AMYGDALA 



1399 



former connections have been demonstrated by elec- 

 trophysiological means, the latter by anatomical 

 methods. 



Several rhinencephalic connections are sources of 

 aflferent fibers to the amygdala, among them the 

 tuberculum olfactorium (118, 122, 137, 157, 206), the 

 piriform cortex, especially its anterior part (62, 109, 

 118, 132, 137, 153, 164, 184, 206, 207, 239, 247), and 

 the hippocampus (104). The existence of these con- 

 nections has been demonstrated by anatomical and 

 electrophysiological methods, except for those from 

 the hippocampus for which so far there is only 

 electrographic evidence. 



Finally there are neocortical afferent connections 

 to the amygdala. Their anatomical description is still 

 incomplete (132, 139, 163), but physiological studies 

 have demonstrated the existence of pathways from 

 the temporal pole, the first temporal convolution, the 

 anterior insula, the posterior orbital cortex and from 

 the motor area to the amygdala (121, 206, 207, 229). 

 Differential effects upon the amygdaloid responses 

 were noted on stimulation at a rate of 4 to 8 cps of 

 the temporal pole and of the first temporal convolu- 

 tion. The former facilitated whereas the latter in- 

 hibited the evoked amvgdaloid respon.ses (229). 



Efferent Connections 



.AN.'VTOMic.^L STUDIES. Subcortical connections^. There are 

 two main efferent subcortical fiber systems of the 

 amygdala : a dorsal one, the stria terminalis, swinging 

 around the bulk of the internal capsule, and a ventral 

 one, passing underneath it (see below). Of these path- 

 ways the best known is the stria terminalis of which 

 several components have been described {26, 62, 63, 

 108, 109, 122, 137, 233, 256). The pattern of origin 

 of this bundle from the amygdaloid complex is not 

 clearly established. 



The stria terminalis fibers end in the septal area, 

 the preoptic region and the hypothalamus. Descrip- 

 tions of the detailed pattern of their termination 

 among the preoptic and hypothalamic nuclei are 

 somewhat contradictory. It seems established how- 

 ever that the septal and preoptic area, the anterior 

 hypothalamus and the ventromedial hypothalamic 

 nucleus are the principal receptors of stria terminalis 

 fibers. However, connections to all other hypothalamic 

 areas and nuclei as far back as the premammillary 

 region have been described by some investigators and 

 denied by others (3, 21, 28, 108, 137). 



' A vast anatomical litcratmc exists on this subject which 

 cannot possibly be reviewed in detail here. For fuller informa- 

 tion consult the references listed in footnote i . 



Our anatomical knowledge of the ventral amyg- 

 clalosubcortical pathways is less precise. Some of the 

 fibers travel with the diagonal band of Broca and end 

 in the tuberculum olfactorium and in the septum 

 (118, 132, 137, 157J, with some fibers entering per- 

 haps the median forebrain bundle (109). Other fibers 

 of this system have a less well-defined course (62, 63, 

 69, 122, 132, 157, 162, 233), .some of them forming a 

 rather diffuse system (62, 132), others being assembled 

 into a bimdle homologous with the ventral olfactory 

 projection tract of lower forms (157, 162), and still 

 others being associated with the longitudinal associa- 

 tion bundle (62, 63) or even with the anterior com- 

 missure (122). These ventral fibers end in the bed 

 nucleus of the stria terminalis, the septal and pre- 

 optic region, and in the anterior hypothalamus. Their 

 projection territory therefore overlaps widely with 

 that of the stria terminalis. Some fibers may connect 

 with the entopeduncular nucleus (62), the subthala- 

 mus (162) or may even descend to the brain-stem 

 tegmentum (162). There probably exist also some 

 connections from the amygdala to the basal ganglia, 

 especially to the putamen and claustrum (109). 



Amygdalothalamic fibers have been described by 

 some anatomists (64, 118), but their existence has 

 also been denied (31). These fibers supposedly end in 

 the pulvinar, the dorsomedial, lateralis posterior and 

 lateralis dorsalis nuclei of the thalamus (64). 



ANATOMICAL STUDIES. Cortical connections. Amygdaloid 

 connections to the cortex are not well known ana- 

 tomically. Most firmly established are the connec- 

 tions from the basolateral amygdaloid complex to the 

 piriform cortex (47, 109, 132, 157, 184). 



Amygdalohippocampal connections are described 

 in normal material by some authors (69, 109, 118, 

 132, 162, 184); but others, on the basis of careful 

 histological work including the use of experimental 

 techniques, come to the conclusion that no direct 

 amygdalohippocampal connections exist (3, 7, 31). 



Still less is known about possible connections to 

 other cortical areas. Some evidence has been cited 

 that the cingulum (118), the tip of the temporal lobe 

 (139) and the insula (163) may receive arnygdaloid 

 fibers. 



ELECTROPHYSIOLOGICAL STUDIES OF EFFERENT AMYG- 

 DALOID CONNECTIONS. The anatomical findings con- 

 cerning the efferent projections of the amygdaloid 

 complex have been essentially confirmed and enlarged 

 by electrophysiological studies in which evoked re- 

 sponses to single .shock and repetitive amygdaloid 



