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Kristoffer H Madsen

Publications and source records attributed to Kristoffer H Madsen.

4 recordsLinked to original sources
PubMed2006

An fMRI study of the neural correlates of graded visual perception.

The neural correlates of clearly perceived visual stimuli have been reported previously in contrast to unperceived stimuli, but it is uncertain whether intermediate or graded perceptual experiences correlate with different patterns of neural activity. In this study, the subjective appearance of briefly presented visual stimuli was rated individually by subjects with respect to perceptual clarity: clear, vague or no experience of a stimulus. Reports of clear experiences correlated with activation in a widespread network of brain areas, including parietal cortex, prefrontal cortex, premotor cortex, supplementary motor areas, insula and thalamus. The reports of graded perceptual clarity were reflected in graded neural activity in a network comprising the precentral gyrus, intraparietal sulcus, basal ganglia and the insula. In addition, the reports of vague experiences demonstrated unique patterns of activation. Different degrees of perceptual clarity were reflected both in the degree to which activation was found within parts of the network serving a clear conscious percept, and additional unique activation patterns for different degrees of perceptual clarity. Our findings support theories proposing the involvement of a widespread network of brain areas during conscious perception.

Adult↗
PubMed2006

Images of illusory motion in primary visual cortex.

Illusory motion can be generated by successively flashing a stationary visual stimulus in two spatial locations separated by several degrees of visual angle. In appropriate conditions, the apparent motion is indistinguishable from real motion: The observer experiences a luminous object traversing a continuous path from one stimulus location to the other through intervening positions where no physical stimuli exist. The phenomenon has been extensively investigated for nearly a century but little is known about its neurophysiological foundation. Here we present images of activations in the primary visual cortex in response to real and apparent motion. The images show that during apparent motion, a path connecting the cortical representations of the stimulus locations is filled in by activation. The activation along the path of apparent motion is similar to the activation found when a stimulus is presented in real motion between the two locations.

Adult↗
PubMed2005

Non-white noise in fMRI: does modelling have an impact?

The sources of non-white noise in Blood Oxygenation Level Dependent (BOLD) functional magnetic resonance imaging (fMRI) are many. Familiar sources include low-frequency drift due to hardware imperfections, oscillatory noise due to respiration and cardiac pulsation and residual movement artefacts not accounted for by rigid body registration. These contributions give rise to temporal autocorrelation in the residuals of the fMRI signal and invalidate the statistical analysis as the errors are no longer independent. The low-frequency drift is often removed by high-pass filtering, and other effects are typically modelled as an autoregressive (AR) process. In this paper, we propose an alternative approach: Nuisance Variable Regression (NVR). By inclusion of confounding effects in a general linear model (GLM), we first confirm that the spatial distribution of the various fMRI noise sources is similar to what has already been described in the literature. Subsequently, we demonstrate, using diagnostic statistics, that removal of these contributions reduces first and higher order autocorrelation as well as non-normality in the residuals, thereby improving the validity of the drawn inferences. In addition, we also compare the performance of the NVR method to the whitening approach implemented in SPM2.

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