As a concept, your third one really strikes me, as I'm a biology major and a monist (the philosophy that "mind" and "brain" are the same, there is no soul). It's title of "wisdom," combined with simple biological forms all squished together in the center there, just seems to be a depiction of that philosophy. Mind? what mind? Wisdom? what wisdom? It's all in the brain, the neurons and glia cells and synapses.
As to your fourth one, I can tell it's much older, but your style is still apparent and use of color, some of the same exactness in representing the biological form (mostly in the arms and hands, as the troll head...thing...is more fantastical), and you clearly like working with imagery which is uncanny. The background is nice in the way that it is aesthetically interesting, yet not distracting from the focus, and ties in with the focus in overlapping with it (the red paint splotches) and also becoming part of the narrative (blood?) if one exists.