THE INTRINSIC SYSTEMS OF THE FOREBRAIN 



1331 



turns out to be more basic than the number of ol)jects 

 in the situation per se. 



Review of Other Data 



The lag in attaining the strategy to sample ex- 

 tensively shown by monkeys with posterior intrinsic 

 sector lesions is correlated with other deficiencies in 

 differentiation that follow such lesions. These de- 

 ficiencies differ in some respects from those produced 

 by lesions of the extrinsic (classical 'primary pro- 

 jection') systems, but the differences are subtle and 

 have repeatedly eluded precise specification (116). 

 The available data may therefore be briefly reviewed 

 in a renewed attempt at such specification, a) Drastic 

 bilateral removal of an extrinsic sector severely limits 

 differentiative behavior in the modality and only in 

 the modality served by that sector. The limitation 

 affects practically all differentiations in the mode: 

 thus, a monkey in which the occipital lobes have been 

 removed reacts only to gross changes in the en- 

 vironment that affect the visual receptors — changes 

 that can be ascribed to variations in total luminous 

 flux (61). Comparably, drastic bilateral removal of a 

 posterior intrinsic sector restricts differentiative be- 

 havior within the mode served by that sector, and 

 only within that mode, but the limitation is not as 

 severe as that produced by drastic removal of the 

 extrinsic sector serving that mode (14, 107). b) Under 

 some conditions, differentiation is unimpaired after 

 drastic posterior intrinsic sector resection : for ex- 

 ample, after such a removal, a monkey can catch a 

 flying gnat in mid-air and can pull in a peanut which 

 is beyond reach but attached to an available fine silk 

 thread (0000 surgical). In these situations, as in situ- 

 ations that necessitate the opening of a single box or 

 depressing of a single lever, the operated animal is 

 indistinguishable from an unoperated control {108). 

 f ) Under other conditions, such as those in the experi- 

 ment described above, differentiation is impaired 

 after posterior intrinsic sector ablations. These con- 

 ditions have in common the requireinent that two or 

 more separate responses be systematically related to 

 the differences between the environmental events that 

 determine the stimulus; i.e. alternatives are available 

 to the organism, alternatives that are specified by 

 environmentally determined stimuli. Such stimuli, 

 for convenience, will hereafter be referred to as "input' 

 variables. Examples of the problems where impair- 

 ment is found in the visual mode are : brightness, color, 

 form, pattern, size and flicker discriminations (go-92) ; 

 successive and simultaneous discriminations (116); 



successions of discriminations ("learning set" j (12, 120); 

 oddity discriminations (50); and matching from 

 sample (50). Although the operated animals may per- 

 form "normally' on particular problems within a prob- 

 lem group, decrement is found on other more 'difficult' 

 problems in that group. Difficulty of problem is inde- 

 pendently defined ijy the number of trials taken by 

 naive unoperated animals to learn the problem. In 

 most instances problem difficulty has also been re- 

 lated to differences between the physical dimensions 

 of the objects, such as size discrimination (91), and to 

 other determinants of the alternatives in the situation, 

 including situational differences (116) and sampling 

 in the multiple-object problem. 



Analysis of Results 



These then are the data. Extensive bilateral abla- 

 tions of both extrinsic and posterior intrinsic sectors 

 impair differentiative behavior, but differences be- 

 tween the impairments exist. Attempts to portray 

 these differences are familiar. Neurologists have spoken 

 of 'defective sensibility' and of 'agnosia' (33, 52), the 

 latter often conceived as a disorder of memory. In so 

 far as this distinction assumes an associationistic model 

 of the functions of the intrinsic sectors, it gains little 

 support from neurological or neuropsychological evi- 

 dence (108). An alternate view can be proposed. 

 Psychologists have spoken of 'existential discrimina- 

 tions' and "differential discriminations' (57), or of 

 'sensibility' and 'intelligibility' (89), distinctions that 

 are made on the basis of whether the organism's 

 actions are determined by 'simple presence or absence' 

 of input variables or by 'some more complex rela- 

 tionship' between these variables, such as the number 

 of "contextual alternatives' in the situation (88). The 

 results of the experiment reported in this presentation 

 warrant an attempt to pursue this conceptualization 

 of the distinction by proposing a formal model of the 

 interaction between the functions of the intrinsic and 

 extrinsic sectors in differentiati\e behavior. 



The defect in differentiative behavior that results 

 from lesions of the extrinsic and posteror intrinsic 

 sectors of the forebrain can be characterized by stating 

 the variety of transformations of the input under 

 which behavior remains invariant. Following exten- 

 sive bilateral resections of the extrinsic sectors, be- 

 havior remains invariant under a great variety of 

 transformations of the input. For instance, for these 

 preparations, even brightness and size of luminant 

 are multiplicatively interchangeable quantities (61), 

 whereas differentiative behavior by organisms with 



