By applying the powerful methodologies of cell biology to the hippocampus, Alden and I easily picked some low-hanging intellectual fruit. Soon after we started we had our first successful experiment. I shall never forget it. I work all morning and part of the afternoon to complete the surgery…I connected the recording electrode to a loudspeaker…We were trying to record from neurons in the hippocampus…Suddenly we heart the loud bang! Bang! Bang! Of action potentials, a sound I recognized immediately from my experiments on crayfish. Alden had penetrated a cell! Every stimulus I applied elicited a beautiful, large action potential…Alden and I were euphoric—we had obtained the first intracellular signals ever recorded from the region of the brain that stores our fondest memories! We almost danced around the lab…
This experiment and the ones that followed were physically exhausting, sometimes lasting twenty-four hours. It was a good thing we had both just finished a medical internship, where working twenty-four hours at a stretch was not uncommon…
As we became more familiar with the hippocampus, we realized that finding out how its neural networks process learned information…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…But it became clear to me…that to make any reasonable progress, 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…But what animal? Here Alden and I parted intellectual company. He was committed to mammalian neurophysiology…
Even though it meant swimming against the tide of current thinking, I yearned for a more radical, reductionist approach…In the 1950s and 1960s, most biologists shared Alden’s reluctance to apply a strictly reductionist strategy to the study of behavior…Not surprisingly, I was discouraged from pursuing this research strategy by a number of senior scientists in neurobiology…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…could be found by focusing on individual nerve cells.
Alden Spencer and I had found few differences in the basic properties of neurons that participate in memory storage and those that do not. Those findings supported the idea that memory relies not on the properties of the nerve cell per se but on the nature of the connections…This led me to think that memory may result from changes in synaptic strength brought about by certain patterns of sensory stimulation…
John Eccles had been very enthusiastic about the possibility that synapses change in response to excessive use, but when he tested the idea, he found that they changed for only a brief period of time..It now dawned on me that perhaps Pavlov was so successful in producing learning because the simple patterns of sensory stimulation he used elicited certain natural patterns of activation that were particularly suited for producing longer changes…With further reflection I decided to try to stimulate in the nerve cells of Aplysia the patterns of sensory stimulation that Pavlov had used in his learning experiments…I would remove the nervous system and work on a single ganglion, a single cluster of about two thousand nerve cells. Second, I would select a single nerve cell—a target cell—in that ganglion to serve as a model of any changes that might occur as a result of learning…I would then apply different patterns of electrical pulses modeled on the different forms of learning to a particular bundle of axons extending from sensory neurons on Aplysia’s body surface to the target cell.
To simulate habituation, I would apply repeated, weak electrical pulses to this neural pathway. To simulate sensitization, I would apply repeated, weak electrical pulses to this neural pathway. To simulate sensitization, I would stimulate a second neural pathway very strongly, one or more times, and see how it affected the target cell’s response to weak stimulation of the first pathway. Finally, to simulate classical conditioning, I would pair the strong stimulus to the second pathway with the weak stimulus to the first pathway in such a way that the strong stimulus would always follow and be associated with the weak stimulus.
I dissected out the abdominal ganglion [of an Aplysia snail], with its two thousand nerve cells, and mounted it in a small chamber bathed with aerated seawater. I placed microelectrodes inside one cell and then recorded that cell’s responses to various sequences of stimuli applied to the neural pathways that converged on it. I used three patterns of stimulation, based on Pavlov’s work in dogs..The experiments proved even more effective than I had anticipated…
I applied a weak electrical stimulus…and then repeated that stimulus ten tens. I found that the cell’s response to the stimulus decreased progressively. By the tenth stimulus, the response was only about one-twentieth as strong as it had been initiatlly, just as an animal’s behavioral response abates when aneutral stimulus is presented repeatedly…Then I a applied a series of five stronger stimuli to a different pathway leading to the same cell. The cells’ response to stimulation of the first pathway was greatly enhanced..up to thirty minutesThen I repeatedly paired a weak stimulis to one neural pathway with a strong stimulus to another pathway. The weak stimulus came first and acted as awarning of the strong stimulus. The pairing of the two stimuli greatly enhanced the cell’s response to the weak stimulus.
By 1985, after more than fifteen years of hard work, we had shown that a simple behavior in Aplysia could be modified by various forms of learning…Next we needed to map the neural circuitry…To my surprise, I found that I could readily map the synaptic connections between cells. By inserting a microelectrode into a target cell and stimulating action potentials in other cells of the ganglion, one cell at a time…it proved possible for the first time in any animal to map the working synaptic connections between individual cells.