Sensorimotor Dynamics and Two Visual Systems: Shades of Skeffington & Brock Part 1


I’ve just finished reading a very nice compilation published last year by Oxford University Press entitled Perception, Action, and Consciousness, subtitled Sensorimotor Dynamics and Two Visual Systems.  Simply put, the two visual systems represent the view of two parallel but largely independent streams of visual information, first introduced by Ungerleider and Mishkin and elaborated by Goodale and Milner as the ventral stream, also known as the “what” pathway traveling to the temporal lobe involved with object identification deriving its main input from parvocellular sources, and the dorsal stream or  “where pathway” traveling to the parietal lobe involved with spatial location deriving its main input from magnocellular sources.  Milner and Goodale have a chapter in this book in which they address cortical visual systems for perception and action.  They suggest that vision for perception is subserved by processing in the ventral stream as distinct from vision for action thought to be subserved by the dorsal stream.  This dual system hypotheses is quite old, with its antecedents dating back at least to the 1960s when Trevarthen published about the parallel processes of ambient and focal vision, nicely summarized here, and further elaborated by our optometric colleague, Dr. William Padula.

Milner and Goodale are careful to point out that there is no such thing as a pure visuomtor task nor a pure perceptual task, and that under most normal circumstances our actions are visually co-determined by complementary processing in both dorsal and ventral streams.  Of the many gems in this book, my favorite is chapter 13 by Alva Noe, titled Vision without representation, and you can gain a feel for Noe’s philosophy in this interview he did with Salon magazine.  Noe’s premise is what he refers to as Actionism, that perceptual consciousness depends on the perceivers’ practical grasp of the significance of movement and action for perceptual experience.  In other words, movement produces sensory change.  Actionism is a sensorimotor view, and another way of supporting the fact that although we parse vision into “visual efficiency” and “visual information processing” for the sake of evaluative convenience, in the real world our visual perceptual system is driven by the recursive integration of action and perception.

As if we needed another remidner, Noe’s chapter reminds us how much Skeffington, considered to be the father of behavioral optometry and the driving force behind the Optometric Extension Program, anticipated interdisciplinary research in visual cognitive neuroscience.  Bob Sanet and I touched on this in our chapter on Spatial Vision in a marvelous new book on Vision Rehabilitation co-edited by Penelope Suter and Lisa Harvey.  Skeffington’s Four Circle Model of Vision is well known to every behavioral optometrist, Centering, Identification, Anti-gravity and Language.

In selecting his terms, Skeffington referred to “Centering” and “Identification”, two terms entirely foreign to classical (and some would say “mainstream”) Optometry.  The closest parallel terms seemed to be vergence and accommodation, but these didn’t imply the same thing.  What Skeffington was really alluding to was the interplay between ambient and focal processes, or more appropos to Noe’s chapter, the ventral stream and dorsal streams.  Most importantly, he anticipated the interplay between the eyes, brain and body to generate the basic what, where and how of visual guidance.  Other optometrists such as Bowan and Harris have touched upon iterations of the constituent circles that formulate vision as an emergent process.

Stay with me, because this quote from Noe’s chapter hits the nail on the head:  “A suggestion made by Jannerod and endorsed by Campbell, and I think by Goodale and Milner too, is that what the ventral system needs to do is to tell the dorsal stream the location of the object on which it is to act.  The ventral stream passes along the object’s coordinates to the visuomotor system.  The dorsal stream doesn’t need to be sensitive to the object itself, merely to its location.”

This really crystallized something that Bob Sanet and I spent hours working on.  It was a quote attributed to the optometrist Larry MacDonald which said: “Eyes don’t tell brains what to see; brains tell eyes what to look for. ”  As you read Noe’s chapter you’ll realize that his model inserts behavioral optometry squarely into the equation.  Actionism is motor, and Language is how we conceptualize what we see.  Noe writes:  “The picture theory of visual experience doesn’t seem to be able to do justice to the role that experience actually needs to play for us, that of selecting targets which we can then grasp, or pick up, or identify, or about which we can think and talk.”

As elegant as Skeffington’s Circles are, they do not fully capture the multi-dimensionality of space.  In Chapter 9 of Perception, Action, and Consciousness, Susanna Schellenberg nicely summarizes this in a section titled Space and the capacity to act.  A theme recurrent throughout the book is that vision is motor not in the sense that vision is equated with motor action, but that it is infused with motor knowledge that helps us predict the consequences of our actions.

Schellenberg reviews this is in the context of egocentric vs. allocentric frames of reference.  Allocentric is a frame of reference centered on points in space distinct from the one that the perceiver is occupying.  The egocentric frame of reference for movement is determined by the direction of intended movement linked to corresponding body parts involved.  Schellenberg continues:  “This might lead one to believe that the frame of reference of visual perception is centered on the eyes, but this cannot be right.  The position of one’s body in relation to the perceived object is at least as important as the position of one’s eyes.  When we turn our head to the left we do not perceive the objects to the left of our body as in front of ourselves.”

It is in this context that Brock, another optometrist, made a significant contribution to the importance of visual frames of reference.  Sue Barry has put up Brock’s Lecture Notes on Strabismus on her website, a very insightful set of notes regarding the training of binocular vision is space.  Although Brock contributed marvelous measurement tools (Brock Posture Board) and therapy equipment (Brock Stereo Motivator), it is one of his simplest tools, the Brock String for physiological diplopia testing and therapy, for which he is best known.  As you’ll notice here, the standard Brock String consists of three beads.  Without knowing the answer for sure, I’m going to suggest that Brock picked the number because he intuitively grasped something that has become commonplace in neuropscyhology, which is the distinction between three crucial and distinct regions of space:  personal, peripersonal, and extrapersonal space.  This theme runs through several chapters in Perception, Action, and Consciousness.  You can also get glimpses of it in a nice online review course from Pacific University College of Optometry, in an article by Suchoff and Ciuffreda, and in a review article by Massucci.

In short, as applied to the visual system, personal space is the space within one’s body schema.  Peripersonal space is outside one’s immediate body schema or egocenter, but within the range of touch.  Extrapersonal space is beyond the range of interaction with one’s body, but within the distance of visual capture.  How can I establish a frame of reference that would give me a continuum through all three regions of space simultaneously?  One way would be to place cord at the tip of my nose that has three distinct regions of space extending to a point well beyond my body.  Voila!  The Brock String.

- Leonard J. Press, O.D., FCOVD, FAAO

4 thoughts on “Sensorimotor Dynamics and Two Visual Systems: Shades of Skeffington & Brock Part 1

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