AMYGDALA 



1397 



amygdala only the olfactory fibers are anatomically 

 well defined. These fibers originate in the olfactory 

 bulb and reach the amygdala mainly via the lateral 

 olfactory tract. They teiminate in the corticomedial 

 complex (2, 8, 42, 109, 122, 130, 132, 157, 162, 180, 

 igi). Some fibers cross the mid-line in the anterior 

 commissure and reach the contralateral amygdala 

 (2, 42). 



In the amygdala evoked potentials to electrical 

 stimulation of the olfactory bulb may be recorded 

 from the cortical nucleus which forms part of a larger 

 primary olfactory projection area covering the whole 

 periamygdaloid cortex (39, 65, 133, 214). 



When the olfactory bulb is stimulated with long 

 pulses (27) or with tripled electrical stimuli (120) 

 evoked potentials can be elicited in the whole extent 

 of the amygdaloid complex. However, under these 

 conditions the amygdala is merely a part of a much 

 larger cortical and subcortical field activated by 

 olfactory bulb stimulation which includes large parts 

 of the striatum, thalainus, hippocampus, brain stem 

 and cerebellum. In all these areas there is extensive 

 overlap of olfactory evoked potentials with responses 

 evoked by other sensory inodalities (52, 120), and 

 therefore these areas have to be considered as an 

 unspecific projection field unrelated to olfaction as a 

 specific sense modality. Only the corticomedial amyg- 

 daloid complex and the medial portion of the basal 

 nucleus, where the olfactory responses have a short 

 latency of about 3 to 5 msec, and where no overlap 

 with other modalities occurs, can be regarded as part 

 of the specific olfactory receiving area {120). 



With microelectrodes, evoked unitary discharges in 

 response to electrical stimulation of the olfactory bulb 

 are found in all subdivisions of the amygdala, except 

 for the cortical and medial nuclei, usually considered 

 to be part of the primary olfactory receiving area 

 (166). The general characteristics of these unitary 

 responses are similar to those evoked by other sensory 

 stimuli to be discussed below (p. 1398). 



With natural olfactory stimuli a rapid rhythm at 

 I 2 to 20 cps or even faster with a tendency to occur 

 in spindles can be evoked in the piriform cortex and 

 amygdala (4, 6, 38, 39, 59, 103, 172, 173). It is un- 

 certain, however, whether this activity is specific for 

 olfaction since it has been observed to occur in re- 

 sponse to other sensory stimuli (173), to stimulation 

 of the mesencephalic reticular formation (59) or even 

 to stimulation of the anterior limbic cortex (133).* 



^ These findings are at variance with those of Carreras et al. 

 (38) who found this response to be specifically olfactory. In 

 their experiments it could not be elicited by other stimuli which 



In view of these anatoinical and electrophysiological 

 observations, there can be no doubt that the amygdala 

 receives an important olfactory inflow. Yet the im- 

 portance of the amygdala for olfaction as a sensory 

 function still remains uncertain. Anatoinical observa- 

 tions suggest that the amygdala cannot be very closely 

 related to the olfactory sense since completely anos- 

 matic aquatic mammals, like the dolphin or the 

 porpoi.se, possess a very well-developed airiygdala in 

 which not even the corticomedial complex shows any 

 signs of regression, except for the nucleus of the lateral 

 olfactory tract, and yet there is no trace of an olfac- 

 tory bulb, tract or prepiriform cortex (i, 30, 156). Fur- 

 thermore in human brains with agenesis of the olfac- 

 tory lobes the amygdala appears well developed (54). 



This anatomical evidence is supported bv phvsio- 

 logical oljservations. Rats (238) and dogs (5) with 

 bilateral destruction of the amygdala still retain the 

 faculty of olfactory discrimination. Allen (5) found 

 that the discrimination between a positive and a 

 negative olfactory conditioned response was lost in 

 dogs with bilateral amygdaloid-piriform lesions. How- 

 ever, in view of more recent evidence stressing the 

 importance of the amygdala in conditioned avoidance 

 behavior (.see p. 141 1), these results may have to be 

 reinterpreted. They may indicate interference with 

 the inotivational forces involved in the particular con- 

 ditioning procedure used rather than a true disturb- 

 ance of olfactory discrimination. 



Conflicting evidence regarding the olfactory func- 

 tion of the amygdala is given by human observations. 

 Some authors (84) found a marked increase in ol- 

 factory threshold on the operated side after temporal 

 lobectomy including the amygdala, whereas others 

 (227) noted no such deficits. 



Stimulation studies add little to the problem of 

 olfactory functions of the amygdala. It is noteworthy 

 that in man olfactory sensation is far less often pro- 

 duced by amygdaloid stimulation than other sensory- 

 experiences (129) (seepage 1398). In animals, sniffing 

 may be produced by amygdaloid stimulation (87, 

 135, 155, 172); but this response has to be considered 

 within the context of other behavioral stimulation 

 effects and is not necessarily evidence for primary 

 olfactory representation in tlie amygdala. 



OTHER AFFERENT SENSORY CONNECTIONS OF AMYG- 

 DALA. There is no convincing anatomical evidence 

 for the presence of nonolfactory sensory afferent 

 connections to the amygdala, although they have 



produced only a low-voltage desynchronization of the electro- 

 gram as is typical for neocortical arousal. 



