Switchboard versus statistical theories of learning and memory.

Early theories of learning were based largely upon studies of condition.ing. In such experiments, techniques were often used that depended basically on the establishment of stimulus substitution between an arbitrarily selected conditioned stimulus (CS) and an unconditioned stimulus (US) which caused some measurable response. The reasoning of many early workers (especially American behaviorists) might be paraphrased as: "There is an input region which receives the CS and an output region which produces the response to the US. During learning, this input somehow comes to produce that output as a conditioned response. Therefore, a new connection must be established between the CS and US regions; some kind of pathway is built." The conviction that training wore a "groove" of increased excitability along specific neural paths fom sensory input to motor output, and that the existence of that groove of cells was the memory of the experience, launched an avalanche of studies of lesions in which investigators sought the locus of the postulated connection. Because the critical event in learning is envisaged as the formation or facilitation of specific connections, such theories are here referred to as switch-board theories. An essential feature of most such theories is that "remembering" requires the discharge of those particular cells w,hich constitute the new line, and of those cells to which the line is directed. In that *eRe, such theories are also place theories, in which it is assumed that a memory is localized in a discrete set of cells reserved for that function.