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arxiv.org arxiv.org
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The chief methodological contribution is using themodel priors to analytically derive a closed-form expression that directly explains (1.1);
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we propose a probabilistic modelof text generation that augments the log-linear topic model of Mnih and Hinton (2007) with dynamics, inthe form of a random walk over a latent discourse space
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- Oct 2023
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postlab.psych.wisc.edu postlab.psych.wisc.edu
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Some experiments which involve conscious perception of external stimuli with reports/tasks have shown activation of prefrontal areas, but this activation may have been related to the reports/tasks rather than the conscious experiences (not indicative of content-specific NCC). Other experiments which involve conscious perception of external stimuli without reports/tasks showed more posterior activation than anterior activation (indicative of content-specific NCC).
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Within-state paradigms comparing conscious individuals to unconscious or minimally conscious individuals have revealed posterior area activity to show the most difference between consciousness and unconsciousness or minimal consciousness (there is a "posterior hot zone" which may be indicative of the NCC).
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However, neuroimaging experiments can sample brain activ-ity systematically and noninvasively in healthy volunteers (Pol-drack and Farah, 2015) and, with appropriate methodologies,they can also provide valuable information about the functionalspecificity of brain regions (Moran and Zaki, 2013; Poldrack andFarah, 2015).
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Compared with case studies (lesions) and electrical stimulation studies, neuroimaging studies are less accurate in determining the exact brain regions that contribute to consciousness. Neuroimaging often covers multiple brain areas, some of which may not be directly involved in modulating content-specific NCC.
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Together, stimulation studies support the idea that some pos-terior cortical regions may contribute directly to specific contentsof experience, but the evidence for prefrontal regions is scarceand indirect.
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Many studies have demonstrated that electrical stimulation of the posterior cortex induces discrete changes in the content-specific NCC more reliably than electrical stimulation of the anterior cortex. Hence, most evidence suggests that posterior regions of the brain contribute more to the content-specific NCC than do anterior regions of the brain.
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For the most part, electrical and TMS stimulation of the frontal cortex does not elicit content-specific changes in experience. Stimulation of the ACC and MCC (posterior areas), however, does elicit some content-specific changes in experience, suggesting that posterior area stimulation is more likely to excite content-specific NCC than frontal area stimulation.
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Several studies have shown that electrical stimulation and EEG activation of posterior cortical areas is effective at restoring consciousness in subjects where it is impaired, demonstrating that the excitability of the full NCC can be modulated through arousal systems.
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Electrical stimulation during neurosurgery is an important source ofevidence for a direct contribution of different brain areas to con-sciousness (Penfield, 1959; Desmurget et al., 2013), as indicatedby its superior value in predicting postoperative deficits com-pared with fMRI or diffusion tensor imaging (Borchers et al.,2011).
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Although frontal injuries can slightlyincrease the threshold for perceiving some brief (16 ms) andmasked visual stimuli, patients still experience them (Del Cul etal., 2009), suggesting that these frontal regions may modulate theNCC (i.e., act as background conditions) rather than contribut-ing directly to consciousness (Kozuch, 2014).
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By contrast, there is little evidence for loss of specific con-scious contents after frontal damage (Penfield and Jasper, 1954).
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With regards to content-specific NCC, there is abundant neu-rological evidence that lesions in the posterior cortex can cause aloss of specific contents of experience (Farah, 2004).
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traumatic lesions to the posterior corpus callosum appear to permanently cause states of VS (coma, or impeded consciousness), whereas traumatic lesions to the frontal lobe do not seem to do this.
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There are many examples of people who have experienced bilateral lesions to the frontal lobe and still retained most, if not all aspects of consciousness. These instances lend credence to the idea that the anterior cortex may not account for the full NCC. Even when such damage causes deficits in cognition, perception, or executive function, consciousness does not appear to be significantly altered and effected individuals still seem capable of living normal lives.
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