Neurophysiology of Consciousness

In my imagination, I visualize the brain as fountain of activity. The base of the fountain is the brain stem that sends an upward stream of activity into a lens-like structure, the thalamus that disperses the stream into all areas of the brain. The fountain stream returns to the thalamus and the cycle begins again with the brain stem acting as the pump.

Consciousness depends on spontaneously emitted pulses from brain stem neurons that ascend in a complex mesh of activating circuits to awaken neurons in the limbic system, thalamus, and cerebral cortex. Without this ascending activation, humans lapse into a coma. Four neurotransmitters appear to be most important in creating consciousness: norepinephrine, serotonin, dopamine and acetylcholine. Drugs such as anesthetics that interrupt consciousness interfere with cortical activation.

Pacemaking neurons in the brainstem fire rhythmically, sending activating pulses upwards into the thalamus. Pulses of electrical activation are accompanied by pulses of chemicals released to tonically activate regions of the brain. The thalamus, in turn, activates the cerebral cortex and links all subsystems in meaningful packages of activity that deliver monitor images of their activity to consciousness. Cortical neurons return signals to the thalamus so that cortical activation can be regarded as a looping system that recurs and resonates.

No one knows what part of the brain produces the conscious monitor images. Likely, the ascending reticular activation system (the brainstem pump) feeds carrier signals to the thalamus and cortex and recursive signal processing from thalamus to cortex, back to thalamus, to cortex, to thalamus… is the neural basis of consciousness. Other modulating information is fed into the mix from the smaller nuclei surrounding the thalamus contributing feelings, mood and other information. Later I will develop the idea that the thalamus is the multi-channel mixer in the brain that combines activity from many brain modules using function-specific templates. The templates are base on innate patterns and are modified by learning.

Consciousness is a monitor image and only samples of other brain activity reach this recursive looping system. The consciousness system sustains a sense of continuity and stability that can be interrupted abruptly by a novel signal from the outside or inside the body. Strong emotions, such as anger and fearare action templates stored in the amygdala that quickly change the entire consciousness mix. Emotions interrupt other activities and “take over” consciousness, sending a volley of signals to many destinations in the body and brain.

In their review of 10 years of studying the connections of thalamic nuclei in rats, Van der Werf et al stated: ”The thalamic midline and intralaminar nuclei, long thought to be a non-specific arousing system in the brain, have been shown to be involved in separate and specific brain functions, such as specific cognitive, sensory and motor functions. Fundamental to the participation of the midline and intralaminar nuclei in such diverse functions seems to be a role in awareness. It is unknown whether the midline and intralaminar nuclei, together often referred to as the 'non-specific' nuclei of the thalamus, act together or whether each nucleus is involved idiosyncratically in separate circuits underlying cortical processes. Detailed knowledge of the connectivity of each of these nuclei is needed to judge the nature of their contribution to cortical functioning. The present account provides an overview of the results of neuroanatomical tracing studies on the connections of the individual intralaminar and midline thalamic nuclei in the rat, that have been performed over the past decade in our laboratory. …These anatomical relationships, in combination with functional studies in animals and in humans, lead us to propose that the midline and intralaminar nuclei as a whole play a role in awareness, with each of the groups subserving a role in a different aspect of awareness. The following groups can be discerned: (1) a dorsal group, consisting of the paraventricular, parataenial and intermediodorsal nuclei, involved in viscero-limbic functions; (2) a lateral group, comprising the central lateral and paracentral nuclei and the anterior part of the central medial nucleus, involved in cognitive functions; (3) a ventral group, made up of the reuniens and rhomboid nucleus and the posterior part of the central medial nucleus, involved in multimodal sensory processing; (4) a posterior group, consisting of the centre median and parafascicular nuclei, involved in limbic motor functions.”

Because the thalamus is so complexly interconnected with all other parts of the brain, a thalamic model of executive function will be misleading to some extent. A combination of frontal lobe and thalamic circuits is essential, for example, for anticipatory planning - one of the more recent and complex attributes of cognition.