Olivia Gosseries | Search bioRxiv

bioRxiv2015

Structural constraints to information flow within cortical circuits: a TMS/EEG-dMRI study

Transcranial magnetic stimulation (TMS) has been used for more than 20 years to investigate brain function by perturbing and observing the consequent behavioral, pathophysiological and electrophysiological modulations. These latter, mainly measured by high-density electroencephalography (hd-EEG), revealed signatures of the functional organization in a brain network. In order to unveil the nature and the underlying mechanism of these signatures, we here mapped TMS-induced hd-EEG changes onto changes in information flow and brain structural architecture, using multimodal modeling of source reconstructed TMS/hd-EEG recordings and diffusion magnetic resonance imaging (dMRI) tractography in a cohort of awake healthy volunteers. We observed that the relationship between information flow and structural connections depend on the stimulation site and on the frequency of the TMS-induced brain rhythms. These findings highlight the importance of taking into account the dynamics of different local oscillations when investigating the mechanisms for integration and segregation of information in the human brain. Our whole-brain analysis sheds light on the function-structure organization of the brain network after TMS, and on the huge variety of information contained in it. TMS/EEG dMRI directed functional connectivity structural connectivity structure-function brain information flow

Neuroscience↗

bioRxiv2014

Stimulus set meaningfulness and neurophysiological differentiation: a functional magnetic resonance imaging study

A meaningful set of stimuli, such as a sequence of frames from a movie, triggers a set of different experiences. By contrast, a meaningless set of stimuli, such as a sequence of TV noise frames, triggers always the same experience - of seeing TV noise - even though the stimuli themselves are as different from each other as the movie frames. We reasoned that the differentiation of cortical responses underlying the subjects experiences, as measured by Lempel-Ziv complexity (incompressibility) of functional MRI images, should reflect the overall meaningfulness of a set of stimuli for the subject, rather than differences among the stimuli. We tested this hypothesis by quantifying the differentiation of brain activity patterns in response to a movie sequence, to the same movie scrambled in time, and to TV noise, where the pixels from each movie frame were scrambled in space. While overall cortical activation was strong and widespread in all conditions, the differentiation (Lempel-Ziv complexity) of brain activation patterns was correlated with the meaningfulness of the stimulus set, being highest in the movie condition, intermediate in the scrambled movie condition, and minimal for TV noise. Stimulus set meaningfulness was also associated with higher information integration among cortical regions. These results suggest that the differentiation of neural responses can be used to assess the meaningfulness of a given set of stimuli for a given subject, without the need to identify the features and categories that are relevant to the subject, nor the precise location of selective neural responses.

Neuroscience↗