Hippocampal theory of consciousness
 | It has been suggested that this article be merged into Consciousness. (Discuss) Proposed since February 2018. |
The hippocampus plays a key role in information processing in the brain. For the major information processing streams engulfing the cerebral cortex, it is arguably the 'heart of the brain'.[1]
Consciousness as a sequence of 'symbols' describing unique self-organized firing patterns[edit]
External information, received and modulated by peripheral sensory organs, is filtered and matched in primary and secondary sensory cortices and higher association cortices (matched, that is, to frontally or parietally provided task- and expectation-related information), before converging onto structures in the medial temporal cortex (parahippocampal and entorhinal cortices) and ultimately the hippocampus,[2] which in CA3 (Cornu Ammonis region 3) has the capacity to rapidly form unique firing patterns by way of self-organization (attractor dynamics).[3] Sensory information (about objects and their context), being thus channeled onto the medial temporal lobe and the hippocampus, is converted from coding in ego-centric frames of reference[4][5] to coding in an allocentric frame of reference,[6] so that representations rapidly formed in CA3 of the hippocampus encode object information within an allocentric spatial context (a spatial context that appears to exist independently of the organism’s orientation within it)[7] as well as within an emotional context[8] (reflecting on the organism’s motivational and physiological state). What is being encoded in CA3 are event memories (representations of events, constituting objects encountered in a particular place and at a particular time), which, pending further consolidation, can be recalled as episodic memory (whereby fragments of the original event codes would be reinstated and recombined in CA3). Consciousness is argued by Behrendt to be an epiphenomenon of this very process of recurrent, self-organizing pattern formation in CA3. It is the sequence of these event memory codes, as they are being formed one after another, that is experienced as the stream of consciousness.[9][10][11]
The recurrent emergence of self-organizing attractor states (i.e., event memory codes) in CA3 solves the ‘binding problem’ that challenges attempts to locate the neural correlate of consciousness in the neocortex.[12] Hippocampal representations of objects are embedded in a spatial and emotional context (owing to the fact that the hippocampus receives information from all sensory processing streams as well as information reflecting the motivational and physiological state of the organism, in part via the anterior insula). These representations, being formed at each cycle of the local theta rhythm, have the informational content of consciousness (contexualized allocentric representations). We can deal with the ‘mind-body problem’ by regarding the stream of consciousness as a sequence of symbols (highly complex numbers) that describe (capture the information content of) neural patterns that arise out of self-organization in CA3. Consciousness itself, as a mere symbolic reflection of a neural process, would have no bearing on reality, however the self-organizing neuronal activity patterns that are reflected in these symbols very much do: by (i) affecting, downstream in CA1 (Cornu Ammonis area 1), the process of self-localization with regard to space and time[13] and (ii) influencing, via subicular output to the medial prefrontal cortex, the selection of a behavior mode appropriate to the current location or situation.[14]
Regarding the evolution of the hippocampus, emotional (needs-reflective and behavior-state-determining) and self-localization information obtained by the hippocmapus of simpler vertebrates from the chemical composition of the environment[8] is likely the forerunner of self-localization (and of social self-orientation) based on contextualized and allocentric visual (or auditory) representations of landmarks (or events), which, in turn, would be the forerunners of consciousness of the world.
Hippocampal connections schema[edit]
The illustration schematically shows the relevant connections of the hippocampus. The hippocampus combines information about objects and their context processed in lateral and medial entorhinal cortices, which are modulated by activity in perirhinal and parahippocampal cortices and, upstream, by reverberating activity in sensory and association thalamocortical systems, which, in turn, is constrained by peripheral sensory input. The hippocampus maps contextualised objects (and landmarks) onto location or situation representations; and it engages, via output to the medial prefrontal cortex, a behavior mode that is appropriate to the present location or situation, whereupon the medial prefrontal cortex shapes activity in the dorsolateral prefrontal cortex. Situation-relevant (and task-relevant) activation of the dorsolateral prefrontal cortex (working memory) provides attentional modulation to posterior sensory and association cortical areas, from where activity changes feed back to the hippocampus via the medial parietal cortex and then the parahippocampal region. Information processed in the parahippocampal region is modulated by the basolateral amygdala and orbitofrontal cortex (which also modulate activity in upstream association cortices) in accordance with behaviorally relevant rewards or punishers. Thus the anterior (ventral) hippocampus integrates reward- and punisher-related information with other object-related information (and this is bound into a unique event code with contextual information arriving at the posterior hippocampus). While hippocampal output to the medial prefrontal cortex and ventral striatum (directly or via the ventral subiculum) implements a location- or situation-relevant behavior mode (and may implement goal-directed behavior) (in conjunction with hippocampal activation, mediated by the ventral subiculum and lateral septum, of relevant motor, autonomic, and endocrine pattern generators in the hypothalamus), hippocampal output to the medial parietal cortex (via the dorsal subiculum or Papez circuit) (subsequently feeding back to the hippocampus via the parahippocampal region) may play a key role in the default-mode network and its functions (including conscious goal anticipation and selection).[15]
Consciousness as neocortical activity coordinated by feedback from hippocampal engrams[edit]
There is a more recent theory of hippocampal simulation of external reality that accepts the widespread notion that conscious experience ultimately has to rely on synchronous activity in neocortical sensory and association areas. It postulates that hippocampal output (from CA1 in particular) back to the neocortex generates conscious experience by simultaneous activation of neocortical modules in accordance with recorded experience by the hippocampus.[16] This theory represents a compromise between currently prevailing attempts to locate the neural correlate of consciousness in oscillatory processes in the neocortex (and thalamocortical system) and the growing recognition of the role of the hippocampus in episodic memory functions, including the rapid and cyclical formation of new event memories, their subsequent consolidation, and their later reconstitution manifesting as conscious recall, such as for the purpose of goal anticipation, consideration of alternative outcomes, and goal selection.[17][18]
Faw and Faw[16] emphasised their position that subjective experience does not take place in the hippocampus itself (if it ever can be said to be taking place in any part of the brain) but arises from an activation pattern across the neocortex that is bound together by an 'episodic memory engram' (event memory code) formed in the hippocampus (and fed back to the neocortex). While being consistent with unceasing efforts to locate the neural correlate of consciousness in neocortical processes, it still raises the question as to how activations in distributed neocortical regions can be synchronized to give rise to a unique and coherent pattern that is experienced consciously (or rather reflected in conscious experience). The global neocortical pattern generated by hippocampal output (from CA1) may well be experienced subjectively, but this experience could still be an epiphenomenon of synchronous neuronal processes that proceed without regard to (or direct feedback from) the consciousness they give rise to.
It would still be a 'cul-de-sac', in Faw and Faw's words, but this should not be a problem if we were to insist on a distinction in nature between the world that is consciously (and fundamentally subjectively) experienced by the individual and the realm of physical processes (the material world), which, as philosophical idealism suggests, exists and evolves independently from consciousness and is fundamentally inaccessible to a conscious observer.
See also[edit]
References[edit]
- ↑ Acknowledgment: this article was written by Ralf-Peter Behrendt, but kindly improved by others
- ↑ Eichenbaum, Howard; Sauvage, Magdalena; Fortin, Norbert; Komorowski, Robert; Lipton, Paul (August 2012). "Towards a functional organization of episodic memory in the medial temporal lobe". Neuroscience & Biobehavioral Reviews. 36 (7): 1597–1608. doi:10.1016/j.neubiorev.2011.07.006.
- ↑ Rolls, E. T. (14 November 2007). "An attractor network in the hippocampus: Theory and neurophysiology". Learning & Memory. 14 (11): 714–731. doi:10.1101/lm.631207.
- ↑ Colby, Carol L.; Goldberg, Michael E. (March 1999). "Space and attention in parietal cortex". Annual Review of Neuroscience. 22 (1): 319–349. doi:10.1146/annurev.neuro.22.1.319.
- ↑ Andersen, Richard A.; Buneo, Christopher A. (March 2002). "Intentional Maps in Posterior Parietal Cortex". Annual Review of Neuroscience. 25 (1): 189–220. doi:10.1146/annurev.neuro.25.112701.142922.
- ↑ Byrne, Patrick; Becker, Suzanna; Burgess, Neil (April 2007). "Remembering the past and imagining the future: A neural model of spatial memory and imagery". Psychological Review. 114 (2): 340–375. doi:10.1037/0033-295X.114.2.340.
- ↑ Rolls, Edmund T.; Xiang, Jianzhong; Franco, Leonardo (July 2005). "Object, Space, and Object-Space Representations in the Primate Hippocampus". Journal of Neurophysiology. 94 (1): 833–844. doi:10.1152/jn.01063.2004.
- ↑ 8.0 8.1 Lathe, R. "Hormones and the hippocampus". The Journal of Endicrinology. 169 (2): 205-231.
- ↑ Behrendt, Ralf-Peter (2013). "Conscious Experience and Episodic Memory: Hippocampus at the Crossroads". Frontiers in Psychology. 4. doi:10.3389/fpsyg.2013.00304.
- ↑ Behrendt, Ralf-Peter (July 2010). "Contribution of hippocampal region CA3 to consciousness and schizophrenic hallucinations". Neuroscience & Biobehavioral Reviews. 34 (8): 1121–1136. doi:10.1016/j.neubiorev.2009.12.009.
- ↑ Behrendt, Ralf-Peter (2016). "Hallucinatory experience as aberrant event memory formation: Implications for the pathophysiology of schizophrenia". Progress in Neuro-Psychopharmacology and Biological Psychiatry. 71: 203–209. doi:10.1016/j.pnpbp.2016.07.009.
- ↑ Behrendt, Ralf-Peter (September 2017). "Mind-body problem and hippocampus". Wiley Interdisciplinary Reviews: Cognitive Science. 8 (5): e1448. doi:10.1002/wcs.1448.
- ↑ Eichenbaum, Howard (August 2017). "On the Integration of Space, Time, and Memory". Neuron. 95 (5): 1007–1018. doi:10.1016/j.neuron.2017.06.036.
- ↑ Behrendt, Ralf-Peter (1 January 2013). "Situationally appropriate behavior: translating situations into appetitive behavior modes". Reviews in the Neurosciences. 24 (6). doi:10.1515/revneuro-2013-0037.
- ↑ Figure and caption have been adapted with several substantial changes from Reference 11.
- ↑ 16.0 16.1 Faw, Matt; Faw, Bill (September 2017). "Neurotypical subjective experience is caused by a hippocampal simulation". Wiley Interdisciplinary Reviews: Cognitive Science. 8 (5): e1412. doi:10.1002/wcs.1412.
- ↑ Addis, Donna Rose; Wong, Alana T.; Schacter, Daniel L. (January 2007). "Remembering the past and imagining the future: Common and distinct neural substrates during event construction and elaboration". Neuropsychologia. 45 (7): 1363–1377. doi:10.1016/j.neuropsychologia.2006.10.016.
- ↑ Schacter, D. L; Addis, D. R. (29 May 2007). "The cognitive neuroscience of constructive memory: remembering the past and imagining the future". Philosophical Transactions of the Royal Society B: Biological Sciences. 362 (1481): 773–786. doi:10.1098/rstb.2007.2087.
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