Regional changes in cerebral haemodynamics as a result of a visual stimulus measured by near infrared spectroscopy

Abstract

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 ($\pm $ s.e.m.) change in [HbO$_{2}$] was 0.54 $\pm $ 0.14 $\mu $mol l$^{-1}$. The change in total haemoglobin concentration, given by [HbO$_{2}$] + [Hb] was 0.61 $\pm $ 0.21 $\mu $mol l$^{-1}$, equivalent to a rise in cerebral blood volume of 0.04 $\pm $ 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.