THE SENSE OF SMELL 



545 



ELECTROPHYSIOLOGICAL INVESTIGATIONS. Responses 

 can generally be recorded in animals exposed to 

 olfactory stimuli from considerably wider areas than 

 those which neuroanatomical studies have indi- 

 cated as being directly connected with the olfactory 

 bulb. Changes in electrical activity have been reported 

 from the olfactory tubercle, the septal region, the 

 prepyriform and periamygdaloid cortex and from 

 the hippocampal formation (4, 14, 53, 65). 



Direct electrical stimulation of the olfactory bulb 

 has provided clearer information than the use of 

 olfactory stimuli, since it allows some assessment of 

 the temporal sequence of spread and mav permit 

 inferences to be drawn concerning the structures in 

 monosynaptic connection with the olfactory bulb (22, 



43. 58, 79)- 



In the cat under pentobarbital anesthesia, the bi- 

 polar record from the prepyriform cortex (with the 

 lead electrode nearer the point of stimulation) shows 

 an initial fast negative spike with a latencv of 2.0 

 msec, and presumably resulting from conduction in 

 the olfactory tract. This is succeeded by a biphasic 

 response with an initial negativity peaking at 6 to 8 

 msec. In records from near the caudal border of the 

 prepyriform cortex the diphasic response appears as 

 a double negative wave. This second peak is elimi- 

 nated by repetitive stimulation, possibly from svn- 

 chronization of cortical activity, in such a way that 

 the same elements which previously fired separately 

 to produce two peaks discharge in unison to produce 

 a single larger response (43). This is supported by 

 anatomical studies in the primary olfactory cortex 

 of the mouse (75) which have disclosed intracortical 

 neuron chains possessing abundant and systematically 

 distributed cells with short axis cylinders within 

 these chains. 



Similar records from the surface of the olfactory 

 tubercle in the cat indicate two negative waves with 

 latencies of 6.0 msec, and 1 1 .0 msec. Records in 

 depth show a single deflection peaking at 8.0 msec. 

 Surface records from the pyriform lobe usually show 

 two peaks, an initial diphasic wave with a latency of 

 8 to 10 msec, and a later deflection at 20 to 35 msec. 

 Since this late respon.se is not abolished by section of 

 the lateral olfactory tract but only by complete tran- 

 section of the prepyriform cortex, it is suggested that 

 the late response in pyriform cortex depends on trans- 

 cortical connexions between prepyriform and more 

 caudally placed pyriform cortical areas (43). 



Potentials similar to those in the cat are obtained 

 in the monke\ from stimulation of the olfactorv 



ALLIGATOR 



MACAQUE 



OLF. B 



FR. PREPYR AREA 



TEMP PREPYR 

 AREA 



OLF TUB 

 AMYG- "I 

 ENTORHINAL AREA 

 FR PREPYR AREA 

 OLF TUB 



TEMP. PREPYR AREA 

 AMYG 

 ENTORHINAL AREA 



FIG. II. The comparative extent of the primary olfactory 

 cortex (shaded area) in alHgator, rat and monkey, indicating 

 the progressive reduction in the proportion of the cortical 

 mantle receiving fibers directly from the olfactory bulb in 

 higher vertebrates. Abbreviations: AMYG., amygdala; FR. 

 PREPYR. AREA, frontal prepyriform area; OLF.B., olfactory 

 bulb; OLF. TUB., olfactory tubercle; PREPYR. AREA, pre- 

 pyriform area; TE.M P. PREPYR. AREA, temporal prepyriform 

 area, [.-^fter .■\llison (15).] 



bulb (58). A fast negative spike can be recorded in 

 the olfactory tract, along the lateral and medial 

 olfactory striae, from the rostrolateral portion of the 

 olfactory tubercle and tip of the hippocampal gyrus. 

 Second and third negative deflections appear after 

 7 to 1 1 msec, and 18 to 45 msec, in the olfactory 

 tract, the cortex of the posterior orbital surface of 

 the frontal lobe just external to the lateral olfactory 

 stria, the rostrolateral posterior of the olfactory 

 tuljercle, the limen insulae and the anterior end of 

 the hippocampal gyrus. In subjects under very light 

 chloralose anesthesia, Kaada also noted responses in 

 the posterior part of the hippocainpal gyrus, the hip- 

 pocampus and the septum lucidum. 



Centrifugal influences may modify both the resting 

 and induced electrical activity of the olfactory bulb 

 (59). Stimulation of the prepyriform cortex, cortical 

 amygdaloid nucleus and olfactory tubercle is followed 

 by a depression of electrical activity in the bulb. 

 Similar effects follow high frequency stimulation of 

 the anterior commissure. These influences are thought 

 to be mediated through the granule cells of the bulb 

 and appear to exert tonic effects siinilar to those ob- 

 served in the modulation of spinal afferent path- 

 ways (51). 



