Source: A Brain-Mind Odyssey, Ch. 11-12
Tags: sensory perception, sensation, perception, naïve realism, chemotaxis, phototaxis, electromagnetic spectrum, visible light, ultraviolet, infrared, echolocation, electroreception, magnetoreception, olfaction, olfactory receptor, GPCR, anosmia, olfactory bulb, pheromone, vomeronasal organ
Sensation is the collection of environmental information via receptors; perception is the brain's interpretation of that information. Different species have access to wildly different slices of the physical world (UV, infrared, electric fields, magnetic fields, ultrasound). In humans, olfaction begins when airborne molecules bind to roughly 350 different GPCR-type receptor proteins in the nasal epithelium, and the signal travels through the olfactory bulb to cortical and limbic regions, which is why smell is so closely linked to emotion and memory.
Sensation
The collection of information from the environment via sensory organs and receptors.
Perception
The analysis and interpretation of sensory information by the nervous system, producing mental states of perceptual awareness.
Naïve realism
The epistemological assumption that what we perceive is identical to what actually exists in the world. Easily disproven by optical illusions.
Chemotaxis
Movement of an organism toward or away from a chemical stimulus. Even bacteria (e.g., E. coli) exhibit this.
Phototaxis
Movement of an entire organism toward light.
Phototropism
Bending or growing toward light (typically in plants).
Echolocation
Biological sonar. Animals emit high-frequency sound and hear its reflection off nearby objects. Used by bats, dolphins, and whales.
Olfactory epithelium
The moist, mucous-lined tissue inside the nasal passages where volatile odorant molecules are caught and detected.
Anosmia
Loss of olfactory sensitivity. Specific anosmia is the inability to smell one particular odour (often genetic). General anosmia is broad or total loss of smell.
Pseudogene
A non-functional gene that does not code for a working receptor protein. Humans have roughly 600 olfactory pseudogenes.
Pheromone
A chemical signal carrying social communication information between members of the same species.
Vomeronasal organ
A specialised olfactory sensory structure in many vertebrates that responds somewhat selectively to pheromone molecules.
Sensation: raw data collection from sensory organs
Perception: the brain's construction of experience from that data
These are distinct processes; perception involves interpretation and can be fooled (illusions)
E. coli uses amino acids interacting with receptor proteins on its outer membrane to swim toward nutrients
Motility pattern: runs (straight swimming, flagella bundled as a propeller) and tumbles (flagella reverse direction, fly apart, cell tumbles randomly)
A simple example of sensation at the single-cell level
Phototaxis: whole organism moves toward light
Phototropism: organism bends or grows toward light
Purposes: energy generation (photosynthesis) or growing toward open regions to disperse spores/seeds
The assumption that perception mirrors reality directly
Optical illusions demonstrate that our perceptual systems construct and sometimes distort our experience
The perceptual system has been tuned over evolutionary time, but it is not a perfect mirror
Vision, hearing, taste, smell, touch
These are not the only senses humans have
Vestibular sense: receptor organ is the semicircular canals of the inner ear; receptor cells are hair cells; stimuli are gravity and acceleration; gives the experience of balance
Proprioception: stretch receptors in muscles and joints detect muscle tension and joint position; tunes body alignment and coordinated movement
Both tend to operate below conscious awareness; you notice them when something goes wrong (dizziness, being off-balance)
Ranges from very high-energy gamma rays and X-rays to very low-energy radio waves
Spans more than 18 orders of magnitude (10¹⁸)
Visible light occupies less than a single order of magnitude: roughly 400 nm to 700 nm
Ultraviolet: slightly higher energy than visible light
Infrared: slightly lower energy than visible light; absorbed by molecules to produce heat
Karl von Frisch (early 20th century) demonstrated that honeybees have colour vision
Bees can detect UV patterns on flowers that are invisible to humans
UV markings act as guides to attract and direct bees to nectar
Pit organs on the snake's head detect infrared radiation (heat)
Allows accurate prey location even in complete darkness
Electromagnetic radiation vibrates in specific planes
The plane and degree of polarisation of sunlight vary depending on the direction you look relative to the sun
Many insects, birds, and reptiles can detect polarised light and use it for navigation
Von Frisch discovered this in honeybees
Human hearing: 20 to 20,000 Hz
Infrasound (below 20 Hz): audible to elephants
Ultrasound (above ~20,000 Hz): audible to bats, dolphins, whales, dogs, cats
Echolocation uses ultrasound because shorter wavelengths permit finer spatial discrimination
Some organisms detect the electrical fields generated by living things
Sharks: electroreceptive structures called ampullae of Lorenzini, dispersed over the head; used to locate camouflaged prey
Platypus: electroreceptors in the bill detect prey in murky water
Electric fish: found in murky river waters (e.g., the Amazon)
All limited to aquatic environments where electric fields propagate well
Many animals (birds, fish, turtles) navigate using the Earth's magnetic field
Homing pigeons: classic example; placing a small magnet on a pigeon disrupts its navigation
Pigeons may use magnetic particles in the eye
Some animals combine magnetic cues with other environmental information
Odorants are airborne, volatile molecules
They enter the nasal passages during normal breathing
Some molecules get trapped in the moist, mucous tissue of the olfactory epithelium
Receptor cells have cilia that increase surface area and are densely packed with olfactory receptor proteins
Olfactory receptor proteins are GPCRs
Each GPCR responds to molecules of a specific molecular shape
Different patterns of activation across the 350 human GPCRs allow discrimination of a very large number of distinct odours (estimated ~100,000)
Receptor protein numbers across species:
Fish: ~100 different olfactory GPCRs
Mammals (general): ~1,000
Mouse: ~1,300
Human: ~350 functional GPCRs, plus ~600 non-functional pseudogenes
Olfactory stem cells are embedded in the olfactory epithelium
Flexible enough to differentiate into various types of olfactory receptor cells
Receptor cells are replaced every 1 to 2 months
Essential oil: the oily concentrate of volatile aromatic molecules from a plant
Often hydrophobic
Originally prepared by distillation (invented for perfume-making)
Aromas are typically mixtures of many molecules interacting with multiple receptors
Sometimes one or two molecules dominate (e.g., cinnamon's aroma is a combination, but certain compounds contribute most strongly)
Geranial smells of lemon; geraniol smells of rose
The structural difference: just two hydrogen atoms and a slightly different bond at one end
Molecules containing the -SH group
Often smell unpleasant (e.g., skunk spray, asparagus-scented urine)
Specific anosmia: inability to detect a particular odour
Example: some people cannot smell the sulphur compounds in urine after eating asparagus
Most likely cause: a genetic variation in one of the 350 olfactory GPCRs
General anosmia: loss of sensitivity to a broad range or all odours
Causes: head trauma, nasal congestion, developmental factors, degenerative brain disease, hypersomnias
Olfactory receptor cells send axons into the olfactory bulb (located just above the nasal cavity)
These fibres constitute cranial nerve I (the olfactory nerve)
In the olfactory bulb, axons synapse with dendrites of mitral cells
Mitral cells project to:
Piriform cortex (deep in the brain) and the amygdala (limbic system)
Piriform cortex then sends axons to the thalamus, and from there to the orbitofrontal cortex (frontal lobe), which is responsible for interpreting olfactory information
Additional connections to the hypothalamus and temporal cortex
The direct link from olfactory bulb to amygdala explains the strong connection between smell and emotional response/memory
Pheromones: chemicals that carry social signals between members of the same species
Used for identity, social status, mate attraction, territorial marking, danger signalling
Vomeronasal organ: a distinct sensory structure and neural pathway in many vertebrates that responds somewhat selectively to pheromones
Well-developed in rodents and many other animals
Vestigial or non-functional in adult humans (debated)
⚠️ Distinguish sensation (data collection) from perception (interpretation). This distinction runs through the entire sensory unit.
⚠️ Olfactory receptors are GPCRs. This connects to the broader theme of GPCRs across chapters (opioid receptors, cannabinoid receptors, rhodopsin in vision, taste receptors).
⚠️ Know the olfactory pathway: olfactory epithelium → olfactory bulb (CN I) → mitral cells → piriform cortex + amygdala → thalamus → orbitofrontal cortex.
⚠️ The direct olfactory bulb-to-amygdala connection explains why smell is uniquely linked to emotion and memory.
⚠️ Olfactory stem cells replace receptor cells every 1-2 months. This is a rare example of ongoing neuronal replacement in the adult nervous system.
⚠️ Specific anosmia is typically caused by genetic variation in a single olfactory GPCR.
⚠️ Be able to give examples of senses beyond the "canonical five": vestibular, proprioception, electroreception, magnetoreception.
Q: Why is olfaction more closely linked to emotion than vision or hearing?
A: The olfactory bulb projects directly to the amygdala and piriform cortex (limbic system) before reaching the thalamus and cortex. Other senses are routed through the thalamus first. This direct limbic connection gives smell a particularly strong link to emotional responses and memory.
Q: How many functional olfactory receptor proteins do humans have, and how can they discriminate so many odours?
A: Humans have approximately 350 functional olfactory GPCRs. Each receptor responds to a specific molecular shape, and a single odorant activates a unique combination of receptors. The combinatorial pattern across all 350 receptors allows discrimination of an estimated 100,000 different odours.
Q: What is specific anosmia, and what is its most likely cause?
A: Specific anosmia is the inability to detect a particular type of smell (e.g., asparagus metabolites in urine). The most likely cause is a genetic variation in one of the 350 olfactory GPCRs, resulting in a non-functional receptor for that odorant's molecular shape.
Q: What is echolocation, and why does it use ultrasound?
A: Echolocation is biological sonar in which an animal emits sound and detects its reflection from objects. It uses ultrasound (very high frequency) because shorter wavelengths allow finer spatial discrimination, enabling the animal to detect small objects and navigate precisely.
Q: How do honeybees use ultraviolet vision?
A: Honeybees can detect UV light that humans cannot see. Many flowers have UV-reflective patterns that act as visual guides, attracting bees and directing them to nectar sources. Karl von Frisch demonstrated honeybee colour vision through careful experimentation.
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