PSYC10003 · Mind, Brain and Behaviour 1
Depth, Size and Auditory Perception
Week 11 completes Sensation & Perception with depth and size perception — monocular (pictorial) and binocular (disparity, convergence) depth cues and how they support size constancy and size illusions — and audition: how the ear transduces sound and how pitch, loudness and location are coded. The examinable skill, within the 25% Sensation & Perception block, is classifying depth cues, explaining size-constancy illusions through misjudged distance, and mapping the binaural cues (ITD and ILD) to the frequencies they serve.
What this chapter covers
- 01Oculomotor cues: convergence and accommodation; monocular vs binocular cues
- 02Monocular pictorial cues: occlusion, relative height/size, perspective, texture gradient, shadows
- 03Movement-based cues: motion parallax and deletion/accretion
- 04Binocular disparity and the horopter; relative vs absolute disparity; the correspondence problem
- 05Size perception: angular size and perceived distance; size constancy S = K × (R × D)
- 06Size illusions from misjudged distance (Ames room; Holway & Boring)
- 07Sound as pressure waves; frequency→pitch, amplitude→loudness; the missing fundamental and timbre
- 08Auditory localisation: interaural time difference (ITD, low frequencies) and level difference (ILD, high frequencies); the cone of confusion
Size constancy, depth cues and sound localisation
- +1(a) Apply the relation. Perceived size S depends on retinal-image size R and perceived distance D (with constant K). The room's construction makes the distant person be misperceived as near, so D is underestimated; with R fixed by the geometry, an underestimated D yields an underestimated S — they look small. Misjudging distance forces misjudging size.
- +1(b) Classify the first pair. Texture gradient is monocular (available to one eye, a pictorial cue); binocular disparity is binocular (requires the two eyes' slightly different images).
- +1Classify the second pair. Occlusion is monocular (one object covering another is a pictorial cue); convergence is binocular/oculomotor (the inward rotation of the two eyes for near objects).
- +1(c) Localise the low tone. A low-frequency tone is localised mainly by the interaural time difference (ITD) — the sound reaches the near ear slightly before the far ear. The interaural level difference (ILD) is poor for low frequencies because their long wavelengths bend around the head, so the head casts little sound shadow; ILD works best for high frequencies.
Key terms
- Monocular vs binocular depth cues
- Monocular cues need only one eye (pictorial cues like occlusion, relative size, texture gradient, plus motion parallax); binocular cues need both (binocular disparity and convergence).
- Binocular disparity
- The difference between the two eyes' retinal images because they view a scene from slightly offset positions; the visual system uses it to compute depth, with relative disparity giving depth order independent of fixation.
- Size constancy
- The tendency for an object's perceived size to stay stable despite changes in distance, achieved by combining retinal-image size with perceived distance: S = K × (R × D).
- Interaural time difference (ITD)
- A binaural localisation cue: a sound reaches the nearer ear slightly before the farther ear; most useful for low-frequency sounds.
- Interaural level difference (ILD)
- A binaural localisation cue: the head casts a sound shadow so the nearer ear receives a higher level; large and most useful for high-frequency sounds.
- Missing fundamental
- The phenomenon in which perceived pitch matches the fundamental frequency even when it is physically absent, because the remaining harmonics are its multiples and the waveform still repeats at the fundamental rate.
Depth, Size and Auditory Perception FAQ
How does misjudging distance cause a size illusion?
Because perceived size is computed from retinal-image size and perceived distance together (S = K × (R × D)). If a display makes an object look closer than it is, its retinal image implies a smaller object and it looks small; make it look farther and it looks large. The Ames room and Holway & Boring's corridor study both exploit this.
What is the difference between monocular and binocular depth cues?
Monocular cues are available to a single eye and include the pictorial cues (occlusion, relative size and height, linear perspective, texture gradient, shadows) and motion parallax. Binocular cues require both eyes — chiefly binocular disparity (the offset between the two retinal images) and convergence (how far the eyes rotate inward).
Why do ITD and ILD serve different frequencies?
The interaural time difference (which ear the sound reaches first) works best for low frequencies. The interaural level difference relies on the head casting a sound shadow, which only happens for high frequencies whose wavelengths are short relative to the head; low-frequency waves diffract around the head, so there is little level difference.
What is the missing fundamental, and why does it matter?
You still hear a complex tone's pitch at its fundamental frequency even when that frequency is physically removed, because the remaining harmonics are integer multiples and the whole waveform still repeats at the fundamental rate. It shows pitch is a perceptual construction, not a direct read-out of the lowest present frequency — for example hearing a low voice over a phone that cannot reproduce low frequencies.
Is any of this quantitative?
The size-constancy relation S = K × (R × D) is conceptual — you reason about how changing perceived distance changes perceived size, not compute a number. The rest is findings and classification (which cue, which frequency, which illusion). Expect conceptual MCQs, not calculations.
Exam move
Learn depth cues as a classified list (monocular pictorial, monocular movement-based, binocular) and be able to sort any named cue instantly. Use S = K × (R × D) as the lens for every size illusion: identify what the display does to perceived distance, and size follows. For audition, pair ITD with low frequencies and ILD with high frequencies and keep the pitch concepts (frequency vs pitch, missing fundamental, timbre) distinct. This block is conceptual, so drill classification and prediction rather than memorising prose, and rehearse a few signature illusions with their mechanisms. Confirm exam details on Canvas.
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