Behavioural state linkage in the ovine fetus near term

Nine fetal sheep were surgically prepared with placement of electrocortical and electro-ocular electrodes for monitoring behavioural state activity to determine the relationship of adjacent low-voltage (LV)/rapid eye movement (REM) and high-voltage (HV)/non-(N)REM epoch durations and the inter-epoch transition time. Animals were subsequently studied over an 8-hour period with behavioural state epoch duration and transition time assessed using paired t-test and regression analysis. For all animals, the duration of LV/REM epochs averaged 14.8 ± 0.8 (SEM) minutes which was significantly greater than that for HV/NREM epochs at 10.1 ± 0.5 min (P < 0.01). The mean duration of LV/REM to HV/NREM transition periods at 93 ± 3 s was also significantly longer than that for the HV/NREM to LV/REM transition periods at 78 ± 6 s (P < 0.05). HV/NREM epoch duration was positively correlated with the prior LV/REM epoch duration with a group mean correlation of 0.59 (P < 0.01). HV/NREM epoch duration was likewise positively correlated with the subsequent LV/REM epoch duration with a group mean correlation of 0.46 (P < 0.01). We conclude that the transition time into HV/NREM is longer than that into LV/REM for the ovine fetus near term which may involve differences in the rate of maturation of cycling control mechanisms for these two behavioural states and earlier development of REM-on versus REM-off pathways. The positive LV/REM-HV/NREM linkage relationships also support a homeostatic model of behavioural state control whereby LV/REM and HV/NREM timings are both controlled by accumulation of propensity for these states during the other state and favours an interactive process between these states in the brain's growth and development.