The roots of many sciences can be found in philosophy. This is hardly surprising since until recent times philosophy and science (and even religion) did not have clear boundaries between them. Psychology is no exception. The topic of memory has been a topic of interest to philosophers since the times of the ancient Greek. Every philosopher since Plato and Aristotle that became interested in the question of mind, had to face the problem of memory. After all, the field of epistemology, or the science of knowledge, is inherently linked with our understanding of memory.
In the beginning of the nineteenth century, the French philosopher Maine de Biran wrote a monograph entitled ``The influence of habit on the faculty of thinking'' [de Biran, 1804]. He observed that habits were developed by repetition and resulted in increased automaticity of execution and decreased conscious awareness. He acknowledged the fact that there could be different kinds of memory. He postulated three different kinds, which he referred to as mechanical memory, sensitive memory and representative memory. Mechanical memory is involved in acquisition of motor and verbal habits that operate at an unconscious level. Sensitive memory is involved in acquiring feelings, affects and fleeting images, and it too operates at an unconscious level. The representative memory is what performs the conscious recollection of facts and ideas. Given the lack of knowledge and empirical data of his time, this was a rather remarkable account.
The phrenologists of the mid nineteenth century, although later dismissed because of their wrong conclusions based on correlating high level behavioral traits with bumps on the skull, actually had some respectable observations and theories about the workings of the mind. Franz Joseph Gall focused on the differences in the type of information handled by different forms of memory. He contrasted between two different types of faculties. First content or domain specific modules that operate on particular kinds of information. The other type cuts across content domains, such as unitary faculties of memory, judgment, perception and so forth. Gall commented that ``Perception and memory are only attributes common to the fundamental faculties, but not the fundamental faculties themselves'' [Gall, 1835]. Spurzheim elaborated Gall's ideas and argued that each mental faculty has a separate memory, relying on observations of within individual variations in memory abilities [Spurzheim, 1834].
Next comes the path breaking studies of physicians Broca and Wernicke on the language areas of the brain based on the studies of patients with brain lesions. Although these studies were influential for neuropsychology in general, they also have relevance as evidence for different faculties of memory. The ability to articulate words is learnt in childhood. In his classic work, Broca commented ``Is it not, after all, a kind of memory, and those who have lost it have lost, not the memory for words, but the memory of the procedures required for articulating words'' [Broca, 1861].
Ewald Hering, in his famous 1870 lecture at the Vienna Academy of Sciences emphasized the role of memory in cognition as a unifying force that holds the self together [Hering, 1920]. He proposed the concept of an ``organic memory'' involved in heredity, development, and habit. William Carpenter, in one of the earliest treatises on physiological psychology [Carpenter, 1874], argued that motor memory or habit should be distinguished from recollection of personal experiences.
The famous American psychologist William James, in his 1890 classic ``Principles of Psychology'' [James, 1890] treated memory and habit in separate chapters. He also distinguished between what he called primary memory, one that endured for a very brief time, and secondary memory, ``the knowledge of a former state of mind after it has already once dropped from the consciousness''. It is surprising that, not until 1958 [Broadbent, 1958]were separate short-term and long-term memories specifically postulated.
Although a number of scientists had been collecting data and theorizing about memory, it is not unfair to say that Hermann Ebbinghaus set the stage for a century long sequence of studies for the analysis of memory. The methodology he presented in his 1885 book ``Uber das Gedachtniss'' - the use of nonsense syllables as to-be-remembered materials, the savings method etc. - influenced generations of researchers. He laid out an experimental agenda that included such problems as repetition effects, remote associations, massed versus distributed practice, and the like. The merits and drawbacks of the Ebbinghausian influence on the psychological analyses of memory have been debated since the early years of memory research. However one should remember that he developed his methodology more than a century ago, in a setting where not much data was available. It is unfair to give him the responsibility if some later researchers were too slow in breaking out of the limits he had set. Ebbinghaus still is regarded as the father of the cognitive science of memory.
At the turn of the century, Freud published his much debated theories on psychoanalysis [Freud, 1900]. Although much of what he had said is dismissed because of being unscientific, and never mentioned in the contemporary cognitive science courses, he did have interesting hypotheses about the workings of the mind. Memory played quite an important role in his psychoanalytic theory. He most clearly emphasized the distinction between conscious and unconscious and drew the attention to the effect of infantile memories on a person's mental health and value judgments. Compared to Ebbinghaus, he tried to theorize about phenomena way ahead of his time scientifically, but this does not mean that the same questions he asked will not turn around and face science again.
In the late 1800s, a Russian physician, S. S. Korsakoff, reported on a syndrome he found with chronic alcoholics. Korsakoff's syndrome was to play an important role in the future studies of memory, since it is one of the most isolated cases of amnesia. Patients with Korsakoff's syndrome have normal IQs, are alert and attentive, appear motivated, and generally lack other neurological signs of cerebral deficit. This is in contrast with their extensive retrograde and anterograde amnesia.
The debate between globalism and localizationism in psychology was still heated in the beginning of the twentieth century. Globalists believed that the brain functioned as a single, integrated whole, localizationists believed that the brain was a collection of distinct organs, each responsible for a separate ability. The neuropsychological study of memory is rooted in this debate. Karl Lashley started a lifetime project in 1915 to identify the neural locations of learnt habits. In most of his experiments he removed portions of or made lesions in the cortex of animals. After decades of experimentation he was unable to localize specific memories in the brain. In 1950 he concluded that ``it is not possible to demonstrate the isolated localization of a memory trace anywhere in the nervous system. Limited regions may be essential for learning or retention of a particular activity, but ... the engram is represented throughout the region'' [Lashley, 1950]. Ironically, only three years later a neurosurgeon named William Scoville made one of the most influential discoveries of neuropsychology. He removed the hippocampus bilaterally from patient H. M. to cure his epilepsy and unexpectedly created the most famous and most studied subject of amnesia in history.
In the meanwhile studies in the cognitive science of memory were independently pursued by researchers such as Miller [Miller, 1956] and Sperling [Sperling, 1960]. These researchers concentrated on short term memory, and were influenced by the Ebbinghausian tradition at the methodological level. The predominant approach to long term memory in this era had been the associative interference theory, which emphasized the importance of relatively automatic and passive formation of stimulus-response bonds. Studies against this hypothesis were seen from time to time such as the work of the British psychologist Frederic Bartlett [Bartlett, 1932]. Bartlett proposed schemas - organized mental structures - as the key element in remembering. He emphasized that memory is a process of active reconstruction rather than a mere revival of previous experience. Associationist approach was finally taken over by organization theory and related cognitive approaches that highlighted the active and constructive nature of the mnemonic processes after the work of researchers such as Tulving [Tulving, 1962] and Mandler [Mandler, 1968]. Until very recently, the cognitive science of memory have been largely developed independent of the neuroscience perspective, and researchers have done little to connect their theories to neural matter. Since this review is focused on the neuroscience, I will not dive into any more detail of the pure cognitive approach.
One last bit of history I would like to go into before I close the section is the work of Hebb [Hebb, 1949]. Hebb went a step further from the behavioral data and developed a theory of the neurological basis of short-term and long-term memory in his classical book ``Organization of Behavior''. He invented Hebbian learning rule that models the change in a neural network by repeated experience. His principles with minor modifications were later verified by the discovery of long term potentiation in real neurons. One generation before Hebb, neuroanatomists Cajal and Golgi were still debating whether neurons are individual units or form a continuous structure. Hebb attempted to explain psychological events by the physiological properties of the nervous system. His work still remains the best attempt to combine the principles of psychological reality and the facts of neuroscience.