As a human, I know that I’m a very visual creature, spending most of my work hours reading papers (cough) and examining interesting objects in my visual field (ahem). A greater proportion of the information that the human brain has to process comes from the visual system. Most of our memories are stored as visual imagery. Taking this into account, when one compares the anatomical structures and connectivity of the visual system to the olfactory system, which processes the sense of smell, one finds a curious imbalance: the olfactory system seems to be directly connected to our emotion-cognitive brain structures such as the Amygdala and the Hypothalamus, as well as the more decision oriented frontal cortex, and even more astonishingly, it is the only sensory system which has – in its sensory layer in the epithelium of our nose – the ability to replace and renew olfactory sensory neurons. The visual system, or for that matter any other sensory system, does not possess these special characteristics. The direct connectivity to the Amygdala probably allows us to assign positive or negative associations to certain smells which directly influences our behavior towards them. We know that the smell of something putrid will invoke an immediate aversive response from us (or any other animal), and a unique smell, like the smell of fresh linen, can instantly and almost involuntarily trigger a recall of memories from the remote past. Here in lies the first of my two questions, why does visual input lag behind olfactory input in stimulating behavior directly and effectively? Does the answer lie in a stronghold maintained by the evolutionary older olfactory system?
It is obvious that the olfactory system is of great importance to many species of animals: insects such as the widely studied fruit fly, rodents such as mice, and our very own best friend the dog, are all highly dependent on their sense of smell for survival, and it is well known that their sense of smell is much more refined and has a much larger range than ours; for example, the sensory epithelium of a dog has a surface area forty times that of humans. Through the course of evolution, as animals moved to land from water bodies, the greater variety of smells they encountered on land required a larger and more complex olfactory system to be able to perceive and discriminate smells. This is the reason why fish possess fewer olfactory coding genes (about 100) than mice (1000 genes, the work of Nobel laureates Axel and Buck). There is a certain trend, in our species and related species such as the Gorilla, to have become less dependent on olfaction and more dependent on vision; evidence comes from studies in loss of gene function (carta.anthropogeny.org). So it seems that evolution is driving us toward becoming more and more visually oriented animals. Here’s a surprise though: a recent study by Bastir and colleagues (Nature communications 2011) reports an increase in olfactory bulb and orbito-frontal cortex size (both involved in olfactory processing) in modern humans compared to Neanderthals. As we all know, Neanderthals are extinct and we are thriving! I wouldn’t want to read too far into the results but they do seem to confuse the evolutionary trend!
- G. Narula