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6A: Sensing the Environment

Every sense from photon to percept: thresholds you can slide, cones you can silence, and a scene that flattens as each depth cue is removed.

25 topics · 100 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

6A. Absolute and Difference Thresholds

  1. How it worksHow does an absolute threshold end up defined as the level you detect exactly half the time?
  2. How it worksWalk me through finding a difference threshold by nudging one tone louder until the change is noticed.
  3. Worked exampleFind the difference threshold when a 100 gram weight must reach 102 grams before it feels heavier.
  4. Common mistakeWhy isn't an absolute threshold a hard wall where detection switches from never to always?
  5. Going furtherWhat happens to a threshold when a subject is warned a faint tone is coming versus told nothing?
Your turneasy1 of 3
In a method-of-constant-stimuli run, a tone at 11 dB is detected on 20 percent of trials and the same tone at 30 dB is detected on 80 percent of trials. Detection rises linearly between those two intensities. At what intensity, in dB, does this listener's absolute threshold sit?

6A. Weber's Law

  1. How it worksHow does Weber's law keep the noticeable difference a fixed fraction of whatever you started from?
  2. Worked exampleFind the just noticeable difference for a 500 gram package when the Weber fraction for weight is 2 percent.
  3. Worked exampleWalk me through why 10 dollars off a 20 dollar shirt feels huge and 10 dollars off a 2000 dollar laptop feels like nothing.
  4. Common mistakeWhy isn't the just noticeable difference a fixed number of grams or decibels you can memorize per sense?
  5. Going furtherWhat happens to Weber's law at the very faintest and the most intense ends of a stimulus range?
Your turneasy1 of 3
The Weber fraction for lifted weight is 4 percent. A parcel weighs 687 grams. What is the smallest increase, in grams, that would be noticed on half the trials?

6A. Signal Detection Theory

  1. How it worksHow do a noise curve and a signal curve overlapping under one criterion line produce hits and false alarms?
  2. How it worksWalk me through hits, misses, false alarms and correct rejections all changing as the criterion slides left.
  3. Worked exampleFind the hit rate and false alarm rate for a screener who catches 18 of 20 weapons and stops 5 of 80 harmless bags.
  4. Going furtherWhat changes about where you set your criterion when a miss costs a life and a false alarm costs a minute?
  5. Common mistakeWhy isn't a high hit rate on its own proof that somebody is good at detecting a signal?
Your turneasy1 of 3
A screener inspects 34 bags that contain a weapon and 99 bags that are harmless. She flags 19 of the weapon bags and 6 of the harmless bags. What is her false alarm rate, as a percentage?

6A. Sensory Adaptation

  1. How it worksHow does a receptor's firing rate fall away while the constant stimulus is still physically there?
  2. Common mistakeWhy isn't sensory adaptation the same thing as deciding to ignore something you can still feel?
  3. Going furtherWhat happens to your vision when an image is held perfectly still on the retina and cannot drift?
Your turnmedium1 of 3
A pressure receptor fires at 78 spikes per second the instant a constant weight is set on the skin. Under that unchanging weight its rate halves every 5 seconds. What is its firing rate, in spikes per second, after 17 seconds?

6A. Psychophysics and the Sensation-Intensity Curve

  1. How it worksHow does a psychophysical function map physical stimulus intensity onto felt magnitude?
  2. Worked exampleWalk me through why a 70 decibel sound carries ten times the power of a 60 decibel sound.
  3. Common mistakeWhy isn't perceived brightness proportional to the number of photons landing on your eye?
Your turnmedium1 of 3
A sound measured at 87 dB is compared with a reference sound at 43 dB. How many times greater is the physical intensity of the louder sound?

6A. Sensory Receptors and Transduction

  1. How it worksHow does a receptor cell turn a physical stimulus into a change in its membrane potential?
  2. How it worksWalk me through what a photoreceptor, a mechanoreceptor, a chemoreceptor and a nociceptor each respond to.
  3. Common mistakeWhy isn't a stronger stimulus coded by a bigger action potential travelling up the nerve?
  4. Going furtherWhat happens to perception when one receptor type is missing but the pathway behind it is intact?
Your turneasy1 of 3
A receptor membrane rests at -75 mV, and the first action potential in its afferent fiber is triggered when the membrane reaches -55 mV. Each additional unit of stimulus energy depolarises the receptor by 5 mV. How many units of stimulus are needed to trigger the first action potential?

6A. Sensory Pathways and the Thalamus

  1. How it worksHow does a sensory signal travel from receptor to thalamus to its own primary cortical area?
  2. Worked exampleTrace the route a pinprick on the left index finger takes from skin receptor to cortex.
  3. Common mistakeWhy doesn't smell take the thalamic detour that sight, sound and touch are all forced through?
  4. Going furtherWhat happens to perception when a primary sensory cortex is destroyed but the sense organ works fine?
Your turneasy1 of 3
A pinprick on the left index finger is carried 87 cm to the spinal cord along an A-delta fiber conducting at 18 metres per second. How many milliseconds does that first leg of the route take?

6A. Structure and Function of the Eye

  1. How it worksHow do the cornea, pupil, lens and retina each do a different job on the light coming in?
  2. How it worksWalk me through the lens changing shape to focus on something near and then something far away.
  3. Worked exampleShow me why a too-long eyeball lands the image in front of the retina and blurs distant objects.
  4. Common mistakeWhy isn't the pupil a muscle you control the way you control a camera aperture?
Your turnhard1 of 3
An eye's retina sits 2.4 cm behind its lens system. To bring an object 46 cm in front of the eye to a sharp focus on the retina, what focal length, in cm, must the lens system take? Use 1/f = 1/(object distance) + 1/(image distance).

6A. Rods, Cones and the Retina

  1. How it worksHow do rods and cones split the work between dim light and fine color detail?
  2. How it worksWalk me through the retina from photoreceptor to bipolar cell to ganglion cell to optic nerve.
  3. Going furtherWhat happens to your sensitivity over twenty minutes in a dark room as rhodopsin regenerates?
  4. Common mistakeWhy isn't the fovea the best part of the retina for spotting a faint star at night?
Your turneasy1 of 3
On walking into a dark room a subject's absolute threshold for light falls by a factor of 12 over the first 10 minutes as the cones adapt, and then by a further factor of 29 by minute thirty as rhodopsin regenerates in the rods. By what overall factor has the threshold fallen?

6A. Visual Pathways in the Brain

  1. How it worksHow do the nasal halves of each retina cross at the optic chiasm while the temporal halves stay put?
  2. How it worksWalk me through retina to optic chiasm to lateral geniculate nucleus to primary visual cortex.
  3. Worked exampleTrace exactly what a person can still see after the optic chiasm is cut straight down the middle.
  4. Common mistakeWhy isn't the left eye's entire output simply shipped to the right hemisphere?
Your turneasy1 of 3
One optic nerve carries 1079 thousand axons. At the chiasm the 55 percent of them that arise from the nasal half of that retina cross to the other side. How many thousand axons from that eye continue to the lateral geniculate nucleus on the same side?

6A. Parallel Processing

  1. How it worksHow does the visual system split one scene into separate streams for color, form, motion and depth?
  2. How it worksWalk me through the dorsal where stream and the ventral what stream doing different jobs on one image.
  3. Common mistakeWhy isn't vision built as one assembly line that finishes each feature before starting the next?
  4. Going furtherWhat happens to perception when the motion stream is destroyed and every other stream survives?
Your turneasy1 of 3
A strictly serial model of vision computes colour in 41 ms, then form in 56 ms, then motion in 117 ms, one stage after another. A parallel model runs all three at once and is finished when the slowest of them, motion, is done. How many milliseconds does the parallel model save?

6A. Feature Detection

  1. How it worksHow do detectors for edges, angles and movement stack up into recognizing a whole face?
  2. Worked exampleShow me which cortical cell fires as a vertical bar sweeps a receptive field and then tilts 45 degrees.
  3. Common mistakeWhy isn't there one grandmother cell that fires for your grandmother and for nothing else?
Your turnmedium1 of 3
A simple cell in primary visual cortex fires at 64 spikes per second for a bar at its preferred orientation of 30 degrees, and its rate drops by 0.512 spikes per second for each degree the bar is rotated away from that. What is its firing rate, in spikes per second, for a bar at 64 degrees?

6A. Trichromatic and Opponent-Process Color Vision

  1. How it worksHow do three cone types with overlapping sensitivity curves code every color you can see?
  2. How it worksWalk me through red-green, blue-yellow and black-white channels firing in opposite directions.
  3. Worked exampleShow me why red-green colorblindness confuses a ripe and an unripe tomato but leaves blue untouched.
  4. Common mistakeWhy isn't trichromatic theory simply wrong given that afterimages show up in the opposite color?
  5. Going furtherWhat happens inside the opponent channel the instant a green stimulus is taken away?
Your turneasy1 of 3
A monochromatic light drives the L cones to a relative response of 84 and the M cones to 34 on the same scale. A red-green opponent ganglion cell computes L minus M. What is its output on that scale?

6A. Structure and Function of the Ear

  1. How it worksHow do the eardrum, the ossicles and the oval window amplify a pressure wave on its way in?
  2. How it worksWalk me through a sound wave going from pinna to cochlea and becoming a travelling wave.
  3. Common mistakeWhy isn't conductive hearing loss the same underlying problem as sensorineural hearing loss?
  4. Going furtherWhat happens to hearing when the stapes fuses to the oval window and stops moving?
Your turneasy1 of 3
The tympanic membrane has an effective vibrating area of 64 mm2 and the oval window an area of 4 mm2. The ossicles pass essentially the same force from one to the other. By what factor is the pressure at the oval window greater than the pressure at the eardrum?

6A. Hair Cells and Auditory Transduction

  1. How it worksHow does the basilar membrane bending stereocilia open ion channels and fire an auditory neuron?
  2. Common mistakeWhy doesn't a dead hair cell simply grow back the way a skin cell or a liver cell does?
  3. Going furtherWhat changes for a person whose hair cells are gone but whose auditory nerve is intact?
Your turneasy1 of 3
A region of one cochlea starts with 3393 hair cells. Years of noise exposure kill 29 percent of them, and mammalian hair cells are not replaced. How many hair cells remain in that region?

6A. Place Theory and Frequency Theory of Pitch

  1. How it worksHow does place theory put high pitches at the stiff base of the cochlea and low pitches at the floppy tip?
  2. How it worksWalk me through frequency theory coding pitch by how fast the whole auditory nerve fires.
  3. Common mistakeWhy isn't frequency theory able to account for a 5000 hertz tone on its own?
  4. Going furtherWhat does the volley principle add once single neurons hit their maximum firing rate?
Your turnmedium1 of 3
A single auditory neuron can fire at most 539 times per second. Firing in a volley, a group of 6 such neurons takes turns so that the group as a whole produces one spike per cycle of the tone. What is the highest tone frequency, in Hz, that this group can follow cycle for cycle?

6A. Sound Localization

  1. How it worksHow do tiny differences in arrival time and loudness between two ears place a sound in space?
  2. Worked exampleFind which ear hears a bark first and louder when the dog sits 40 degrees to your right.
  3. Common mistakeWhy isn't a sound directly in front of you easy to tell apart from one directly behind you?
Your turnhard1 of 3
A listener's ears are 19 cm apart. A dog barks from 52 degrees to the right of straight ahead. Taking the interaural time difference as (d/343)·sin(a) with d in metres and sound travelling at 343 metres per second, how many microseconds earlier does the bark reach the right ear?

6A. Somatosensation and the Gate Theory of Pain

  1. How it worksHow do touch, pressure, temperature and pain travel the skin on four separate receptor systems?
  2. How it worksWalk me through the spinal gate opening for thin pain fibers and slamming shut when thick touch fibers fire.
  3. Common mistakeWhy isn't pain intensity simply proportional to how much tissue damage there actually is?
  4. Going furtherWhat happens to the gate when signals come down from the brain already expecting relief?
  5. Ties togetherHow does the pain gate connect to top-down processing changing what you end up perceiving?
Your turneasy1 of 3
A participant's two-point threshold is 4 mm on the fingertip and 48 mm on the upper back. How many times finer is the fingertip's spatial resolution than the back's?

6A. Taste

  1. How it worksHow do taste buds with chemoreceptors for five basic tastes signal the gustatory cortex?
  2. Common mistakeWhy isn't there a tongue map with sweet at the tip and bitter across the back?
  3. Going furtherWhat changes about taste for a supertaster carrying far more fungiform papillae than average?
Your turneasy1 of 3
A supertaster carries 62 fungiform papillae per square centimetre at the tip of the tongue, against 14 for a nontaster. How many times as many papillae does the supertaster carry?

6A. Smell, Olfactory Pathways and Pheromones

  1. How it worksHow do olfactory receptor cells in the nasal epithelium send axons straight up into the olfactory bulb?
  2. How it worksWalk me through smell reaching the amygdala and hippocampus without a thalamic stop on the way.
  3. Going furtherWhat happens to a smell you have been sitting inside of for twenty straight minutes?
  4. Common mistakeWhy isn't the evidence for human pheromones as solid as the evidence in moths and mice?
Your turnmedium1 of 3
A panellist rates the intensity of a steady odour 84 on a 100 point scale the moment it is switched on. Under continuous exposure the rating falls by 33 percent of its current value each minute. What rating does the panellist give after 6 minutes?

6A. Kinesthetic and Vestibular Senses

  1. How it worksHow do proprioceptors buried in muscles and joints report limb position with no visual check?
  2. How it worksWalk me through semicircular canals catching rotation while otolith organs catch tilt and acceleration.
  3. Common mistakeWhy isn't the dizziness after spinning caused by your head still physically moving?
  4. Going furtherWhat happens when the vestibular system reports motion and the eyes report stillness, as on a boat?
Your turneasy1 of 3
A rotating chair speeds up smoothly from rest to 305 degrees per second over 7 seconds. What angular acceleration, in degrees per second squared, does the horizontal semicircular canal register during the spin-up?

6A. Bottom-Up and Top-Down Processing

  1. How it worksHow does bottom-up processing assemble a percept out of raw features with no expectations involved?
  2. How it worksWalk me through top-down expectation filling in a smudged letter from the word around it.
  3. Going furtherWhat happens to an ambiguous figure the moment you are told in advance what to look for?
  4. Common mistakeWhy isn't perception purely bottom-up when all the information does arrive at the senses?
Your turnmedium1 of 3
A degraded letter is identified in 645 ms when it is presented on its own and in 391 ms when the same degraded letter appears inside a familiar word. By what percentage does the word context speed identification?

6A. Gestalt Principles of Perceptual Organization

  1. How it worksHow do proximity, similarity, continuity, closure and common fate each regroup the same scattered elements?
  2. How it worksWalk me through figure-ground reversal turning one edge into a vase and then into two faces.
  3. Worked exampleShow me a grid of dots snapping from rows into columns the moment the spacing changes.
  4. Common mistakeWhy isn't the whole just the sum of the parts when none of the parts have changed?
Your turneasy1 of 3
A display holds 6 rows of 7 identical dots. Dots within a row sit 6 mm apart, while successive rows sit 17 mm apart. Grouping the display by proximity alone, how many groups does a viewer see?

6A. Depth Perception: Monocular and Binocular Cues

  1. How it worksHow do retinal disparity and convergence supply depth that only two eyes working together can give?
  2. How it worksWalk me through interposition, relative size, linear perspective, texture gradient and motion parallax.
  3. Worked exampleShow me a street scene flattening as each depth cue is stripped out of it one at a time.
  4. Common mistakeWhy isn't depth information actually present anywhere on the flat two-dimensional retina?
  5. Going furtherWhat happens to depth perception in someone who permanently loses the use of one eye?
Your turnhard1 of 3
A viewer's eyes are 6.54 cm apart and both fixate a point 74 cm straight ahead. What is the total convergence angle between the two lines of sight, in degrees? Use 2·atan((i/2)/d) and convert to degrees.

6A. Form, Motion and Perceptual Constancy

  1. How it worksHow do size, shape, color and brightness constancy hold an object steady while its image changes?
  2. How it worksWalk me through the phi phenomenon turning two blinking lights into one light that moves.
  3. Common mistakeWhy isn't the moon genuinely larger sitting on the horizon than it is overhead?
  4. Going furtherWhat happens to size constancy once the depth cues telling you distance are taken away?
Your turneasy1 of 3
A sign is carried from 4 metres away to 20 metres away while an observer reports its size as unchanged. By what factor does the height of its retinal image shrink?

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