Paralimbic Cortex and Heteromodal Cortex

The role played by the neocortex in the generation of consciousness was long assumed essential. Recent work suggests that if one pays careful attention to a distinction between conscious state versus conscious content, the role of the cortex, particularly the neocortex, appears to be limited to its underpinning conscious cognitive contents, with these conceptualized as cognitive extensions of core consciousness. An exception to this general principle may be paleocortex, and possibly other heteromodal systems anteriorly and posteriorly, particularly parietal regions and prefrontal regions. The region of paralimbic cortex most consistently linked to consciousness is the anterior cingulate, long thought essential to attentional function, response arbitration, and motivational salience of virtually all stimuli.

The respective roles of heteromodal neocortex versus paleocortex in generating core or primary consciousness is incompletely understood, but research suggests that some paleocortex and heteromodal cortex is likely essential for the creation of phenomenal content ("the movie in the brain"). The ability to have coherent sensory content in ANY specific modality may require (counterintuitively) heteromodal systems. Functional imaging studies (see Rees et al., 2002, for summary) comparing conscious and unconscious visual stimuli demonstrate that conscious stimuli show activation of various heteromodal regions in prefrontal and parietal cortex, plus visual cortices, while unconscious stimuli only activate visual pathways. Lesion correlates of hyperkinetic mutism (associated with bilateral destruction of posterior temporal-parietal association cortex) also suggest that posterior heteromodal fields are likely essential for core consciousness. Hyperkinetic mutism resembles an extremely severe delirium, with severe fragmentation of intention, no evidence of working memory, and severe attentional collapse. (Perhaps the most puzzling aspect of the syndrome of hyperkinetic mutism is the question of why these patients are mute and do not produce Wernicke's aphasic speech output.) Interestingly, bilateral extensive damage to dorsolateral prefrontal heteromodal fields produces a very similar state, and also resembles a severe unremitting delirium, although without the puzzling mutism of hyperkinetic mutism if Broca's area is preserved.

This suggests that the posterior and anterior heteromodal fields and their reciprocal connectivities enable integrated action-perception linkages or cycles essential to coherent agency, and the meaningful organization of both behavior and perception in consciousness. A reasonable hypothesis from these lesion correlates is that extensive bilateral damage to posterior or anterior heteromodal cortical fields may prevent the organization of any kind of coherent perceptual object in any modality, or coherent working memory or task organization/procedural memory, respectively. Behavior is disorganized in either very extensive anterior or posterior bilateral heteromodal disease possibly because action-perception cycles (Fuster, 1991), linking behavioral organization to coherent perceptions of the world, are devastated in either instance.

The evidence argues though that consciousness is about "intrabrain relations and integrative communication." An ongoing dialog between paralimbic-heteromodal cortex and unimodal-idiotypic systems appears essential for normal sensory (and presumably motor content and the sense of agency also, although this has been much less closely studied). This communication between paralimbic-heteromodal and more unimodal-idiotypic systems seems to allow "category-specific". regions (e.g., those dedicated to faces, words, or objects) to get their processing into conscious workspace, possibly by virtue of critical gating and selective modulation performed by the heteromodal regions. Differential forms of gating and selective enhancement and modulation may also characterize the roles of many deep mesodiencephalic brainstem regions vis a vis the cortex. However, much in terms of the critical interactions between cortex and these multiple deep mesodiencephalic regions in constructing the core functional envelopes of intention, attention, emotion, and specific sensory content remains to be fully elucidated. Table 3.2 summarizes evidence for such a global workspace or global access theory, leaving many fundamental mechanisms still to be elucidated by future research.

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