REGIONAL CHANGES IN CEREBRAL HEMODYNAMICS AS A RESULT OF A VISUAL STIMULUS MEASURED BY NEAR-INFRARED SPECTROSCOPY

Near infrared spectroscopy (NIRS) is used to measure global changes in cerebral haemodynamics. We have adapted the technique to measure regional changes in response to a visual stimulus. Ten volunteers were exposed to a computer generated visual stimulus designed to activate a large area of the visual cortex, including V1, V2, V3, V4 and V5. The stimulus was on for 30 s and off for 30 s. Changes in the concentrations of oxyhaemoglobin ([HbO(2)]) and deoxyhaemoglobin ([Hb]) were measured using a commercial spectrometer (NIRO500), over the occipital cortex. The data were summed over ten cycles. As a control, the experiment was repeated over the frontal cortex. For each subject [HbO(2)] increased during stimulation, and decreased when the stimulus was off. The mean (+/-s.e.m.) change in [HbO(2)] was 0.54 +/- 0.14 mu mol l(-1). The change in total haemoglobin concentration, given by [HbO(2)] + [Hb] was 0.61 +/- 0.21 mu mol l(-1), equivalent to a rise in cerebral blood volume of 0.04 +/- 0.01 ml 100 g(-1) which is about 2% of the total cerebral blood volume. There was no significant change in [HbO(2)] over the frontal cortex, implying that the changes in blood volume originated in the occipital lobe. This demonstrates that NIRS provides a non-invasive method of measuring regional changes in cerebral haemodynamics as a result of visual stimulation.