PSYC 212 – Perception
Chapter 1 :Principles of Perceptual Measurement – Notes
- In general, ALL visual, auditory, tactile, or chemosensory stimulation
that we are able to perceive can be measured.
- Perception is a very private experience and since it cannot be exposed
to anyone else, it cannot be measured by anyone else either. Most of
us are able to correctly identify the color of a stoplight – but are we all
experiencing an identical physiological event or could it be slightly
different for everyone? We may never know the answer to this.
- Many psychologists have asserted that even making the attempt at
measuring a perceptual event is fruitless because it is not a
measurable thing and therefore can never be verified.
- This has generated lots of debate – opposite views held by some
experimental psychologists that the perceived intensity of sensations
can be reliably estimated by the perceiver. Can also be scientifically
validated since information obtained is generally consistent across
A. Scientific Basis of Perceptual Measurement
- Advantage of knowing this scientific relationship: might give us an
insight about the nature of the brain, the way it processes info and how
the biological operations within it lead to sensation and perception
Quantitative relationships and their benefits
- Provide an estimate of the perceptual quality of a stimulus in numerical
terms and allow comparisons with other stimuli
- Allow comparison among individuals and species (comparison among
the different sensory modalities, aka cross-modal comparison)
Must answer this basic question first: Is there a general relationship between
physical stimulus and perception?
- Early experimentalists were looking for a formula/function that could
describe all sensory systems - Increasing function (as physical intensity of the stimuli increases so
does our perception of it)
- Could be a linear function. For any given increase in physical intensity,
there is a certain increment in the perceived intensity. The
proportion/slope is constant.
- Could be an exponential function. Perceived sensation intensity
changes very slowly at the low values of physical intensity, but after a
certain point small changes in stimulus intensity produce a dramatic
increase in perception. Slope progressively increases with physical
- Could be a logarithmic function. Very large slope at the beginning.
Perceived intensities change dramatically with small changes in
stimulus intensity. However at higher stimulus intensities, this effect
diminishes and the function trails off. Displays a decreasing slope over
its entire range.
2 approaches to get precise relationship b/w physical events and
1. Ask human subjects to rate perceived intensity of a certain stimulus
at various physical intensities (eg. Loudness of a sound)
- Plot the values and determine which of the 3 functions it best
2. Measure the smallest change in stimulus output that still causes a
change in sensation
To understand how this is important, must examine ideas of experimental
psychologists of the 19 century. They understood that a mathematical
relationship b/w physical and perceptual qualities needed 2 descriptions of
that function – the starting point and the slope.
B. Classical Psychophysics
- The 3 possible functions do not start at the origin because it is
impossible to detect very low levels of stimulus intensity
- Even if a stimulus is physically present, the biological elements
involved with capturing stimuli and transforming it into a sensory
experience cannot function well if the intensity is too low - Intensity must reach a certain minimum level (absolute threshold)
before it is registered in the brain as a sensory event.
- Stimulus intensities below this point will not produce detectable
- Stimulus intensities above this point will produce sensation
- The suprathreshold points are the ones forming the slope. One way to
obtain this info is by knowing just how small a change in stimulus
intensity is required to produce a discriminable change in sensation
- Once this is known, the best suitable function can be determined
Can be used to
- This approach was developed by Gustav Fechner in 1860, calledate how slope
psychophysics. Believed that there existed a general relationship
between physical and perceptual qualities and that it could behreshold levels
determined by knowing the stimulus energy at which the output can be
1. Psychophysical Methods
3 methods to obtain absolute and difference threshold by Fechner
1. Method of Adjusment = simplest method. Human is told to adjust
physical intensity of stimulus until it is barely detectable. Very fast and
actively engages subject.
2. Method of Limits = more reliable estimates than method of
adjustment. Subject is presented with chosen stimulus whose intensity
is ascending or descending. Intensity stimulus is initially set and
increases/decreases by a fixed amount until the subject reports that it
is perceived/disappeared. Provides a better estimation of threshold if
time is not an issue.
3. Method of Constant Stimuli = best method. In contrast to the other
2 methods, this method has randomly chosen intensity values. Neither
the experimenter nor the subject knows the intensity of the next
stimulus. Subject simply replies whether it was felt or not. Best method since subject has no idea what the next stimulus will be in comparison
to the last one (predictability is less accurate)
2. Absolute Threshold
- Results should look like a step function – pg. 7 figure 1.2
- HOWEVER, humans are not ideal detectors. Our sensory systems deal
with several factors of uncertainty; Stimuli aren’t always perfect every
time, our nervous system is inherently noisy, noise interferes with
signal detection, and after all that we still have to judge whether it was
- Pg. 8 figure 1.3 shows actual results, which looks like an S-shape
Conventional approaches to threshold estimation
- Which value do we use to represent absolute threshold? Actual results
have no well-defined point that can serve as threshold
- Must adopt arbitrary response level (most psychophysicists use 50%
YES value). The physical intensity at 50% is taken to be the absolute
- In reality, there are no all-or-none stimulus detection; our thresholds
can fluctuate a little
- Over time, some values have been obtained for different sensory
Touch – skin dimpling of 10 is sufficient to detected
Smell – absorption of 40 molecules by nose detectors produces a smell
Hearing – tiny threshold; eardrum movement of 10 -10cm produces a
Vision – 54-148 photons are necessary to produce a sensation of light
3. Difference Threshold
- For the whole function to be determined, Fechner needed to know what
the slope of the function was at suprathreshold levels and how that
slope changed with increasing intensity - Ernst Weber How much does a stimulus need to change in order to
Constant slope = linear function
produce a detectable change in sensatioIncreasing slope = exponential
intensity needs to be added or subtractfunctiona target light in order
Decreasing slope = logarithm
for there to be a just noticeable difference (JND) in sensation?
A difference threshold experiment on the visual system
- Experiment: 2 lights, one target light (intensity increases or
decreases), one reference light (intensity remains the same). Subject
says which one is dimmer or brighter.
- Experiment performed using Fechner’s Method of Constant Stimuli.
- Just like the other experiments, humans do not behave as ideal
detectors when it comes to difference judgments either (gives an s-
One of the shape function/ogive)
equations - (increment threshold) or (decrement threshold)where b is
in the 75% brightness intensity, a is the 50% brightness intensity and
perceptual c is the 25% brightness intensity. ∆I is the difference threshold.
4. Weber’s Law
Weber what happens if we change the reference light in the previous
experiment to a higher level, will the old difference threshold (∆I) stay the
Multiple discrimination threshold experiments
pg. 11 figure 1.5 the greater the intensity level at which we have to
make a JND judgment, the greater the difference threshold (∆I) needed to
attain that JND.
Weber’s Law: The difference threshold is not constant but increases
in a linear fashion with stimulus intensity.
where k = proportion (Weber’s fraction) and I =
JND requirement: The incremental amount must be in proportion to the
stimulus intensity. How do we find k? Experimentally determined (refer to Table 1.1 pg. 11
for Weber fractions for different sensory systems)
Weber fractions are accurate for a broad range of stimulus intensities
except for the extremities (eg. very high and very low intensities). At
these extremes, Weber’s law no longer applies.
5. Fechner’ Law
Fechner’s central goal was to find out the relationship between sensory
magnitude and stimulus intensity. As we know, Weber’s law asserts
that higher levels of suprathreshold intensity require a correspondingly
greater change in intensity (∆I) to produce a change in sensation (∆S) that
is just distinguishable (JND). Fechner wanted to know more about ∆S.
Fechner’s assumption: All JNDs are produced by equal increments in
sensation regardless of the operating level.
In other words, exactly the same ∆S value was needed at all sensory
magnitudes because JND is standard unit of change (psychological