6 receives strong input from V2, V3, and V3A (see: Wish I coulda had a V8). V6 projects to bimodal V6A (visual & somatosensory), MIP (medial intraparietal), and VIP (ventral intraparietal), as well as LIP (lateral intreparietal), MT (middle temporal), and MST (medial superior temporal) areas. Yet as the authors emphasize, there is much co-mingling and interaction among these pathways.
As to function, the occipito-parietal circuit serving these three pathways appears to integrate information equally well between central and peripheral fields. It represents space largely through egocentric frames of reference. In the old days, our good buddy Dr. Iwrin Suchoff would refer to this as the invariant, in Piagetian terms. How do we establish this body image? Although initial visual representations are retinotopic, the occipito-parietal circuit transforms them into additional reference frames relative to parts of the body as well as the eye. As noted in the article, the posterior parietal region has been broadly implicated in the long-term memory and retrieval of spatial information both within and across modalities. Bilateral parietal damage in humans can lead to impairments in recall and imagination of events (particularly their spatial aspects) and to deficits in mental rotation.
So here’s where I’ll leave you with Press’s Paradox: How is it that two optometric vision therapy programs can look so different, yet achieve equally successful outcomes? Hint: It’s all in the streams; you just have to know where to go fishing, and how, and when.
– Leonard J. Press, O.D., FCOVD, FAAO