The EEG and behavioral continuum of the crocodilian, Caiman sclerops

Abstract

Caiman were implanted with chronic electrodes in forebrain, midbrain, hindbrain, orbital cavities, and in jaw, nuchal and forelimb muscles. Strain gauges measured throat palpitations (“sniffing”) and respiration. Recordings and photographs were obtained for 6–12 consecutive days. Four characteristic postures were associated with levels of behavioral arousal: (1) limbs flexed, body raised from floor, head and neck elevated, eyes open; animals responded immediately and aggressively to stimulation; (2) posture similar to above, but with trunk resting on floor; animals responded as above, but less aggressively; (3) limbs slightly flexed with one to three legs relaxed and extended backward against body, head rested on floor, eyes were mostly closed; responses to stimulation were sluggish, especially when eyes were closed; (4) all limbs extended backward against body, head on floor, eyes closed; animals could be handled for several seconds prior to eyes opening and movement (the latter was poorly coordinated and very sluggish). Posture 4 and, in certain instances, Posture 3 denote behavioral sleep, i.e., motor quiescence, stereotypic posture, increased response threshold and rapid state reversibility. EEGs (polymorphic and irregular in form) were somewhat higher in frequencies and larger in amplitudes during behavioral arousal, but changes across the posture-arousal continuum were minimal. Neither epochs of slow wave nor paradoxical sleep were recorded. Large amplitude (up to 200 μV), arrhythmic spikes (50 msec duration) characterized caiman EEG and were most prevalent in the telencephalon. Spiking increased with behavioral quiescence and decreased or transiently vanished with induced or spontaneous arousal and respiration. Spike incidence substantially increased during recovery from 24 or 48 h of induced behavioral arousal. The association of spikes with quiescence and their apparent compensatory rebound after enforced arousal encourage the hypothesis that they are analogs of EEG spike activity reported to accompany mammalian and avian behavioral sleep.