femme Neuroscience Notes

Some Topics

Consciousness

Consciousness is the experience of monitor images in the mind. At any moment, you are and the world are revealed in your own separate consciousness. Everything that exists is manifest in consciousness.

Paradoxically, we also know that much, if not most, brain activity occurs without the benefit of consciousness. Psychoanalytic and other metaphysical descriptions of the mind invented the “unconscious” or the “subconscious” to try to explain some of the more peculiar aspects of human behavior. Often consciousness and the subconscious were set apart as adversaries in a subterranean battle of mind. Modern neuroscience would suggest that all brain activity carries on below the surface and only a glimmer of this continuous brain processing is projected into consciousness as a monitor image.

The study of the brain has not revealed exactly how consciousness is produced but we know some important and practical things about consciousness. If we thought in terms of a consciousness module in the brain, we could assume that the module might have high-level executive functions and at the same time creates a monitor image. The deep question is that if I am that consciousness that is the monitor image of my brain activity, where exactly do I exist?

As a monitor image of brain activity, consciousness will support different content. The contents of consciousness vary continuously and mostly involuntarily. We can refer to the contents of consciousness as "awareness" and you are more aware when the contents of consciousness are rich and varied. The underlying process of consciousness involves bringing monitor images of the outside world together with monitor images of inside the body. Images of the outside tend to be detailed and explicit in consciousness.

When you can see, a marvelously detailed and interesting moving picture of what is out there dominates consciousness. The information content of the picture is enormous. If you try to record all the visual information in a few seconds of visual scanning your environment, you would consume gigabytes of computer memory. Consciousness consists of realtime monitor images that are not recorded in memory.

Neurophysiology of Consciousness

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.

In my imagination, I visualize a fountain-like activity in the brain that produces consciousness. 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.

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 images depend on the recursive interaction of the thalamus and cortex. 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.

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 fear are 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."

They proposed that the midline and intralaminar nuclei mediate awareness. Each of the groups have a distinct role in a different aspect of awareness:

(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.

Early Buddhist Philosophers declared that consciousness is a fundamental property of the universe equivalent to gravity and electromagnetic radiation. In their view, consciousness enters and leaves an animal or human body and is neither created nor destroyed. I like the idea and wish it were true. I cannot imagine a way to refute the idea of consciousness as an independent force in the universe. There is no need to refute the idea. Consciousness cannot be constrained by definition, cannot be captured by description, and cannot be limited by measurement.