Probably it doesn't change us that much "directly", but indirectly. For example, by triggering conditions like OCD (anxiety/neurological brain disorder) it causes rise in anxiety, which then impacts our perception of the world. The anxiety attacks are debiliating and frustrating, which then makes us suffer emotional trauma. We become prisoners, mentally and physically, of the anxiety. We get so tired from the panic attacks, or the rituals, and so absorbed in them, that we can no longer enjoy all the things we did. It impacts our ability to go out and make friends (or even to go out in some cases of aggoraphobia), which in turns leads to isolation, which in turn leads to loneliness, which in turn leads to depression. It can impact personal relationships either because the person is unable to make that attachment because of the anxiety or problem, or can impact people because of the infertility mercury can cause, hence triggering depression. Then there is the other indirect impacts i.e. the fatigue of mercury poisoning might make someone have to think twice about a favourite sport, or the muscle pains can do the same. Being light sensitive might impact a person enjoyment of sunny weather or cinema, etc.

So, while the actual effect of mercury on the brain may be pretty standard (with various degrees of severity depending on sensitivity) the consequences and manifestations vary enormously depending on a person's lifestyle, circumstances, and the other effects that mercury has. If someone is already married or in a relationship, for example, and the partner is supportive, then that might help avoid some of the deprssion that a single person might go through because they have an additional isolation of being alone and not able to commit to relationships or dates because of the anxiety. But on the other hand, if the marriage is not good, or the partner not supportive, it could make the person just as depressed. And if the person is not depressed about their relationship, then they might be depressed about something else.

These are just my own thoughts, why I think it's hard for some medical people to accept there is mercury poisoning. Maybe they expect a set pattern i.e. a + b - c always = xy when with mercury poisoning, there isn't such a patterns, not only because the things that mercury attacks are dependent on variables like genetic dispositons, but that the mental areas affected can manifest differently in different people.