273 Near Infrared Spectrophotometry Detects the Neuronal Activation Induced by Tactile, Auditory and Visual Stimulation in term Neonates

Abstract

Background: Near infrared spectrophotometry (NIRS) can be used to non-invasively measure changes in optical properties during functional activation. Because these changes originate from differences in optical properties between activated and non-activated neurons, the spatial localization and temporal resolution of this signal make it a promising tool for studying the time course of neuronal activity in localized brain areas. So far no normative data exist in neonates.Aim: 1. To detect neuronal activation and observe its time course in healthy term and preterm infants. 2. To get normative data in order to establish a NIRS optical encephalogram.Methods: The NIRS instrument consists of a measuring system, stimulation unit and sensors. To allow for mapping of different brain regions, 8 locations were analyzed simultaneously. The following stimulation tasks were performed: tactile (vibration motor), auditory (headphones), and visual (light emitting diodes). Four sensors were placed over either the C3 or C4 area (tactile), T3 or T4 area (auditory), or the occipital O1 area (visual). Left and right sides were alternately stimulated for 20s. Stimuli were pulsed using random on/off cycles ranging from 300 to 500ms. Each stimulation period was followed by 10 seconds of rest. Stimulation-rest cycles were repeated for a minimum of 10min. Novel adaptive spectral filters where applied to extract the neuronal signal form the optical intensity signal. A time triggered, folded average allowed isolation of the time course of the neuronal signal.Results: For tactile stimulation a total of 29 measurements in 16 neonates were recorded. For auditory stimulation a total of 27 measurements in 14 neonates, and for visual stimulation a total of 30 measurements in 10 neonates were recorded. Preliminary results indicate that visualization of the neuronal response to tactile stimulation was possible in 13 infants and to auditory stimulation in 11 infants. No clear signal could be observed during visual stimulation. The latter might be due to immaturity, as only the subcortical visual region is activated in term newborns.Conclusions: Our observations show that it is possible to detect neuronal activation from the optical NIRS signal in healthy term infants. The NIRS system thus, may provide an inexpensive tool for cerebral functional diagnostics in the neonatal intensive care unit.