Glossary

amygdala A group of nuclei in the anterior-medial part of the brain that are involved in emotional memory.

declarative memory Memory that involves the conscious recollection of events.

DNA (deoxyribonucleic acid) The central library of an organism's genetic information.

DNA microarray Technique used to investigate thousands of genes from the same tissue sample.

engram The memory trace laid down in the brain. It is believed to be formed by changes in the synapse.

Hebbian synapse When a signal between two neurons is strengthened due to the simultaneous activation of the presynaptic and postsynaptic neurons.

hippocampus A structure located in the medial temporal lobe that is involved in spatial learning.

learning A change in behavior as a result of experience.

long-term potentiation A form of synaptic plasticity induced by brief high-frequency stimulation.

memory The ability to store and recall an experience.

nondeclarative memory Memory usually measured by performance, such as automatic motor skills.

RNA (ribonucleic acid) Short-lived molecule that contains information about the DNA.

transduction The change of physical energy into neural signals.

The neuron is the principal functional unit of the brain.

Neurons both transform physical energy such as pressure and temperature into neural energy and conduct light and sound energy transformed by the eyes and ears. This transformation is the first step in information processing. Because we all have slightly different perceptual worlds and experiences, our individual identity is based on our memories of perceptions. This article presents learning and memory as an example of an information processing system. The article takes a cognitive and molecular biology approach in understanding how memories are formed.

The world first exists, and then the states of mind; and these gain a cognizance of the world which gets gradually more and more complete.

William James (1892)

Understanding movement of sensation from the external environment to the inside of one's head as useful information is a process that has consumed philosophers and scientists for hundreds of years. Think of the Challenger space shuttle explosion. In your mind's eye you can visualize the twin plumes of smoke streaking higher into the sky that represented the end of the mission. How did this image get into your brain and why will it be with you for the rest of your life? These are two questions this article seeks to address.

The human body exists in a sea of physical stimuli, only a tiny fraction of which we are aware. Consider

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the electromagnetic spectrum, which includes X-rays able to permeate solid objects and ultra-low-frequency radio waves with which we can contact submerged submarines thousands of miles away. We see only a tiny fraction, from 400 to 760 nm. We do not see cosmic rays or television transmissions. Perhaps our tiny range of vision allows us to focus on those external objects necessary for our survival. Our senses, then, are transducers that convert physical energy to neural energy. Ears convert sound, nerve endings convert temperature and pressure, the labyrinth converts gravity, the tongue converts acids and sugars, and the nose captures molecules from the air and gives them each a unique sensation. Consider the curious life of synasthets, people who hear light or see sound when their neural energies are crossed at the level of the thalamus so that hearing neural energy goes to the visual areas and vice versa.

We do not all perceive the same world because physical energy is transduced by biological systems that have many states and conditions. A person who walks into a darkened movie theater does not see nearly as clearly as the person who has been in the theater for a long time and who is dark adapted. At that moment their perceptual worlds differ. A person unable to distinguish red and green, a dichromatic, sees a different world, and a blind person does not see the world at all. Because we all have slightly different perceptual worlds and experiences, our individual identity is based on our memories of perceptions. As William James said, cognizance of the world is an iterative process that depends on perception and memory; together, perception and memory allow information processing. Everyone agrees that the brain is the organ that perceives the world and stores our individual memories, and the computational unit of a brain is the neuron (Fig. 1). As a cell, the neuron exhibits continuous metabolic activity but exists in only one of two transmission states, "on" and "off." Thus, it is analogous to a computer memory address, which contains a bit of information—either 1 or 0. Although the content of a computer memory address is set by a central processor, a neuron is much more complicated, gathering information from its dendrites and deciding whether and how many on states, called action potentials, it will generate. Neurons connect through special contacts called synapses at the end of axons. As depicted in the Fig. 1, a neuron connects with many other neurons, usually on a part of the neuron called a dendrite.

Figure 2 shows that neurons are organized into units called nuclei and these nuclei perform functions and communicate with other nuclei. Neuroanatomy is the study of the organization of nuclei. Continuing our computer analogy, nuclei function like integrated circuits. Note that in Fig. 2, neurons become more complex as one moves up the phylogenetic scale from mouse to human. Because it is more complex, the human neuron has more computational power. This is the basis of superior information processing in humans. Note that rat and human brains are similar in architecture. Both have cerebral hemispheres, both have a thalamus, and both have a cerebellum. Based on this similarity, we expect humans and rats to have similar functions, and both are capable of learning mazes, for example.

Now we can ask a difficult question: How can a perception become a memory and vice versa? Figure 3 shows a most intriguing result. In this experiment, a monkey was trained to look at a fixed point in the center of the circular pattern, and this was followed by

Figure 1 The neuron is the basic signaling unit of the nervous system. There are 1012neuronsinthe brain and most have a cell body that gives rise to the axon, dendrites, and synaptic boutons or terminals [reproduced with permission from Rosenzweig, M. R., Leiman, A. L., and Breedlove, S. M. (1996). Biological Psychology. Sinauer Associates, Sunderland, MA].

Figure 1 The neuron is the basic signaling unit of the nervous system. There are 1012neuronsinthe brain and most have a cell body that gives rise to the axon, dendrites, and synaptic boutons or terminals [reproduced with permission from Rosenzweig, M. R., Leiman, A. L., and Breedlove, S. M. (1996). Biological Psychology. Sinauer Associates, Sunderland, MA].

Rat Human Brain
Figure 2 Midsagittal view of human and rat brains. The structures observed contain neurons and each performs different functions [reproduced with permission from Rosenzweig, M. R., Leiman, A. L., and Breedlove, S. M. (1996). Biological Psychology. Sinauer Associates, Sunderland, MA].

Figure 3 (A) A pattern of flickering lights shown in the visual field of a macaque monkey (B) Pattern is mapped in the striate cortex using 2-deoxyglucose (reproduced with permission from Tootell et al, 1988. Deoxyglucose analysis of retinotopic organization in primate striate cortex. Science 218, 902-904).

Figure 3 (A) A pattern of flickering lights shown in the visual field of a macaque monkey (B) Pattern is mapped in the striate cortex using 2-deoxyglucose (reproduced with permission from Tootell et al, 1988. Deoxyglucose analysis of retinotopic organization in primate striate cortex. Science 218, 902-904).

an injection with a synthetic form of glucose. Neurons that were metabolically active look darker. The circular pattern is recreated on the cortex of the monkey. A network of interconnected neurons represents the perception. The pattern can be stored in the monkey brain and made into a memory if the network that represents the circle can be made permanent. Hebb proposed this idea in 1949, but he called the network of neurons a cell assembly. In order for the monkey to re-experience the pattern, the network has to be reactivated in its brain.

Hebb proposed that the pattern could be made permanent if the connections between neurons were made stronger in a functional sense. That is, if neuron A, the first in the network representing the circle pattern, persistently fires neuron B, and so on, then some process takes place in either A or B and all the neurons that represent the pattern so that the efficiency of A firing B is increased. In other words, when neuron A fires the pattern is recreated. Today, the phenom enon by which one cell creates a larger response in the second by repeated stimulation is called long-term potentiation (LTP), and it is considered to be the best model of how the brain may store information.

Are there any nuclei in the brain whose function is to make memories? In the 1950s, a patient H.M. had his hippocampus removed on both sides of his brain to control his grand mal epilepsy. Following the surgery, H.M. could remember the past, but he could not remember anything new. He was said to have a deficit in his ability to consolidate or make permanent new memories. Interestingly, Fig. 4 shows that H.M. could learn a backward mirror drawing task, even though he did not remember the apparatus from day to day. It is thought that the hippocampus is a brain structure whose function is to lay down new declarative memories or "knowing that'' (the apparatus) rather than "knowing how'' (to draw backwards). In Section III.A, we take a closer look at the hippocampus and determine whether its organization or arrangement of

(a) The mirror-tracing task

(a) The mirror-tracing task

(b) Performance of H.M. on mirror-lracing task

Trials

Figure 4 (a) The mirror-tracing task used to test memory in patient H.M. (b) Performance of H.M. on this task. Despite the improvement over days in this task, H.M. did not remember having performed this task in the previous days [reproduced with permission from Rosenzweig, M. R., Leiman, A. L., and Breedlove, S. M. (1996). Biological Psychology. Sinauer Associates, Sunderland, MA].

Trials

Figure 4 (a) The mirror-tracing task used to test memory in patient H.M. (b) Performance of H.M. on this task. Despite the improvement over days in this task, H.M. did not remember having performed this task in the previous days [reproduced with permission from Rosenzweig, M. R., Leiman, A. L., and Breedlove, S. M. (1996). Biological Psychology. Sinauer Associates, Sunderland, MA].

cells provide clues as to how it might lay down memories. In Section III.G, we examine the phenomenon of LTP in the hippocampus. If the hippocampus really is a nucleus designed to lay down memories, then LTP should abound in this structure. Finally, we examine an interesting phenomenon of memory: Many are permanent. What could possibly change in a neuron so that you remember the first time you rode a bicycle?

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