Convergence Subtypes Activate Unique Brain Regions

Accommodative, proximal, disparity and voluntary convergence paradigms trigger distinct areas in the brain, according to a study published in Ophthalmic & Physiological Optics. Proximal and voluntary conditions activate more areas compared with accommodative and disparity conditions.  

Investigators included 10 right-handed participants (mean age, 24.4 years; 6 women) with normal binocular vision and analyzed blood oxygenation level-dependent brain signals using functional magnetic resonance imaging (fMRI) scans during 4 different vergence eye movements. The team assessed accommodative convergence by having participants alternately view letters monocularly with a −2.00D lens. Proximal convergence was evaluated as patients alternately viewed difference of Gaussian targets monocularly at either near or distance. Disparity convergence assessment involved viewing random-dot stereograms with increasing disparity, and voluntary convergence was measured as participants voluntarily converged their eyes with binocular viewing.

The accommodative convergence paradigm activated the right side of the brain in the right fusiform cortex, as well as the right middle occipital cortex. The right proximal convergence stimulus, however, mostly triggered areas in the right occipital lobe. The disparity stimulus activated areas in the left occipital and left frontal cortices. The voluntary convergence paradigm primarily activated the occipital lobe, mostly bilaterally. The highest number of areas of activation were noted for the proximal convergence and voluntary convergence conditions, according to the report. 

“When vergence stimuli that are likely to result in activation of multiple convergence subtypes are used, the results of the current study suggest that two outcomes are possible,” according to the researchers. “First, it may be that more areas of cortical (or cerebellar) activation appear compared with the vergence subtype-isolating stimuli used in the current study. Second, it may be that stimuli that drive multiple vergence subtypes result in a stronger BOLD signal from areas that are common to different vergence subtypes.”

Study limitations include the use of convenience sampling and failure to distinguish between sensory and motor activation responses.