The thalamus and the cortex are extensively interconnected and the partnership between them underlies our conscious experience; there is no consciousness without it. The thalamus has been identified as a major target of most anesthetic agents (
2). Additionally, localized thalamic strokes are associated with persistent loss of consciousness ( 3). All sensory information that reaches the cortex does so trans-thalamically. The only exception here is olfaction. Thus, thalamocortical interactions are important to describe in any exploration of how conscious experience, including perception, arises.
The inputs to the thalamus can be subdivided into two categories, driver and modulator (
4, 5). Broadly speaking, the former transmit information while the latter modulate how that information is transmitted. They are distinguished based on morphological, neuroanatomical and neurophysiological criteria. We will focus here on the driver inputs. They have thicker axons, larger and more proximal terminals; they utilize ionotropic glutamatergic receptors only and produce larger excitatory post-synaptic potentials. All of these features are consistent with their role as the main information transmitting inputs.
The neurons that provide driver inputs to the thalamus, regardless of their source, branch and innervate lower motor centers in the central nervous system. This suggests that all information that arrives in the thalamus for relay to the cortex contains copies of motor commands to those motor centers. To use an example from the visual system, axons of the optic tract, on their way from the optic chiasm to the thalamus, branch and 1, provide driver inputs to the lateral geniculate nucleus (LGN) and 2, innervate the superior colliculus and the pretectal nucleus in the midbrain, which are motor centers that control eye movements and the pupillary response, respectively. To use another example, again from vision, the primary visual cortex, which receives visual information from the LGN, contains layer VB neurons that likewise branch and provide 1, driver inputs to the thalamus and 2, innervation to the superior colliculus and the pretectal nucleus. We could give similar examples from other sensory modalities with the exception of olfaction, which does not reach the cortex trans-thalamically.
Thalamic relays differ with respect to whether their driver inputs arrive from the cortex (higher order HO or associational relays such as the pulvinar nucleus) or from subcortical sources (first order nuclei FO such as the LGN) (see
Figure 1). Thus, FO relays transmit copies of motor commands issued by subcortical inputs to the thalamus, while the HO thalamic relays transmit copies of motor commands issued by cortical layer VB neurons. Another way to think about this is that the FO nuclei transmit information that has not been processed by a cortical area yet while the HO relays transmit information that has been processed by at least one cortical area.
Figure 1. Comparison Between the Traditional View of Thalamocortical Functioning (A) and an Alternative View Proposed by Murray Sherman and Ray Guillery (B) (Adapted Adapted from (
Interactions between the thalamus and the cortex correspond to how our mind experiences reality. According to the conventional view of thalamocortical functioning, the thalamus serves as a relay for sensory information from the bodily periphery to reach the cortex. The cortex then processes this information sequentially as it passes from sensory cortical areas to sensorimotor areas and then to motor areas. Motor cortical areas then issue commands to the body. In contrast, according to the alternative view based on the microarchitecture of thalamocortical networks, the main information-carrying inputs that reach the thalamus and then the cortex are actually copies of motor commands issued to lower motor centers in the central nervous system. Such information, that reaches the cortex via the first order thalamic nuclei (FO), has not been processed by any cortical region yet but nonetheless contains copies of motor commands already issued as this information is travelling to the thalamus. Higher order thalamic nuclei (HO) relay copies of motor commands issued to lower motor centers by cortical areas at all levels of sensory and motor cortical hierarchies. Because every cortical area issues motor commands, as well as copies of those commands to the thalamus, there is no qualitative division between sensory and motor processing in the cortex.
All cortical areas, regardless of their classification as sensory, associational or motor, contain layer VB neurons with branching axons that issue motor commands to lower motor centers in the CNS and provide driver inputs to thalamic relays, which in turn relay this information to other cortical areas. We discussed one example of this earlier, namely, that of the primary visual cortex, which contains layer VB neurons that branch and simultaneously innervate the thalamus and lower motor centers. Therefore, one way to describe cortico-thalamo-cortical loops is that they are a substrate for an ongoing elaboration of copies of motor commands issued at all levels of the sensorimotor hierarchy.
These findings together challenge the sensory versus motor dichotomy within the thalamus and the cortex. Qualitatively speaking, all of the main information-carrying inputs to the thalamus transmit information that is both sensory and motor. This means that no purely sensory information reaches the cortex trans-thalamically. The world of experience is an active representation. Our experience of the world emerges as motor commands are continually issued, copied, and then relayed trans-thalamically thereby creating both perceptual experience and behavior simultaneously, which are inextricably linked. The same neurons that issue motor commands to lower motor centers in the CNS branch and innervate thalamic relays that then provide their target cortical areas with their most direct access to the information about the body and the world. The mind perceiving the world corresponds to the brain detecting the copies of motor commands that it issues to the body.
The brain is therefore not merely an information-processing but also an information-generating structure (
6). What it generates, among other things, is the human experience, which is, therefore, a model. Importantly, a hallmark of all models is that they have pre-conditions and purposes not contained within them. As an analogy, let us consider maps. What enables a map to function as such is not contained within it. It, by itself, does not contain its relation to a larger reality. Something else, not contained within a map, needs to decide how the map is related to what it is depicting and how to use it. For this reason, maps and other models usually function within the framework of something else (such as another more comprehensive model), which in turn provides the necessary pre-conditions and purposes.
Thus, because human experience of reality is necessarily a model, it is not an end in itself. As with other models and representations, its pre-conditions and purposes are not contained within. For example, sociologists argue that it is not possible to understand our sense of reality without appreciating how societal forces shape an individual’s sense of what is real. Similarly, psychoanalysts hold that our sense of reality emerges from unconscious forces, and that conscious experiences are in the service of those forces. To use yet another example, dreams are also models whose preconditions lie within a larger framework provided by a more comprehensive awareness during wakefulness.
Our brains and our experience that it generates have evolved in order to allow our species to survive and to ultimately propagate our genetic material. Evolution, however, is itself a model, which is embedded in a more basic model provided by genetics and molecular biology. Molecular biology is in turn embedded into chemistry and chemistry into physics. Thus, to account for the human experience we need increasingly basic models, which science has been remarkably successful in providing. However, before we explore this further, let us investigate an instance of when human experience as a model malfunctions.