Apical Amplification – A Cellular Mechanism of Conscious Perception?

Abstract

We present a theoretical view of the cellular foundations fornetwork-level processes involved in producing our conscious experience.Inputs to apical synapses in layer 1 of a large subset of neocortical cellsare summed at an integration zone near the top of their apical trunk. Theseinputs come from diverse sources, and provide a context within which thetransmission of information abstracted from sensory input to their basal andperisomatic synapses can be amplified when relevant. We argue that apicalamplification (AA) enables conscious perceptual experience and makes it moreflexible, and thus more adaptive, by being sensitive to context. AA providesa possible mechanism for recurrent processing theory that avoids strongloops. It makes the broadcasting hypothesized by global neuronal workspacetheories feasible while preserving the distinct contributions of theindividual cells receiving the broadcast. It also provides mechanisms thatcontribute to the holistic aspects of integrated information theory. As AAis highly dependent on cholinergic, aminergic, and other neuromodulators, itrelates the specific contents of conscious experience to global mental statesand to fluctuations in arousal when awake. We conclude that apical dendritesprovide a cellular mechanism for the context-sensitive selectiveamplification that is a cardinal prerequisite of conscious perception.

Characterizing top-down microcircuitry of complex human behavior across different levels of the visual hierarchy

“…Marvan et al 40 argue that perceptual content represented in the activity of pyramidal cells located in deep layers remains "unconscious" until it is amplified by apical amplification and that this mechanism can explain differences in the magnitude of activation to identical stimuli based on their perceptual status, reported here (Figures 2,3 and 4) and elsewhere 14,42 . While to directly test this hypothesis we would require, for example, faster sampling to be able to infer the temporal evolution of top-down effects across layers, it is in line with what is reported in this work.…”

“…For example, it has been reported that complex task demands, such as alphabetizing characters, modulates the outer layers 9 in the prefrontal cortex, which may indicate the presence of apical amplification 38 . This effect of increased dendritic activity has been associated with conscious perception, increasing the flexibility of neural coding by rendering it sensitive to context 40 . It has further been suggested that this mechanism of apical amplification contributes to holistic integration of information 40 , a crucial process in face perception 41 .…”

“…This effect of increased dendritic activity has been associated with conscious perception, increasing the flexibility of neural coding by rendering it sensitive to context 40 . It has further been suggested that this mechanism of apical amplification contributes to holistic integration of information 40 , a crucial process in face perception 41 . Accordingly, the FFA may be receiving feedback from a higher level face processing region, such as the anterior inferior temporal cortex, which appears to be highly sensitive to the perceived faceness of stimuli 14 .…”

“…The results of larger top-down modulations in the outer depths of FFA and in inner depths of V1 is also consistent with the reverse hierarchy theory 43,44 . The outer layer, context specific top-down modulations in the FFA (resulting from apical amplification), would then be fed back to inner depths of V1, where pyramidal cells are located and somatic integration occurs 40 . This would serve to “select” the relevant low-level information stored in early sensory areas (in this case V1) that will subsequently be fed-forward to higher level areas to fulfill task demands.…”

Excitatory and inhibitory effects of HCN channel modulation on excitability of layer V pyramidal cells

“…Recent theoretical and empirical work suggestss the mechanism of “apical amplification” [Marvan et al, 2021] or “dendritic integration” [Aru et al, 2020b] to be at the core of conscious processing. According to these views, only stimuli that are prioritized by contextual information reach the status of being consciously perceived.…”