title: Cortical excitability and the shape of the haemodynamic response.
abstract: 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.
authors.affiliation: Department of Psychology, University of Essex, Wivenhoe Park, Colchester, Essex CO4 3SQ, UK. Electronic address: haighsm@upmc.edu.
authors.forename: S M
authors.initials: SM
authors.lastname: Haigh
authors.affiliation: Department of Psychology, University of Essex, Wivenhoe Park, Colchester, Essex CO4 3SQ, UK. Electronic address: ncooper@essex.ac.uk.
authors.forename: N R
authors.initials: NR
authors.lastname: Cooper
authors.affiliation: Department of Psychology, University of Essex, Wivenhoe Park, Colchester, Essex CO4 3SQ, UK. Electronic address: arnold@essex.ac.uk.
authors.forename: A J
authors.initials: AJ
authors.lastname: Wilkins
mesh_terms: D000293:Adolescent; D000328:Adult; D005260:Female; D006439:Hemodynamics / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D010108:Oxyhemoglobins; D010364:Pattern Recognition, Visual / Q000502:physiology*; D019265:Spectroscopy, Near-Infrared / Q000379:methods*; D014793:Visual Cortex / Q000502:physiology*; D055815:Young Adult
chemical_list: D010108:Oxyhemoglobins
keywords: Excitability; Haemodynamic response; NIRS; Oxyhaemoglobin