Source: Module 4, Chapter 11 | Course: A Brain-Mind Odyssey (UC Berkeley)
Tags: sensory perception, sensation, chemotaxis, phototaxis, phototropism, naive realism, electromagnetic spectrum, visible light, ultraviolet, infrared, echolocation, electroreception, magnetoreception, audition, polarisation of light
Sensory perception is the brain's two-part process of collecting environmental information through receptors and then interpreting that information into conscious experience. Different organisms have evolved radically different sensory capabilities, from bacterial chemotaxis to pit-viper infrared detection to shark electroreception. Humans perceive only a narrow slice of the electromagnetic spectrum and a limited range of sound frequencies, while other animals sense UV light, polarised light, electric fields, and magnetic fields.
Sensory perception
The process by which the brain receives signal information from the body's environment and uses it to form a mental experience of the world. Two components: (1) collection of information via sensory organs and receptors, and (2) analysis and interpretation by the nervous system, producing perceptual awareness.
Bacterial chemotaxis
Behaviour in which amino acids (e.g. aspartic acid, serine) interact with receptor proteins on a bacterium's outer membrane, directing the organism to swim toward nutrients. Nutrient binding reduces tumbling and keeps the bacterium moving toward the food source.
Runs (E. coli chemotaxis)
Straight-line swimming lasting roughly one second. Occurs when the bacterium's flagella rotate in one direction, bundling together like a propeller to produce coherent forward motion.
Tumbles (E. coli chemotaxis)
Brief episodes where the bacterium stops swimming and flops about randomly. Occurs when flagellar rotation reverses, causing individual flagella to fly apart and forward motion to cease.
Phototaxis
Movement of an entire organism toward light, e.g. a swimming bacterium orienting toward a light source.
Phototropism
Growth or bending toward light, e.g. a plant leaning toward a window. Distinct from phototaxis because the organism does not relocate, it reorients by growing.
Naive realism
The assumption that what we perceive is identical to what exists in the external world. Limited by phenomena such as optical illusions, which show that perception can diverge from physical reality.
Electromagnetic spectrum
The full range of electromagnetic energy, from very high-energy gamma rays and X-rays, through ultraviolet, visible light, infrared, microwaves, down to low-energy radio waves.
Visible light
A narrow band of the electromagnetic spectrum (roughly 400–700 nm) that falls within the sensitivity range of the human visual system.
Ultraviolet (UV)
Electromagnetic radiation slightly higher in energy than visible light. Some animals (e.g. honeybees) can detect UV, and many flowers have UV-visible patterns invisible to human eyes.
Infrared (IR)
Electromagnetic radiation slightly lower in energy than visible light. Absorbed by many molecules in a way that sets them vibrating. Pit vipers detect infrared to locate warm-blooded prey.
Infrasound
Sound frequencies below 20 Hz. Elephants can both generate and hear infrasonic frequencies.
Ultrasound
Sound frequencies above human hearing range (~20,000 Hz). Dogs and cats can hear up to around 40,000 Hz and above. Bats use ultrasonic frequencies above 100,000 Hz for echolocation.
Echolocation
Technique used by bats, dolphins, and whales: emitting high-frequency sounds and detecting their reflections off nearby objects to navigate and hunt.
Electroreception
Detection of electric fields generated by living organisms. In sharks, electroreceptive structures called ampullae of Lorenzini are distributed across the head.
Electrolocation
Used by fish in murky water (e.g. Amazon river species). The fish generate oscillating electric fields that propagate into the environment; distortions in these fields reveal nearby animals and obstacles. Analogous in principle to echolocation.
Ampullae of Lorenzini
Electroreceptive organs densely distributed over a shark's head, enabling detection of bioelectric fields produced by prey.
Magnetoreception
The ability to sense Earth's magnetic field. Used by migratory animals such as pigeons to navigate long distances with high accuracy. Field intensity is strongest near the poles and weakest near the equator.
Light polarisation
A property of electromagnetic radiation where the field vibrates along specific angles relative to the direction of travel. Sunlight scattering in Earth's atmosphere produces a polarisation pattern across the sky that some animals can read to determine the sun's position.
Sensory perception breaks into two stages: (1) the collection of environmental information through sensory organs and receptors, and (2) the nervous system's interpretation of that information into conscious experience
This distinction matters because raw sensory data and the subjective experience of it are not the same thing
Amino acids such as aspartic acid and serine bind to receptor proteins on the bacterial outer membrane
Nutrient binding reduces the probability of tumbling and biases the bacterium's movement toward higher nutrient concentrations
E. coli alternates between runs (straight-line swimming, flagella bundled as a propeller) and tumbles (random reorientation, flagella flying apart when rotation reverses)
The net effect: a biased random walk toward food
Phototaxis (whole-organism movement toward light) and phototropism (growth/bending toward light) both increase exposure to light
Useful both for organisms that photosynthesise and for those that disperse spores or seeds toward open, well-lit areas
The common-sense view that perception mirrors reality is called naive realism
Optical illusions demonstrate that what we perceive can diverge substantially from physical stimuli
Humans see visible light (~400–700 nm), a tiny fraction of the full electromagnetic spectrum
Karl von Frisch demonstrated honeybee colour vision through training experiments with coloured papers and sugar syrup. Bee vision extends into UV, allowing them to see flower patterns invisible to humans.
Pit vipers (including rattlesnakes) have pit organs positioned below the eyes that detect infrared radiation, enabling prey detection in total darkness
Many animals perceive polarised light; even a small patch of blue sky can reveal the sun's position through polarisation patterns
Human hearing: approximately 20–20,000 Hz
Elephants generate and detect infrasound (below 20 Hz)
Dogs and cats hear up to roughly 40,000 Hz and above
Bats use frequencies exceeding 100,000 Hz for echolocation; very short wavelengths give extremely fine spatial acuity
Dolphins and whales also use echolocation
Sharks detect bioelectric fields from prey using ampullae of Lorenzini
Every living creature produces electric fields from the movement of charged ions
Platypus and echidna are the only mammals known to use electroreception (via bill-mounted electroreceptors)
Some Amazon river fish generate their own oscillating electric fields for communication and electrolocation
Migratory animals (e.g. pigeons) sense Earth's magnetic field to return to exact locations year after year
Magnetic field intensity varies with latitude: strongest near the poles, weakest at the equator
⚠️ Be clear on the distinction between phototaxis (whole-organism movement) and phototropism (bending/growth toward light).
⚠️ Know the difference between echolocation (sound-based) and electrolocation (electric-field-based).
⚠️ Naive realism is a philosophical starting point that the course challenges. Expect questions on why it fails (illusions, species-specific sensory windows).
⚠️ Karl von Frisch's honeybee experiments are a classic example of rigorous experimental design disproving a widely held assumption (that insects are colour-blind).
⚠️ Runs and tumbles in E. coli chemotaxis: know what each looks like mechanistically (flagella bundling vs. reversal).
Q: What are the two basic components of sensory perception?
A: (1) Collection of information from the environment via sensory organs and receptors, and (2) analysis and interpretation of that information by the nervous system to produce perceptual awareness.
Q: How does E. coli use runs and tumbles to navigate toward nutrients?
A: During runs, flagella rotate in one direction and bundle together, producing straight-line swimming. During tumbles, flagellar rotation reverses, flagella fly apart, and the cell reorients randomly. Nutrient binding to membrane receptors reduces tumbling frequency, biasing overall movement toward the nutrient source.
Q: What is the difference between phototaxis and phototropism?
A: Phototaxis is the movement of an entire organism toward light (e.g. a swimming bacterium). Phototropism is the bending or growth of an organism toward light (e.g. a plant leaning toward a window).
Q: How do pit vipers detect prey in complete darkness?
A: Using pit organs located below their eyes and near their nostrils, which detect infrared radiation emitted by warm-blooded prey.
Q: Name two mammals that use electroreception and describe how they use it.
A: The platypus and the echidna. The platypus has electroreceptors in its bill that detect bioelectric fields generated by small crustaceans and molluscs, allowing it to locate prey.
Q: What is naive realism, and why does the course treat it as insufficient?
A: Naive realism holds that perception is identical to external reality. It is insufficient because illusions and the existence of sensory modalities beyond human perception (UV, infrared, electric fields) demonstrate that our experience is a constructed representation, not a direct copy of the world.
sensation, perception, sensory modality, chemotaxis, E. coli, runs and tumbles, flagella, phototaxis, phototropism, naive realism, electromagnetic spectrum, visible light, ultraviolet, infrared, Karl von Frisch, honeybee vision, pit vipers, pit organs, light polarisation, infrasound, ultrasound, echolocation, electroreception, electrolocation, ampullae of Lorenzini, magnetoreception, magnetic navigation, pigeons, platypus, auditory range, sensory ecology