I recently received the following question in an email. I started to answer - in email - and realized (a) I needed illustrations, (b) the question fit the scope of this blog, and (c) I needed to get off of my duff and get back to blogging...
Speaker, you may or may not know the answer to this question, but: Can a stroke be survivable but severe enough to leave the adult at little more than a young child’s capabilities? Motor movement mostly unimpaired (a listing shamble, maybe), perhaps with an ability to obey simple commands, but intelligence, emotional control, and higher reasoning abilities destroyed? I *believe* I’ve heard of such cases, but at least a few cursory searches haven’t given me much in the way of conclusive results.
Great question! I think I can answer this with a number of different possibilities, but first let's get the brain parts back up here...
Let's talk about the functions, briefly, then get to the question of what could happen with a stroke in each of the different areas.
Occipital lobe is the primary vision center. Parietal lobe has a lot of the processing areas for the senses - not necessarily what we call primary sensory cortex - but the regions that interpret complex information and combine senses. The Central Sulcus is an important landmark - on the Parietal side is the Somatosensory Cortex - the region that handles the sense of touch for the body (i.e. "tactile" sensation) and knowledge of body/limb position ("proprioception"). On the Frontal side of the sulcus is the Primary Motor Cortex that controls voluntary muscle movement. The Frontal lobe is involved in motor and behavior planning, decision making, assessing risk vs. reward, and is also involved in personality. The Temporal lobe contains the Primary Auditory Cortex along the "top" surface (Sylvian Fissure). Deep inside the Temporal lobe are the Hippocampus - responsible for memory processing, Amygdala - involved in emotional and "historical" context for memory, and the Pyriform Cortex - involved in taste and smell.
The striped gray area at the back of brain is the Cerebellum, involved in movement coordination and coordinating motion with sensation (sight & sound). The white "stick" at the bottom is the lowest extent of the Brainstem and Midbrain, which contains most of the "automatic" (technically termed the "Autonomic") functions of involuntary muscle movement. The latter includes breathing, swallowing, reflexes, heart rate, blood pressure, plus body temperature, and other "maintenance" functions. There are also some "junctional" specializations: At the junction of Temporal and Parietal lobes is Wernicke's area involved in language; while at the junction of Temporal and Frontal lobe is Broca's area involved in speech.
A stroke can occur anywhere in the brain. The process usually involves blood clot forming somewhere else, breaking loose, then traveling through the larger blood vessels until it lodges in and blocks a smaller blood vessel. The effects of the stroke can be predicted by understanding the principal functions handled by each candidate brain area. Thus a stroke in the Temporal lobe would likely involve memory - in the Occipital lobe may affect vision - along the central sulcus would affect movement and touch sensation.
Some brain regions are *more* likely to suffer from stroke due to function, blood flow and size of the blood vessels - these include the motor cortical and somatosensory areas, Temporal lobe and Brainstem. Severity of the stroke also depends on the brain area and how large a region is affected. Remember that small blood vessels generally only cover a small area. Quite frequently "ministroke" or Transient Ischemic Attacks (TIAs) result from very small clots and blockages, or even spasms of the small muscles surrounding blood vessel (to regulate blood pressure). These can indeed be quite minor and even temporary.
Major strokes - such as to a large vessel or just about anything involving brainstem - are likely to result in DRT - "dead right there" with little hope of survivability (or recovery if survived). TIAs and small stroke can be almost unnoticeable unless they affect a major function that itself only occupies a small area (such as the center of the visual field or a speech/language center). Even large strokes to the Parietal or Frontal lobe may not have apparent effects if they do not directly involve the motor and somatosensory regions.
And thus we come to the heart of the question - is there a type of stroke that is survivable, and basically just affects the "mind" or the patient?
Yes. The information above should help the reader build up to the understanding that much of what we consider "mental" capacity is based on memory, decision-making, "executive function" (sort of like decision-making, but more broadly including judgement, selection, assessment), and personality. Frontal and Temporal lobe strokes fall directly into this category. Such a stroke in Frontal lobe would have to cover a large area to be noticeable - increasing the risk that it would include motor cortex, Broca's area or other lobes of the brain. Temporal lobe strokes can also impinge on language and speech areas in addition to memory and emotional processing. Parietal strokes can result in the inability to fully process and integrate sensory information even though the senses themselves are unaffected. Such as result could manifest as seeing an apple, describing it as red and round, but unable to speak the name (while possibly even being able to write the name!).
The probability of a stroke severe enough to affect personality, memory and decision-making, but not affecting the more noticeable triad of motor-speech-tactile is low. Generally a stroke severe enough to affect the "mind" is pretty severe and will be evident in other signs. That likely results in the inability to find much in internet searches. The secondary cause for lack of information goes back to the bloodflow into those areas of the brain - some areas get blood flow from two different directions - reducing chance of a single clot causing a stroke. Frontal cortex is one of those areas, so again, the incidence of something severe enough to be noticeable and survivable is low.
SO, I hope that answers the question. As always, I welcome questions and will answer them here when I get the chance.
Please check back for the final installments of The Lab Rats Guide to the Brain: Interfacing the Brain - coming soon to this blog!