Oscillatory phenomena have gained increasing importance in the field of neuroscience, particularly because improvements in analysis methods have revealed how oscillatory activity is both a highly efficient and also information-rich signal. One paradigmatic shift in the conceptualization of oscillatory activity has been to consider not only changes within a particular frequency band, but also the interactions and synchronizations between frequencies of brain activity that are in turn thought to coordinate responses between widespread brain areas and may represent a key “binding” mechanism necessary for perception, consciousness and actions.
This project focuses on the development of methods to non-invasively and quantitatively assess such oscillatory activity and applies these methods to identify the spatio-temporal mechanisms underlying visual object processing. From a methodological standpoint, this project combines and builds upon recent developments both in the field of signal analysis and also in the field of source modeling of surface-recorded electromagnetic signals (i.e. the so-called bioelectromagnetic inverse problem), which was a focus of an earlier SNSF-funded project led by our group (grant #3200B0-100606). Oscillatory couplings will be detected by means of analysis methods based on advanced signal processing and machine learning. Not only will surface-recorded electroencephalography (EEG) signals be analyzed, but also estimates of intracranial local field potentials within a 3-dimensional matrix of points distributed throughout the brain volume. In this way, the analytical and methodological aspects of this project are geared to facilitate translational research with intracranial studies in animals as well as similar studies in human patients undergoing pre-surgical evaluation.
From a neurophysiological standpoint, this project will address the mechanisms by which objects are differentiated from background information, perceived as forms, and discriminated to engender a behavioral response. The robustness and importance of object perception is attested to by our ability to withstand degraded visual conditions (e.g. poor illumination, occlusion and scotoma) and to accurately interpret the two-dimensional nature of the retinal projection that will produce object images with discontinuous boundaries or boundaries where no luminance difference is physically present.
In experimental settings these conditions have been reproduced with illusory contour (IC) stimuli, which give the perception of borders or surfaces that span inducing elements across regions of homogenous luminance. Such stimuli and experimental conditions are thought to tap brain mechanisms mediating the binding of information. Importantly, this binding refers not only to how information dispersed throughout a visual scene is demarcated as belonging to the same object, but also to how incoming sensory information is conjoined with ongoing brain activity, discrimination processes, task demands, and performance outcome.
Previous research has shown that the perception and discrimination of ICs is a multi-stage processes that (minimally) involves an analysis of the spatial arrangement of elements within the scene, sensitivity to object borders (both real and perceptual), and active discrimination of perceived shapes in parallel with access to stored mental representations. Furthermore, some, but not all, of these stages can be affected by task requirements and performance accuracy. While our prior work regarding these processes has largely focused on event-related measures of brain activity in healthy individuals as well as psychiatric populations, it is increasingly evident that oscillatory activity also plays a major role. A principal objective of this project is therefore to use ICs as an experimental model from which to evaluate more general mechanisms for the cross-coupling of oscillatory activity either in alternative paradigms or in clinical populations.
In the context of this project, both top-down and bottom-up influences will be experimentally manipulated by changing the facility of task- performance and the dominant oscillation frequency at stimulus onset, respectively. The experiments of this proposal, which is conceived as a PhD thesis project, combine psychophysical and EEG measures from healthy participants, using infrastructure existing within the Lemanic Center for Biomedical Imaging (CIBM). Preliminary results demonstrate the feasibility of this project. Extensions of this project to combinations with other brain imaging methods (diffusion MRI, fMRI, TMS, etc.) are readily foreseen. Working Hypotheses & Specific Aims • Oscillatory brain activity and its cross-coupling are hierarchically structured. We will use object discrimination during degraded viewing conditions to identify cross-frequency interdependencies and their evolution as a function of time/processing stage. • Stimulus-related oscillatory cross-coupling is modulated by so-called top-down influences including task and performance accuracy.
The to-be-performed task as well as its difficulty will be parametrically varied to identify the manner in which such top-down influences affect the hierarchical structure of oscillatory cross-coupling at each temporal stage of object processing. Stimulus-related oscillatory cross-coupling is modulated by ongoing pre-stimulus activity. We will use steady-state activity to induce ongoing oscillatory activity with task-irrelevant stimuli at specific frequencies that will then be subsequently perturbed by the presentation of a task-relevant stimulus.