Neurodynamicist Walter Freeman on globalist vs. modularist approaches to EEG

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Walter Freeman (http://sulcus.berkeley.edu/) is very prominent within the neurodynamics  world, but is perhaps not as well known to the neurophenomenology and emobodied cognition communities as he should be. This is possibly because of the forebodingly technical nature of the physics concepts he employs. He told me at a conference some years ago that his views were very close to those of Francisco Varela, who himself was a dynamical systems neuroscientist. He is quite possibly the world’s foremost expert modeling cognitive neurodynamics with EEG. I am examining his work again as I am in the process of designing an EEG study.  Our Science Club in Austin has been wrestling with his paper “Metastability, Instability, and State Tranistions in Neocortex” (Freeman and Holmes, 2005) where he presents a “globalist” alternative to researchers who focus on “modules lighting up:”

“Humans observe and grasp complex events and situations by means of expectations that have the form of theories. A theory determines the techniques of observation, which in turn shape what is observed and
understood. The classic case in physics is the wave–particle duality, in which the choice of one or two slits determines the outcome of the observation. A similar situation holds for the classic debates among proponents of competing theories about neocortical dynamics: localization vs. mass action. In one view, cortex is a collection of modules like a piano keyboard, each with its structure, signal, and contribution to behavior. In the other view, the neocortex is a continuous sheet of neuropil in each cerebral hemisphere, which embeds specialized architectures that were induced by axon tips arriving from extracortical sources during embryological development. Cooperative domains of varying size emerge within each hemisphere during behavior that includes the specialized.

Observers of both kinds use electroencephalograms (EEGs) and units to test their models. Localizationists (e.g. Calvin, 1996; Houk, 2001; Llina´s & Ribary, 1993; Makeig et al., 2002; Singer & Gray, 1995) analogize the neocortex to a cocktail party with standing speakers; each
module gives a signal that, when activated like a voice in a room, by volume conduction occupies the whole head and overlaps other signals. On the assumption of stationarity, the signals can be separated by independent components analysis (ICA) of multichannel EEG recordings. Globalists (e.g. Amit, 1989; Basar, 1998; Freeman, 2000) analogize neocortex to a planetary surface, the storms of which are generated by intrinsic dynamics and modified by the structural features of the surface.

These analogies throw into sharp relief the contrasting assumptions and inferences on which the two theories are based. Further, they justify the different methods by which the EEGs are processed, so that after the processing the two forms of the postprocessed EEG data differ dramatically, each legitimately in support of the parent theory. This is
why any description of a brain theory should be prefaced by a review of the methods used to get the data that supports the theory

Raw EEG data must be preprocessed prior to measurement. Here six decisions are summarized that have to be made by localizationists and globalists before they acquire EEG data. The choices are diametrically opposed (Freeman, Burke, & Holmes, 2003; Freeman & Holmes, 2005).

(i) According to localizationists, specified behaviors require activation of selected cortical modules that give signals at specific stages of the behaviors and are otherwise silent. The background EEG is incompatible
with this expectation, so they adopt the theory established years ago by Bullock (1969) and Elul (1972) that background EEG is dendritic noise, which is so smoothed by volume conduction, particularly at the scalp, that it has no identifiable spatiotemporal structure. They use time ensemble averaging (TEA) to attenuate the noise in proportion to the square root of the number of repeated stimuli that activate the modules, and to extract the expected signals as event related
potentials (ERPs). Globalists view the background
activity as the necessary pre-condition for execution of the specified behavior. That activity is modified by conditioned stimuli in differing ways in various areas of neocortex. The induced modifications are not time-locked to triggering stimuli, so that TEA cannot be used. Instead, spatial ensemble averaging (SEA) is used to extract reference values for sets of
phase and amplitude values from multiple EEGs.

(ii) The sensor of choice for localization is the depth microelectrode, because the size of the tip determines the acuity of spatial resolution. For globalization the spatial resolution is determined by the interelectrode
distances, so the electrode face to minimize noise should be as large as possible without touching neighbor electrodes.

(iii) Both observers use as many electrodes as possible. Localizationists space their electrodes as far apart as possible to sample from as many modules as they can. Globalists space them closely to avoid spatial aliasing and undersampling of spatial patterns of cortical activity.

(iv) Localizationists sharpen the spatial focus of the signals by high-pass spatial filters such as the Laplacian to correct the smoothing by volume
conduction. Globalists use low-pass spatial filters to attenuate contributions that are unique to individual electrodes and enhance the sampling of synchronized field potential activity.

(v) Narrow band-pass filters are favored by localizationists on the premise that modular signals are likely to be bursts at definite frequencies such as 40 Hz. Globalists prefer broad-band filters in expectation that oscillatory signals in EEGs are aperiodic (chaotic).

(vi) Signal sources are localized to modules by fitting equivalent dipoles to the filtered data in order to solve the inverse problem. Global signals are not confined to specific anatomical sites; they are localized not in
the Euclidean space of the forebrain but in multidimensional N-space, where N is the number of available electrodes. These diametrically opposed choices in data processing lead to widely divergent EEG data, and the data lead to theories that are skew. The two theoretical positions are more complementary than conflicting”

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Modularism vs. globalism in cognitive neuroscience: implications for a science of body-knowledge

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Models of how people are able to access physiological state information should take into account a long-running divide in cognitive neuroscience about to what extent explanations, models, and purported mechanisms privilege local, reductionistic, and/or modular theories, as opposed to global and holistic theories that emphasize connectedness with and interdependence of particular systems to the entire brain. The debate is described by the dynamicist Walter Freeman (Freeman and Holmes, 2005) :

“In one view, cortex is a collection of modules like a piano keyboard, each with its structure, signal, and contribution to behavior. In the other view, the neocortex is a continuous sheet of neuropil in each cerebral hemisphere, which embeds specialized architectures that were induced by axon tips arriving from extracortical sources during embryological development. Localizationists analogize the neocortex to a cocktail party with standing speakers; each module gives a signal that, when activated like a voice in a room, by volume conduction occupies the whole head and overlaps other signals… Globalists analogize neocortex to a planetary surface, the storms of which are generated by intrinsic dynamics and modified by the structural features of the surface”

The issue of “module activation” vs. “global pattern dynamics” should be kept in mind while reviewing the evidence for specific regions as crucial to biological models of sensation or perception. Nonetheless, for researchers investigating the neurophysiological basis of access to interoceptive information or body-knowledge focus on a number of cortical areas of interest, particularly somatosensory cortex, orbitofrontal cortex, insular cortex, and cingular cortex/cingulate gyrus. The somatosensory cortex or (S1) is conceived as containing “maps” of body surface areas. A standard interpretation would explain the perception of touch, temperature, and pain as occurring through sensory nerves, which are joined into the spinal cord, and which eventually route through the thalamus, and then the cortical region known as the postcentral gyrus.

One standard refinement to the traditional model gives the label “primary somatosensory cortex” only to the area shown in red, Brodmann area 3 (Kaas, 1983). In any event, primary somatosensory cortex/S1 is conventionally modeled as having four complete maps of the body surface. Arguably, the biological/anatomical grounding of this concept allows one to state that the somatosensory cortex/SI contains “multiple representations of the sensory surface of the body,” without running the risk of invoking representationalist epistemologies, with their polymorphous and “mentalistic” significations. Over time, a picture has emerged of sensation occurring on the outer surface of the body, and then activating S1: neurons in these regions are firing (generating electrical discharges and secreting “signaling” molecules across synapses) at a higher amplitude. Any model that accounts for how perception and awareness of the body is possible will likely need to reference the role of somatosensory cortex.

Another cortical region implicated in interoception or internal perception is that part of the frontal lobes known as the orbitofrontal cortex, which can be defined as that part of the prefrontal cortex that receives certain key afferent projections from the thalamus (the so-called “gateway to the cortex), which receives afferent projections from the body, including the visceral organs. In theory, enhanced activation of physiological state (such as heart rate increase) should be reflected in increased activation of orbitofrontal cortices.

Studies of the role of cortex in processing internal body state often emphasize the role of the (formerly) obscure structure known as the insula, a cortical structure which is nonetheless tucked away underneath the visible cortical layers. The anterior portion of the insula is especially implicated in interoception and internal body-state “information gain”.

Yet another specialized brain area becomes more active in those psychophysiological processes involving internal body state dynamics: a collection of white-matter fibers known as the cingulate gyrus of the cortex.

Again, it should be stressed that neuroscientists may debate the extent to which any one region’s activity should be privileged against global overall processes. Certainly, S1, orbitofrontal cortices, anterior insula and anterior cingulate gyrus are only one of a series of regions that play a part in allowing visceral perception, interoception, or a gain in information about the inside of the body. Emphasizing the contribution of such discreet areas carries forward the “modularist” tradition, while other models will stress more of a global or holistic system of interactions, which is a classic debate in psychology and neurology (Gardner, 1985). Arguably, the pre-understanding of how much processing is done by local “modules” as opposed to collective and global activities influences the very means of data collection (Freeman and Holmes, 2005).