It’s an intriguing question, and a recent article published in the Journal of Vision suggests that the answer may be yes.
Let’s first briefly review what we know about visual spatial neglect, otherwise known as visual inattention or extinction. In its classic form, inattention occurs after a stroke where there is a lesion or other type of insult than results in one region of space being accentuated in consciousness at the expense of the contralateral region of space being tuned out. Awareness, orientation, movement, or action has become asymmetrical, causing a functional disability. This inattention or neglect most commonly occurs in leftward space after damage to the right brain, and is most commonly encountered in the visual domain although it can occur in other modalities and even multi-modally.
Whereas hemianopia occurs due to a lesion in the visual pathway that is retinotopic – meaning referenced to retinal projection all the way through to the occipital or visual cortex, hemi-neglect is referenced to the body. Neglect can therefore occur as a result of damage to any of the brain’s lobes (parietal, temporal, or frontal); from limbic regions such as the anterior cingulate; or from subcortical regions such as the thalamus or the putamen. Chances are you know what an absolute visual field loss looks like – it’s obvious that things are dark or missing in one region of the visual field. In contrast, neglect or inattention is a relative extinction of one side of space, accentuated where there are competing stimuli occurring simultaneously on both sides of the field. Here’s how it is unmasked chair-side:
When the examiner wiggles her fingers or just the right side or left side, the patient has not trouble pointing to which set of fingers wiggled. But when the examiner wiggles both fingers simultaneously, the patient with left neglect perceives only the fingers wiggling on the right side.
Neuroscientists speak of space-based attention which is primarily subserved by the parietal cortex, interfacing with visual cortex as well as frontal cortex, directing the eyes where to look. This has led neuroscientists and therapists to experiment with a variety of techniques in neglect that mirror what we’ve done in therapy with selective occlusion to alter inter hemispheric balance. Anything done to accentuate inattention is called suppression, and anything done to aid inattention is called facilitation.
So here’s where the relationship between the two eyes, the brain, and the rest of body becomes particularly interesting. Recall that in previous blogs we’ve identified strabismus as an asymmetry between the two eyes with implications from the head to the toes. That is why for example we no longer refer to sensory-motor alterations in strabismus as “anomalous retinal correspondence”, which is retinotopic, but rather as anomalous correspondence which is mediated by regions well beyond the eyes.
My argument therefore, and one bolstered by the Journal of Vision article above, is that suppression in strabismus is a form of inattention. That inattention or extinction is a natural adaptation to avoiding diplopia or confusion. Suppression does not occur when the fixating eye is covered and is only operative as a relative condition when both eyes view simultaneously. However, when each eye is tested separately, effects of this asymmetry are exhibited by both eyes, not just the turned eye – as compared to performance by patients without strabismus. That is why we engage patients in therapy under monocular conditions as well as with both eyes functioning simultaneously as a complement to fusion activities. That is why anti-suppression activities are useful in providing feedback, but pro-facilitation procedures (divided attention, quadrant loading, etc.) are more useful in gaining transfer to space-based perception. That is why many of the same stimuli effective in countering extinction or inattention are useful in strabismus therapy. That is also why we sequence activities from basic to complex, since suppression or selective inattention becomes more pronounced when the visual field is busier or more complex.
We are not just interested in what the patient can do with visual targets when the eyes are aligned, but in how well the patient can approach symmetry between right and left sides of space – both object centered and body centered. We previously referenced this with regard to Infinity Walk, and that can be extended to activities such as Chalkboard Circles, Space Fixator, SVI, Brock String, Press Lites, TBI bulbs, Tactus Visual Attention Therapy, and Racethe8s.