Thoughts, Consciousness, and Memory
Our most difficult problem in discussing consciousness, thoughts, memory, and learning is that we do not know the neural mechanisms of a thought and we know little about the mechanisms of memory. We know that destruction of large portions of the cerebral cortex does not prevent a person from having thoughts, but it does reduce the depth of the thoughts and also the degree of awareness of the surroundings.
Each thought certainly involves simultaneous signals in many portions of the cerebral cortex, thala-mus, limbic system, and reticular formation of the brain stem. Some crude thoughts probably depend almost entirely on lower centers; the thought of pain is probably a good example because electrical stimu-lation of the human cortex seldom elicits anything more than mild pain, whereas stimulation of certain areas of the hypothalamus, amygdala, and mesen-cephalon can cause excruciating pain. Conversely, a type of thought pattern that does require large involvement of the cerebral cortex is that of vision, because loss of the visual cortex causes complete inability to perceive visual form or color.
We might formulate a provisional definition of a thought in terms of neural activity as follows: A thought results from a “pattern” of stimulation of many parts of the nervous system at the same time, probably involving most importantly the cerebral cortex, thalamus, limbic system, and upper reticular formation of the brain stem. This is called the holistictheory of thoughts. The stimulated areas of the limbicsystem, thalamus, and reticular formation are believed to determine the general nature of the thought, giving it such qualities as pleasure, displeasure, pain, comfort, crude modalities of sensation, localization to gross areas of the body, and other general characteristics. However, specific stimulated areas of the cerebral cortex determine discrete characteristics of the thought, such as (1) specific localization of sensations on the surface of the body and of objects in the fields of vision, (2) the feeling of the texture of silk, (3) visual recognition of the rectangular pattern of a concrete block wall, and (4) other individual characteristics that enter into one’s overall awareness of a particular instant. Consciousness can perhaps be described as our continuing stream of awareness of either our sur-roundings or our sequential thoughts.
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