Time and Mind II - Information Processing Perspectives

Chapter 9: Time and conscious visual processing (p. 141-142)

ANDREAS K. ENGEL

Abstract

Cognitive functions like perception, memory, language or consciousness are based on highly parallel and distributed information processing by the brain. One of the major unresolved questions is how information can be integrated and how coherent representational states can be established in the distributed neuronal systems subserving these functions. It has been suggested that this so-called "binding problem" may be solved in the temporal domain. The hypothesis is that synchronization of neuronal discharges can serve for the integration of distributed neurons into cell assemblies and that this process may underlie the selection of perceptually and behaviorally relevant information. The chapter will review experimental results, obtained in sensory systems of both animals and humans, which support this notion of temporal binding and suggest that synchrony is related to the buildup of states of sensory awareness.

Introduction: Binding and awareness

This chapter intends to contribute to the ongoing debate about the neural correlates of consciousness from the viewpoint of a particular experimental approach: the study of distributed neuronal processing and of dynamic interactions which implement specific bindings in neural network architectures. The notion of binding and the search for potential binding mechanisms has received increasing attention during the past decade. Having been introduced first in the psychological discourse, the issue of binding has now advanced into the focus of research also in other disciplines within cognitive science such as neural network modeling, philosophy of mind and cognitive neuroscience.

In all these domains, the problem has been identified that encoding and retrieval of information in neuronal networks requires some sort of mechanism which allows the expression of specific relationships between elementary processors. This socalled binding problem arises for several reasons: First, information processing underlying cognitive functions is typically distributed across many network elements and, thus, one needs to identify those neurons or network nodes that currently participate in the same cognitive process. Second, perception of and action in a complex environment usually requires the parallel processing of information related to different objects or events that have to be kept apart to allow sensory segmentation and goal-directed behavior. Thus, neuronal activity pertaining e.g. to a particular object needs to be distinguished from unrelated information in order to avoid confusion and erroneous conjunctions (von der Malsburg, 1981). Third, it has been claimed that specific yet flexible binding is required within distributed activation patterns to allow the generation of syntactic structures and to account for the systematicity and productivity of cognitive processes (Fodor & Pylyshyn, 1988).

Fourth, many cognitive functions imply the context-dependent selection of relevant information from a richer set of available data. It has been suggested that appropriate binding may be a prerequisite for the selection and further joint processing of subsets of information (Singer et al., 1997; Singer, 1999). These arguments suggest that cognitive functions require the implementation of binding mechanisms in the distributed networks subserving these functions.

In what follows, I want to focus on the idea that some kind of binding mechanism may also be critical for the establishment of conscious mental states. In recent years, several authors have emphasized a close link between binding and consciousness, following the intuition that consciousness requires some kind of integration, or coherence, of mental contents. Damasio (1990) has suggested that conscious recall of memory contents requires the binding of distributed information stored in spatially separate cortical areas. In various publications, Crick and Koch have discussed the idea that binding may be intimately related to the neural mechanisms of sensory awareness (Crick & Koch, 1990). According to their view, only appropriately bound neuronal activity can enter short-term memory and, hence, become available for access to phenomenal consciousness. Llinas and coworkers (1994) have proposed that arousal and awareness require binding of sensory information which is implemented by interactions between specific and nonspecific thalamocortical loops. Metzinger (1995) has extended this discussion by speculating that binding mechanisms might not only account for low-level properties of phenomenal consciousness like the holistic character of perceptual objects, but also for the formation of a phenomenal selfmodel and its embedding into a global world-model.

In the present paper, the discussion will be restricted to one particular aspect of consciousness, namely, sensory awareness. With many authors, I share the view that sensory awareness is one of those facets of consciousness that is probably most easily accessible both in terms of experimental quantification and theoretical explanation (Crick & Koch, 1990). Furthermore, there can be little doubt that we have this basic form of phenomenal consciousness in common with many other species (presumably with at least most other higher mammals). Thus, it is conceivable that research on animals can contribute substantially to explaining this aspect of consciousness, which may not hold for many higher-order features of consciousness which, for instance, require a language system or an elaborated self-model.