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How does the mind search our memory?

This question originally appeared on Quora. Answer by Paul King, Computational Neuroscientist.

Understanding how the mind works is still very much the subject of active research in both neuroscience and cognitive science. The first thing to note is that the mind is not very fast. A computer can search millions of pages of documents much more quickly and accurately than the brain can. What the brain is good at is intelligence: Making sense of the world in a way that facilitates problem-solving, prediction, flexible action, language, and complex social organization.

The second thing is that the way memory works in the brain is very different from how it works in a computer. In a computer, information is represented in a literal way in memory, and searched by scanning memory locations in sequence, or sometimes using shortcuts like indexes to locate something.

Memory in the brain evolved to serve survival. Instead of storing memories in a "memory bank," the brain incorporates experiences into associational networks so that "lessons" from past experience can inform effective action when the time is right.

One type of information that the human brain can store is "declarative knowledge," which are facts and beliefs about the world inferred from observation or from what other people tell us. The facts one learns in school are forms of declarative knowledge, and while a computer can store facts like the names of all the countries of the world in milliseconds, humans often need days or weeks of practice to store this same quantity of information, and then it is often forgotten weeks or months later.

So while the human ability to store declarative knowledge is fairly unique in the animal world, it is also fairly unnatural for the brain compared to a digital computer. Where the human brain excels is in perception and learning flexible skills, whereas computers are particularly bad at this.

The brain does not have much innate ability to "search" its memory. When concepts enter working memory, often prompted by statistical matching against sensory input, related concepts are automatically activated by association and incorporated into perception or brought into working memory.

When you notice a familiar face, your brain is not scanning a memory bank of faces to find a match; instead the brain is processing all elements of the visual scene in parallel through a statistical model to make predictions about the future. Instead of perceiving a face that matches against memory, you instead see the familiar person. In this way memory is incorporated directly into the process of perception and does not happen as a separate "search and recall" step.

When we attempt to search our own memory, for example to produce an answer for an exam, we often use "cognitive strategies" to exploit the innate associational machinery of the brain. For example you might reflect on things you believe to be related to the answer, or you might think back to the moment when you learned a fact, or imagine the person who told it to you and replay how that conversation went.

Recalling memories can thus be broken into two different aspects:

  1. How the brain works naturally, which is by parallel statistical matching, conceptual association, and episodic recall.
  2. The "cognitive strategies" and skills we learn to exploit these natural mechanisms, for example the memory tricks we learn in school in order to achieve greater cognitive control over declarative memory storage and recall.

One of the "technologies" that humans have invented to aid declarative memory and recall is language.

Helen Keller, who was deaf and blind from the age of three and rediscovered language at the age of seven, recalls her life before language as a blur of experience like being "at sea in a dense fog." After acquiring her first words, "suddenly I felt a misty consciousness as of something forgotten - a thrill of returning thought; and somehow the mystery of language was revealed to me. Everything had a name, and each name gave birth to a new thought. As we returned to the house every object which I touched seemed to quiver with life. That was because I saw everything with the strange, new sight that had come to me."

Human language, which evolved over many tens of thousands of years, is itself a cognitive technology, shared by society at large, and transmitted from parents to children. Language provides the brain with a synthetic structure to aid thought and the accumulation and use of knowledge, including memory.

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