1492 



HANDBOOK OF PHYs JOY 



NEUROPHYSIOLOGY III 



the ideas thai is presently under active experimental 

 study. 



One Neural Correlate? 



In the literature repeated references arc found to 

 the 'neural event' that accounts for learning, as if a 

 single one was envisaged. Yet many theories, as we 

 have seen, postulate two or more to occur (e.g. a 

 synaptic change and a new neural circuit). There 

 seems to lie no way to settle the point of single vs. 

 multiple possibilities except by further experiments 

 that will define where in the brain, and when in time, 

 the essential changes occur. Such explanations must 

 harmonize a number of disparate facts about CR's 

 which we will enumerate here. 



STRUCTl RES INVOLVED. Some Type I habits can be 

 abolished by decortication and then be relearned. 

 The pre- and postoperative ( 'R's seem to be the same 

 and, if we make the plausible assumption that the 

 cortex participated in the original habit, it is evident 

 that the second one is mediated by extracortical 

 structures. The cortex, however, is clearly the site of 

 the durable change in certain kinds of Type II habits. 

 I his conclusion emerges, for example, from the ex- 

 periments in which the corpus callosum was sec- 

 tioned: in that situation learning is localized to the 

 cerebral hemisphere, and to one side only, for habits 

 involving both tactile and visual discriminations. 



Consideration of the time course of acquisition 

 raises another set of problems. A monkey learning to 

 avoid shocks bv pressing a lever at a signal appears 

 in pass through a series of behavioral stages in the 

 process. At firsl the signal arouses much 'emotional' 

 activity, such ,1' piloerection and vocalization. Later, 

 when learning has progressed to 50 per cent correct 

 responses, ibis 'emotional' behavior can be partly 

 replaced bv an 'alert or attentive' attitude. The 

 fully trained animal, in final complete command of 

 the situation, seems undisturbed by the signal and 

 often del. iv- making the comet response until the 



\ei\ las) moment. 



rhese successive behavioral stages presumably re- 

 flect -i progressive reorganization of brain structures 

 or processes during acquisition. II this is true, brain 

 events measured al a particular place in the earl) 



- ol learning might be absent there later, while 



.11 another brain locus, characteristic brain events 



might appear Old) when learning has become com- 

 plete. 



COMPLEXITY. Different amounts of brain seem to be 

 required according to the degree of complexity of Un- 

 learning problem. If a two-tone pitch discrimination 

 and a three-tone pattern discrimination are taught 

 to a cat, removal of its auditory cortex abolishes both 

 and only the "simple" pitch discrimination can be re- 

 learned. Some neural events responsible for the 'com- 

 plex' tone-pattern CR have been eliminated Im- 

 partial decortication but the regions necessary for a 

 'simple' CR remain. 



phylogenetic evidence. Learning is common to the 

 octopus and the cat despite the large differences in 

 their neural apparatus (17). In particular the mam- 

 malian cerebral cortex is obviously not needed for 

 learning per se. 



'emotional' learning. In the case of the rat trained 

 both 11) to press a lever to get a drop of water and 6) 

 to expect' a shock at the termination of a signal, the 

 animal loses only the second of these habits after 

 experiencing a number of convulsive seizures. This 

 experiment defines a clear operational difference 

 between CR's in the intact animal, and it suggests 

 that a corresponding difference exists in the neural 

 b.isi- of "emotional' as opposed to other CR's. 



ease of learning. Ordinarily many or verv many 

 combinations of CS and US are required to establish 

 a CR, but in the case of imprinting, a single exposure 

 to CS alone produces lifetime retention. The neural 

 events in this exceptional instance, where for a few 

 hours the brain is 'primed' to make a particular set 

 of functional connections might, if understood, also 

 serve for the general case. Perhaps instinctual be- 

 havior, like that of newly hatched birds which scatter 

 for cover at the first presentation of specific sounds or 

 moving shapes (238), represents simply the ultimate 

 with respect to such neural processes, namely build- 

 ing them into the organism at the outset so that they 

 need not be formed bv le. lining at all. 



Maturation and Learning 



This brings us to tin- question of whether or not 

 the neural changes taking place in the normal em- 

 bryological and postnatal growth of an organism dif- 

 fer from those taking place in learning. Is it possible 



ih.it 1 1 1<\ .in- basically the same and require only 



slightlv different environmental conditions to bring 

 I liem about? 



Innate behavior is distinguished from learned be- 



