Smell has apparently always been a peculiar sense. The sensory pathway of smell information to the brain runs completely independent from the other senses. The pathways for the other senses run through the midbrain and thalamus and are then relayed to cortical regions. But smell goes to the olfactory bulb behind the nose, and from there directly to various forebrain regions such as the amygdala, hippocampus, and prefrontal cortex).
This independent pathway is ancient. From the earliest vertebrates, it appears that smell has always gone directly to the telencephalon (the forebrain) while the other senses went through the optic tectum (midbrain) region.
This is strange, because while other sensory information, such as vision, hearing, and touch are routed to the forebrain in mammals, allowing the formation of sensory images in the cortex, this does not appear to happen in amphibians and reptiles. The creation of sensory images in the forebrain, other than smell, appears to be an innovation of mammals and birds (perhaps independently, making it an example of convergent evolution).
This has led many biologists to conclude that the telencephalon in fish and reptiles is basically just a “smell brain”. This seems borne out by experiments with fish, where their telencephalon was destroyed, and the fish seemed able to go about their normal lives. However, the fish did lose the ability to learn or anticipate consequences, including new spatial navigation. In other words, the fish lost the ability to remember and imagine, which seems to indicate that there is more at work in their forebrain than just smell.
But if you think about it, smell, unlike the other senses, is much more entangled with memory. Smells of predators and prey linger after they’ve departed. For an animal to make use of smell information requires memory and imagination, accessing past associations of the smell, whether it indicated a predator or some food source, and thinking about what the smell in the current situation means. This isn’t necessarily true for vision, hearing, touch, or taste, where reacting reflexively to current stimuli can still be adaptive.
In other words, the rise of smell might have led to the rise of memory and imagination. And as I’ve written before, sentience, the ability to feel, is only adaptive if it can be used for something. This is why most neuroscientists see feelings as we consciously perceive them being linked to the same regions where imagination is coordinated, the frontal lobes in mammals, or more broadly the forebrain in non-mammalian vertebrates. Which is to say, that smell may have been what led to the evolution of sentience.
Todd Feinberg and Jon Mallatt, in their book The Ancient Origins of Consciousness, discuss and argue against this proposition. For them, it seems far more reasonable to see vision as the sense which drove consciousness. And strictly in terms of image based consciousness, they may be right. But in early vertebrates, most of that image based consciousness seemed focused on the midbrain region, a region that doesn’t appear capable of memory and nonreflexive learning, behaviors typically associated with sentient consciousness.
An interesting question to ponder is, if vision, hearing, and the other senses are processed primarily in the optic tectum, the midbrain region for fish and reptiles, how much of that sensory information actually makes it to their telencephalon, that is, into their memories and imagination? Humans have no introspective access to the low resolution images formed in our own midbrain region, only to the ones we form in our cortex. But is the telencephalon of an amphibian or reptile able to access the visual information from its optic tectum?
John Dowling in his book, Understanding the Brain, points out that a frog, which can catch and eat flies with its tongue, can only see a fly if it is moving. A frog in a cage stocked with fresh but dead flies, will starve. Dowling asks what the frog is actually “seeing” in that case. It may be that the frog’s optic tectum can generate reflexive tongue movements, but that the frog itself has no conscious access to a visual image of the fly, or any other visual images.
And yet, the telencephalon of these species can inhibit the reflexive reactions from their tectum. In order to do so effectively, it seems like they should get some information from their tectum, their midbrain region. In fact, Feinberg and Mallatt in their book indicate that some visual and audio information has been shown to make it to the telencephalon. But it seems likely, similar to how we receive processed information from our midbrain, that this comes in the form of feelings rather than detailed sensory information.
This has led many biologists to conclude that amphibians and reptiles aren’t conscious. As I’ve noted before, whether to call a particular species “conscious” is ultimately a matter of interpretation. However, we can say that their experience of the world is very different from ours. That experience does not appear to include visual and auditory images, although it may well include olfactory ones.
So it’s possible that we are conscious today because of smell. This proposal is strange and counter-intuitive to us because we’re primates, a group of mammalian species where the sense of smell has atrophied. But for most animals, smell is a major part of their worldview.
What do you think? Did we use smell to climb the ladder of sentience and then, as primates, kick that ladder loose? Does the lack of visual images mean fish aren’t conscious?