In Search of Memory: The Emergence of a New Science of Mind (19 page)

I was therefore taken aback—and hurt—when the day after I gave the seminar on our work, Felix stopped talking to me. I could not understand what had happened. Only with time did I realize that science is filled not simply with a passion for ideas but also with the ambition and strivings of people at different stages of their careers. Many years later, Felix renewed our friendship and explained that he had been chagrined when two relatively inexperienced scientists—incompetents, in his eyes—had been able to produce interesting and important experimental results.

As the afterglow of our beginner’s luck began to fade, Alden and I realized that as fascinating as our findings were, they were leading us in directions unrelated to memory. In fact, we found that the properties of hippocampal neurons were not sufficiently different from those of spinal motor neurons to account for the ability of the hippocampus to store memories. It took us a year to realize what should have been obvious from the start: the cellular mechanisms of learning and memory reside not in the special properties of the neuron itself, but in the connections it receives and makes with other cells in the neuronal circuit to which it belongs. As we learned through reading and discussions with each other to think more deeply about the biological mechanisms of learning and memory, we concluded that the role of the hippocampus in memory must arise in some other way, perhaps from the nature of the information it receives, the way its cells are interconnected, and from how that circuitry and the information it carries are affected by learning.

That change in our thinking led us to change our experimental approach. To understand how the neural circuitry of the hippocampus affects memory storage, we needed to know how sensory information reaches the hippocampus, what happens to it there, and where it goes after it leaves the hippocampus. This was a formidable challenge. Practically nothing was known then about how sensory stimuli reach the hippocampus or how the hippocampus sends information to other areas of the brain.

We therefore carried out a series of experiments to examine how various sensory stimuli—tactile, auditory, and visual—affect the firing pattern of the pyramidal neurons of the hippocampus. We saw only occasional, sluggish responses—nothing comparable to the brisk responses reported by other investigators in the neural pathways of the somatosensory, auditory, and visual cortices. In a final attempt to understand how the hippocampus might participate in memory storage, we explored the properties of the synapses that incoming axons of the perforant pathway form with the nerve cells in the hippocampus. We stimulated those axons repetitively at the rate of 10 impulses per second and observed an increase in synaptic strength that lasted about 10 to 15 seconds. We then stimulated them at the rate of 60 to 100 impulses per second and produced an epileptic seizure. These were all interesting findings, but they were not what we were looking for!

As we became more familiar with the hippocampus, we realized that finding out how its neural networks process learned information and how learning and memory storage change those networks was an extraordinarily difficult task that would take a very long time.

I was initially drawn to the hippocampus because of my interest in psychoanalysis, which tempted me to tackle the biology of memory in its most complex and intriguing form. But it became clear to me that the reductionist strategy used by Hodgkin, Katz, and Kuffler to study the action potential and synaptic transmission also applied to research on learning. To make any reasonable progress toward understanding how memory storage occurs, it would be desirable, at least initially, to study the simplest instance of memory storage and to study it in an animal with the simplest possible nervous system, so I could trace the flow of information from sensory input to motor output. I therefore searched for an experimental animal—perhaps an invertebrate such as a worm, a fly, or a snail—in which simple but modifiable behaviors were controlled by simple neuronal circuits made up of a small number of nerve cells.

But what animal? Here Alden and I parted intellectual company. He was committed to mammalian neurophysiology and wanted to stay with the mammalian brain. He felt that although invertebrates are instructive, the organization of the invertebrate brain is so fundamentally different from that of the vertebrate brain that he did not want to work on them. Moreover, the components of the vertebrate brain were already well described. Biological solutions valid for the rest of the animal kingdom would draw his interest and his admiration, but unless they were true for the vertebrate brain, the human brain, they would not draw his effort. Alden therefore turned to one of the simple subsystems of the spinal cord of the cat and examined spinal reflexes, which are modified through learning. Over the next five years Alden made important contributions in this area of research in collaboration with the psychologist Richard Thompson. However, even the relatively simple reflex circuits in the spinal cord proved too difficult for a detailed cellular analysis of learning, and by 1965 Alden had turned from the spinal cord and the study of learning to other areas of research.

EVEN THOUGH IT MEANT SWIMMING AGAINST THE TIDE OF
current thinking, I yearned for a more radical, reductionist approach to the biology of learning and memory storage. I was convinced that the biological basis of learning should be studied first at the level of individual cells and, moreover, that the approach was most likely to succeed if it focused on the simplest behavior of a simple animal. Many years later, Sydney Brenner, a pioneer of molecular genetics who introduced the worm
Caenorhabditis elegans
to biology, was to write:

In the 1950s and 1960s, however, most biologists shared Alden’s reluctance to apply a strictly reductionist strategy to the study of behavior because they thought it would have no relevance for human behavior. People have mental abilities not found in simpler animals, and these biologists believed that the functional organization of the human brain must be quite different from that of simpler animals. Although this view holds some truth, I thought it overlooked the fact—amply demonstrated in the fieldwork of ethologists like Konrad Lorenz, Niko Tinbergen, and Karl von Frisch—that certain elementary forms of learning are common to all animals. It seemed likely to me that, in the course of evolution, humans had retained some of the cellular mechanisms of learning and memory storage found in simpler animals.

Not surprisingly, I was discouraged from pursuing this research strategy by a number of senior scientists in neurobiology, including Eccles. His concern reflected in part the hierarchy of acceptable research questions in neurobiology at that time. Although some scientists were studying behavior in invertebrates, that work was not considered important—indeed, it was largely ignored—by most people working on the mammalian brain. Of even greater concern to me was the skepticism of knowledgeable psychologists and psychoanalysts that anything interesting about higher-order mental processes like learning and memory could be found by focusing on individual nerve cells—particularly the cells of an invertebrate. I had made up my mind, however. The only remaining question was which invertebrate would best suit the cellular study of learning and memory.

Besides being a good place to do research, NIH was also a good place to learn about new developments in biology. During the course of any given year, most good scientists working on the brain visit the NIH campus. As a result, I was able to speak with many people and to attend seminars in which I learned about the experimental advantages of various invertebrate animals, such as crayfish, lobsters, honeybees, flies, land snails, and the nematode worm
Ascaris
.

I remembered vividly Kuffler’s description of the advantages of the crayfish’s sensory neuron for studying the properties of dendrites. But I ruled out crayfish: although they have a few very large axons, their nerve cell bodies are not very large. I wanted an animal with a simple reflex that could be modified by learning and that was controlled by a small number of large nerve cells whose pathway from input to output could be identified. In that way, I could relate changes in the reflex to changes in the cells.

After about six months of careful consideration, I settled on the giant marine snail
Aplysia
as a suitable animal for my studies. I had been greatly impressed with two lectures I had heard about the snail, one given by Angelique Arvanitaki-Chalazonitis, a senior, highly accomplished scientist who had discovered the usefulness of
Aplysia
for studying the signaling characteristics of nerve cells, and the other by Ladislav Tauc, a younger person who brought a new biophysical perspective to the study of how nerve cells function.

Aplysia
was first mentioned by Pliny the Elder in his encyclopedic study
Historia Naturalis
, written during the first century
A.D
. It was mentioned again in the second century by Galen. These ancient scholars called it
lepus marinus
, or sea hare, because, when sitting still and contracted, it resembles a rabbit. When I began to examine
Aplysia
myself, I found, as others had before me, that it releases copious amounts of purple ink when disturbed. That ink was once erroneously thought to be the royal purple used to dye the stripe on the togas of the Roman emperors. (The royal purple dye is actually secreted by the clam
Murex
.) Because of
Aplysia
’s tendency to ink so profusely, some ancient naturalists also thought it was holy.

The American species of
Aplysia
that lives off the California coast (
A. californica
), and which I have spent most of my career studying, measures more than one foot in length and weighs several pounds (figure 9–2). It assumes the reddish brown coloration of the seaweed it feeds on. It is a large, proud, attractive, and obviously highly intelligent beast—just the sort of animal one would select for studies of learning!

What drew my attention to
Aplysia
was not its natural history or physical beauty but several other features outlined by Arvanitaki-Chalazonitis and by Tauc in their lectures on the European species (
A. depilans
). They both emphasized that the brain of
Aplysia
has a small number of cells, about 20,000, compared with about 100 billion in the mammalian brain. Most of these cells are grouped into nine clusters, or ganglia (figure 9–3). Since individual ganglia were thought to control several simple reflex responses, it seemed to me that the number of cells committed to a single simple behavior was likely to be small. In addition, some of
Aplysia
’s cells are the largest in the animal kingdom, making it relatively easy to insert microelectrodes into them to record electrical activity. The pyramidal cells of the cat hippocampus, whose activity Alden and I had recorded, are among the largest nerve cells in the mammalian brain, yet they are only 20 micrometers in diameter (1/1250 of an inch) and can be seen only under a high-powered microscope. Some cells in
Aplysia
’s nervous system are fifty times that size and can be seen with the naked eye.

9–2
Aplysia californica
: the giant marine snail.
(Courtesy of Thomas Teyke.)