Here is a series of visuals I sketched in response to several discussions about depression and the effect of antidepressants. These are visualisations of shared anecdotal experiences only and should be read as such.
In a non-depressed person (normal as in statistically), their emotional state fluctuates between positive and negative in response to life events. A wedding may cause a large positive spike. A death may cause a large negative spike. The depth of feelings is more reasonably accounted for - we are happy with cake than we are with celery. The death of a pet is felt more keenly than losing $10. Generally, their emotional state is fairly stable.
A depressed person still feels a range of emotional states, but the entire emotional range is shifted downward into the negative. Fewer events are able to shift the emotional state into the positive. Cake may not be enough. And as everything is already shifted in the negative, negative events, even minor ones, can cause deep downward spikes. Losing $10 can feel like losing $100,000.
Antidepressants seem to do two things. They shift the emotional state back up towards the normal. And they dampened the extreme emotional spikes, both positive and negative, leading to a numbing effect.