Neural correlates of consciousness and related disorders: From phenotypic descriptors of behavioral and relative consciousness to cortico-subcortical circuitry

We report a review of medical aspects of the consciousness. The behavioral dimension, phenotypic descriptors, relative consciousness and neural correlates of consciousness and related disorders were addressed successively in a holistic and chronological approach. Consciousness is relative, specific to each individual across time and space. Historically defined as the perception of the self and the environment, it cannot be separated from behaviors, entailing an idea of conscious behavior with metapractic and metagnostic aspects. Observation of spontaneous and evoked overt behavior distinguishes three main types of disorder of consciousness (DoC): coma, vegetative state or unresponsive wakefulness, and minimally conscious or relationally impoverished state. Modern functional exploration techniques, such as imaging, increase the understanding of DoCs and consciousness. Whether consciousness is a superior function and/or an instrumental function is discussed. Neural correlates can be subdivided into two wakefulness pathways (superior thalamic cholinergic and inferior extra-thalamic), and cortico-subcortical circuitry. The deep brain structures are those described in the well-known sensorimotor, associative and limbic loops, as illustrated in the mesolimbic model of DoC. The cortices can be segregated into several overlapping networks: (1) a global workspace including thalamo-cortical loops; (2) the default mode network (DMN) and related intrinsic connectivity networks (i.e., central executive, medial DMN and salience networks); (3) a 3-fold network comprising the fronto-parietal control system and its dorsal and ventral attentional sub-networks, the fronto-parietal executive control network, and the cingulo-opercular salience network; (4) the internal and external cortices, respectively medial, turned toward the self, and lateral, turned toward the environment. The network dynamics is the reflection of consciousness, notably anticorrelations such as the decrease in activity of the posterior cingulate-precuneus regions during attentional tasks. Thanks to recent advances in DoC pathophysiology, further significative therapeutic progress is expected, taking into account the societal context. This depends notably on the dissemination of medical knowledge and its transfer to a wider public.

Progressive prosopagnosia: Clinical and neuroimaging results

The authors report the longitudinal case study of a patient with the right temporal variant of frontotemporal lobar degeneration. His deficit, initially limited to visuoperceptual disturbances, progressed 2 years later to a severe semantic breakdown. Neuroimaging data indicate that the underlying degenerative process, initially confined to unimodal visual associative cortices, progressed along the ventral pathways to multimodal areas in charge of integrating knowledge from various modalities (the anterior temporal lobes).

[The human perirhinal cortex]

The perirhinal cortex is a structure that lies within the medial temporal lobe. In the present paper, we review current knowledge of the anatomical boundaries and functional correlates of this structure. In the past decade, numerous animal studies have attempted to understand the contribution of the perirhinal cortex to memory. Taken together, they suggest that the perirhinal cortex is crucially involved in recognition memory. This function appears to be independent from those assumed to be subserved by the hippocampus. In humans, data are scarce but tend to corroborate results found in the animal literature. The perirhinal cortex appears to support context-free (non-episodic) knowledge, such as general knowledge about the world and "item-specific" memories. Models of declarative memory that take into account the specific contribution of the perirhinal cortex are discussed, along with their potential application to early cortical neurodegenerative disorders.

Evaluation of visual recognition memory in MCI patients

BACKGROUND

Neurofibrillary tangles seen early in Alzheimer disease (AD) initially appear in a subregion of the perirhinal cortex. In the monkey, damage to the perirhinal cortex impairs performance on visual recognition memory tasks. The authors evaluated impairment of visual recognition memory as a potential early diagnostic marker of AD.

METHODS

The authors developed a visual delayed matching-to-sample task (DMS48) designed to assess visual recognition memory in humans. Twenty-three patients fulfilling the criteria of amnestic mild cognitive impairment (MCI) (mean Mini-Mental State Examination [MMSE]: 26.6, SD = 1.6) were recruited. All underwent a full neuropsychological evaluation, which included the Free and Cued Selective Reminding (FCSR) test. Their performance was compared with that of 10 patients with mild AD, 20 patients with moderate AD, 20 patients with Parkinson disease (PD), and 40 age-matched controls.

RESULTS

Control subjects and patients with PD performed close to ceiling. Patients with mild AD had very low scores, while patients with moderate AD answered at random. MCI patients obtained scores that were between those of control subjects and patients with mild AD (78%, SD = 16%). MCI patients who failed on the DMS48 had lower scores on free recall (p < 0.05) and received less benefit from cueing (p < 0.01) on the FCSR than the other MCI, suggesting a profile of genuine memory impairment related to medial temporal lobe lesions.

CONCLUSION

The DMS48, a test of visual recognition memory, is impaired early in the course of patients with MCI. Further studies are necessary to determine whether the evaluation of visual recognition memory may contribute to the identification of patients with AD.