Sensorimotor Dynamics and Two Visual Systems: Shades of Skeffington & Brock Part 2


In Part 1 we introduced Brock’s  “string theory”, setting up a relative versus absolute partitioning of three fundamental regions of space.  Let’s briefly review.  In the typical string arrangement, the far bead is placed at the end of the rope, which is approximately 10 feet.  The near bead is placed at the patient’s Harmon distance, which is the normal near work space.  The intermediate bead occupies a distance midway between the near bead and the far bead.  Set up this way, standard instructional set focuses on the patient maintaining the strings meeting at the bead.   Binocular abnormalities center on vergence insufficiency or excess (strings meeting in front or behind of where the patient visually aims) or suppression (absence of part or all of one of the ropes and/or one or more of the beads that should be seen in physiological diplopia).  These are absolute anomalies based on geometric regions of space in classical terms of vergence, or postural shifts in binocularity in behavioral terms.

An additional way to think about the Brock String is that it partitions relative regions of visual space.  A framework for the specification of functional spaces is laid out in Chapter 12 of the book we introduced in Part 1 – Perception, Action, and Consciousness, in what the authors refer to as a neuro-cognitive model for space boundaries.  Instead of thinking of the spacing of the three beads as representing maximum convergence, an intermediate level of convergence, and a minimal level of convergence, think of three relative zones or regions of space as depicted below.  This now positions difficulties with the Brock String in a different light, more within the framework of visual neglect or inattention.

Bottom line:  the Brock String is a task of divided binocular attention, requiring the patient to report awareness of one region of space while simultaneously fixated on another region of space.  For example, a patient may have difficulty with awareness of “normal” projection beyond the plane of the near bead not because she is having “divergence problems”, but because she is having relative neglect of peri-personal or extra-personal space.  In this context, suppression is a relative cortical neglect or inattention.  Instead of thinking of suppression of a region of the rope as active inhibition to avoid diplopia, think of the possibility (at least for some patients) that it is a form of binocular neglect/inattention to a region of space.

The degree to which patients exhibit neglect or inattention can vary depending on whether the tasks in which they are asked to engage primarily involve personal space, peri-personal space, or extra-personal space.  The same strategies that work to reduce inattention for spatial neglect can be used to gain desired responses on the Brock String.  These include the application of yoked prisms, and cross-modal procedures such as touch and balance.

At a very basic level, this can help explain why some patients must begin with just one bead, and work on locating the position of the bead that will help them partition two broad regions of visual space, whether we call it egocentric vs. allocentric, or personal space vs. space beyond the body schema.  As the patient is able to appreciate an extended binocular continuum of physiological diplopia along a reasonable length of the rope, more complexity can be added by introducing the other beads thereby partitioning the three principal regions of space.

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

2 thoughts on “Sensorimotor Dynamics and Two Visual Systems: Shades of Skeffington & Brock Part 2

  1. Len, really enjoyed these posts. It occurred to me that applying this model of the “inattention” or “avoidance” of personal space, seen in CI for example, might go some way towards understanding prism adaptation and the failure for BI glasses and pencil pushups to be effective in many cases.

  2. Thanks, Paul – so glad that you found it of interest. I believer you are correct, in that these approaches used reductively in such cases are chasing signs and symptoms of relative inattention rather than the origin of the problem. We understand this well when working with “brain injury” patients, neglecting (pardon the pun) that many other patients have forms of visual stress resulting in difficulties achieving uniformity or coherence in the continuum that is visual space.

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