June 27, 2013 — A series of studies conducted by Randy Bruno, PhD, and Christine Constantinople, PhD, of Columbia University’s Department of Neuroscience, topples convention by showing that sensory information travels to two places at once: not only to the brain’s mid-layer (where most axons lead), but also directly to its deeper layers. The study appears in the June 28, 2013, edition of the journal Science.
For decades, scientists have thought that sensory information is relayed from the skin, eyes, and ears to the thalamus and then processed in the six-layered cerebral cortex in serial fashion: first in the middle layer (layer 4), then in the upper layers (2 and 3), and finally in the deeper layers (5 and 6.) This model of signals moving through a layered “column” was largely based on anatomy, following the direction of axons — the wires of the nervous system.
“Our findings challenge dogma,” said Dr. Bruno, assistant professor of neuroscience and a faculty member at Columbia’s new Mortimer B. Zuckerman Mind Brain Behavior Institute and the Kavli Institute for Brain Science. “They open up a different way of thinking about how the cerebral cortex does what it does, which includes not only processing sight, sound, and touch but higher functions such as speech, decision-making, and abstract thought.”
It sounds like we know both more and less than we did before, but that is probably a good place to begin, when studying the brain.
Note this however:
The study suggests that upper and lower layers of the cerebral cortex form separate circuits and play separate roles in processing sensory information. Researchers think that the deeper layers are evolutionarily older — they are found in reptiles, for example, while the upper and middle layers, appear in more evolved species and are thickest in humans.
Oh dear, where is that Kevin Padian when we need him? In what sense are modern mammals “more evolved” than modern reptiles?
Since we are here anyway: Reptiles vary greatly in intelligence; some reptiles are actually fairly smart. For that matter, so are some molluscs (but not others). Whatever this new find about the six layers of the cerebral cortex ends up meaning, we should avoid the unbidden mental image of a “tree of intelligence” (= all mammals are smarter than all reptiles; all vertebrates are smarter than all invertebrates, etc.) As far as intelligence is concerned, evolution simply does not work that way.
Journal Reference:
C. M. Constantinople, R. M. Bruno. Deep Cortical Layers Are Activated Directly by Thalamus. Science, 2013; 340 (6140): 1591 DOI: 10.1126/science.1236425