Wild Thing

Researchers are one step closer to understanding the origins of brain mapping diversity in eye dominance. A study has found evidence that the diversity of ocular dominance patterns relates to the amount of cortex available to represent each binocular point – something that varies between species as well as individual animals. The project, led by postdoctoral researchers Jianzhong Jin and Sohrab Najafian, and Jose Manuel Alonso, Professor at the College of Optometry, Laboratory of Visual Neuroscience, New York, USA, studied humans, macaques, and cats.

In humans and macaques, the cortex splits the map of visual space in intercalated pairs of stripes for the left and right eyes forming a Zebra pattern. In carnivores, the cortex splits the map in blobs forming a Dalmatian pattern. In rodents and lagomorphs, the afferents from the two eyes mix and do not form any specific pattern.

The researchers found that the pattern of ocular dominance columns in primary visual cortex was strongly associated with the amount of visual cortex devoted to process a binocular point across perpendicular axes of the visual field. “Our results predict that differences in the pattern of ocular dominance columns should be associated with differences in binocular processing. For example, stripe patterns should be associated with more pronounced differences in binocular processing across perpendicular axes of the visual field than blob patterns. However, these predictions have not yet been tested with psychophysical measurements,” says Alonso.

But there was more. Alonso and his team noticed that when cortical resources decrease to represent points that are increasingly farther from the point of visual fixation, the half eye that is closest to the nose (nasal retina) dominates and gains access to more cortical space than the other half eye (temporal retina). Interestingly, this nasal retina dominance only increases with distance from the point of fixation within the focus plane, not across depth. “In other words, the nasal retina dominates vision when you fixate at one letter of this page, but read adjacent letters using peripheral vision,” says Alonso.

“The temporal retina never dominates, but its contribution to visual processing nearly matches the contralateral retina at the point of fixation.” Alonso and his team are hoping to further their research by developing a model of cortical topography that aims to explain how the visual cortex represents and process multiple stimulus dimensions – including not only spatial position and eye dominance, but also light-dark contrast polarity, stimulus orientation, spatial frequency and direction.