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