Revisiting BOP and BIM – Part 4: The Role of Vergence
If Degrees of Freedom are the proverbial plane ticket to visual paradise, then binocularity can easily be considered the pilot for our journey – without whom our flight would become nothing more than a heap of metal on the tarmac. Similar to the plane and pilot relationship, solid binocularity is needed to ensure degrees of freedom can occur. Without it, our trip to paradise may detour to somewhere less exotic – like the garbage dump.
By definition, binocularity is the ability to achieve three levels of fusion. Assuming our patients can achieve simultaneous perception – sometimes a BIG assumption – first degree fusion consists of simultaneous perception with the ability to overlap, or “fuse” dissimilar targets. Second degree fusion is the fusion of two similar targets, with corresponding contours, which do not offer depth. Third degree fusion is the ability to derive three dimensional depth (or stereopsis) by way fusion of two targets similar contours, yet the disparity to make stereopsis possible.
Good binocularity means good spatial awareness. The patient needs to gain the concept of where they are in space in relation to the object of regard. These concepts can be employed with both strabismic and non-strabismic patients; however, the main goal is to achieve clear and single vision because we should always work to better integrate and build degrees of freedom between the vergence system and the accommodative system.
Most theories and practices will usually begin working binocularity in second and third degree fusion, if there is no strabismus or amblyopia. Second degree fusion targets – i.e. Brock String, MFBF activities like Red/Green Mazes – strive for flexibility and stability with less emphasis on accommodation. When using third degree fusion targets – i.e aperture rule, lifesaver card, eccentric circles, vectograms – pay particular attention to accommodative vergence. For the purposes of this explanation, let’s examine task common to most VT practices – Vectograms – as an example of how we work in the therapy room to improve binocularity. One of the strongest and most successful activities we will use in building binocularity is the rope circle target (Quoits). Without much in the way of accommodative demand, this target allows for work more specifically on the convergence and divergence ranges on a gross peripheral basis, allows the patient to experience feeling tone with the different postures, allows for an initial localization attempt, and provides an opportunity for parallax. The Quoits also do not offer much in the way of a central, or focal, target which helps build strong peripheral fusion, which is a critical component of binocularity. Without “much to look at” centrally, this target offers patients an opportunity to rely on their periphery to appreciate the changes in size and depth, or SILO. SILO (smaller in, larger out) occurs when a patient is asked to maintain fusion as the vergence demand is increased or decreased, and a perceptual change is experienced. Appreciation of SILO is important because it demonstrates the patient’s ability to trust the input they are receiving from their visual system, without interjecting logic of real life experiences since in real life objects increase in size as they become closer, and vice versa when objects get farther away. From there, we will move in the second round of vectograms with a new target, the clown. With more detail and more central stimulus, the clown offers the patient the opportunity to integrate the detail oriented central vision with the depth perceptive peripheral vision which will assist binocularity, and ultimately with the improvement of the patient’s depth perception. The Spirangle allows for more work in spatial localization and degrees of freedom between accommodation and vergence. Chicago skyline works degrees of freedom even more.
An increase in demand to build degrees of freedom between accommodation and vergence is working with what is commonly referred to as BOP and BIM (BOP = Base Out Plus, BIM = Base In Minus). We can either introduce +/- lens flippers or introduce BI/BO prism flippers to drive the two systems in opposite fashion, stimulate one while relaxing the other.
Part One contains diagrams illustrating a base in vectogram target which requires the patient to diverge the target to keep it single and clear. Since minus lenses stimulate the accommodative system, first vergence will reflexively be stimulated to converge, but then to maintain a single target, it must actively diverge. Conversely, if BO prism is introduced while looking at a vectogram set for BI, it further relaxes the vergence system to keep the target single while asking the accommodative system to stimulate to kept the target single on the plane of regard.
Part Two illustrates a base out vectogram target requires the patient to converge on the target to keep it single and clear. Since plus lenses relax the accommodative system, first vergence will reflexively relax, but then to maintain a single target, it must actively converge. Conversely, if BI prism is introduced to a vectogram set for BO, it further stimulates more convergence to keep the target single while asking the accommodative system to relax to keep the target single on the plane of regard.
A good measure of the ownership and effectiveness of these new found skills is to use the vectograms for jump vergences by setting one set of cards in the Base In posture and the other in a Base out posture. The goal of these jump vergences is for the patient to understand the feeling tone of switching back and forth. The patient is then asked to use the feeling tone information to make the jump from one target to another more efficient.
As my experience has grown, my tendency is to also place the vectograms on a window to work more in real space divergence, we will have patient walk closer and farther while maintaining a single and clear image. This “tromboning” action furthers the patient’s ability to appreciate SILO, increases the flexibility in accommodative vergence as the patient travels closer and further from the plane of regard, as well as assists the patient with learning to appreciate the changes in real space which ultimately sets the stage for these new skills to transfer into their real world. Other methods to increase the demand or load of an activity will be by adding auditory interference, increasing the stress level with unnecessary and unneeded visual stimulation that the patient must ignore, or at the highest level as the patient to multi-task while performing the activity at hand.
Since peripheral vision is directly responsible for answering the “where am I” and the “where is it” questions which is the equation whose product is depth perception, as modeled by Dr. A.M Skeffington, stimulating and expanding a patient’s peripheral awareness is a crucial step in the process of successfully achieving binocularity. In order to achieve good binocularity and appreciate stereopsis by way of third degree fusion, a person must be able to understand the relative distance between themselves and the object, or objects, they are looking at. Appreciating the spatial difference between the “where am I” and the “where is it” is the backbone of appreciating depth.
Now that we have covered the parts involved in BOP and BIM, and building Degrees of Freedom between Accommodation and Vergence, our pilot has turned on the “Fasten Seatbelts” sign, and we are approaching our destination.
Some final thoughts coming soon 🙂