Tuesday, February 5, 2013

Excerpts from "Experimental and theoretical approaches to conscious processing"

See this previous post for the link. Quote:

Theoretical Modeling of Conscious Access

The above experiments provide a convergent database of observations. In the present section, we examine which theoretical principles may account for these findings. We briefly survey the major theories of conscious processing, with the goal to try to isolate a core set of principles that are common to most theories and begin to make sense of existing observations. We then describe in more detail a specific theory, the Global Neuronal Workspace (GNW), whose simulations coarsely capture the contrasting physiological states underlying nonconscious versus conscious processing.

Convergence toward a Set of Core Concepts for Conscious Access

Although consciousness research includes wildly speculative proposals (Eccles, 1994; Jaynes, 1976; Penrose, 1990), research of the past decades has led to an increasing degree of convergence toward a set of concepts considered essential in most theories (for review, see Seth, 2007). Four such concepts can be isolated.
A supervision system.

In the words of William James, ‘‘consciousness’’ appears as ‘‘an organ added for the sake of
steering a nervous system grown too complex to regulate itself’’ (James, 1890, chapter 5). Posner (Posner and Rothbart, 1998; Posner and Snyder, 1975) and Shallice (Shallice, 1972, 1988; Norman and Shallice, 1980) first proposed that information is conscious when it is represented in an ‘‘executive attention’’ or ‘‘supervisory attentional’’ system that controls the activities of lower-level sensory-motor routines and is associated with prefrontal cortex (Figure 6). In other words, a chain of sensory, semantic, and motor processors can unfold without our awareness, as reviewed in the previous section, but conscious perception seems needed for the flexible control of their execution, such as their onset, termination, inhibition, repetition, or serial chaining.

A serial processing system.

Descartes (1648) first observed that ‘‘ideas impede each other.’’ Broadbent (1958) theorized conscious perception as involving access to a limited-capacity channel where processing is serial, one object at a time. The attentional blink and psychological refractory period effects indeed confirm that conscious processing of a first stimulus renders us temporarily unable to consciously perceive other stimuli presently shortly thereafter. Several psychological models now incorporate the idea that initial perceptual processing is parallel and nonconscious and that conscious access is serial and occurs at the level of a later central bottleneck (Pashler, 1994) or second processing stage of working memory consolidation (Chun and Potter, 1995).

A coherent assembly formed by re-entrant or top-down loops.

In the context of the maintenance of invariant representations of the body/world through reafference (von Holst and Mittelstaedt, 1950), Edelman (1987) proposed re-entry as an essential component
of the creation of a unified percept: the bidirectional exchange of signals across parallel cortical maps coding for different aspects of the same object. More recently, the dynamic core hypothesis (Tononi and Edelman, 1998) proposes that information encoded by a group of neurons is conscious only if it achieves not only differentiation (i.e., the isolation of one specific content out of a vast repertoire of potential internal representations) but also integration (i.e., the formation of a single, coherent, and unified representation, where the whole carries more information than each part alone). A notable feature of the dynamic core hypothesis is the proposal of a quantitative mathematical measure of information integration called F, high values of which are achieved only through a hierarchical recurrent connectivity and would be necessary and sufficient to sustain conscious experience: ‘‘consciousness is integrated information’’ (Tononi, 2008). This measure has been shown to be operative for some conscious/nonconscious distinctions such as anesthesia (e.g., Lee et al., 2009b; Schrouff et al., 2011), but it is computationally complicated and, as a result, has not yet been broadly applied to most of the minimal empirical contrasts reviewed above.

In related proposals, Crick and Koch (1995, 2003, 2005) suggested that conscious access involves forming a stable global neural coalition. They initially introduced reverberating gamma band oscillations around 40 Hz as a crucial component, then proposed an essential role of connections to prefrontal cortex. Lamme and colleagues (Lamme and Roelfsema, 2000; Super et al., 2001) produced data strongly suggesting that feed forward or bottom-up processing alone is not sufficient for conscious access and that top-down or feedback signals forming recurrent loops are essential to conscious visual perception. Llinas and colleagues (Llina´s et al., 1998; Llina´s and Pare, 1991) have also argued that consciousness is fundamentally a thalamocortical closed-loop property in which the ability of cells to be intrinsically active plays a central role.

A global workspace for information sharing.

The theater metaphor (Taine, 1870) compares consciousness to a narrow scene that allows a single actor to diffuse his message. This view has been criticized because, at face value, it implies a conscious
homunculus watching the scene, thus leading to infinite regress (Dennett, 1991). However, capitalizing on the earlier concept of a blackboard system in artificial intelligence (a common data structure shared and updated by many specialized modules), Baars (1989) proposed a homunculus-free psychological model where the current conscious content is represented within a distinct mental space called global workspace, with the capacity to broadcast this information to a set of other processors (Figure 6). Anatomically, Baars speculated that the neural bases of his global workspace might comprise the ‘‘ascending reticular formation of the brain stem and midbrain, the outer shell of the thalamus and the set of neurons projecting upward diffusely from the thalamus to the cerebral cortex.’’

We introduced the Global Neuronal Workspace (GNW) model as an alternative cortical mechanism capable of integrating the supervision, limited-capacity, and re-entry properties (Changeux and Dehaene, 2008; Dehaene and Changeux, 2005; Dehaene et al., 1998a, 2003b, 2006; Dehaene and Naccache, 2001). Our proposal is that a subset of cortical pyramidal cells with long-range excitatory axons, particularly dense in prefrontal, cingulate, and parietal regions, together with the relevant thalamocortical loops, form a horizontal ‘‘neuronal workspace’’ interconnecting the multiple specialized, automatic, and nonconscious processors (Figure 6). A conscious content is assumed to be encoded by the sustained activity of a fraction of GNW neurons, the rest being inhibited. Through their numerous reciprocal connections, GNW neurons amplify and maintain a specific neural representation. The long-distance axons of GNW neurons then broadcast it to many other processors brain-wide. Global broadcasting allows information to be more efficiently processed (because it is no longer confined to a subset of nonconscious circuits but can be flexibly shared by many cortical processors) and to be verbally reported (because these processors include those involved in formulating verbal messages). Nonconscious stimuli can be quickly and efficiently processed along automatized or preinstructed processing routes before quickly decaying within a few seconds. By contrast, conscious stimuli would be distinguished by their lack of ‘‘encapsulation’’ in specialized processes and their flexible circulation to various processes of verbal report, evaluation, memory, planning, and intentional action, many seconds after their disappearance (Baars, 1989; Dehaene and Naccache, 2001). Dehaene and Naccache (2001) postulate that ‘‘this global availability of information (.) is what we subjectively experience as a conscious state.’’ 

“More generally, these simulations provide a partial neural implementation of the psychophysical framework according to which conscious access corresponds to a ‘‘decision’’ based on the accumulation of stimulus-based evidence, prior knowledge, and biases (Dehaene, 2008; for specific implementations, see Lau, 2008, and the mathematical appendix in Del Cul et al., 2009).

Modeling Spontaneous Activity and Serial Goal-Driven Processing

An original feature of the GNW model, absent from many other formal neural network models, is the occurence of highly structured spontaneous activity (Dehaene and Changeux, 2005). Even in the absence of external inputs, the simulated GNW neurons are assumed to fire spontaneously, in a top-down manner, starting from the highest hierarchical levels of the simulation and propagating downward to form globally synchronized ignited states. (209-212)

Conclusion and Future Research Directions

The present review was deliberately limited to conscious access. Several authors argue, however, for additional, higher-order concepts of consciousness. For Damasio and Meyer (2009), core consciousness of incoming sensory information requires integrating it with a sense of self (the specific subjective point of view of the perceiving organism) to form a representation of how the organism is modified by the information; extended consciousness occurs when this representation is additionally related to the memorized past and anticipated future (see also Edelman, 1989). For Rosenthal (2004), a higher-order thought, coding for the very fact that the organism is currently representing a piece of information, is needed for that information to be conscious. Indeed, metacognition, or the ability to reflect upon thoughts and draw judgements upon them, is often proposed as a crucial ingredient of consciousness (Cleeremans et al., 2007; Lau, 2008) (although see Kanai et al., 2010, for evidence that metacognitive judgements can occur without conscious perception). In humans, as opposed to other animals, consciousness may also involve the construction of a verbal narrative of the reasons for our behavior (Gazzaniga et al., 1977). Although this narrative can be fictitious (Wegner, 2003), it would be indispensable to interindividual communication (Bahrami et al., 2010; Frith, 2007).

Metacognition and self-representation have only recently begun to be studied behaviorally with paradigms simple enough to extend to nonhuman species (Kiani and Shadlen, 2009; Terrace and Son, 2009) and to be related to specific brain measurements, notably anterior prefrontal cortex (Fleming et al., 2010). Thus, our view is that these concepts, although essential, have not yet received a sufficient empirical and neurophysiological definition to figure in this review. Following Crick and Koch (1990), we focused solely here on the simpler and well-studied question of what neurophysiological mechanisms differentiate conscious access to some information from nonconscious processing of the same information. Additional work will be needed to explore, in the future, these important aspects of higher-order consciousness. (218-19).

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