Online research in philosophy

Phillips & Singer (P&S) extend ideas derived from the observation eight years ago that the coherence (synchronization) of cortical oscillations can be modulated by the structure of visual stimuli. As described in the target article, a large part of the continued interest in this finding is related to independent theoretical work suggesting that synchronized cell firing could help solve the problem of binding together within cortex neuronal activity associated with different attributes of visual stimuli. The authors present an abstract “proof of concept” model describing how their cortical processing scheme could work, but our biologically realistic models of cortical relationships suggest that the proposal is biologically implausible. Our realistic models lead to a very different interpretation of the significance of cortical oscillations.

Temporal binding via 40-Hz synchronization of neuronal discharges in sensory cortices has been hypothesized to be a necessary condition for the rapid selection of perceptually relevant information for further processing in working memory. Binocular rivalry experiments have shown that late stage visual processing associated with the recognition of a stimulus object is highly correlated with discharge rates in inferotemporal cortex. The hippocampus is the primary recipient of inferotemporal outputs and is known to be the substrate for the consolidation of working memories to long-term, episodic memories. The prefrontal cortex, on the other hand, is widely thought to mediate working memory processes, per se. This article reviews accumulated evidence for the role of a subcortical matrix in linking frontal and hippocampal systems to select and ''stream'' conscious episodes across time (hundreds of milliseconds to several seconds). ''Streaming'' is hypothesized to be mediated by the selective gating of reentrant flows of information between these cortical systems and the subcortical matrix. The physiological mechanism proposed for this temporally extended form of binding is synchronous oscillations in the slower EEG spectrum (< 8 Hz).

Our work supports synchronization for binding within Phillips & Singer's “contextual field” (CF) as well as the type of its lateral interaction they propose. Both firmly agree with our “association field” (AF) and its modulatory influences (Eckhorn et al. 1990). However, the CF connections seem to produce anticorrelation among assemblies representing unrelated structures, whereas experimental evidence indicates decoupling. Finally, it is unclear how the cortex can have access to the logistic function used in the “coherent infomax” approach.

Some patients with a lesion to the striate cortex (V1), when assessed through forced-choice paradigms, are able to detect stimuli presented in the blind field, despite reporting a complete lack of visual experience. This phenomenon, known as blindsight, strongly implicates V1 in visual awareness. However, the view that V1 is indispensable for conscious visual perception is challenged by a recent finding that the blindsight subject GY can be aware of visual qualia in his blind field, implying that V1may not be critical under all circumstances. This apparent contradiction raises the following question: if V1 is not always necessary for phenomenal awareness, why do V1 lesions have such a detrimental effect on conscious perception? It is suggested here that this contradiction can be resolved by considering the impact of V1 lesions on the functioning of the whole visual cortex.