Saturation in Phosphene Size with Increasing Current Levels Delivered to Human Visual Cortex

Area of Science:

• Neuroscience
• Biomedical Engineering
• Ophthalmology

Background:

• Electrical stimulation of the early visual cortex elicits phosphenes, which are artificial visual percepts.
• Phosphenes are typically described as small, regardless of stimulation parameters.
• Understanding phosphene generation is crucial for developing visual cortical prosthetics.

Purpose of the Study:

• To investigate the relationship between electrical stimulation parameters and phosphene size in human subjects.
• To develop a predictive model for phosphene size based on cortical activation and location.
• To assess the implications of phosphene size saturation for visual prosthetic design.

Main Methods:

• Electrical stimulation of 93 electrodes in the visual cortex of 13 human subjects.
• Subjects reported phosphene size while stimulation current was systematically varied.
• Development of a model incorporating cortical activation (sigmoidal function) and cortical magnification factor.

Main Results:

• Phosphene size increased with stimulation current up to a saturation point.
• Phosphene size was dependent on the stimulated cortical location, increasing with distance from the foveal representation.
• The developed model accurately predicted observed phosphene sizes across various conditions.

Conclusions:

• Phosphene perception involves a large population of activated cortical cells.
• Saturation of phosphene size indicates fundamental restrictions in artificially evoked cortical activity spread.
• Current stimulation methods may limit the dynamic range for spatial form control in visual prosthetics, necessitating advanced techniques.