Knowledge clips
Sensory thresholds
Clip 1. Law of Weber, Fechner and Stevens
→ This clip is an introduction to psychophysics: the relation between concentration and perceived
intensity (Ch 2.2).
3 general laws/rules:
1. Weber’s law: the amount of a physical stimulus which in needed to increase to be just
perceivably different in a constant linear ratio. Delta intensity or concentration / intensity =
constant over the range of possible concentrations.
Just Noticeable Difference (JND) = Difference detected in 50% of the cases.
2. Fechner’s law: by adding up JND’s, one can construct a psychophysical relationship between
physical stimulus intensity and sensory intensity. Perceived intensity is a linear function of
logarithm of the intensity (S = k log I). Fechnerian psychophysical function in linear in semi-
log plot (x-as).
3. Stevens’ Law: magnitude estimation: assign the intensity a number in proportion to the
intensity of the standard stimulus. E.g. standard is 10 then half as sweet is 5 and twice as
sweet is 20. Perceived intensity is a constant times the physical intensity to the power of n (S
= k In) Stevens power function is linear in log-log plot. n > 1 accelerating, n < 1 decelerating
psychophysical function.
Scaling and psychophysical functions:
Magnitude estimation methods Category, Visual
Analogue and JND scales
Shape of a power function Log-linear shape
Steven's law Fechner's law
Clip 2. Threshold measurements
→ This clip explains the principles, applications and different methods of threshold measurements
(Ch 6). You will use these methods during one of the practicals.
ASTM: chemical sense threshold → A concentration range exists below which the odour or taste of a
substance will not be detectable under any practical circumstances, and above which individuals with
a normal sense of smell or taste would readily detect the presence of the substance. 3 types:
1. Detection threshold → the minimum level that is different from water or air (e.g. sucrose
taste different then water but not clear that sucrose taste sweet).
2. Recognition threshold → the minimum levels that take on the characteristic taste or smell of
the stimulus (e.g. sucrose has a sweet taste).
3. Difference threshold → the minimum physical change necessary in order for a person to
sense the change 50% of the time (equivalent to JND).
Conceptual notion of threshold → an energy level below which detection never occurs and above
which detection always occurs (so yes/no, does not happen in practice).
Empirical threshold → the level at which detection occurs 50% of the time (response bias → 50% of
the time you are right) or 50% above chance level (75% correct). So when concentration increases
the chance of detection also increases.
Sensory thresholds
Clip 1. Law of Weber, Fechner and Stevens
→ This clip is an introduction to psychophysics: the relation between concentration and perceived
intensity (Ch 2.2).
3 general laws/rules:
1. Weber’s law: the amount of a physical stimulus which in needed to increase to be just
perceivably different in a constant linear ratio. Delta intensity or concentration / intensity =
constant over the range of possible concentrations.
Just Noticeable Difference (JND) = Difference detected in 50% of the cases.
2. Fechner’s law: by adding up JND’s, one can construct a psychophysical relationship between
physical stimulus intensity and sensory intensity. Perceived intensity is a linear function of
logarithm of the intensity (S = k log I). Fechnerian psychophysical function in linear in semi-
log plot (x-as).
3. Stevens’ Law: magnitude estimation: assign the intensity a number in proportion to the
intensity of the standard stimulus. E.g. standard is 10 then half as sweet is 5 and twice as
sweet is 20. Perceived intensity is a constant times the physical intensity to the power of n (S
= k In) Stevens power function is linear in log-log plot. n > 1 accelerating, n < 1 decelerating
psychophysical function.
Scaling and psychophysical functions:
Magnitude estimation methods Category, Visual
Analogue and JND scales
Shape of a power function Log-linear shape
Steven's law Fechner's law
Clip 2. Threshold measurements
→ This clip explains the principles, applications and different methods of threshold measurements
(Ch 6). You will use these methods during one of the practicals.
ASTM: chemical sense threshold → A concentration range exists below which the odour or taste of a
substance will not be detectable under any practical circumstances, and above which individuals with
a normal sense of smell or taste would readily detect the presence of the substance. 3 types:
1. Detection threshold → the minimum level that is different from water or air (e.g. sucrose
taste different then water but not clear that sucrose taste sweet).
2. Recognition threshold → the minimum levels that take on the characteristic taste or smell of
the stimulus (e.g. sucrose has a sweet taste).
3. Difference threshold → the minimum physical change necessary in order for a person to
sense the change 50% of the time (equivalent to JND).
Conceptual notion of threshold → an energy level below which detection never occurs and above
which detection always occurs (so yes/no, does not happen in practice).
Empirical threshold → the level at which detection occurs 50% of the time (response bias → 50% of
the time you are right) or 50% above chance level (75% correct). So when concentration increases
the chance of detection also increases.
