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End Notes: The Six Physical Human Senses
The Cheat Sheet provides the detailed required by the analysis conducted in Abstract Senses. This looks at how we form an image in our heads about what is going on in the outside world.
Observation: The Six Physical Human Senses
Observation: Human vs. State of the Art Senses
Sight (a)
Humans have good eyes: 20/20 visual acuity, with trichromatic colour vision via three cone types. Each retina contains ~6 million cones and ~120 million rods. Cones provide most spatial detail (like “pixels”), while rods extend sensitivity in dim light (like “ISO” in a camera). For a mammal this is impressive, but not the best in the animal kingdom. By the measure of distance vision, eagles are champions: with 20/5 acuity they see ~2–4× sharper than humans, depending on the method of measurement.
Both eagles and mantis shrimp have colour vision, but in very different ways. Eagles are tetrachromatic, with an extra cone type that allows them to see ultraviolet and finer colour contrasts. Mantis shrimp have 12–16 receptor types compared to our 3, and can detect polarised light. They don’t necessarily perceive “more colours” in the way humans do; instead, they process wavelengths differently, optimised for speed in complex reef environments.
Humans, with cones tuned to red, green, and blue, can differentiate up to ~10 million colours. About 12% of women carry a fourth cone variant, a form of tetrachromacy that may extend this range, though functional tetrachromacy is uncommon. In practice, human vision is good enough for survival, but not exceptional; eagles see sharper, and mantis shrimp view the world through an alien palette.
Hearing (b)
Young humans hear roughly 20 Hz–20 kHz, with peak sensitivity around 2–5 kHz (speech). By animal standards the range is modest, which is striking given how much we achieve with hearing:
- Communication
- Music and pleasure
- Sound localisation
The greater wax moth detects ultrasound up to ~300 kHz, exceeding the highest frequencies used by most bats.
Motion (c)
Flies and other haltere-equipped insects are the champions of motion sense, detecting body rotation at wingbeat frequencies (~200 Hz). Among vertebrates, birds—especially fast flyers—are highly sensitive, with fish close behind thanks to their lateral line system, which detects water displacements smaller than 1 μm. Humans detect angular acceleration of ~0.5°/s² and linear accelerations of ~0.007–0.01 g, modest compared to these specialists.
Smell (d)
Elephants lead in olfactory receptor gene count, while dogs and bears are elite trackers. Humans, with ~6–10 million olfactory neurons and ~400 receptor types, are far less sensitive.
Taste (e)
Taste is strongly related to smell, and humans do not have a strong sense of taste alone. Humans have ~5k–10k taste buds; cows, rabbits, and pigs have 15k–25k; catfish have >100k taste buds, distributed over their entire body surface.
Touch (f)
Among primates, humans have the most sensitive touch—especially in the fingers. Fingertips pack hundreds to thousands of mechanoreceptors per cm² (method-dependent; classic estimates ~241/cm²), yielding two-point discrimination thresholds of ~1–2 mm. The star-nosed mole has ~25,000 Eimer’s organs in its nasal star, allowing prey detection and consumption in ~120–230 ms. Octopus suckers contain chemotactile receptors, allowing them to “taste by touch.”
Observation: The Physiology of the Human Senses
Sight(g)
After the optic chiasm, most retinal output travels to the lateral geniculate nucleus (LGN) of the thalamus, and from there to the visual cortex. Collateral branches project to the superior colliculus and hypothalamus for reflexes and circadian timing. The retina itself performs substantial preprocessing (centre–surround contrast, colour-opponent channels, motion onset), so the signal is already structured when it reaches the cortex.
Hearing(h)
In the cochlea, vibrations become electrical signals that travel via the auditory nerve to the brainstem (cochlear nuclei → superior olive → inferior colliculus), then to the thalamus and cortex. Early reflexes—like startle—are triggered along the way.
Motion(i)
The vestibular system in the inner ear detects rotation and acceleration. Signals pass first through the brainstem, then to the thalamus and cortex, where they are integrated with visual and proprioceptive cues.
Smell(j)
Olfaction bypasses a first-order thalamic relay: olfactory bulb → piriform cortex, amygdala, hippocampus. Later projections reach the orbitofrontal cortex. This direct limbic access explains smell’s intense links to memory and emotion.
Taste(k)
Taste buds detect chemical stimuli via cranial nerves VII, IX, and X. Signals reach the brainstem (nucleus of the solitary tract), then the thalamus and limbic system, before reaching cortex. Reflexes (salivation, gagging) are triggered at lower levels.
Touch(l)
Mechanoreceptors in the skin, muscles, and joints send signals via the spinal cord and brainstem, through the thalamus, and on to the somatosensory cortex. Reflex arcs in the spinal cord and brainstem provide rapid protective responses.
Observation: Wiring of the Human Brain(m)
- Vision: After the optic chiasm, signals are relayed via the LGN to visual cortex, with branches to superior colliculus and hypothalamus. Heavy retinal preprocessing means the signal is structured, not raw.
- Smell: Olfactory bulb projects directly to limbic regions, bypassing thalamus at first.
- Other senses: Hearing, touch, motion, taste route through brainstem and thalamus before reaching cortex, enabling early reflexes and filtering.
Consequences Drawn: The Six Physical Human Senses
Consequences Drawn: Observations of Human vs. State of the Art Senses(n)
When Homo sapiens shopped for senses, he didn’t buy the luxury package. He chose good-enough sight, fit-for-purpose hearing, modest smell and taste, and high-fidelity touch in the fingers. Later he built telescopes, microphones, flavourings, and instruments to upgrade the set.
Compared to the champions of the animal kingdom, humans are mid-tier in every raw sense. What set us apart was not sensory supremacy, but how we used “good-enough” data.
Colour vision illustrates this: although less rich than eagles or mantis shrimp, our trichromatic retina assigns ~1 of 10 million colours to each “pixel” of the image before it even leaves the eye. This preprocessing provides context that helps separate objects, easing the load on cortex and saving energy. Evolution traded maximal detail for efficient, context-rich input.
Consequences Drawn: The Physiology of the Human Senses(o)
All the senses interface with the old brain, priming reflexes, emotions, and survival responses. If you hear a clap of thunder, you may hunch reflexively to protect vital organs before conscious thought. Reflexes are fast, but crude; the cortex adds refinement.
Consequences Drawn: Wiring of the Human Brain(p)
Because of both wiring and importance, ~30–50% of cortex is devoted to vision. Hearing takes ~3–4%, somatosensory ~8–10%, and smell <1% (all rough estimates). No wonder vision dominates our inner sense of reality. If I had to place my “self,” it feels as if I live behind my eyes—even though most visual processing happens at the back of my head.
Smell is the outlier: projecting straight to the limbic system, it bypasses thalamic filtering. This is why odours can hijack mood and memory instantly—pleasant or foul, they shift demeanour before thought catches up.
Action: Based on the Consequences of Earth History Data(q)
Observation → consequence → action is the through-line from evolutionary wiring to modern effectiveness:
- Observation: The old brain fires first, biasing reflexes
- Consequence: Helpful if aligned with reality; dangerous if not.
- Action: Cortex reconciles—success means adaptation, failure means miscommunication or poor decisions.
This bias kept our ancestors alive: jump at the rustle first, think later. Today the same reflexes can misfire, creating overreaction or misunderstanding. The cortex is our repair mechanism.
📖 Series Roadmap
- Forward: A Little Background
- Introduction: Action, Reaction, and the Human Paradox (16.09.2025)
- Looking Back in Time: The Development of the Human Brain (23.09.2025)
- Abstract Senses: Enhancing the way we see the world outside (30.09.2025)
- Bias as a Concept & Climbing the Stairs: Pattern Recognition & Everyday Tasks (07.10.2025)
- Abstract Feelings and Abstract Senses (14.10.2025)
- Motivation (04.11.2025)
- The Social Knowledge Base (11.11.2025)
- Potential (18.11.2025)
- The Subliminal Way We Go Through Life (26.11.2025)
- Taking Responsibility (02.12.2025)
- Fishing for Complements (22.12.2025)
- Peter and Fermi (22.12.2025)
🔗 R&R Navigation
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End Notes: The Six Physical Human Senses
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