As the race to decipher the genetic code came to a close in 1965, Nirenberg sought out new scientific puzzles. Many minds were still trying to unravel the mysteries of protein synthesis, but Nirenberg's mind was on another mystery--that of the mind itself. From 1965 to 1969 Nirenberg turned his attention and his laboratory over to the field of neurobiology.
Neurobiology is the study of the brain and the nervous system. Neurobiologists seek to understand the many facets of this system by considering its development, physiology, activities, and malfunctions. Neurons, the cells of the nervous system, interact with each other at molecular, chemical, cellular, and electrical levels to form an intricate and complicated web called the "neural network". In 1913, physiologist Edgar Douglas Adrian studied the neural network and discovered that neurons transfer information throughout the nervous system by sending electrical impulses in discrete units, creating "spikes" of electrical discharge. Modern neurobiologists still study the neural network by investigating how the nervous system processes and transfers the large amount of information that is stored within the complex web of neurons.
Nirenberg's move to neurobiology may at first seem to be a peculiar shift from molecular biology, but it makes sense when information processing is considered. There are only two biological systems that process information by receiving it, storing it, and then relaying it: the DNA- RNA-protein system, which processes heritable, genetic information; and the brain, which processes sensory and mental information. The analogy between the genetic code and the brain attracted many molecular biologists who had studied genetics. For example, Seymour Benzer studied the genetics of viruses, a subject for molecular biology, in the 1940s, and later investigated the relationship between genes and behavior in the fruit fly Drosophila melanogaster in the 1960s and 1970s. Similarly, Sydney Brenner helped pioneer the field of molecular biology in the 1940s and 1950s but then studied Caenorhabditis elegans, a small roundworm, to examine its genetics and behavior in the 1960s and 1970s. Julius Adler also worked extensively in molecular biology and then moved on in the 1960s to investigate the chemical basis of motion in bacteria, a topic in neurobiology. Other molecular biologists, such as Gunther Stent, Max Delbruck, Francis Crick, and Cyrus Levinthal, joined these scientists in the molecular migration to neurobiology in the late 1960s and early 1970s.
Neurobiology offered Nirenberg the freedom to investigate a whole new field of science. He has called this period of his scientific career one of the happiest times of his life. But the transition to neurobiology and the newfound freedom it offered was not entirely free of problems. Nirenberg had made a similar transition in the late 1950s when he left the comfort and familiarity of biochemistry to enter the unknown world of molecular biology. A decade later, he was making yet another transition from the familiar to the unknown. Nirenberg also faced other dilemmas. Neurobiology was still evolving and experienced growing pains of its own in the 1960s; as Nirenberg perceived, entering neurobiology was "like walking up to your neck in a swamp."
One of the "swampy" areas of neurobiology in 1967 revolved around the nature of the neural code. In the 1950s and 1960s, neurobiologists thought that Adrian's spikes might carry and process the neural information in the form of a neural code. Nirenberg studied the neural code by employing the conceptual and experimental approaches to science that proved so successful for him in his work on the genetic code. He looked for the general rules of the code: he sought to identify the basic units of information used by the system; he hunted for the underlying logic of the system; and he immediately began considering what biological systems he could utilize to study the neural network and its code.
Nirenberg considered the various facets of the neural code for more than a year, but these reflections never made it into a published form. Nirenberg did draw the analogy between the genetic code and the neural code in a draft of his 1968 Nobel acceptance speech, but he edited this material from the final version. Nirenberg also presented his ideas at Howard University in a 1969 lecture titled "Genetic Versus Neural Information Processing Systems", but he was only willing to endorse the thoughts as "speculations". While Nirenberg's interest in the similarities between the genetic and the neural code never evolved past the conceptual stage, these ideas helped to spark his curiosity in how the brain and the nervous system develop. This curiosity would drive much of his research in neurobiology for the next thirty years.