Say you are walking down the same street you have walked every day for a decade. Before you realize what you are looking at, you notice something is different. Maybe the McDonald’s has been replaced by Burger King. What’s going on ?
Our internal representation of space has come under the spotlight with the discovery of spatial maps in the hippocampus and entorhinal cortex. But how does this internal model affect our sensory processing ? This recent paper looks at the activity of visual cortex neurons and how they are influenced by the internal representation of an environment.
The authors expressed the calcium sensor GCamp6 in layer 2/3 neurons of the visual cortex of mice. The animals were then head fixed for 2-photon imaging, and were free to run on a spherical treadmill to navigate in a virtual environment. This environment was a tunnel with distinct landmarks simulated on the tunnel walls. The landmarks were interspersed with orthogonal gratings, labelled stimulus “A” and stimulus “B”. At the end of the tunnel, the animal was given a water reward.
As expected, the group observed increased activity of orientation selective neurons. They also observed that the neurons selective for “A” or “B” responded differently to the same stimulus presented elsewhere in the tunnel. They inferred that these orientation selective cells also contained spatial information.
After a couple of runs in the virtual tunnel, some interesting cells emerged that fired before the stimulus was presented. They were selective for the orientation of the stimulus presented and predicted the upcoming stimulus, hence they were termed predictive neurons.
Once emerged, the predictive neurons were found to be stable. In case of a mismatch between the stimulus predicted and the stimulus encountered by the animal, an error signal was generated. The prediction appeared to arrive from the anterior cingulate cortex. Complete omission of the stimulus led to an increase in the average activity of the entire population. Additionally, a subset of cells that were selective for this omission developed with experience.
Where we are in an environment generates an internal representation of space which seems to predict the visual stimulus one can expect in that location. When we are walking down a familiar street, a prediction of the visual stimulus (McDonald’s) might be generated in our visual cortex.
This prediction seems to be compared to the feed-forward visual stimulus (Burger King) and any mismatch generates an error signal to update expectations and guide learning.