The olfactory system detects molecular odorants. It has both similarities and differences from the other senses.

We often talk about five major senses: vision, hearing, touch, smell, and taste. Actually, there are more than five, like pain, sensory pleasure, and even a tickle sense in early childhood.

The famous five senses all appear in our stream of consciousness, but vision, for example, is believed to have at least a third of a second of unconscious processing before a simple visual stimulus becomes conscious. This mix of conscious and unconscious elements also applies to hearing, speech, music perception and touch. 

Smell and taste are not as well understood, leaving many unanswered questions. 

In this issue, neurobiologist Stuart Firestein, one of the world’s leading experts on olfaction, discusses some new work, from his lab and others, that suggests the way we code olfactory inputs in the receptors may be fundamentally different from, say, vision or hearing. He explores the commonalities and surprising differences of smell and taste in relation to vision, hearing, and somatosensory systems, like touch.

New Questions about Smell, Taste & Consciousness with Neurobiologist, Stuart Firestein. 

Episode 26 features Dr. Stuart Firestein, an esteemed neurobiologist who specializes in the olfactory system – why and how our brains and noses sense smell. Stuart is the former chair of Columbia University’s Department of Biological Sciences, and a professor whose lab studies the vertebrate olfactory system, possibly the best chemical detector on the face of the planet. His lab uses the vertebrate olfactory receptor neuron as a model for investigating general principles and mechanisms of signal transduction.

Stuart is devoted to the history, evolution and future of science. Author of “Ignorance: How it Drives Science” and “Failure: Why Science is so Successful” (OUP), his books cast science as an unending quest to illuminate ignorance and failure as an essential component in that process.

We discuss:

• Is Olfaction an Idiosyncratic or Unique Brain System?
• The Connection Between Smell, Memory & Emotion
• The Immune System Recognizes a Repertoire of Molecules Just Like the Olfactory System
• Regenerative Capabilities of Olfactory Neurons
• Are Olfactory Receptors Like Other Chemoreceptors?
• The Integration of Olfactory Stimuli into a Chemical Panorama
• Is Olfaction a Low- or High-Dimensional Input System?
• How to Improve Science Learning
• The Value of Ignorance and Failure in Science

How do airborne plumes of odorant molecules emerge as vivid conscious smells and tastes in the brain? 

Most of us have conscious memories of important tastes and smells from childhood. Why do certain smells – rapidly, and sometimes vividly – flood our conscious brain with memories of the past, of a specific person or an event?

• Ramón y Cajal, often considered to be the father of modern neuroscience, was the first to observe under a microscope that only two synapses (connections) link the olfactory cortex to the outside world.

• There is a 3 way relationship here between olfaction, emotion and memory, which may be because memory involves strengthened connectivities. One hypothesis that Stuart poses could be related to the unique wiring of the olfactory system.

Is the olfactory system a kind of “Darwinian” selectionist system that is designed to recognize an open set of molecules? This is a question famously raised by Nobelist Gerald Edelman.

• The immune system has cell surface molecules that sense incoming toxins and so on, and produce a kind of Neural Darwinist response in such a way in that the set of toxins that the immune system can recognize is an open (unending) set, in perhaps the same way as the olfactory system recognizes an potentially endless set of molecules.

Olfactory receptors seem to be unique among brain cells for their ability to regenerate throughout an animal’s life. All specialized cells in the body begin as unspecialized stem cells that then differentiate, and olfactory cells are specialized for recognizing odorants.

• “But most neurons don’t regenerate,” says Stuart. “When you lose them, they’re dead and they’re gone, and you don’t get new ones to replace them. However, in the olfactory system and the hippocampus, you can regrow these neurons and the animal’s sense of smell will be restored. This happens in humans as well. It’s a very robust phenomenon.”

Chemoreceptors are distinctive in that they’re found in lots of other tissues. 

• Originally they were thought to be unique to the nose, but they turn out to be found in many other tissues, including the heart, in liver, in muscle, in esophagus, in the gut, and in particular in the kidney, where we actually know what the functions are.

There is even the possibility that olfactory receptors did not evolve as olfactory receptors. They began as chemoreceptors in other tissues in the body and eventually coalesced, along with the rest of the brain, into a single organ in the head where they became the nose and expanded significantly. We don’t know which came first, the olfactory receptors in the nose or the olfactory receptors in the kidney.”

• One of the great stories is that olfactory receptors are a certain kind of receptor called G-protein coupled receptors (GPCRs), which are very closely related to receptors for dopamine and serotonin.

The olfactory system presents us with alternative views of how sensory stimuli are parsed in the brain and how we integrate those into some sort of conscious world view. 

• Indeed, the olfactory system forces us to think of alternatives to the dominant models developed for vision. We have lived under the tyranny of visual feature detectors for 70 years – there are many ways the brain may parse sensory stimuli besides the visual.

Olfaction presents some interesting questions in higher level studies. It presents a contrast to other sensory systems in that the input is high dimensional and discrete, rather than continuous. Molecules have many dimensions.

How does the brain deal with that?

To listen to the full episode, CLICK HERE!

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Talking Points

• 0:00 – Intro
• 3:59 – The Ups and Downs of Consciousness in Science 
• 12:30 – Stuart’s Journey from Theater to Neuroscience
• 19:14 – Is Olfaction an Idiosyncratic or Unique Brain System?
• 25:30 – The Connection Between Smell, Memory & Emotion
• 34:37 – The Immune System Recognizes a Repertoire of Molecules Just Like the Olfactory System
• 42:51 – The Regenerative Capabilities of Olfactory Neurons
• 51:48 – Are Olfactory Receptors Like Other Chemoreceptors?
• 01:02:34 – The Integration of Olfactory Stimuli into a Chemical Panorama
• 01:10:24 – Is Olfaction a Low- or High-Dimensional Input System?
• 01:16:56 – How to Improve Science Learning
• 01:22:25 – The Value of Ignorance and Failure in Science

Bios

 

 

Global Workspace Theory (GWT) began with this question: “How does a serial, integrated and very limited stream of consciousness emerge from a nervous system that is mostly unconscious, distributed, parallel and of enormous capacity?”

GWT is a widely used framework for the role of conscious and unconscious experiences in the functioning of the brain, as Baars first suggested in 1983.

A set of explicit assumptions that can be tested, as many of them have been. These updated works by Bernie Baars, the recipient of the 2019 Hermann von Helmholtz Life Contribution Award by International Neural Network Society form a coherent effort to organize a large and growing body of scientific evidence about conscious brains.