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updated 10/19/2005 10:00:10 PM ET 2005-10-20T02:00:10

As you dash outdoors in the middle of winter, you might make it halfway down the block before realizing that your ears are freezing because you forgot your hat.

Now, scientists have shown that even though you've had an apparent memory lapse, your brain never forgot what you should have done.

Memory works mainly by association. For example, as you try to remember where you left your keys, you might recall you last had them in the living room, which reminds you that there was a commercial for soap on television, which reminds you that you need soap, and so on. And then, as you're heading out the door to buy soap, you remember that your keys are on the kitchen counter.

Your brain knew where the keys were all along, it just took a round-about way to get there.

Now, scientists at the Salk Institute for Biological Studies are studying associative memory in monkeys to figure out just how this complicated process works.

First, the researchers trained a group of rhesus monkeys to remember arbitrary pairs of symbols. The researchers showed the monkeys one symbol (cold weather) and then gave them the choice of two other symbols, one of which (a hat) would be associated with the first. A correct choice would earn them a sip of their favorite juice.

Most of the monkeys performed the test flawlessly, but one kept making mistakes.

"We wondered what happened in the brain when the monkeys made the wrong choice, although they apparently learned the right pairing of symbols," said study leader Thomas Albright.

Albright and his team observed signals from the nerve cells in the monkey's inferior temporal cortex (ITC), an area of its brain used for visual pattern recognition and for storing this type of memory.

As the monkey was deciding which symbol to choose, about a quarter of the activity in the ITC was due to the choice behavior.

Meanwhile, more than half of the activity was in a different group of nerve cells, which scientists believe represent the monkey's memory of the correct symbol pairing, and surprisingly, these cells continued to fire even when the monkey chose the wrong symbol.

"In this sense, the cells 'knew' more than the monkeys let on in their behavior," Albright said. "Thus, behavior may vary, but knowledge endures."

This study is detailed in the Oct 20 issue of the journal Neuron.

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