Revisiting BOP and BIM – Part 2
In Part One, we saw illustrations of what occurs when Base In targets are mixed with minus lenses, also known as BIM. Our goal in this process is to teach our patients how to stimulate accommodation, while relaxing vergence – which is one half the process of building Degrees of Freedom between the two systems. As a brief repeat from Part One, Degrees of Freedom is a measurement used to indicate the amount of separation available between vergence and accommodation within the visual system.
Part Two will illustrate what occurs when Base Out targets are mixed with plus lenses. The goal of this process is the opposite of BIM, in that we are trying to teach our patients how to stimulate vergence, while relaxing accommodation. As with Part One, all diagrams can be clicked on and enlarged for better viewing.
Same as with Part One, Diagram A illustrates vision in a “normal” state of rest. If we were having our patient perform Vectograms, this illustration shows where their vision would be when we are on the zero setting. As you can see, vergence (illustrated by the orange star), and accommodation (illustrated by the green star) are both settled on or near the plane of regard (a.k.a. “the target”) as we achieve clear and single vision.
Diagram B illustrates vision under Base Out, or convergent, conditions. As you can see, vergence (illustrated by the orange star) is ahead the plane of regard (POR), and accommodation (illustrated by the green star) has settled on or near the POR to achieve clear and single vision. The dotted line illustrates the current degrees of freedom, or amount of separation between the two systems. As the demand is increased (we move the Vectogram into higher numbers) the point of convergence travels further from the POR, and closer to our eyeballs. In order to maintain a clear and single image, the patient must hold accommodation on the POR. Patients who struggle in this situation may either report single and blurry because convergence is appropriate, but accommodation cannot relax back to the POR – or double and clear, which would indicate accommodation is appropriate, but vergence cannot properly align.
Diagram C illustrates the results of plus lenses relaxing the accommodative system and vergence reflexively being relaxed and diverging, while looking at a Base Out target. This is commonly referred to as BOP, or Base Out Plus. Note both the green and orange stars have recessed, in equal increments due to the relaxation of accommodation caused by the plus lens, but the length of the dotted line has not yet changed. To maintain a single target, our patient must now actively converge, or stimulate vergence to the point of fusion, while holding accommodation in the relaxed position.
Diagram D illustrates the increased distance between the two stars and length of the dotted line. This demonstrates successful separation between the fusion point of vergence (illustrated by the orange star) and the focal point of accommodation (illustrated by the green star), showing an increase in the degrees of freedom between the two systems. If, and when, the patient is able to simultaneously meet the demands of the Base Out target and the plus lenses, they will achieve clear and single viewing – which again, should always be our goal with Vectograms.