Chapter 4: Sensory Processes — Quick Reference

• Psychophysics explores the relationship between physical stimuli and psychological experiences, focusing on thresholds.

Absolute Threshold

• The minimum stimulus energy that can be reliably detected, typically 50% of the time, measured using psychophysical methods.

• Example: Hecht et al.'s research showed that human vision can detect light flashes with as few as 100 photons, with a remarkable 7 photons actually reaching the photoreceptors, demonstrating single-photon sensitivity.

Just-Noticeable Difference (JND)

• The minimum difference in stimulus intensity that can be reliably detected.

• It is measured using a standard stimulus and comparing it to other stimuli, often calculated as $JND \approx (I<em>{75\%} - I</em>{25\%})/2$. A smaller JND indicates higher sensitivity.

Weber-Fechner Law (Weber’s law)

• Proposed by Ernst Heinrich Weber, this law states that the size of the detectable increment ($\Delta I$) is proportional to the baseline intensity (I), expressed as $\frac{\Delta I}{I} = k$. The constant $k$ (Weber fraction) varies by sensory modality.

• Example: If a person's Weber fraction for light is 0.08 (8%), detecting a change in a room lit by 'I' candles would require an intensity change of $\Delta I = 0.08 \times I$.

• Weber fractions vary significantly across modalities: e.g., light 8%, sound 5%, odor 15%, salt 20%, lifting weight 2%, electric shock 1%.

Stevens’ Power Law

• Developed by S.S. Stevens, this law describes suprathreshold sensation, where perceived magnitude (P) is related to physical magnitude (D) by the formula $P = a \;D^{r}$. Here, 'a' is a constant and 'r' is a modality-specific exponent.

• Example: The exponent 'r' can vary, with curves showing $r \approx 0.5$ for certain modalities (e.g., brightness) and r > 1 for others (e.g., pain or electric shock).

Signal Detection Theory (SDT)

• This theory acknowledges that sensation involves both the presence of a signal and background noise. Detection is characterized by:

  • Hits: Correctly detecting a signal when it is present.

  • False Alarms: Incorrectly reporting a signal when none was present.

• SDT separates perceptual sensitivity ($d'$) from the decision criterion (bias, $\beta$), clarifying whether detection differences are due to actual sensory ability or decision-making thresholds.

• Example: A radiologist interpreting an X-ray for a tumor; SDT helps determine if a higher rate of tumor detection is due to better visual sensitivity or a more cautious decision bias.

Sensory Coding

• Addresses two main problems for each sense:

  1. How physical energy is transduced into neural signals by receptors.

  2. How intensity and quality of stimuli are encoded into distinct neural patterns.

Sensation vs. Perception

• Sensation: Refers to the raw, immediate experiences from stimuli, primarily a biological process.

• Perception: Involves the meaningful interpretation and organization of sensory information within the cortex, a psychological process.