Understanding Prism Adaptation
Prism adaptation, a sensorimotor phenomenon, was recognized long before its clinical application. Prism adaptation requires an individual to wear prism lenses while making arm-reaching movements toward visual targets.
For treating left-sided spatial neglect, an individual wears the prism lenses that shift the visual field – including the target images and their own reaching arm – certain degrees to the right depending on the diopter of the lens (e.g., 20-diopter lens shifts the visual field by 11.4 degrees).
At the onset of prism exposure, the individual’s motor system has not yet registered that visual information has shifted to the right, and thus the arm reaches to the shifted image while the actual target remains in its original position (i.e., to the left of the shifted image). The visual system sends feedback regarding the missed target to the motor system, which adapts over repetitive practice of arm reaching toward the target. The individual eventually achieves success with a leftward movement reaching the actual target. This process is prism adaptation.
Upon removing the prisms, the visual information is now normal (actual target and its image are in the same location), but the motor system has adapted to a movement reaching toward the left of the visual image. Thus, the arm moves toward the left of a visible target. This inaccurate performance is called the “after-effect.” The leftward after-effect facilitates greater awareness of the left side of space in individuals with left-sided neglect who seldom initiate exploration of the left side of space on their own.
Prism adaptation and its after-effect occur implicitly, requiring no effortful top-down strategy or conscious self-monitoring, which is a great advantage in integrating it into rehabilitation of individuals who have limited cognitive capacity to participate in therapies that require explicit learning or low self-awareness of their own deficits. It can be a preparatory treatment before carrying out regular therapy activities planned for individuals.
Prism lenses using prism adaptation treatment are not prescribed or personalized. Rather, they are used to facilitate stimulation, modification, and even long-term changes of brain connectivity among the cerebellum, parietal cortex, temporal cortex, and frontal cortex. Neural mechanistic studies are still ongoing to reveal how this deceivingly simple treatment impacts cognitive and motor functions. In other words, high-strength prisms (e.g., 20-diopter) used in prism adaptation treatment are for the purpose of stimulating neural networks and can only be used during a therapist-guided session.
On the other hand, prism lenses prescribed for individual use (such as 3M Press-On Fresnel prisms or Peli Lens™) are assistive devices to correct vision when vision is required to perform a task. For example, prescribed prisms are for expanding users’ visual field or to eliminate double vision.
