The picture above illustrates the discredited phrenological model. Crudely speaking the idea was that the mind correlated with the brain and that inferences about the brain could be made from measurements of the skull. In this manner, higher mental functions could be mapped to certain parts of the skull. We now know that this is simply wrong. While very broad functions can indeed be mapped to certain brain regions, more complex mental functions need a more sophisticated understanding. At a certain level though there is truth in some of the assumptions in this model. We know that some functions can indeed be mapped to certain regions of the brain. The correlation of visual perceptions to the occipital cortex is a simple case in point. However if we talk more specifically about visual perceptions of a cup then we will get lost in semantics. A simple object might be represented in one part of the visual cortex. The same object might also be represented in the temporal lobe. Stimulation therefore of the temporal lobe might reproduce this visual experience. We don’t have a one to one correlation between experience and brain location. There are many further subtle arguments to complicate matters…
However there is a gem of a paper by two neurologists who being rather intrigued by over a century of electrical brain stimulation have decided to summarise the findings in a paper. Electrical stimulation of the brain typically happens when somebody undergoes certain types of neurosurgical procedure. For instance, some people with chronic epilepsy that hasn’t responded well to medication might undergo surgery to remove part of the brain which causes the seizures – the so-called seizure focus. Needless to say this is a very delicate procedure as the neurosurgeon will need to remove the maximum amount of dysfunctional brain tissue and the minimum amount of functioning brain tissue. In order to do this, surgeons and neurophysiologists have developed a remarkable procedure which is known as intraoperative electrode recording. The patient will remain awake while their brain is exposed and stimulated with electrodes. In this manner, the surgeon will stimulate brain tissue around the area which will be operated on. The patient will then be able to describe their experiences while the brain is being stimulated in this way. This gives the surgeon an idea of the ‘boundaries’ of the viable tissue.
Aslihan Selimbeyoglu and Josef Parvizi have gone to considerable lengths to identify the relevant literature in a paper which is freely available here. They have searched through the medical literature using the medline database in combination with relevant search terms. They identified 9272 reports and read through all of the abstracts. After completing this search they were left with 93 papers. They gathered additional papers from handsearching the reference sections of the papers they had retrieved. They then document the corresponding experiences and observations of patients when certain areas of the brain are stimulated. The essence of the paper is contained in Table 1. Many of the findings will be unsurprising to those with a background in neuropsychology, psychiatry, neurology or neurosurgery. However a number of findings will be surprising.
I will give a few examples of the types of associations that were found
Anterior Cingulate Cortex – ‘sensation of whole body swaying or rocking’
Insular – ‘Feeling like “out of this world” ‘
Dorsomedial Parietal and Precuneus – ‘Feeling of levitation’
Temporooccipital Junction – ‘Seeing people’
Hippocampus – ‘Hearing water dripping’
Internal Capsule – ‘Crying uncontrollably without sadness’
Orbitofrontal and Ventromedial Frontal Cortex – ‘Twitching’
There are some general conclusions that the authors have made. Roughly half of the stimulations produced no corresponding experiences or observations that the researchers were aware of. There are explanations for this and developments in tests have allowed for insights in this regards. Simple experiences are easier to elicit than complex experiences while some of the areas have not readily been mapped due to the difficulty in safely recording activity. I think that this type of data is not easy to summarise or to use in making generalisations. There are many regions that have been stimulated and each offers us esoteric insights. The brain remains complex and whole realms of functioning remain distant. The results may not relate to the area directly stimulated but instead to a modulation of the activity from neurons synapsing on this area – afferent neurons.
Nevertheless Selimbeyoglu and Parvizi have ‘quietly’ produced a masterpiece. They have taken over 100 years of electrical stimulation data and put it into a single table that can be easily accessed. This table allows us to ask fascinating questions about the brain. The results are able to fill us with awe and humility as we realise how little and how much we know about the brain and how the journey began by Galvani in clarifying the biological importance of electrical impulses continues with further deep insights over 200 years later. The recognition that electrical impulses causes us to experience our innermost feelings, our most nostalgic memories, our very perception of the outside world which changes from moment to moment is nowhere more clearly demonstrated than in the findings of these two authors.
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