The Functional Electrical Neuroimaging Laboratory

Functional neuroimaging of the human brain involves determining the spatial and temporal pattern of active networks subserving sensation, perception, and cognition. To do this, we use non-invasive techniques of electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). The Division is equipped with a 128-channel EEG system and our fMRI investigations are conducted in collaboration with the Service de Radiodiagnostic et Radiologie Interventionnelle.

Major research themes include:

Auditory processing pathways and their functional plasticity

Recent evidence shows that the auditory system may contain specialized pathways for the treatment of sounds' identities (green) and of their location/motion (red). This project examines the spatio-temporal patterns of brain activity supporting these parallel functions in healthy individuals as well as the breakdown and recovery of these functions in brain damaged individuals (see e.g. Maeder et al., 2001 Neuroimage; Adriani et al., 2003 Neuroimage and Experimental Brain Research).

Current projects examine the spatiotemporal brain dynamics of specific functions within each of these pathways.

One example is the discrimination of sounds of living versus non-living objects (Murray et al., submitted). Our data indicate that this discrimination occurs within 70ms following stimulus onset within right auditory and bilateral frontal cortices.

Additional studies are examining non-linear response interactions in the treatment of inter-aural intensity (IID) and temporal difference (ITD) cues that are used to define sounds' locations. Psychophysical results from healthy individuals indicate that subjects are significantly faster and more accurate when both cues are combined than when either is used alone. Furthermore, this facilitation exceeded predictions based on probability summation, consistent with an explanation in terms of neural response interactions between IID and ITD cues.

Multisensory interactions

There is growing evidence indicating that information conveyed to one sensory system can dramatically impact processing of information of another sensory system. Moreover, instead of such multisensory interactions being limited to occurring in higher-order association cortices, recent work has demonstrated that these interactions occur early in time post-stimulus onset as well as within brain regions traditionally held to be unisensory in function. Current projects are further examining the basic neurophysiolgy of multisensory interactions.

Fig. 1. In one experiment (see Murray et al., 2004 Cerebral Cortex), subjects sat comfortably in a darkened room, centrally fixating a computer monitor and responding to stimulus detection via a foot pedal. Vibrotactile stimulators were held between the thumb and index finger of each hand, as subjects rested their arms on those of the chair. Speakers were placed next to each hand. Reaction Times were faster for multisensory events no matter how they were spatially distributed. Electrophysiological results indicate that responses to the multisensory 'whole' are greater than the summed responses of the unisensory 'parts' at just 50ms post-stimulus. Moreover, these interactions occur within auditory areas traditionally thought to be unisensory in function.

Additional multisensory experiments are investigating the functional impact of these interactions on perception and cognition. Our data indicate that multisensory events can impact later unisensory processes, even without our explicitly knowing.

Fig. 2. In this experiment (see Murray et al., 2004 Neuroimage), subjects performed a continuous recognition task, discriminating repeated versus initial image presentations. Corresponding, but task-irrelevant, sounds accompanied half of the initial presentations during a given block of trials. On repeated presentations within a block of trials, only images appeared, yielding two situations - the image's prior presentation was only visual or with a sound. Image repetitions that had been accompanied by sounds yielded improved memory performance accuracy (old/new discrimination) and were differentiated as early as ~60-136ms from images that had not been accompanied by sounds, through generator changes in areas of the right lateral-occipital complex. It thus appears that unisensory percepts trigger multisensory representations associated with them. The collective data support the hypothesis that perceptual/memory traces for multisensory auditory-visual events involve a distinct cortical network that is rapidly activated by subsequent repetition of just the unisensory visual component.

Visual perceptual grouping and object recognition

The visual system readily overcomes both quantitative and qualitative variations in visual scenes to achieve object recognition. For example, object borders can be perceived even though physically absent from the retinal images. A series of studies investigated the neurophysiological bases of these perceptual filling-in processes (see Murray et al., 2002 and 2004 Journal of Neuroscience; Doniger et al., 2000 Journal of Cognitive Neuroscience).

The neurophysiological coupling of EEG and fMRI

This joint project with the Service de Radiodiagnostic et Radiologie Interventionnelle (CHUV), Institut de Traitement des Signaux (EPFL), and the Electrical Neuroimaging Group (HCUGE) is targeted at determining the neural basis of the signals measured by functional magnetic resonance imaging (fMRI) and their correspondence with surface-recorded electrophysiolgical measures (EEG). This project addresses the fundamental assumptions of non-invasive neuroimaging techniques that currently limit our ability to map the spatio-temporal dynamics of the brain.

The development of EEG analysis methods

Electromagnetic recordings have a major advantage in their high temporal resolution. However, it remains debated how to use such non-invasive recordings to glean neurophysiological interpretations of response modulations. This project is directed at developing analysis techniques benefit from the added information of high-density electrode montages and readily distinguish between modulations of the brain's generator configuration and modulations of the strength of responses. An additional objective is the development of inverse solution methods based on biophysical properties of intracranial generators (see e.g. Michel et al., 2004 Clinical Neurophysiology). Grave de Peralta Menendez et al., 2004 Neuroimage; Andino et al., 2005 Experimental Brain Research).

These projects are conducted in close collaboration with the Functional Brain Mapping Laboratory (Geneva, Switzerland) headed by Prof. Christoph Michel. LAURA and ELECTRA inverse solutions have been developed by Rolando Grave de Peralta Menendez and Sara Gonzalez Andino of the Electrical Neuroimaging Group (Neurology Department, Geneva, Switzerland).