In order to elucidate whether relatively primitive primates show a similar topological organization of substrates mediating defense and flight within the ventromedial hypothalamus as has been found in other mammals, 15 marmosets (Callithrix jacchus) were chronically implanted with a total of 57 electrodes for electrical stimulation. Experiments took place in a familiar environment and under systematically altered conditions. All anatomical sites yielding vocal threat and short attacks were found to be situated in the n. ventromedialis. Flight was elicited from a region immediately lateral to this nucleus and from sites in the posterior hypothalamus. Thus, it appears that the topological organization of defense and flight behavior in cat, oppossum and marmoset are essentially similar.

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Information about the strength and position of one or more points of mechanical contact with the skin may be transmitted in a number of nerve fibres acting as an organized group. The coding of such information in a group of receptor fibres from the cat's pad has been investigated by C. J. Armett, J. A. B. Gray, R. W. Hunsperger and S. Lal (unpublished), who have also investigated the way in which this code is modified at the first synaptic junction. A single stimulus, having a particular position and strength, sets up in the primary fibres a pattern of activity which is, within limits, uniquely related to that stimulus. The mechanism which determines this pattern is naturally of interest, because an understanding of it would enable the patterns of activity set up by more complicated combinations of stimuli to be predicted. Basic points in this problem are the size of the receptive fields and the amount of overlap between them. Such receptive fields might be determined by a branching of the receptor nerve fibre to cover the area; on the other hand a single receptor in the centre of the field would be all that would be required if the stimulus set up a mechanical wave that travelled across the pad. The experiments described in this paper are concerned with this problem, and the results indicate that a mechanical wave does travel across the pad as a result of each stimulus and that this wave is important in determining the pattern of activity that results in the primary fibres.The first section of the results deals with experiments in which single stimuli were used to excite single receptor units. The second section deals with the effects of a mechanical pulse on the excitability of the units to a second pulse. METHODSThe experiments were performed on cats anaesthetized with chloralose, 005 g/kg, and urethane, 0-5 g/kg. The lumbar cord was exposed to give access to the dorsal roots, and the animal was then mounted rigidly in a frame (cf. Fernandez de Molina & Gray, 1957); steel * Present address:

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