Cortical excitability and the shape of the haemodynamic response.

Individual differences in the temporal dynamics of the haemodynamic response can reflect cortical excitation and can reveal underlying cortical physiology. Here, we show differences in the shape of the haemodynamic response that are dependent on stimulus parameters. Two sets of visual stimuli were used varying in parameters that are known to manipulate the haemodynamic response in the visual cortex. We measured the oxyhaemoglobin response using near infrared spectroscopy. The first set of stimuli comprised chromatic square-wave gratings that varied with respect to the separation in the CIE UCS chromaticities of the alternating bars. The gratings with large separations in chromaticity evoked an oxyhaemoglobin response with greater amplitude, consistent with greater activation of the visual cortex. The second set of stimuli comprised horizontal achromatic gratings that (1) were static, (2) drifted at a constant velocity towards fixation, or (3) reversed direction every half spatial cycle to create a vertical vibrating motion. Although the three types of grating had a similar effect on the amplitude of the oxyhaemoglobin response, the moving gratings (2 and 3) evoked a steeper decrease in oxyhaemoglobin concentration after stimulus-offset. The steeper slope appears to reflect the post-stimulus undershoot and the slope may provide a correlate of cortical excitability when the amplitude of the haemodynamic response has saturated.

Chromaticity separation and the alpha response.

Chromatic gratings can be uncomfortable to view and can evoke a large haemodynamic response. Both the discomfort and the amplitude of the haemodynamic response increase monotonically with the perceptual difference in the colour of the component bars of the grating, as registered by the separation in their chromaticity in the CIE 1976 UCS diagram. Individuals with photosensitive epilepsy exhibit epileptiform EEG activity in response to flickering light of alternate colours. The probability of the epileptiform response again increases monotonically with the separation of the colours in the CIE UCS diagram. We investigated whether alpha power, which is known to reflect the excitation of large populations of neurons, is similarly affected by the separation in chromaticity. Chromatic square-wave gratings with bars that differed in CIE UCS chromaticity were presented, together with a central fixation cross. In 18 non-clinical participants, alpha responses were recorded over the visual cortex (O1, Oz, O2, PO3, POz, PO4, P1, P2) and compared to responses in prefrontal cortex (F1, F2). Gratings comprised bars of two alternate colours that either had a small difference in chromaticity (mean CIE UCS separation of 0.03), a medium difference (mean separation of 0.19), or a large difference (mean separation of 0.43). The colour pairs had chromaticities that lay on the red-green, red-blue, or blue-green borders of the screen gamut. Regardless of the hue, the larger the separation in chromaticity, the greater the alpha desynchronization and the lower the alpha power (p = 0.004), but only in posterior electrodes (p < 0.001). Together this indicates that differences in colour evoke a cortical excitation that increases monotonically with the colour difference. In this respect the alpha response resembles the haemodynamic response.