Sleep ontogeny in the chimpanzee: From two months to forty-one months

Abstract

Over a one year period, 14 and 24 h EEG records were obtained with scalp electrodes from 14 infant chimpanzees aged from 2 to 41 months, sleeping on a couch in a soundproof room. They started to crawl at 3 months, sit and stand at 4–5 months, walk at 6–7 months, climb at 8–9 months and run and swing around 15 months. They napped until 9–10 months. The EEG morphology changed constantly until 2–3 years. Each stage had its own individual evolution. The waking EEG was composed of low voltage fast activity. Alpha rhythm appeared at 3 months, increased in amplitude until 20 months and became well stabilized around 30–36 months. Drowsiness with EEG hypersynchrony was seen at 11 months, the hypersynchrony became maximal in amplitude and duration at 18 months and then decreased. Stage 1 increased with age in number of occurrences, duration and EEG frequency. Sharp wave activity was seen in the parieto-occipital areas after 2 months and then increased in incidence and amplitude until around 18 months after which it declined and was rarely seen again. In stage 2 spindles declined in amplitude, occurrence and frequency with age, unlike the V waves and K complexes which increased in occurrence and definition with age. Stage 3 had few spindles, but more K complexes. High voltage sharp waves at 4–5 c/sec occurred in bursts between 1 and 2 years and then disappeared. Slow waves in the delta band marked the pattern of stage 3 after 2 years. Stage 4 was not seen before 4–5 months. High voltage sharp waves were maximal in amplitude and occurrence around 18 months. Delta activity was synchronous and without overriding faster activity after 2 years. REM showed an increase in frequency of background activity with age but decrease in percentage along with decreases in centro-parietal spike activity. The EEG pattern tended to reach maturity around 3 years.