Animal Senses #13: Combinations

In the last but one chapter of Immense World, Ed Yong reminds us of something that was easy to forget given the level of detail each chapter had on one particular sense. Animals’ don’t use just one sense, they use a combo. How do mosquitoes identify the individual to suck blood from? By sensing their body heat and the release of carbon dioxide. Scientists tried masking one of the two signals, and the mosquito would still home in on its, er, food source.

 

It is obvious why animals use multiple senses:

“Each sense has pros and cons, and each stimulus is useful in some circumstances and useless in others.”

Beyond complementing each other, senses can also “combine”. In humans, it is rare and we call it “synesthesia” (For people with this ability/condition, sounds have colors, words have tastes and so on).

“This perceptual blurring is special among humans, but standard to other creatures.”

How do we know this? Because the neurons in their brain that receive signals from one sense also receive signals from another sense, so their brain almost certainly processes a combo of the two types of signals. A platypus, for example, probably combines its electroreception with its touch signals to have “electrotouch”. Mosquitoes have neurons that can respond to both temperature and chemicals. It could mean that they can taste heat. Receptors in many species aren’t “tidy”, i.e., they don’t just handle one type of signals only. Nature doesn’t design for engineering elegance; it goes with whatever works.

 

Then there are the inward facing senses, informing animals of the state of their own bodies. Examples include the sense of position and movement; of balance.

“(They’re) so important that we take them for granted.”

 

Which brings us to the problem we don’t even think of. There are two types of signals: (1) exafference – signals produced by stuff outside the body; and (2) reafference – signals produced by stuff inside the body. Now comes the catch:

“These signals are the same from the point of view of the sense organs.”

Put differently:

“How does it know if it is touching something, or if it is being touched?... If a fish’s lateral line deflects flowing water, is that because something is swimming towards it, or because it is itself swimming?”

 

This is such a fundamental problem for all animals that they all have the same solution. When a motor command is issued to an organ to do something (move, turn, open, whatever), a “copy” of that command is used by the brain to predict the consequence of that command. The brain then compares the output (actual) v/s predicted to differentiate the change in the world outside v/s the change due to its own action. This is why our vision is stable even when we are moving, why chirping crickets can block out the sound of their own calls.

“These feats are so profound that they don’t feel like feats at all.”

 

Then there’s the octopus. Each of its 8 arms has an almost-complete nervous system, and then there is its brain. What that means is that in effect, it has “nine brains that have their own agendas”. Even more strangely, its 8 arm brains deal with touch and taste-based signals, while its head brain is about vision signals. Surely there must be cross-talk and contradictory “ideas” among the brains?

“Its unusual senses challenge our imagination, but so does the way it brings those senses together. Its component threads are unfamiliar, the weave is exotic, and the tapestry that results is utterly alien.”

(Ability to explain aside, Yong’s English is awesome).