Sensory Psychology and Psychophysics

Understanding Sensory Psychology and Psychophysics

Psychology’s sensory branch explores how physical stimuli are transformed into perceptual experiences. This course unpacks core concepts such as stimulus‑perception relationships, sensory thresholds, psychophysical laws, and color theory. By the end, you will be able to explain key principles, apply them to real‑world examples, and recognize common misconceptions.

1. The Mapping Between Physical Stimuli and Perception

One frequent error is assuming a one‑to‑one correspondence between a physical stimulus and a unique perceptual experience. This mistaken belief is known as the constant stimulus assumption. In reality, the brain interprets stimuli through complex, context‑dependent processes, meaning the same physical input can lead to different perceptions depending on factors like adaptation, attention, and prior experience.

• Example: Two people looking at the same shade of gray may describe it as “light gray” or “dark gray” based on surrounding colors.
• Key takeaway: Perception is not a simple mirror of the external world; it is constructed.

2. Visible Light Spectrum and Stimulus Classification

The human visual system detects wavelengths between approximately 380 nm and 780 nm. Light outside this range is invisible to the eye, though it can still affect the body (e.g., ultraviolet causing skin damage). Therefore, the correct statement is:

Wavelengths shorter than 380 nm are invisible ultraviolet rays.

Understanding these limits is essential for fields ranging from ergonomics to lighting design.

3. Auditory Thresholds and Subliminal Stimuli

Human hearing typically ranges from 20 Hz to 20 kHz. A sound presented at 15 Hz falls below the audible threshold, making it a subliminal auditory stimulus. Such low‑frequency vibrations may be felt rather than heard, influencing environments like industrial settings where low‑frequency noise can cause discomfort without being consciously perceived.

4. Psychophysical Laws: Weber’s Law vs. Stevens’ Power Law

Weber’s law states that the just noticeable difference (JND) is proportional to the baseline intensity of a stimulus. In formula form:

ΔI / I = k, where ΔI is the JND, I is the initial intensity, and k is a constant specific to the sensory modality.

This principle explains why it is easier to notice a change from 1 kg to 2 kg than from 100 kg to 101 kg.

In contrast, Stevens’ power law describes the relationship between perceived magnitude (P) and physical intensity (I) as:

P = k·Iⁿ, where n is an exponent that varies by stimulus type. When n > 1, perception grows faster than intensity; when n < 1, it grows more slowly.

5. Dark Adaptation and the Purkinje Phenomenon

During dark adaptation, the eye becomes most sensitive around ≈510 nm, a wavelength corresponding to green‑blue light. This shift illustrates the Purkinje phenomenon, where the peak sensitivity of the retina moves toward shorter wavelengths as illumination decreases. Recognizing this effect is crucial for designing low‑light displays and safety signage.

6. Stimulus Intensity Limits: Terminal Stimuli

When stimulus intensity is so high that the sensory response reaches a plateau, the stimulus is termed a terminal stimulus. At this point, increasing intensity no longer produces a stronger perceptual response, reflecting the saturation of sensory receptors. Examples include extremely bright lights that no longer appear brighter to the observer.

7. Applying Stevens’ Law to Squared Intensities

According to Stevens’ law, if the physical intensity (I) of a stimulus is squared, the perceived magnitude increases by a factor of k·(I)ⁿ where n > 1. This means perception grows more than proportionally when the exponent exceeds one, highlighting the non‑linear nature of many sensory experiences.

8. Color Mixing: Additive vs. Subtractive

Color mixing can be categorized into two distinct processes:

• Additive mixing combines light wavelengths. When red, green, and blue light are overlapped, they produce white light. This principle underlies digital screens and stage lighting.
• Subtractive mixing involves pigments or dyes that absorb (subtract) certain wavelengths. Mixing cyan, magenta, and yellow pigments removes more light, eventually yielding black.

The correct description is:

Additive mixing combines light wavelengths; subtractive mixing combines pigment absorptions.

9. Summary of Key Concepts

• The constant stimulus assumption is a common misconception about stimulus‑perception mapping.
• Visible light spans 380–780 nm; wavelengths outside this range are invisible.
• Sounds below ~20 Hz are subliminal; 15 Hz is an example of a subliminal auditory stimulus.
• Weber’s law explains JND proportionality; Stevens’ power law captures non‑linear perception.
• During dark adaptation, sensitivity peaks near 510 nm, illustrating the Purkinje phenomenon.
• Extremely high intensities produce a terminal stimulus, where perception plateaus.
• Stevens’ law predicts greater-than‑linear growth when n > 1, such as squaring intensity.
• Additive mixing uses light; subtractive mixing uses pigments.

10. Frequently Asked Questions (FAQ)

What is the difference between subliminal and supraliminal stimuli?

Subliminal stimuli fall below the sensory threshold and are not consciously perceived, whereas supraliminal stimuli exceed the threshold and are consciously detectable.

Why does the Purkinje shift occur?

It results from the differing sensitivities of rod and cone cells. In low light, rods (most sensitive around 510 nm) dominate, shifting peak sensitivity toward shorter wavelengths.

Can Stevens’ law be applied to all senses?

While Stevens’ law provides a flexible framework, the exponent n varies by modality (e.g., n ≈ 0.6 for brightness, n ≈ 2.0 for electric shock). Not every sensory system follows a perfect power function, but the law offers a useful approximation.