Question

What sets the limits on the stimuli that an animal can sense? What limits the movements an organism makes? How do sensory and motor systems compare across different types of animals?

Answer 1

Every human has limits. You can only run so fast, jump so high, and go for so long without water. But what about restrictions upon our five senses, those tools that we use to perceive and understand our surroundings? Here are ten limitations on human perception that have a direct impact on how we understand the world.

Vision

About one quarter of the human brain is involved in visual processing - more than any other sense. Arguably the most closely studied of the five main senses, the Society for Neuroscience claims that more is known about vision than any other vertebrate sensory system.

10. Field of View
A pair of healthy human eyes has a total field of view of approximately 200 degrees horizontally — about 120 degrees of which are shared by both eyes, giving rise to what's known as binocular vision — and 135 degrees vertically, (though these values tend to decrease with age). This is due to the fact that both of our eyes are positioned more or less on the front of our heads, as opposed to the sides.

9. Angular Resolution
Angular resolution is one of the terms used to describe an optical device's ability to distinguish very small details. If you want to talk about the smallest thing perceivable by the human eye, it makes sense to do so in terms of angular resolution.

8. The Blind Spot
The human eye is lined with photoreceptor cells that it uses to perceive light. Visual information received by these photoreceptor cells is relayed to the brain via the optic nerve. The only problem is that the optic nerve actually passes through part of photoreceptors lining the inside of the eye, creating a small, receptor-less patch where it's impossible to detect light.

7. The "Visible" Spectrum
Probably the most well-known of human sensory limitations, the typical human eye is only capable of perceiving light at wavelengths between 390 and 750 nanometers. Of course, calling it the "visible" spectrum is a bit of a misnomer, as plenty of animals are capable of perceiving light with frequencies outside this relatively narrow band of electromagnetic radiation.

Hearing

Commonly listed alongside vision as one of the most important of the human senses, hearing is is a vital part of everything from communication to risk-avoidance.

6. Hearing Range
Among young, healthy humans, the range of frequencies that can be picked up by the human ear is usually cited as 20 — 20,000 Hz; however, the upper limit on that range tends to decrease pretty steadily with age.

5. Absolute Threshold of Hearing
Your absolute threshold of hearing is the quietest sound your ears are capable of picking up when there are no other sounds around to mask its perception. This threshold varies from person to person, changes with age, and is largely dependent on the frequency of the noise being perceived. It's also quieter than you might think.

Taste & Smell

These two senses rely on different sensory organs, but are very closely related; when someone loses his or her sense of smell, for example, their sense of taste is dramatically diminished.

4. Limitations in Wine Tasting
The sense of taste is arguably the weakest of the human senses. This is something we've talked about before; your ability to "taste" wine, for example, is actually more dependent upon your sense of smell. Here's what we had to say about the limitations of taste back in March:

3. Supertasters
However, there is also evidence for the existence of so-called "supertasters" — i.e. people who are unusually sensitive to what are known as the "basic tastes," namely bitter, sweet, sour and salty.

2. Odor Detection Threshold
Like taste thresholds, the limits of odor detection have proven difficult to pin down.

1. Two-point Discrimination
Here's something we do know for sure about the sense of touch: it tends to be keenest in regions of the body that are densely populated with sensory neurons. One of the simplest ways of demonstrating this relationship is with something called the two point discrimination test.