November 6, 12
PSYCH – Week 9 Online Readings
Week 9: Perception
Focus Question: How does our brain create meaning from what the body senses?
We are not conscious of the processes involved in creating our perceptions. Our
perception depends not just on sensation (sensory input) but also on our particular
knowledge and experience.
Scientific Study of Perception: How do we measure perception?
Psychophysics: The study of how sensation relates to perception.
Sensory thresholds: The point at which a stimulus triggers the start of an afferent nerve
Difference threshold: The just-noticeable difference between two stimuli.
Weber's Law: The size of the just-noticeable difference (JND) of a stimulus divided by
its initial intensity is a constant. In general, across all sensory domains (loudness, weight,
etc) the just-noticeable difference threshold for a stimulus goes up as its intensity goes up.
Fechner's Law: In every sensory domain, each just-noticeable difference represents an
equal step in the psychological magnitude of a sensation. This means that changes in
stimulus can be compared across sensory domains, for example, between vision and
touch, although each different domain could have a different JND.
Stevens' Power Law: Proposed relationship between the magnitude of a stimulus and its
perceived intensity or strength
Absolute Threshold: Minimum value of a stimulus that can be detected. Ie: faintest
sound, gentlest touch
How can we detect faint signals? It depends on:
Sensitivity:Ability to detect a given stimulus.
Cognitive factors: Factors that involve thought processes, including changes in
attention, expectation, and alertness.
Influencing perceptual decisions: noise and bias November 6, 12
External noise (which researchers try to control for) and a participant’s response bias
influence perception. Neurons, of course, have a minimum rate of firing called
spontaneous firing, or internal/neural noise.
Response bias:Aperson's tendency to say "yes" or "no" when he is not sure whether he
detected the stimulus.
Spontaneous rate: Neuron minimum rate of firing
Signal Detection Theory (SDT): Mathematical theory of the detection of stimulation in
which every stimulus event requires discrimination between signal (stimulus) and noise
(consisting of both background stimuli and random activity of the nervous system). It
allows independent assessment of sensitivity and bias, shown in receiver-operating
characteristic (ROS) plots (Graph of hits and false alarms of participants under different
motivational conditions; indicates people's ability to detect a particular stimulus.)
Psychophysics of Sound
Pitch: how high or low a sound is, depends on frequency of sound waves, or how rapidly
they alternate between compression and rarefaction.
Fundamental frequency: The lowest frequency of a periodic waveform. Think moving
up and down the neck of a violin.
How does the brain get from a vibrating string to perceiving a high or low pitch?
The cochlea analyzes complex sounds, breaking them down into their component
frequencies. The presence of each component and its relative intensity are relayed to the
brain through the individual fibres of the auditory nerve. Each auditory nerve fibre is
most sensitive to a particular frequency.
How does the brain interpret frequencies?
The fact that frequencies are anatomically separated and organized in the ear is called
tonotopic organization. This tonotopic organization is maintained all the way to the
primary auditory cortex. This can serve like a place code to signal which frequencies are
present in the sound.
Alternatively, a temporal code may be used, up to frequencies of 5000 Hz, at least. How
so? Well, it turns out that auditory nerve fibres (the afferent axons from auditory sensory November 6, 12
neurons in the cochlea) work pretty fast – they can fire at up to 1000 times a second.
Also, they tend to fire in synchrony with a stimulating waveform – every time there is a
compression in the sound wave, the hair cell depolarizes, and an action potential is
generated. Temporal codes are used for lower-frequency sounds.
Any individual nerve axon can’t fire more than 1000 times a second (1000 Hz). The
auditory nerve can comfortably follow – generate action potentials in synchrony with –
auditory frequencies up to 5000 Hz, or five times the maximum firing rate.
The Volley Principle
Although any individual fibre can’t fire more than 1000 times a second (1000 Hz), fibres
can take turns generating action principles in order to send temporal codes in synchrony
with auditory frequencies up to 5000 Hz. This is known as the volley principle. For it to
work, the auditory cells must create responses in a precise sequence.
This figure illustrates the volley principle. The top graph labeled
‘Stimulus’shows a sound – we know this is a single frequency (a pure
tone) because it is a sine wave. The numbers along the top label the
stimulating sound waves. These trigger action potentials. Lines a – e
are different auditory nerve fibres, and the vertical lines on the fibres
indicate the action potentials. Any one fibre doesn’t fire on all the compressions. But the
aggregate across the several fibres (‘Total response’) is in time with the stimulating
Although the neural code for pitch is still uncertain, the consensus view these days is that
both codes are used: a place code is used particularly for higher frequencies, and a
temporal code is used for lower frequencies. November 6, 12
Let’s see how these components combine to make up sound. For example,
in this figure the complex sound wave is shown at the bottom. Let’s say it
has a pitch of 100 Hz. The fact that it has a pitch tells us that all the
components bear a mathematical relationship to each other. The lowest-
frequency component, called the fundamental frequency, is shown at the
top. Then all other components – the harmonics – are 2x, 3x, and 4x this
frequency – “integer multiples” of the fundamental. The relationship among the
frequency components – the fact that they are all 100 Hz apart – determines the pitch of
the complex sound.
A complex sound is made by adding a series of harmonics to a fundamental frequency.
Pattern recognition: Identification of particular sound sources by the auditory system.
We can tell a guitar from a violin by identifying the timbre of the sound. The timbre
depends on the relative amplitudes of the components – the amount of each component in
the mixture, just like the perceived colour of light depends on the wavelengths of the
component lights in a mixture. Discrete parts of the basilar membrane respond to each of
the components, detecting their frequencies and amplitudes. The brain can put this
information together again to determine the timbre. Timbre perception is a kind of pattern
Sound intensity (what we hear as loudness) is coded by the degree to which each auditory
nerve fibre fires on every cycle of a stimulating waveform. If a sound is intense, then
each nerve fibre attached to the responsive place on the basilar membrane will fire as
much as it can. Look back to the figure on the volley principle on page 5 of this lesson. If
the sound is intense, fibres a – e will be driven as hard as they can be and each individual
fibre will look as much like the ‘total response’as it can. If the sound is very soft,
individual fibres will still fire in synchrony with the stimulating waveform but
sporadically, such that the ‘total response’might not be as faithful to the stimulating
Loudness depends not just on the intensity of a sound, but also on its frequency. Humans
are most sensitive to the middle frequencies (from 1,000 to 4,000 Hz), which overlap a
lot with the range of frequencies used in speech.
Where is that sound coming from?
Suppose someone called your name from across campus – how do you know which
direction to look?A: You probably heard the voice more loudly in one ear than the other.
This intensity cue is helped by the shape of your pinna, or outer ears, which changes the
timbre of a sound based on its location. In addition, each ear heard the sound at very
slightly different times, telling you which side of your head was closer to the sound
Pinna, Timbre, and Location November 6, 12
Spatial information is not directly preserved in the inner hair cell receptor array. Instead,
locations of sound sources have to be inferred based on frequency and timing information
in the sound. There are three ways we use our ears to locate sounds.
First, our pinnae are unique to each of us. Their precise shape determines how sounds are
transmitted into the ear canal, affecting the timbre of a sound systematically in ways that
we can interpret as information about the elevation of a sound source – whether it’s
coming from over our heads, below us, or in front or behind us.
Timing and Intensity
The timing cue works because sound is relatively slow.
For example if we hear a sound from the left side of us, the
sound reaches our left ear first before our right ear. If we hear it
directly from the middle we hear the sound in both ears at the
same time.Also to figure out location, intensity of a
sound is measured by our ears and brain. Sounds are less
intense in our far ear. Both timing and intensity cues are
used by different structures early in the auditory neural pathway in the brainstem to
compute sound locations. This information is then conveyed to the parietal lobe (which is
important for our sense of space and where objects are in space).
Midline vs. Side Hearing
Humans are more sensitive to changes in sound location near the midline (straight ahead)
compared to locations off to the side (opposite one ear). In other words, the JND in sound
position is smaller near the midline than at the side.
In general, though, our ability to determine the location of objects using hearing is poorer
than our ability to locate them using touch or vision. If a small object is touching you,
you can figure out where it is easily and with great confidence. Similarly if you see it,
you know exactly where it is. If you hear it, you will probably estimate its location
auditorily, and then turn your head in that direction so that you can see it and determine
its location more precisely.
The Psychophysics of Vision
How does the brain create meaning from what the body senses?
As you learned previously, the sensations of pitch, loudness, and timbre relate to the
physical characteristics of sound, and can be measured using the different psychophysical
Similarly, your brain takes the wavelength, amplitude, and purity of light entering the
eye, and ‘sees’these as colour, brightness, and hue, respectively. The location on the
retina where light is detected tells us about the location of the object reflecting/emitting
Vision and Location
Visual localization is more precise than auditory localization. When we see, the brain
has to interpret light signals collected on a flat surface – the retina.Although we perceive
the world in 3D, vision information passes through the retina, which processes in 2D. We
use our knowledge and experience derived from moving around the world in three November 6, 12
dimensions to infer the third dimension from what we see. Perceptual systems are often
working to interpret degraded or ambiguous sensory information, and the role of
knowledge and experience in perception is relevant to all sensory domains.
We perceive the world in three dimensions through a number of cues, including
monocular depth cues (Cues to distance that depend on input from only one eye).
Think about traveling in a car and looking out the window. The world moves past, and
objects that are further away (on the horizon) seem to be moving more slowly than things
that are close to you. Furthermore, the direction of apparent motion of objects in the
world depends on where you are looking – things farther away than the point you’re
looking at will appear to be moving rather slowly, and in the same direction you are.
Things closer than the point you’re looking at will appear to be moving faster and in the
Your eyes also let your brain determine the location of an object through binocular
cues, such as the motions that each eye makes in order to focus on a specific location and
the retinal disparity, which is the degree to which light from an object falls on a different
location on your left retina compared to your right. (When you focus at a given distance,
objects at that distance have no retinal disparity; light from objects closer to and further
away from that that focus distance will fall on different locations on your two retinas).
The accuracy of these binocular cues is determined by the distance between your eyes
and is most accurate at relatively close distances. Most 3D illusions, including those
using special glasses, use binocular cues to trick your brain into seeing distances within a
Vision and the Brain
The visual processing system of the brain is ridiculously complex. Information flows
through this system, visual images are analyzed into their component features (e.g., form,
colour, spatial location, and movement). Then these features are synthesized, organized
into meaningful arrays of objects. This complex system takes us from sensation to
Ventral and Dorsal
Researchers proposed in the early 1980s that the visual
processing system could be thought of as two interconnected
‘streams’of processing. November 6, 12
The ventral stream (“the ‘what’stream”) involves areas at the bottom of the temporal
lobe (shown here in blue). The processing of this stream enables us to see form, colour,
and motion. It allows us to identify objects and to attach meaning and significance to
The dorsal stream (“the ‘where’or ‘how’stream”) involves the parietal cortex (shown
here in red) and enables us to perceive the location of objects such that we can direct
appropriate actions (like reaching to grasp a Frisbee or to post a letter in a long narrow
mail slot) to those objects.
The two streams must work together in the production of useful, adaptive behaviour.
Damage and Recognition
Damage along the visual pathways, especially the ventral pathway, can result in visual
agnosia – the inability of a person, in whom elementary sensory processing is intact (e.g.,
they aren’t blind) to recognize what they are seeing as a meaningful, recognizable object.
An object tha