1 — introduction But in fact, the probability that we perceive a
difference is not all-or-nothing. The probability
For psychology, sensation and percepiton is important because it that we perceive the difference increases as the
is how we see and understand the world around us. This affects difference increases. So the just noticeable
how we behave in response to our environment, and how we difference is not a single point, but a range over
understand and interact with everything in our world. which we go from no perceived difference to
always telling perfectly between two stimuli.
Sense and its sensory organ. Vision (sight): eyes (retina). Hearing
Most psychophysics experiments rely on a 2-
(audition): ears (cochlea). Taste (gustation): tongue (taste buds).
alternative forced choice design. We present two
Smell (olfaction): nose (olfactory epithelium). Touch
stimuli and participants must make a choice
(somatosensation): skin etc. (many). Balance (vestibular):
between two alternatives. We quantify how well
vestibular labyrinths.
they can make this choice as a function of the
Vision consists of form (shape), motion, color, depth and distance, difference between the stimuli.
If we see perception as a translation of the physical environment into a
cognitive influences (attention and awareness). Vision is the Most simply, in the method of constant stimuli,
pattern of neural activity that can be used by our brain to guide behavior,
primary sense in humans. Almost every action we make is guided we test every possible difference over a wide
then to study perception experimentally we will first have to change the
by vision. We do a lot of analysis of vision. Object recognition range. Plotting the choice made as a function of
physical environment to change what we perceive. We need to change our
(form and color), space and motion (location, motion, distance, the difference. This reveals the psychometric
sensory input or stimulus. Then we can see either how this changes
and depth). As a result, a huge proportion of our brain is devoted function, a sigmoid (s-shaped) curve.
behaviour responding to the change in perception or we can see how this
to visual processing.
change in the stimulus changes patterns of neural activity. Normally we care about the 75% threshold,
Neural computations: in the early stages of vision, it is possible to
Weber and Fechner were the pioneers of modern perceptual studies. In because this is the middle of the psychometric
examine the inputs and responses of neurons in so much detail
their time, it was not possible to measure neural activity well, but it was function, its steepest point. This steepest point is
that we can follow how a pattern of activity in one set of neurons
possible to study how humans perceive and respond to things. They came most accurate to measure because there is the
is analyzed by the next neuron to give a new type of response. We
up with the framework of psychophysics. This term comes from the view largest change in perception for the smallest
can see how individual neurons are processing information to
of perception as in interaction of the brain/nervous system (psyche) with stimulus change. For some research questions, we
help us understand the world.
physical stimuli (physics). They saw that sensory stimuli have physical also quantify the slope of the function, how
Dualism is the idea that humans have mind or spirit as a separate effects on sensors, and they wanted to investigate these using quickly we change from missing the difference to
entity from the physical body. psychophysics. perceiving it. However, building a whole
Monoism is the idea that the mind is an aspect of the body, held in psychometric function takes a lot of
The Just Noticeable Difference (JND): psychophysics is best understood as measurement, particularly if we are only
the brain and nervous system.
testing whether subjects perceive a different between two stimuli. If we interested in one point, the threshold. Adaptive
Sensation is a translation of the external physical environment determine the just noticeable difference, the change in the environment staircases include some kind of ongoing estimate
into a pattern of neural activity (by a sensory organ). that we can only just perceive, its size depends on the stimulus that is of the threshold, and focus the measurements on
Perception is the analysis of this neural activity to understnad the being compared. that stimulus difference. Change stimulus
environment and guide behavior. Or, the subjective conscious
Weber-Fechned (psychophysical) law described the relationship between difference intensity depending on pattern of
experience of the outside world.
a physical intensity and its perceived intensity (sensation). Each doubling previous responses.
Sensation and perception reflect interaction between our sensory
in intensity can be described as one unit of increase, i.e. we perceive
organs and physical properties of the world so they are dependent
intensity in units of doublings, or orders of magnitude. Therefore, there is
on physical properties of the world and limited by the physical
a logarithmic relationship between the stimulus intensity and perceived
properties of our sensors.
intensity. This arises because the output of sensory receptors tends to
Sensation and perception have evolved to help us survive and follow this logarithmic scaling. This means we typically only need to
reproduce, so they are optimized for useful representations of the study the just noticeable difference at a single stimulus intensity, and
environment, influenced by interpretations (context and compare that difference to the stimulus intensity. Using the resulting
experience) and dependent on limited resources of attention and ratio, we can then accurately predict noticeable differences at a large
awareness. range of intensities.
, Biological approach to study perception: correlate a neural There are two types of neural activity measured, neural
1 — Introduction activity measure with a change in the presented stimulus or firing (MUA) and synaptic activity (LFP). BOLD responses are
the animal's behavior. Neural activity is either spiking activity a little more correlated with LFP than MUA, but LFP and
Electro-encephalography (EEG) records the field potential from
(action potentials), synaptic activity (synaptic potentials) or MUA are typically correlated. This tells us that BOLD signals
the scalp, so is non-invasive. Because of this, it only captures very
metabolic activity (oxygen and glucose consumption). These reflect this synaptic activity. Together with the
large changes in synchronized activity, and with poor spatial
measures of neural activity often follow each other. However, overcompensation of blood flow, this suggests that BOLD
resolution. Advantages: cheap, high temporal resolution, moves
sometimes they are separable, so it's important to understand signals do not directly reflect increases in metabolic activity.
with the subject, silent (good for auditory perception).
which you are measuring. Instead, BOLD signals arise from neurotransmitter release.
Disadvantages: poor spatial resolution, poor signal-to-noise ratio,
Glutamate is released by neural excitation and causes
only senses activity near the scalp (cortex), slow to set up Measuring spiking activity. Spiking activity is often seen as vasodilation (increase in blood flow). GABA is released by
(particularly for large electrode numbers). the gold standard of neural activity. Spikes are very small neural inhibition and causes vasoconstriction (by inhibition
changes, so they must be measured directly from the neuron. ongoing glutamate release). These changes anticipate
Functional MRI (fMRI) is by far the most common neuroimaging
Invasive recordings inside the brain of an animal or human. upcoming chagens in neural metabolism. Therefore, fMRI
method used in perception. Advantages: high spatial resolution,
straightforward analysis/interpretation, safe and non-invasive, So if we have three cups of water in a magnetic field, and these responses are quite closely linked to neural activity.
easy access. Disadvantages: indirect measure of neural activity, each contain different amounts of water atoms, the energy Measuring synaptic activity. Several measures at different
low signal to noise ratio, awkward environment, poor temopral they release will depend on the amount of water they contain. scales and resolutions. Smallest: local field potential (LFP).
resolution, expensive. fMRI imaging relies on an effect of deoxygenated
haemoglobin, the body's oxygen carrier, on the T2 MRI image. Finally, we can investigate the neural basis of perception by
The physics of magnetic resonance. Because we are mostly made
Oxyhemoglobin concentration increases due to increased changing neural activity. We can see how behavior changes
from water, our tissues contain a lot of hydrogen atoms. In these
blood flow. It does not decrease due to oxygen use. with this change in neural activity. Again, the behaviour
atoms, the electron moving around the proton acts like a tiny
measured is linked to the animal's perception. This approach
magnet. Normally, the orientation of these atoms is random, but a
Blood Oxygenation Level Dependent (BOLD) signal. At time 0, is rarer, but can conclusively demonstrate that the altered
large magnetic field can align them all in one direction. Adding a
with resting neural activity, there is low blood flow and some neural activity was necessary for perception.
smaller magnetic field briefly in another direction (input RF)
of the red blood cells are oxygenated while others are
changes this atom spin direction. When this is removed, the atom In lesion studies, we can characterise perception and
deoxygenated. At time 1, some neural activity happends. This
goes back to its original orientation. This change releases energy behaviour after damage to specific parts of a patient's brain.
uses oxygen, so reduces the amount of oxygenated blood cells.
(exit RF) that we measure. The amount of energy released at each In patient H.M., MT was damaged in both hemispheres. The
This has a small effect on the MRI signal. But then there is a
location determines that location's intensity in the image. patient became completely unable to see visual motion.
large increase in blood flow at time 6 to deliver more oxygen.
Therefore, this demonstrates that area MT is necessary for
If we have a bunch of traffic cones (atoms) in zero gravity (no This greatly increases the amount of oxygenated red blood
motion perception, which is quite different from showing
magnetic field), they will float around with random orientations. if cells, and this is the main signal in the BOLD response. So
that area MT is active when we see motion. Fortunately,
we add gravity (static magnetic field) they all fall the same way there is an early small dip in the BOLD signal following neural
such patients are rare. Also, the brain damage is rarely
because they are heavier at the bottom (protons have a directional activity, but then a far larger increase.
restricted to a single area. Typically quite large areas are
spin). Once they are all standing, we can apply a force like a small
damaged, leading to less specific problems and less specific
kick (the gradient). This will push them away from the orientation
ties between the damaged area and the damaged perception.
fo the gravity. When we release this force, they will then fall back
It is much more common that the lesion damages the whole
to the orientation of the gravity. When they hit the floor, this
visual cortex, leaving the patient blind. Lesions can also be
makes a sound, and if we know the loudness and direction of that
created in animals by damaging the brain deliberately. This
sound, we know where the cones are and also how heavy they are.
can be done more specifically.
So if we had several microphones in different places (received
magnetic coils) we could then make an image of the cones in the Transcranial magnetic stimulation (TMS) is a technique that
room. This is known as a T1 image. But atoms, like traffic cones, uses changing magnetic fields to disrupt the electrical
wobble around when they land, releasing more energy but weaker activity in a specific part of the brain. The device is a coil
and later. This wobble is sensitive to smaller forces like which produces the magnetic field. This temporarily
interactions between atoms. If we recorded just the sound of this disrupts activity in the targeted area in a harmless way.
wobble, we would get another image, of poorer quality, that is Again, it can show that activity in the targeted area is
sensitive to the interactions between atoms. This is the T2 image. necessary to perform the tested perceptual process.
