Scientists discover a hidden brain circuit that rewrites vision

"That's the major conclusion of this paper: There are targeted projections for targeted impact," said senior author Mriganka Sur, Paul and Lilah Newton Professor in The Picower Institute for Learning and Memory and MIT's Department of Brain and Cognitive Sciences.

Investigating Customized Prefrontal Signals

Scientists have long proposed, including Sur's colleague Earl K. Miller at MIT, that the prefrontal cortex can guide the activity of more posterior areas of the brain. While anatomical evidence has supported this idea, the goal of the new study was to determine whether the prefrontal cortex sends one broad type of signal or instead crafts distinct messages for different target regions. Lead author and Sur Lab postdoctoral researcher Sofie Ährlund-Richter also sought to identify which specific neurons receive these signals and how the communication influences downstream processing.

Different Prefrontal Regions Serve Different Roles

The team identified a number of new insights. Two areas in the prefrontal cortex, the orbitofrontal cortex (ORB) and the anterior cingulate area (ACA), were found to relay information about both arousal and movement to two other regions: the primary visual cortex (VISp) and the primary motor cortex (MOp). These messages appear to have unique effects. For example, higher arousal increased ACA's tendency to help VISp sharpen its visual representations. ORB, however, became influential only when arousal was very high, and its involvement appeared to decrease the clarity of visual encoding. According to Ährlund-Richter, ACA may help the brain focus on potentially meaningful visual details as arousal rises, while ORB may act to reduce attention to distracting or overly strong stimuli.

"These two PFC subregions are kind of balancing each other," Ährlund-Richter said. "While one will enhance stimuli that might be more uncertain or more difficult to detect, the other one kind of dampens strong stimuli that might be irrelevant."

Mapping and Monitoring Brain Circuits

To better understand the involved pathways, Ährlund-Richter performed detailed anatomical tracing of the connections ACA and ORB form with VISp and MOp. In additional experiments, mice ran freely on a wheel while viewing structured images or naturalistic movies at different contrast levels. At certain moments, small air puffs increased the animals' arousal level. Throughout these tasks, researchers recorded the activity of neurons in ACA, ORB, VISp and MOp, with particular attention to the signals traveling along the axons linking prefrontal and posterior areas.

The tracing work showed that ACA and ORB each communicate with a variety of cell types in their target regions rather than a single cell class. They also connect in distinct spatial patterns. In VISp, ACA primarily targeted layer 6, while ORB communicated mainly with layer 5.

How Arousal and Movement Shift Visual Processing

When the team examined the transmitted information and neural activity, several consistent patterns emerged. ACA neurons conveyed more detailed visual information than ORB neurons and were more responsive to changes in contrast. ACA activity also tracked closely with arousal level, while ORB responded only when arousal reached a high threshold. When signaling to MOp, both regions conveyed information about running speed. When signaling to VISp, however, they only indicated whether the mouse was moving or still. The two prefrontal regions also carried information about arousal and a small amount of visual detail to MOp.

To see how this communication affects visual processing, the researchers temporarily blocked the pathways leading from ACA and ORB to VISp. This allowed them to measure how VISp neurons responded without these inputs. They found that ACA and ORB exerted specific and opposing effects on visual encoding depending on the mouse's movement and level of arousal.

A Specialized Model of Prefrontal Feedback

"Our data support a model of PFC feedback that is specialized at both the level of PFC subregions and their targets, enabling each region to selectively shape target-specific cortical activity rather than modulating it globally," the authors wrote in Neuron.

In addition to Sur and Ährlund-Richter, the research team included Yuma Osako, Kyle R. Jenks, Emma Odom, Haoyang Huang, and Don B. Arnold.

The work was supported by a Wenner-Gren foundations Postdoctoral Fellowship, the National Institutes of Health, and the Freedom Together Foundation.