A current model of neural circuitry active in forming mental images

Abstract

My aim here is to formulate a compact, intuitively understandable model of neural circuits active in imagination that would be consistent with the current state of knowledge, but that would be simple enough to be able to use for teaching. I argue that such a model should be based on the recent idea of "concept neurons" and circuits of 2 separate loops necessary for recalling mental images and consolidation of memory traces of long-term memory. This paper discusses the role of the hippocampus and temporal lobe, emphasizing the essential importance of recurrent pathways and oscillations occurring in the upper layers of hierarchical neural structures, as well as oscillations in thalamo-cortical loops. The elaborated model helps explain specific processes such as imagining future situations, novel objects, and anticipated action, as well as imagination concerning oneself, which is indispensable for the sense of identity and self-awareness. I attempt to present this compact, simple model of neural circuitry active in imagination by using some intuitive, demonstrative figures.

Figure 1

The visual pathways does not end in the occipital region, but is prolonged by superior structures placed the anterior part of the temporal lobe, where so called ‘object neuron or concept neuron’ can be found. These neurons are connected with the recalling loop of the working memory system.

Figure 2

The perceived object is represented by multiple ‘object or concept neurons’, which are functionally equivalent. We should realize, that apart of this multiple representation some different aspects of the object, such as shape, size, color or texture are perceived, as result of the parallel processing.

Figure 3

Two kinds of connections reach so called ‘object neurons’. One of them, going from the side of hypothalamus and amygdale is active in the case of emotional arousal and take part in the consolidation of the long- term memory traces. The second connection constitutes the oscillating loop, which cause that the evoked mental image is aroused for the time necessary for the action of the working memory.

Figure 4

Three layers of the hierarchical structure of neurons integrating sensory information, for instance – in the visual pathways. The traces of long term memory are consolidated under the influence of connections from hypothalamus and amygdale (A). There are consolidated through two mappings: weights of receiving, ascending synapses (s₁₀, s₁₁, s₁₂) and synaptic weights of reproducing connections (s₁, s₂, s₃). The second, cortico – hippocampal loop (B) is necessary for temporary recalling of the mental image. The activation of the object neuron by this indexing loop causes the recurrent reactivation of neurons in lower layers. The doted line indicate the pathways of repetitive circulations of stimuli in the upper layers of the hierarchical structure, what is the essence of mental imaginary. The object neuron can be activated also from the side of speech area (C). Mental imagery is the essence of episodic memory, short term memory and is used for working memory activity.

Figure 5

Intuitive illustration of the process of so called construction phase of the imagination of a future, possible situation. The working memory system (mainly left hippocampus) is accessing the episodic memory and recombine appropriate details. Afterwards mainly the anterior part of the right hippocampus encodes the pattern of a possible future situation. The encoding consists on alignment of elements, their integration and memorization at least for a certain period of time.

Figure 6

The process of elaboration of the image of a possible, future situation. The construction consists on selection of neurons representing images of past events and of structures responsible for movements and actions, which are able to transform the known, past situation.

Figure 7

The hippocampus (working memory system) realize goal oriented thinking and problem solving, what requires the use of associative connections that are here marked in the horizontal plane as opposed to perceptive hierarchical structures seen here in the vertical plane.

Figure 8

The functional, local map of the associative cortex. We attempt here to illustrate intuitively one of the mechanisms involved in problem solving. The juxtaposition of current and desired situation incites the configuration performed by the working memory system of a “chain of possible passages”. Triggering of nearby concept neurons, representing the current and desired situations stand out these neurons, which starts the search for a “chain of passage”. Working memory system is “looking for transitions” from s¹_(a-2) to s¹_d, by browsing “actions” a¹_(t-1) … a¹_(t) able to transform – step by step the actual situation (a) for a desired situation (d). It is necessary to realize that cortical neurons of the considered chain are depicted here as a map of the horizontal plane as opposed to perceptive hierarchical structures shown on the Figure 7 in the vertical plane.

Figure 9

Intuitive and symbolic depiction of some layers of the hierarchical structure of neurons integrating sensory information and of three coordinating networks. The drawings is similar to the Figure 4. It also illustrates the circuits responsible for perceptions and recalling of mental images. The significance of depicted elements are determined in the legend of the Figure 4. However, here we highlight the existence of the thalamo – cortical loops. Neural activity in the period of a deep sleep is limited to the oscillations of thalamo – cortical loops reaching only the lower parts of the cortex. The frequency of these oscillations are in the theta and delta range. In this situations only a core element of the imagery process is active. The figure enable to understand why perceptions and imagery stimulate further the oscillations in these loops, what increase the frequency of oscillations. The core element of the imagery process is acting permanently without external stimuli.