# Are big brains for processing sensory input?

Elephants have bigger brains than human beings. But human beings are smarter. What's all the cranial matter doing?

I used to believe that a larger organism's brain needs to generate larger electrical impulses to move its muscles. But that's not how electrical engineering works[^1]. It's not like your nerves power your muscles. Your brain just sends a signal to the muscles telling your muscles to activate.

[^1]: The lightswitches in your house do combine power and signal. Lightswitches are a crude. When designing integrated circuits, my electrical engineering textbook recommends controlling power supplies with much smaller signal voltages via a transistor.

Muscular motor units aren't even voltage coded. They're frequency coded. When your brain wants a muscle to contract harder it sends nerve impulses more frequently. Trying to raise the output voltage of a motor neuron is like trying to push the number "2" through a single bit of a binary computer.

It's true that I wouldn't want to use a tiny microcontroller to control a Predator drone but that has nothing to do with the power output of its motor control pins. It's because I want more processing power, fine motor control and robustness against damage.

Blue whales have gigantic brains. I don't think the brain size is for robustness against damage. It would be more efficient for evolution to invest in a thicker skull than metabolically-expensive brain matter. I don't think the brain size is for fine motor control either. What is a blue whale going to do with fine motor control? It almost doesn't even have limbs.

![](https://s3.us-west-2.amazonaws.com/www.lsusr.com/lesswrong/sensory-input-or-motor-control/blue-whale.jpeg)

# Encephalization

An organism's **encephalization quotient** is the ratio of its brain size to its body size. Mammals of similar encephalization quotients tend to have similar intelligence levels.

[![](https://s3.us-west-2.amazonaws.com/www.lsusr.com/lesswrong/sensory-input-or-motor-control/EQ.jpg)](https://www.reed.edu/biology/courses/BIO342/2015_syllabus/2015_WEBSITES/Huang_Vanderhooft_Website_ToolUse/phylogeny.html)

Big brains don't just consume lots of energy. They have lower reaction speeds too[^2] (though slow reaction speeds may less important to large organisms because large muscles take longer to move). I think the slow reaction speed of large brains is related to the time it takes a signal to travel from one end of the brain to the other.

[^2]: The "bigger brains cause lower reaction speeds" is my personal theory. It hasn't (to my knowledge) been proven. It is plausible that larger organisms have slower reactions speeds and that brain size is irrelevant.

Big brains are expensive in multiple dimensions. If big organisms have big brains then the big brains must be doing something useful. Since the encephalization baseline is orthogonal to intelligence, the encephalization baseline must be driven by something other than intelligence. Mammals with bigger bodies require larger brains just to break even. A blue whale has a large brain *despite* its low need for intelligence. Why?

# Sensory Input

If a blue whale's bigger brain isn't driven by damage resistance, motor control or intelligence needs then the only thing I can think of is processing sensory input.

In machine learning, processing an image datastream takes lots of compute. I'd be surprised if evolution didn't operate under a similar constraint—even with all its optimizations. A larger organism afford to collect more sensory data and afford to support more brainpower to process sensory data. It's easy to overlook all the compute evolution puts into processing sensory data for us. Our conscious mind doesn't think "retina cone #4,594,047 just activated". Our occipital lobe just tells us "cute African elephant".

![](https://s3.us-west-2.amazonaws.com/www.lsusr.com/lesswrong/sensory-input-or-motor-control/elephant.jpg)

~~African elephants are the largest land animals on Earth and they have the strongest sense of smell ever identified in a single species. I don't think this is a coincidence. In human brains, the sense of smell is handled by the piriform cortex in the cerebellum. Elephants have giant cerebellums. I don't think that is a coincidence either.~~

~~Cerebellums are circled in red. The relative size of a cerebellum within a brain is accurate but the brains are not to scale relative to each other. (Elephant brains are bigger than human brains.)~~

[See [comment](https://www.lesswrong.com/posts/MxbXroGxXg7yEDFX7/are-big-brains-for-processing-sensory-input?commentId=gFMnyebHsouZYprmX).]

![](https://s3.us-west-2.amazonaws.com/www.lsusr.com/lesswrong/sensory-input-or-motor-control/cerebellum-comparison.png)

Reality check. Don't mice have a better sense of smell than human beings?

![](https://s3.us-west-2.amazonaws.com/www.lsusr.com/lesswrong/sensory-input-or-motor-control/field-mouse.jpg)

Not necessarily. Mice are notoriously good sniffers and yet human beings beat them on a odor sensitivity test by biologist Matthias Laska at Linköping University in Sweden.

[![](https://s3.us-west-2.amazonaws.com/www.lsusr.com/lesswrong/sensory-input-or-motor-control/nose-to-nose.jpg)](https://www.discovermagazine.com/mind/the-sense-of-smell-in-humans-is-more-powerful-than-we-think)

# Conclusion

I think larger organisms have larger brains in order to process more sensory data. If this is true then human beings aren't anywhere close to the maximum intelligence permissible by biology. It may be possible to trade some of our high-resolution sensory input for general cognitive power.

This isn't to say other factors don't matter. Some of our brain size probably goes toward our exceptional fine motor control. But I think the encephalization quotient baseline is driven by how much neurological hardware is necessary to handle sensory input.