Early theories of learning were based largely upon studies of con-dition. 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 work-ers (especially American behaviorists) might be paraphrased as:" There is an input region which receives the CS and an output region which produces the responseto the US. During learning, this input somehow comes to produce that output as a conditioned response. Therefore, a new connec-tion 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 ex-perience, launched an avalanche of studies of lesions in which investiga-tors sought the locus of the postulated connection. Because the critical event in learning is envisaged as the formation or facilitation of specific con-nections, such theories are here re-ferred to as switch-board theories. An essential feature of most such theories is that" remembering" requires the dis-charge 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 isassumed that a memory is localized in a discrete set of cells reserved for that function.