The Paradoxical Brain

Anything that Oliver Sacks writes commands my attention.  That is the case even when it is something as brief as a page and a half foreword in a book, which he has done for The Paradoxical Brain.  Dr. Sacks makes the observation that classical neurology was founded on a deficit/lesion model, and that this clinico-pathological model remains the foundation of neurology today.  Yet John Hughlings Jackson was a British neurologist who  saw the brain as more — as having many functional levels, hierarchically arranged, with activity at higher levels making use of but also constraining the activities of lower levels.  More than a static mosaic of fixed representations or points, the brain is incessantly busy, with certain potentials being actively suppressed or inhibited in a dynamic balance – potentials that could be released if inhibition is lifted.  The heightening of potentials that are normally inhibited can be maladaptive, or may result in a paradoxical functional facilitation.

Kapur, Pascual-Leone, Manly and Cole use this cartoon to illustrate that states of action can be good or bad depending on one’s frame of reference.  Paradoxes may be evident when what normally hinders may help, and what normally helps may hinder.  Among the more familiar aspects of this in developmental optometry is the increasing attention paid to the inhibition of primitive reflexes and their integration with postural reflexes.  The drive toward incorporating activities in vision therapy to help this process of inhibiting retained primitive reflexes stems from the work of Blythe and Goddard.  The interrelationship of visual development and the pruning of primitive reflexes is laid out in Goddard’s book on Attention, Balance and Coordination.

There are several excellent chapters in The Paradoxical Brain, one of which concerns paradoxes in neurorehabilitaiton, specifically acquired brain injury (ABI).  One of particular interest is unilateral spatial neglect, typically occurring on the left side of visual space because of the higher probability of right brain damage.

My good colleague at Kessler Rehabilitation System in NJ, Dr. Vin Vicci, has established a protocol for the use of yoked prisms in ABI.  When an individual without ABI puts on yoked prism bases left causing a rightward shift, there is initially a misreaching to the right by about the same spatial amount as the value contained in the prism.  Once the individual has recalibrated and reaches correctly, if you take the yoked prisms off and re-grasp the target there is now a misreaching in the opposite direction (to the left) by about the same spatial amount as the value contained in the prism.  The paradox is that even though the individual knows that the prisms are gone, the brain has to go through this recalibration process of eyes working in concert with touch in order to overcome this rebound effect.

So what would happen if a patient showing a left visual neglect adapted to yoked prism bases left?  Would they show a similar rebound effect when removing the y0ked prisms and now be more sensitive to left visual space.  Rossetti et al (1998) reported that after just 5 minutes of reaching activity while wearing yoked prisms, the subjects with ABI showed significantly greater awareness of left space when the prisms were removed.  Now here’s the amazing paradox:   Rather than the effect fading, as it does with non-ABI patients, it was if anything stronger after 2 hours of the prism exposure!   Subsequent work has shown that demonstrably positive effects of two hours of yoked prism bases right adaptation were still persisted for at least up to 6 weeks.  Although the technique does not work for all patients, when it does a further paradox is that it helps recalibrate spatial awareness even for non-reaching tasks, such as reading.

Most recently, Rossetti’s group published a paper in the Journal of Neuroscience (2009) utilizing fMRI to pinpoint where the spatial reclaibration with yoked prisms was occurring in the brain.  During the earliest phase the anterior intraparietal sulcus was involved in error detection of past-pointing.  The parieto-occiptial sulcus was involved in error correction.  Cerebellear activity increased as spatial realignment occurred to make correct responses.  The time course of this recalibration suggests that the cerebellum modifies activity in the  superior temporal cortex, mediating the effects of prism adaptation on cognitive spatial representation, thereby reducing neglect.

Bultitude and colleagues published a paper in Brain (2009) consistent with this anatomic localization, but positing the changes due to yoked prism as alleviating difficulties in filtering out and disengaging from local detail in comparison to global form.  Frankly I haven’t fully digested this, but their LI (local interference) to GI (global interference) ratio sounds alot like the PCR ratio or center/periphery issues we’ve discussed recently.    A case report in Cortex (2010) by Bultitude indicates that this same mechanism may be operative in the more rare instances of right spatial neglect.

Ultimately the effect of yoked prisms appears to harken back to the dynamic balance between inhibition and release systems in the brain that John Hughlings Jackson anticipated over one hundred years ago.

– Leonard J. Press, O.D., FCOVD, FAAO

6 thoughts on “The Paradoxical Brain

  1. I think that prism adaptation work equal on ABI and normal subject.
    Different are the effect, that depend on a normal or a pathological (injured) brain.
    So the process is the same, but the effect is very different.
    It’s important to note that there are other sensory tecniques to rehabilitate patient with USN, like TENS, CVS, TMS and rTDCS.
    But this is neuropsychology not optometry (see the journals where research on PA are published)

    • Sorry, Alex, but I don’t agree this is neuropsychology. Prism adaptation is as optometric as you can get. Lenses and prisms are the tools that optomerists have pioneered. If you were to read optometric literature you’d see that we’ve been publishing about this since the 1970s. The main reason I’m citing these multidisciplinary journals like Cortex and Brain is that it lends support to, and helps elaborate the understanding of what optometrists do in practice in rehab environments every day. And let me emphasize that: we don’t do it in theory, we do it in practice.

  2. “difficulties in filtering out and disengaging from local detail in comparison to global form” = skeffington’s figure-ground

    “the dynamic balance between inhibition and release systems in the brain ” = the ‘mechanism’ of partial neglect and in time ‘aquiring’ low levels of awareness in amblyopia instead of ‘aquiring’ improvement in awareness = normal?
    inhibitory levels as a part of the scale that we could use to validate visual function of the human visual system

    great article! thanks Leonard, for the links too

    and yes i have a stack of books from O.Sacks and read and appreciate much of the work of P.Blythe and S.Goddard whom I both met at a conference in Chester 1998. Their work should be mandatory reading material for all optom’s in their education.

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