People are far more aware of focal brain injury rather than diffuse brain injury. Focal brain injury will often result in a profound deficiency related to the portion of the brain affected. In the case, the focal injury is often readily apparent even to those with casual contact.
Diffuse brain injury is often hard to discern to the casual observer. With subtle and cumulative injuries even the affected individual may not be particularly aware of the problem. It is often those close to them that notice personality changes, changes of habit, loss of function. There also seems to be a delayed effect where mental deterioration associated with aging will compound the problem. Mental decline well after the effect can be severe.
Symptoms of diffuse brain injury are; mental slowness, short term memory problems, brain fog, severe apathy, severe depression, anhedonia, problems with executive function, severe social withdrawal, increased impulsive behavior, impaired judgment, poor personal habits becoming much more pronounced. Sometimes it’s hard to tell because weak areas in a persons personality are often where damage will show itself. Example: Someone impulsive will become very impulsive, or someone naturally solitary will become extremely so. It is also common for someone social to become withdrawn especially as they become older.
There are also limits to how much a neurologist can tell you. Profound changes can be easily picked out, more subtle problems can’t be easily discerned from everything else going on with a person. It’s often not easy to tell what the origin of a problem like depression is. Nature vs. nurture, is the problem the result events in the persons life, a genetic predisposition, toxicity (alcohol or heavy metal poisoning), hypoxia or vascular issues, personality issues, growing up issues, or just plain getting hit in the head too much.