Is Strabismic Suppression a Form of Attentional Neglect?


It’s an intriguing question, and a recent article published in the Journal of Vision suggests that the answer may be yes.

JOV Cover

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.

Puzzle

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:

Extinction

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.

 

15 thoughts on “Is Strabismic Suppression a Form of Attentional Neglect?

  1. Nice piece, Dr. P. The use of ‘inattention’ rather than suppression in the context of strabismus would suggest brain damage such as from stroke, as you have described it. How do you reconcile this?

    • Thanks, Dr. B. That’s precisely the point — what I’m suggesting is that it doesn’t require reconciliation but a wider understanding of the brain and body’s tradeoffs in dealing with simultaneous stimuli competing for attention. For example, we suppress physiological diplopia under normal seeing conditions. From an early age, the brain renders it as something that should be neglected within the visual field, yet that form of inattention is not secondary to any form of brain damage. To the contrary, it is a sign of a healthy brain – as is the fact that the grad student in the gorilla suit who walks by the elevator while his lab mates bounce a basketball escapes detection due to suppression of attention or “inattentional blindness”. My contention is that inattention is subconscious. Whether it meets a need or reflects a disability depends on its cause and origin.

      • I could restate my question thus: What is the preferred nomenclature for inattention/neglect due to organic factors (i.e damage) vs. that due to subconscious, aka neuro-functional, causes? Strabismic/refractive amblyopia are in that grey area where we can show structural (organic) changes, which would suggest inattention of a different nature than physiologic diplopia suppression where there is no organic variance from neurotypical. You say inattention is subconscious, but so is the degraded cortical patterning in amblyopia – the mechanism of the two are quite different. The gorilla example is certainly an active process of selective attention, and active cognitive processing, as I recall the demo, both of which push aside the perception of other stimuli in the field. Were we to ask the viewer not to count the passes of the ball, but to simply observe the basketball players, we would expect a different result (more people would notice the gorilla, and likely earlier). The amblyope might experience hemifield effects in visual perception, an organic effect, but also experience the gorilla effect quite apart from the organic/structural neglect. My inattention might be leading to the perception that this is clarifying my point, hope so. Food for thought.

  2. Thank you Dr. Press! — This is a post I will enjoy sharing with friends not familiar with strabismus, as it hits so many issues. I noticed Dr. P reconciled the “inattention” issue by calling it “a form of attentional neglect” in the title … I certainly feel he is onto something, whether it’s “a form of inattention” or neglect. I have a hunch (from Dr. P’s many autism posts) that there are many underlying reasons the brain would choose to limit binocular visual input, of which we are only just now scratching the surface.

  3. Very good food for thought, Dr. B — and there has to be some sort of continuum from the questionably maladaptive divided or selective attention patterns in ADHD to the self-protective pro-adaptive patterns to avoid diplopia in strabismus — neither of which is presumably volitional but whose etiology in most cases remains obscure. I don’t mean to imply that all types of inattention are equal, but as the article in JOV shows there may be elements of neglect or extinction that help the strabismic amblyope make the most use of visual attention and provide an economy of scale to make the best use of attentional resources under condition which nature presumably did not intend.

    • I believe so, Michael. The key word you said is “apparent” – and I gather you’re using apparent as a type of relative field loss, in contract to absolute bilateral concentric, field loss as might occur with damage to photoreceptors in RP. Yes, concentration to center with shutting down of the periphery — that shutting down is a type of transient extinction, isn’t it.

  4. Bravo! This is completely consistent with other forms of neuro-rehabilitation. It also explains why creating demand for sight (simple patching) will not necessarily stimulate complex visual skills and inteferes with development of binocular vision.

  5. Len, Great post. I would agree that much “suppression” is inattention. We attend to what is useful for action. Seeing the Gorilla is not useful for counting the number of basketball passes so we do not attend to them. Physiological diplopia is also not useful to action so we do not attend to it. The strabismic learns to attend to what eyes and hands and bodies have in common for action, not to illusions of the visual system, whose only value rests in pleasing clinicians. When I speak to groups, I have the participant place a target 5 inches before one eye. I have them and look back and forth between me in the distance and the target up close. Since the target is in front of one eye, the convergence is asymmetric. The participants see the target in its natural position and me in my position. The pay attention to the action. With the target in front of one eye and between them and me, if they were paying attention to “first degree fusion” they would see me inside the target (if the target is a finger, inside the finger). this is an example of anomalous binocular correspondence (ABC). The same behavior Brock explained in 1941. Instead of a finger he used a red ring in front of one eye and a light in the distance. Rather than see the light inside the red ring, the strabismics saw the ring in its correct position and the light in its correct position. Strabismics pay attention to what is needed for action unless they are beguiled by vision therapists to do otherwise. If you have a hundred people each look at a Brock string for 5 seconds, then remove the strings and ask what they saw, most will report one string straight ahead. As Flax hypothesized year ago, we all probably have ABC. Attention is key, and attention related to what is of value. ABC strabismics find no value in luster, rivalry, confusion, physiological diplopia,, diplopia, and disparity caused stereopsis. By arranging conditions, actions, and instruction sets to make these illusions of value, they are no longer ignored.

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