Part Of: Attention sequence
Followup To: Attention As Gatekeeper

Part Of: Neuroeconomics sequence
Followup To: Because vs As-If

Topographic Maps

Recall that cerebral cortex is like a sheet: stretched flat, it covers an area of 2.5 square feet.

Mental modules are clusters of functionally-homogenous cortex. If the cortex is a map, modules are the borders of its nation-states. For example, the Fusiform Face Area (FFA) is a well-known example of a specialized module: it performs face recognition.

Mental modules often contain topographic maps. Let’s imagine viewing the FEF topographic map from above, and seeing two hills of activity (electrical storms). These represent different choices:

Our topographic map encodes different saccade vectors. Specifically,

• Saccade A represents looking at the mirror: moving the foveal spotlight horizontally (0°) a moderate distance (10°).
• Saccade B represents looking at Lena’s hat:  moving the foveal spotlight up-right (60°) a small distance (5°).

The closer the two hills of activity, the more similar the saccade vectors. More concisely, in topographic maps, proximity encodes similarity.

The Machinery of Choice

These two peaks of activity (electrical storms) encode two choices under consideration. The brain is considering whether to look at the hat, or the mirror. How does the brain select the best option? 

Topographic maps implement choice via removing all unchosen options from the topographic map. It preserves the winner via a Winner-Take-All (WTA) process, sometimes called exponentiation.

When a topographic map “makes a choice”, its activity peaks transmit inhibitory neurochemicals (e.g., GABA) to one another. The process is not unlike arm wrestling. The option with most vibrant activity is almost always selected. Muscles matter. 🙂

So in the above, since Choice A is the more intense electrical storm, the person chose to look at the mirror.

A Universal Process

Human beings perform more complex behaviors than shifting their gaze. However, WTA has been shown to underlie nearly all of them.

Sometimes, topographic maps comfortably share space without engaging in WTA. How does the brain decide to decide? The resolution threshold is the point of no return: if an electrical storm becomes more intense than that value, it is off to the races. And, of course, the brain has several mechanisms for dynamically altering this threshold.

Wrapping Up

• Economists like to talk about utility maximization.
• Mathematicians like to talk about the argmax operator.  
• Cognitive psychologists like to talk about decision making.

WTA is the unifying thread. It allows meat to make decisions.

References

This writeup was, of course, heavily simplified. For technical details, see:

• Cisek (2006). Integrated Neural Processes for Defining Potential Actions and Deciding between Them: A Computational Model
• Glimcher (2010). Foundations of Neuroeconomics