Mid-brain responses of the auditory and somatic vibration systems in snakes.

This paper presents data on the effects of the parameters of temperature, stimulus intensity and frequency, and stimulus pairing interval, on characteristics of mid-brain responses of snakes to sound and vibration. The results are relevant not only to the understanding of the biology of snakes but also to general aspects of central sensory physiology. The paper also fills a gap in our comparative knowledge of central representation of sensory information in the lower vertebrates, and may shed light on fundamental processes underlying responses to vibration and to sound, and on their phylogenetic development. Evidence concerning the sensory physiology of snakes has only recently become available (Wever & Vernon, i960; Bullock & Cowles, 1952; Bullock & Diecke, 1956; Proske, 1969). Particularly as regards vibratory senses, this may be due to the dearth of behavioural information about the sensory capabilities of these reptiles and the consequent lack of interest in their sensory physiology. In a separate communication, Hartline (1971) demonstrated that in snakes vibration-sensitive organs in the ear and in the skin respond to both air-borne sound and substrate vibration. Effective stimuli for the auditory system can be delivered as sound through the air or as vibration through the substrate, and can be localized at the head or delivered to the body excluding the head. As shown by frequency-response curves, the auditory system (VIII nerve system) was reported to be sensitive to a narrow band of frequencies (150-600 Hz) with greatest sensitivity at a frequency between 200 and 400 Hz. The somatic system (spinal system) responded to air-borne sound or substrate-borne vibrations if either was delivered to the body but not if delivered to the head. It was reported to have a greater frequency range, from less than 100 Hz to greater than 800 Hz in most snakes. The frequency-response curve of the spinal system lacked a distinct peak of sensitivity and was c. 20 dB less sensitive to either air-borne sound or substrate vibration than the auditory system. The responses of the two systems were found to be differently localized in the mid-brain, the spinal system showing a larger area of responsiveness. The peripheral mechanisms and origins of these responses are covered in the above-mentioned report. In this paper, stimuli previously found to be appropriate are used to explore central physiology and to characterize the responses of the two systems.