Asymmetries around the visual field: From retina to cortex to behavior

Area of Science:

• Neuroscience
• Computational Vision
• Visual Perception

Background:

• Visual performance declines with eccentricity and shows polar angle asymmetries.
• Existing models suggest optical quality and cone density explain only a small part of these behavioral variations.

Purpose of the Study:

• To investigate how visual processing beyond initial cone photon absorption contributes to polar angle performance asymmetries.
• To quantify the role of cone phototransduction and retinal ganglion cell (RGC) spatial filtering in visual asymmetries.

Main Methods:

• Quantified asymmetries in cone density, midget RGC density, and V1 cortical magnification factor (CMF).
• Extended a computational observer model simulating human optics, cone isomerizations, phototransduction, and mRGC spatial filtering.
• Used a linear support vector machine classifier on mRGC responses for orientation discrimination.

Main Results:

• Polar angle asymmetries and eccentricity gradients increased from cones to mRGCs, and from mRGCs to the cortex.
• Computational model showed greater asymmetries in photocurrents than isomerizations, and greater still in mRGC signals.
• However, mRGC signal asymmetries were smaller than observed human performance asymmetries.

Conclusions:

• Cone isomerizations, phototransduction, and mRGC spatial filtering contribute to visual performance differences across the visual field.
• Additional contributions from cortical representations are necessary to fully explain observed polar angle performance asymmetries.