The History of the Concept of Modifiable Synapses
Objective 1: Describe the historical perspective underlying our understanding of modifiable synapses.
As of the early part of the 20th century, there was really good evidence that learning and memory systems would be located in a single brain region. After all, Phineas Gage’s unfortunate accident (as described by Harlow) had shown convincingly that something as complex as human emotion resided in a limited area of the brain, namely the prefrontal cortex. Paul Broca had characterized Broca’s area, and showed that the complexity of human language production was in an even smaller area of cortex. All the major sensory and motor systems had been localized to a part of cortex as described in Korbinian Brodmann’s famous map, and there were still plenty of the 48 Brodmann’s areas in human cortex that had still not been identified with a single function in lesion studies.
With characteristic hubris, early 20th century neuroscientists named the concept before they found it. Richard Semon coined the German term Engramm, which became engram in English, based on the conception that learning and memory are “written on the brain”. (Semon’s hubris came to a particularly tragic end and he is now mostly forgotten.)
A Harvard psychologist named Karl Lashley set out to find the location of learning and memory systems in the brain. His preferred model system was to use rodents in a maze, a method pioneered by Edward Tolman. Lashley’s contribution was to ablate different regions of the rat brain and then test them on a maze to see the effect on the rat’s ability to locate a reward at the end of a maze. His attempt to locate the engram had ended in failure (or so it was believed). Now Tolman’s maze exists only as a meme in the public imagination, but a modified version is still used in studies of the hippocampus, an area of the brain which contains place cells, neurons that code for the location of the animal. As we will see, the hippocampus is also the site of critical processes in learning and memory. Although not realized at the time, the work of Lashley was instrumental in helping us understand how these place cells have been adapted to encode what neuropsychologists call declarative memory — memory for facts, dates, and events, and the kind of memory you will use when you take an exam on this material.
In 1949, Donald Hebb published a monograph (i.e. long paper; short book) entitled The Organization of Behavior: A Neuropsychological Theory. This is one of those books that everyone makes reference to but very few have read. Through some difficult passages, the upshot of Hebb’s monograph was an explosive idea: maybe we were all looking for the wrong thing in our search for the engram. Maybe it’s not a place, but rather a process, that we should be looking for.
As often happens in science, it was both. The place is the hippocampus; the process is what has come to be known as a Hebbian synapse in Hebb’s honor. A Hebb