A thalamic reticular networking model of consciousness

Abstract

[BACKGROUND]: It is reasonable to consider the thalamus a primary candidate for the location of consciousness, given that the thalamus has been referred to as the gateway of nearly all sensory inputs to the corresponding cortical areas. Interestingly, in an early stage of brain development, communicative innervations between the dorsal thalamus and telencephalon must pass through the ventral thalamus, the major derivative of which is the thalamic reticular nucleus (TRN). The TRN occupies a striking control position in the brain, sending inhibitory axons back to the thalamus, roughly to the same region where they receive afferents. [HYPOTHESES]: The present study hypothesizes that the TRN plays a pivotal role in dynamic attention by controlling thalamocortical synchronization. The TRN is thus viewed as a functional networking filter to regulate conscious perception, which is possibly embedded in thalamocortical networks. Based on the anatomical structures and connections, modality-specific sectors of the TRN and the thalamus appear to be responsible for modality-specific perceptual representation. Furthermore, the coarsely overlapped topographic maps of the TRN appear to be associated with cross-modal or unitary conscious awareness. Throughout the latticework structure of the TRN, conscious perception could be accomplished and elaborated through accumulating intercommunicative processing across the first-order input signal and the higher-order signals from its functionally associated cortices. As the higher-order relay signals run cumulatively through the relevant thalamocortical loops, conscious awareness becomes more refined and sophisticated. [CONCLUSIONS]: I propose that the thalamocortical integrative communication across first- and higher-order information circuits and repeated feedback looping may account for our conscious awareness. This TRN-modulation hypothesis for conscious awareness provides a comprehensive rationale regarding previously reported psychological phenomena and neurological symptoms such as blindsight, neglect, the priming effect, the threshold/duration problem, and TRN-impairment resembling coma. This hypothesis can be tested by neurosurgical investigations of thalamocortical loops via the TRN, while simultaneously evaluating the degree to which conscious perception depends on the severity of impairment in a TRN-modulated network.

Figure 1

A schematic diagram of the connections between thalamic relay nuclei and their corresponding cortical areas (of the same color) through the thalamic reticular nucleus. Black lines indicate corticothalamic connections, and colored lines indicate thalamocortical connections. A: anterior thalamic nucleus, M: medial thalamic nucleus, VA: ventral anterior nucleus, VL: ventral lateral nucleus, VP: ventral posterior nucleus, LP: lateral posterior nucleus, Pu: pulvinar, C: centromedial nucleus, P: parafascicular nucleus, LGN: lateral geniculate nucleus, MGN: medial geniculate nucleus, TRN: thalamic reticular nucleus (courtesy of Wolfgang Klimesch, with permission).

Figure 2

Schematic drawing of the TRN-modulated thalamocortical looping model of conscious awareness. THL: thalamus, V1: the primary visual cortex. As the color of processing-flow lines gets darker (from yellow to orange, and finally to red), more elaborated information processing is being produced by means of iterating thalamocortical loops through the TRN. Cortical networks tie together neuronal assemblies in widespread cortical regions, and the TRN may play a central role in organizing all of the networks.