The visual system is probably the best understood part of the brain. Over the past 75 years, neuroscientists have assembled a detailed account of how light waves entering your eyes allow you to recognize your grandmother's face, to track a hawk in flight, or to read this sentence. But a new study by UC San Francisco researchers is calling a fundamental aspect of vision science into question, showing that even the best-studied parts of the brain can still hold plenty of surprises.

The new study — published online in Science — shows for the first time that one of these supposedly higher-order visual areas, which is involved in the perception of moving objects, does not depend on information from V1 at all. Instead, this region, known as the post-rhinal cortex (POR), appears to obtain visual data directly from an evolutionarily ancient sensory processing center at the base of the brain called the superior colliculus.

"It's as if we've discovered a second primary visual cortex," said study senior author Massimo Scanzian. "This undermines the whole concept of the visual system in mammalian cortex as a perfect hierarchy with V1 as the gatekeeper and raises a multitude of questions, including how these two parallel visual systems evolved and how they cooperate to produce a unified visual experience."

The research was launched when lead author was recording neural responses to moving visual stimuli in the mouse POR. He used a technique called optogenetics to temporarily silence activity in V1 with light. "It was absolutely remarkable," Beltramo said. "We silenced the main visual area in the cortex and visual responses in POR remained unaffected. That was the first big 'wow' moment that told us we were on to something completely unexpected."

Superior colliculus was driving POR responses to movement

To confirm that the superior colliculus was driving POR responses to movement, Beltramo used optogenetics to systematically silence either V1 or the superior colliculus while recording from POR. More research is needed to test whether visual responses in POR-like areas of the brains of primates like ourselves also depend on input from the superior colliculs, as well as how V1-driven and colliculus-driven activity interact to influence animals' behavior, the authors say.

"We hypothesize that POR, which was previously considered a higher order visual area, could be a sort of 'primal' visual cortex, similar to a simple early amphibian, reptilian or avian visual cortex, and that it may be dedicated to the detection that something is moving in the environment, whether it is small, close prey or a large distant predator," Beltramo said.

Based on studies in primates, blindsight is thought depend on the superior colliculus, but the new study suggests it could involve POR-like areas of the cortex as well. "This is one of those findings that raises a lot of questions rather than answering any, because, as many discoveries, it presents questions no one knew to ask," Scanziani said.