Haemodynamic responses to radial motion in the visual cortex

Abstract

Functional near-infrared spectroscopy (fNIRS) is an optical imaging technique that relies on emitting near-infrared light into cortical tissue to measure changes in haemoglobin concentrations as a result of stimulation. The purpose of this study was to observe haemodynamic changes in response to moving stimuli over the primary visual cortex. The test stimuli were radially expanding and contracting concentric gratings and the control stimulus was a matched stationary pattern. A two-channel oximeter (Oxiplex TS) was used to record changes in oxyhaemoglobin (HbO), de-oxyhaemoglobin (Hb) and total haemoglobin concentrations (THb). An increase in haemodynamic activation was observed 20s after the onset of motion and maintained for up to 20s following motion offset (30s time window). This was compared to a time window of 10s before the offset of the stationary presentation. Differences between expanding and contracting motion did not achieve significance at any of the locations. However, a significantly larger HbO response was observed across the 30s time window when compared to the baseline window at both occipital locations. Preliminary results from more temporal locations also showed a similar trend. It is suggested that the delayed rise in HbO levels could be due to an inconsistent maintenance of luminance and contrast-related features of the moving stimuli in V1 receptive fields. Furthermore, motion-after effects could have contributed to delaying the drop to baseline levels. The current study has shown some evidence that fNIRS can be used to record haemodynamic responses to moving stimuli from the visual cortex. We suggest that longer durations of randomised moving, stationary and grey screen presentations would be useful in teasing apart pure motion responses and after-effects. fNIRS could also potentially be used to investigate haemodynamic changes in disorders such as amblyopia that present with motion processing deficits.