It's long been known that specific types of brain damage can cause massive personality and mental changes. Granted, other parts of the brain can be removed without noticeable ill effect on the mind, but so can relatively unimportant parts of other systems be damaged--the knees, heart, etc.--without causing those to fail. And even those "unimportant" parts, when removed, often impair the system's function in more subtle ways than can be easily detected.
In general, the nervous system provides very strong evidence for complete mind-brain dependence (http://www.ebonmusings.org/atheism/ghost.html#part2). Conditions like Alzheimer's disease and amnesia can damage or even destroy parts of the mind in perfect unison with the appropriate brain sections.
"This patient, who suffered damage to both his hippocampus and his temporal lobes (thought to be important for storing memories) at age 46, has total anterograde and near-total retrograde amnesia: he cannot form new memories or recall old ones. He is trapped in a permanent present, a void of consciousness without memory.
Indeed, he has no sense of time at all. He cannot tell us the date, and when asked to guess, his responses are wild--as disparate [as] 1942 and 2013.... This patient cannot state his age, either. He can guess, but the guess tends to be wrong. Two of the few specific things he knows for certain are that he was married and that he is the father of two children. But when did he get married? He cannot say. When were the children born? He does not know. He cannot place himself in the time line of his family life. (Damasio 2002, p. 69-71)
(As Dr. Damasio tells us, the patient's wife divorced him over 20 years ago, and his children are long since grown up and married.) Does this man still have a soul? In what sense is he conscious? He is adrift in a world of darkness, a blank void with neither past nor future, merely an ever-moving present that continually fades from sight."
Damage to the frontal lobes can produce massive changes in both personality and mental abilities. Brain damage can even produce a person who's incapable of acquiring new memories - in effect, a mind trapped in the same time and place, one which will revert to his or her old memories every 15 minutes and nonchalantly ask his loved ones why they've aged so much after 20 years of asking them the same question.
A young priest once suffered a stroke that rendered him incapable of feeling sadness. Formerly compassionate and empathetic to his leukemia-stricken sister, he now made jokes about it and didn't understand why he should feel guilty about it. As his father commented, "... He looks like our son and has the same voice as our son, but he is not the same person we knew and loved... He's not the same person he was before he had this stroke. Our son was a warm, caring, and sensitive person. All that is gone. He now sounds like a robot."
"This wrenching story illustrates how a human property as fundamental as compassion arises from the brain and can be destroyed by altering the brain. A warm, caring, intelligent young man of God, as the result of brain damage, underwent a complete and drastic personality change. He became indifferent to his duties, unconcerned about the potentially fatal illness of a loved one, even light-heartedly joking about it with his grief-stricken parents, who said that he was "not the same person [they] knew and loved", not the same person he had been before his stroke. "
The author of that article, which explains a mass of other difficulties and cites many case studies, closes with this apt statement:
"The materialist can explain the effects of frontotemporal dementia without difficulty. How does the dualist explain it? What is happening to these people's souls? Is the deterioration of the brain causing changes to the soul - or are personality traits a quality of the brain and not the soul? But that implies that these traits will be lost upon death. In that case, in what sense will the soul in the afterlife be the same person it was during life?"
Not only does brain damage harm the mind, but certain bizarre conditions can even produce, for all intents and purposes, two damaged minds for the price of one healthy one.
"Research shows that in such split-brain cases, the brain generates what seems to be two separate consciousnesses. Research on split-brain patients led brain scientist and Nobel laureate Roger Sperry to conclude, 'Everything we have seen indicates that the surgery has left these people with two separate minds, that is, two separate spheres of consciousness. What is experienced in the right hemisphere seems to lie entirely outside the realm of the left hemisphere.'" I will expand on this particular point below
"Research shows that in such split-brain cases, the brain generates what seems to be two separate consciousnesses. Research on split-brain patients led brain scientist and Nobel laureate Roger Sperry to conclude, 'Everything we have seen indicates that the surgery has left these people with two separate minds, that is, two separate spheres of consciousness. What is experienced in the right hemisphere seems to lie entirely outside the realm of the left hemisphere.'"
I will expand on this particular point below.
Case studies in severed corpus callosum (http://physics.weber.edu/carroll/Wonder/split_brain.html) (the "split brain experiment" alluded to above) more or less spell the death knell for the soul. First, a bit of background on what we can learn from the different hemispheres in healthy people:
http://physics.weber.edu/carroll/Wonder/images/split2.gif" width="90" height="90 (http://physics.weber.edu/carroll/Wonder/images/split2.gif">Left brain dominates for language, speech, and problem solving
Right brain dominates for visual-motor tasks[/b]
[i]"1. Each hemisphere was presented a picture that related to one of four pictures placed in front of the split-brain subject.
2. The left and right hemispheres easily picked the right card. The left hand pointed to the right hemisphere's choice, and the right hand pointed to the left hemisphere's choice.
3. The patient was then asked why the left hand was pointing to the shovel. Only the left hemisphere can talk, and it did not know the answer because the decision to point to the shovel was made in the right hemisphere."
This experiment indicates both sides of the brain are capable of individual thought in some capacity, as if each one had an independent mind. Now we just need to find out whether this curious effect is merely an artifact of our consciousness, or really at odds with self-awareness being the result of a single, indivisible paranormal spirit.
Certain epileptic patients that don't respond to conventional treatment sometimes get the brain halves severed from each other. Amazingly, both halves can go on to develop unique tastes, preferences and beliefs. This indicates once the data link is cut, both can effectively function as "half a soul." In turn, this is quite difficult to reconcile with any remotely traditional model of dualism.
Courtesy of the <A HREF="http://www.macalester.edu/~psych/whathap/UBNRP/Split_Brain/Behavior.html]Macalester College psychology department:
"Before the operation he integrated information between the two hemispheres freely, but after the operation he had two separate minds or mental systems, each with its own abilities to learn, remember, and experience emotion and behavior. Yet, WJ, was not completely aware of the changes in his brain. As Gazzaniga put it: "WJ lives happily in Downey, California, with no sense of the enormity of the findings or for that matter any awareness that he had changed." As previously explained (experiments), words flashed to the right field of vision of patients like WJ could be said and written with the right hand. In contrast, patients couldn't say or write words flashed to their left field of vision [even though they could pick out the object with their hand]."
One brain hemisphere is verbal but has difficulty with certain other functions, while the other can't really talk but has other traits that make up for it. Each of those can, in their own way, identify and describe reality around them, but neither hemisphere has access to the self-awareness or thoughts of the other. Splitting them produces all kinds of anomalous results, like this:
"The patients give evidence of having two differing minds. The best example of this is patient Paul S., whom you read about on the home page. Paul's right hemisphere developed considerable language ability sometime previous to the operation. Although it is uncommon, occasionally the right hemisphere may share substantial neural circuits with, or even dominate, the left hemisphere's centers for language comprehension and production. The fact that Paul's right hemisphere was so well developed in it's verbal capacity opened a closed door for researchers. For almost all split brain patients, the thoughts and perceptions of the right hemisphere are locked away from expression. Researchers were finally able to interview both hemispheres on their views about friendship, love, hate and aspirations.
Paul's right hemisphere stated that he wanted to be an automobile racer while his left hemisphere wanted to be a draftsman. Both hemispheres were asked to write whether they liked or disliked a series of items. The study was performed during the Watergate scandal, and one of the items was Richard Nixon. Paul's right hemisphere expressed 'dislike,' while his left expressed 'like.'"
In light of these and other facts, the existence of the soul is effectively falsified unless one postulates an enormous number of ad hoc hypotheses to salvage it from the data. A modus operandi that tells us nothing about truth, and in fact usually obscures it.
If the soul existed, people wouldn't suffer Alzheimer's disease, couldn't be anesthetized, wouldn't have radical personality changes caused by tumors, and would, if brain hemispheres were split, either die or show a mysterious, spooky data link was still operating at a distance to make both hemispheres consistent with a single mind.
The difference can best be described as thin-client/mainframe vs. personal computing. In one device, the "consciousness" would run on an inaccessible device some distance away from the client, getting its instructions from a network connection. Damaging the client (i.e. body) would leave the files and processes (consciousness) on the mainframe as safe as ever, but it would only produce erratic results in the client.
If a part of the client's processor was damaged, you would feel as fine and clear-headed as you usually would, but your sources of input from the physical world would progressively fail until the link was severed, at which point you would experience conscious, total sensory deprivation (assuming no other source of input was provided, this is a nightmarish scenario).
You couldn't lose any memories, personality and self-awareness, because it would be safe and indestructible on the server. At worst, you could only lose the ability to express it to others successfully as the body went, but it would affect all memories equally, not apparently destroy some while leaving others entirely untouched.
As a further analogy, you could destroy your client's ability to present Microsoft Word documents to others, but you could never find that a specific .DOC was missing on the mainframe from damage entirely limited to the client side.
This is not what occurs--in fact, the exact opposite is observed. People really forget things because of brain damage. Chemical changes in the brain can induce depression and other personality changes. Self-awareness itself goes bye-bye if you're knocked on the head, anesthetized or asleep. And, of course, the "soul" is somehow split in two, directly correlated with physical splits to the brain itself. Thus, there's only one conclusion you can honestly draw from the neurological evidence. You're not an indestructible entity using a fragile gateway to the physical world--you are the gateway, on which every single aspect of yourself is stored. Once it goes, so do "you." So enjoy it while it lasts.