For most animals it is fairly clear which areas of the cortex handle which kinds of motor or sensory function. In cetaceans like dolphins however, these kinds of things are not so obvious. That’s primarily because electrophysiological experiments on live dolphins simply don’t happen. Fortunately, researchers now have a new tool known as diffusion tensor imaging (DTI) which can potentially map out how brains are wired up in unprecedented detail.
The technique is basically a modification of the standard MRI scan which creates maps of the average diffusion of water in the brain. The idea is that water diffuses faster along axons and therefore reveals their connectivity. In a paper that appeared yesterday in Proceedings of the Royal Society B researchers used DTI to try to map out the auditory cortex in a dolphin brain specimen that was preserved 15 years ago. They found that dolphins appear to have evolved several new areas of auditory cortex, presumably to meet the demands of sonar processing.
When you take a look at a dolphin brain, those particular findings are probably not all that surprising. Compared to the human brain, the dolphin cortex has vastly more surface area compacted into an only slightly larger footprint. What that means is that the dolphin cortex ends up much thinner than ours. The real question then becomes, is a thinner, more expansive cortex an optimization for sonar, and if so, how?
For one thing, more cortex gives you relatively more output neurons. These are the pyramidal cells located in the bottom level layer that project a myelinated axon out of the cortex to connect to regions afar. One good way to think about the cortex is to consider it as pizza dough. If you have a pound of dough you can either make a deep dish pie, or a thin crust pie. The difference is that the thin crust pie will let you have a lot more toppings. The abundant toppings here in this case, are those output neurons.
So what does the deep dish pie give you? Well, one answer is that a thick cortex means that the basic cortical unit — the pyramidal cell — gets to enjoy a long, thoughtful apical dendrite. That’s the dendrite that extends up from the deep layer all the way to the top, accumulating information through the full thickness it encounters. In the picture of a pyramidal cell, the thin projection coming out of the bottom is actually the axon, along with a few ‘collaterals’ it sends back to its local cortical homeland.
Long apical dendrites may be great for pondering the secrets of the universe, but the side effect is a necessarily longer integration time. In other words, if the spikes that the pyramidal cell produces are presupposed to take into consideration the activity found on the neuron’s entire network of dendritic feelers, then either the neuron needs to slow down its firing rate in a thick cortex, or miss out on something in generating its primary output.
What does this all mean for sound processing? In contrast to vision, the sound business moves pretty fast. Echolocation, which has gained recent notoriety in the few persevering humans able to master it, is not something for the feint of heart. At this point in the game there is now little scientific resistance against the idea that bat practitioners of the trade routinely make nanosecond timing discriminations with their particular brand of neural hardware.
This need for speed has kept the auditory cortex tight. By that I mean that after sensory information comes into the brainstem from the ear and projects up to the inferior colliculus and thalamic midbrain, it appears to route to the nearest cortex it can find. Through evolutionary time that has caused the major fold of the brain — the Sylvian fissure — to crumple around the auditory terminations. Now there is still much debate about the finer details of where auditory cortex sits. The older method of putting a dye into the auditory midbrain and looking for what parts of the cortex it eventually labels leaves much to be desired.
That’s one reason why new DTI methods, if the can prove accurate, have so much appeal. Scanning a post-mortem brain takes much longer than a living brains since it contains considerable less water, and may not be ideal. Nonetheless the researchers were able to show that dolphin has auditory projections to ‘new’ area both above and below this deep Syvian cleft. The dorsal region they found (upper and back region) abuts up to the dolphin visual area. In most mammals the visual area is at the extreme back of the brain while in dolphins it has been compressed to the top and more centrally as the temporal or side regions of their brains ballooned.
The researchers call this the ‘suprasylvian region’ in keeping with the nomenclature of simpler mammalian brains. In dolphins however, there doesn’t seem to be much sense in trying to assign consistent names to all their many gyri and sulci. They also reported auditory connections within the upper temporal region, the place more traditionally associated with audition in terrestrial animals.
To really understand sound processing and the evolution of brains it will be necessary to look closely at several other species. Elephants, for example, have brains with a thin expansive cortex like dolphins, but also have retained complex hippocampal structures (a region with deep evolutionary connection to olfaction) in contrast to the dolphin. Elephants clearly have done a lot with their noses while dolphins only get to smell when they surface.
Dugongs (or manatees) on the other hand, which in many ways parallel dolphin adaption to a marine lifestyle instead went with the deep dish model, and have little or no folding in their thick cortex. Pooling together many anecdotes like these will help narrow the range of the possible in assigning form to function for cortical metrology, and hopefully help crack some of its mystery.
