Brains are connecty


Brains have different types of cells. We care most about neurons. Here are some mouse neurons:

Mouse neurons

The green blob in the center is the body of a neuron. Cute, huh?

Here’s a diagram of a neuron:


Neurons are connected in a network in your brain. You have millions of neurons. Estimates vary. Anyway, there are a lot of them. Each neuron can be connected to anywhere from a few to thousands of other neurons.

The dendrites are the inputs of the neuron. The synapses are the outputs. When the overall input reaches some level, the neuron fires. The signal goes over synapses to the dendrites of other neurons. Synapses are small gaps between neurons. “Small” means about 30 nanometers.

How do signals cross the gap? The sending neuron spits out chemicals called neurotransmitters. They’re grabbed by receptors on the other side.

Some neurons are connected to your senses. For example, your optic nerve connects to neurons at the back of your brain, as this not-at-all-creepy image shows:

Optic nerve connecting to neurons

Spreading activation, connection strength, and firing frequency

When a neuron fires, activation spreads to other neurons. That’s how memories are triggered. Something reminds you of something else. The more neurons that are connected to a firing neuron, the wider the activation spreads.

Some connections are stronger than others. When a connection is strong, it doesn’t take much for one neuron to trigger another. For example, memories associated with emotional events are particularly strong. A slight smell can evoke memories of an emotionally laden childhood event.

Connections become stronger the more frequently they are triggered. Neurons that frequently activate at the same time will tend to build stronger connections. There’s a saying in neuroscience: “Cells that fire together, wire together.”


What happens when there are few, weak, rarely activated connections? You fuhgeddaboudit.


When a brain sees something, neurons fire, and activation spreads. The spread is limited. Activation only spreads to neurons that have strong connections. Except…

What if some neurons are already partially activated? An example. N0 and N1 are two neurons:


N1 fires when total inputs reach 0.5. (Real neurons use more complex functions than summing, but that doesn’t matter for this discussion.)

There’s a link between N0 and N1 with a strength of 0.3. Other neurons linked to N1 have a total strength of 0.4.

If N0 fires and the others don’t, then N1 will not fire. However, if the others are also active, then N0 firing will trigger N1. The other neurons are said to “prime” N1, making it more likely to activate.

You can see priming at work in surveys. For example, let’s say you ask a question about gay marriage. Responses will differ, depending on the question asked before the question on gay marriage.

  • If the prior question is about equal opportunity, then more people will say they support gay marriage.
  • If the prior question is about religion, then fewer people will say they support gay marriage.


Priming is a source of context effects in learning. For example, Sam is in a physics class. The prof says:

“If you drop a 10 kilo weight and a 1 kilo weight, they’ll hit the ground at the same time.”

Later that day, Sam is in the cafeteria. Someone says,

“If you drop a watermelon and an egg, the watermelon will hit the ground first.”

Sam might agree with both predictions. Why? Sam uses non-intuitive beliefs about gravity in the physics context, and only in that context. The context is triggered when she walks into the physics lecture hall. The context fades when she leaves the lecture hall. In “regular” day-to-day contexts, she uses the more intuitive belief.

Context effects are normal for human brains. That’s just the way they work. Students often need help if they are to transfer ideas from one context to another.


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