title: Characterizing stimulus-response functions using nonlinear regressors in parametric fMRI experiments.
abstract: Parametric study designs proved very useful in characterizing the relationship between experimental parameters (e.g., word presentation rate) and regional cerebral blood flow in positron emission tomography studies. In a previous paper we presented a method that fits nonlinear functions of stimulus or task parameters to hemodynamic responses, using second-order polynomial expansions. Here we expand this approach to model nonlinear relationships between BOLD responses and experimental parameters, using fMRI. We present a framework that allows this technique to be implemented in the context of the general linear model employed by statistical parametric mapping (SPM). Statistical inferences, in this instance, are based on F statistics and in this respect we emphasize the use of corrected P values for F fields (i.e., SPM¿F¿). The approach is illustrated with a fMRI study that looked at the effect of increasing auditory word-presentation rate. Our parametric design allowed us to characterize different forms of rate-dependent responses in three critical regions: (i) bilateral frontal regions showed a categorical response to the presence of words irrespective of rate, suggesting a role for this region in establishing cognitive (e.g., attentional) set; (ii) in bilateral occipitotemporal regions activations increased linearly with increasing word rate; and (iii) posterior auditory association cortex exhibited a nonlinear (inverted U) relationship to word rate.
authors.affiliation: The Wellcome Department of Cognitive Neurology, Institute of Neurology, Queen Square, London, WC1N 3BG, United Kingdom.
authors.forename: C
authors.initials: C
authors.lastname: Büchel
authors.forename: A P
authors.initials: AP
authors.lastname: Holmes
authors.forename: G
authors.initials: G
authors.lastname: Rees
authors.forename: K J
authors.initials: KJ
authors.lastname: Friston
mesh_terms: D001143:Arousal / Q000502:physiology*; D001288:Attention / Q000502:physiology*; D001931:Brain Mapping / Q000379:methods*; D002540:Cerebral Cortex / Q000098:blood supply*; D005625:Frontal Lobe / Q000098:blood supply; D006801:Humans; D016014:Linear Models; D008279:Magnetic Resonance Imaging / Q000706:statistics & numerical data*; D009778:Occipital Lobe / Q000098:blood supply; D011930:Reaction Time / Q000502:physiology; D012039:Regional Blood Flow / Q000502:physiology; D012044:Regression Analysis; D013067:Speech Perception / Q000502:physiology; D013702:Temporal Lobe / Q000098:blood supply