@Vincent BergVB, I would agree that the power of metaphors or similes comes from how we process them. The metaphor of a "mental pathway" doesn't help me much, however. My thinking becomes more useful when I consider things from a psychological, rather than a neurological, perspective.
To me, the power of these devices comes from the familiar/unfamiliar conflict, and the power depends on both, plus the conflict. So, think of
hand/snake
The first time this metaphor was used (probably by Ugg to his brother Zugg while sitting in a cave), it carried power. Take the sentence, "His hand was a snake striking to kill."
This stops the reader with its blatant declaration of a clear untruth. "Hand" generates one mental framework. "Snake" generates another. All the benign connotations of "hand" are eliminated (a hand up, hand out, reaching out a hand, hand shake), leaving only the negatives. In addition, since "snake" is closely associated with threat for most of us, it generates emotions in the fear family, and that might well work for the author. (For some, snakes are also associated with thoughts of "unclean" and generate emotions in the family of disgust, so that might work also.)
Both the familiarity and the unfamiliarity are important. For example, consider:
"The boorishness of his behavior struck him like a bowstring on unprotected forearm - painful and humiliating."
For me, that image is powerful because I've done that - shot a bow without an armguard despite knowing better and gotten my forearm burned by it. The pain was followed by the instant humiliation that I'd done something stupid. For most people, it would be more like, "Huh? What does that mean?" That response might well pull them too far out of the story, but at least it would have the right emotional connotations.
To me, the main problem with your "diarrhea in a meeting" metaphor is that it has the wrong emotional connotations - disgust, shame, humiliation rather than threat, danger, fear. Yes, both aspects of hand/diarrhea are familiar, and they are in an unusual or unfamiliar arrangement, but they don't "work" for the point of the story. Metaphors that become cliches do so because the "work."
Some even work too well; we forget that they are metaphors, not reality. "Mental pathways" would be an example. We don't have any real idea of how thoughts, even repeated patterns of thought, are represented in the structures and operations of the brain, but "mental pathways" feels so right (and, it even fits with some of the bits and pieces we do understand) that we start to think of it as reality. The power of such metaphors, for good and ill, is best shown by the work of Carol Dweck and the colleagues and researchers who have followed after her. Viewing intelligence or talent as a "fixed" thing which we have only so much of sets us up to fail by not trying, practicing, and improving when things get difficult. But, if we can begin to think of intelligence or talent as something we can increase through effort, something that can be grown and developed, we improve and succeed.
Acck! Your post really got me going, and I appreciate your efforts in raising thoughtful topics!