College aantekeningen
Hoorcollege aantekeningen van Sensation and Perception, UU, 2022/2023
- Instelling
- Universiteit Utrecht (UU)
Alle hoorcolleges uitgebreid samengevat van sensation and perception.
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Lecture 1: IntroductionSensation and perception are important because it is how we see and understand the world around
us. This affects how we behave in response to our environment, and how we understand and
interact with everything in our world.
Interactions between physical forces in the world and our sensory organs produce neural activity that
carries information about the world around us. The brain needs to analyse the patterns in these
neural responses.
We all know that we have five senses: sight , sound, taste, smell and touch. Each of these has a
sensory organ associated with it. Within these sensory organs, there are specific sensory receptor
neurone that interact with something physical in the outside world.
We don’t study things like sense of time or familiarity. Also sensations from the internal state of the
body isn’t studied → only true senses are studied coming from a particular sensory organ.
However, there is a sixth sense that is often missed but does have a sensory organ interacting with
external physical forces in the world. This is our sense of balance, or vestibular sense. This relies on
interactions of our vestibular labyrinths of the inner ear with gravity and inertia.
Although much of our sense of touch relies on the skin, there are many distinct sensory receptors
involved, each producing distinct sensations. We have a sense of texture, pressure, stretch, heat,
cold or pain. The main thing that unites these very different sensations is that they mainly rely on
one sensory organ, the skin, and similar processing in the brain.
In our other senses, a single type of sensory receptor’s activity can be analysed in multiple ways, so
that there are multiple parts within each sense.
So within vision, all of these different aspects begin in the same place, the eye, and share early
processing stages. But then the are separated into different functions (form, motion, colour,
depth/distance and attention/awareness) performed by different pathways. Similarly, hearing all
starts with the ear, but then has separate analyses for the sounds location and frequency structure.
Vision is the primary sense in humans. Almost every action we make is guided
by our vision. We do a lot of analysis of our vision for example: object
recognition (form/colour) and space and motion. A big part of our brain is
involved with visual processing.
,Vision is easy to study. With modern computers, we can make any image easily. We can see exactly
where the eye is looking, so we know exactly how the retina is being stimulated. We can even take
measurements from the visual cortex, even at very high resolution. It is easy to access because the
visual cortex is at the back of the head.
So in vision, it is particularly easy to control the inputs to our brain, and to understand the how the
brain is processing these, and test what the observer is perceiving. As a result, we understand vision
in far more detail that the other senses.
In the early stages of vision, it is possible to examine the inputs and responses of neurons in so
much detail that we can follow how a pattern of activity in one set of neuron is analysed by the next
neuron to give a new type of response. In other words, we can see how individual neurons are
processing information to help us understand the world. This field, neural computation and its basis
in neural physiology, involves a more technical way of thinking than almost any area of psychology.
It has often been considered humans have mind or spirit is a separate entity from the physical body.
This concept is called dualism.
Monism is the idea that the mind is an aspect of the body, held in the brain and nervous system. So
monism takes the view that the mind and body are manifestations of the same physical thing.
Our perception is part of our mind, and in this course we work from the fundamental principle that
our perception ACTUALLY IS the activity of the neurons in our brain and nervous system.
Sensation: A translation of the external physical environment into a pattern of neural activity (by a
sensory organ)
Perception: Analysis of this neural activity to understand the environment and guide behaviour or:
The subjective conscious experience of the outside world. Perception is used more broadly and it can
be seen as an analysis of neural activity from the senses to understand the outside world.
Sensation and perception reflect interactions between our sensory organs and physical properties of
the world, so they are:
- Dependent on physical properties of the world. Example: Visible light has a short
wavelength and does not reflect from most surfaces, so it does not travel around corners
However, sound has a longer wavelength and reflects more easily, so we can generally hear
around corners fairly well: we can hear this car around a corner, but can only see around the
corner with a mirror
- Limited by the physical properties of our sensors. Example: The human eye is only sensitive
to a short range of wavelengths of light, part of a much larger electromagnetic spectrum. We
miss a lot of information.
Beyond physics, sensation and perception rely on the nervous system, a biological system that has
not been designed but results from evolution.
,Sensation and perception have evolved to help us survive and reproduce, so they are:
- Optimised for useful representations of the environment. Our brains optimise perception to
understand useful aspects of the environment. As a result of this, perception is often
inaccurate.
- Influenced by interpretation: context and experience. Perception is often inaccurate
because it is strongly influenced by experience and expectations. Some things are very hard
to see for example a face that concaves (naar binnen gekeerd is)
- Dependent on limited resources of attention and awareness. Perception is sometimes
inaccurate because our sensory organs receive more input than we can process. Therefore,
we use attention to focus our limited resources on the input that seems important for our
goals. As a result, we do not perceive parts of our sensory input, typically those that don’t
seem important to us.
All of these features of perception suggest to us that our perception is limited by our body and our
brain, so is a manifestation of neural activity in the brain. → monism idea
To study perception, we first have to change the physical environment to change what we perceive.
So we need to change our sensory input or stimulus. Then we can see either how this changes
behaviour responding to the change in perception OR we can see how this change in the stimulus
changes patterns of neural activity.
The psychological approach → quantitative measurements of behaviour resulting from perception.
This results is psychophysics, this is the scientific study of the relation between stimulus and
sensation.
The just noticeable difference (JND) has been studied by psychophysics. If we determine the just
noticeable difference, the change in the environment that we can only just perceive, its size depends
on the stimulus that is being compared. We can just notice the extra light of one candle when added
to three candles, but not when added to the light of six candles. Against six candles, we need two
extra candles. So the variability or error with which our perception represents a stimulus feature
increases with the magnitude of that feature, or the strength of the stimulus.
Weber-Fechner Law
To put this yet another way, the perceived intensity of a stimulus doesn’t increase with its physical
intensity. Instead, each doubling in intensity can be described as one unit of increase, i.e. we
perceive intensity in units of doublings, or orders of magnitude. Therefore, there is a logarithmic
relationship between the stimulus intensity and perceived intensity. This is a detectable difference
increases with average stimulus intensity.
Most psychophysics experiment rely on a 2- alternative forced choice design. We present two
stimuli and participants must make a choice between 2 alternatives. We quantify how well they can
make this choice as a function of the difference between the stimuli.
, Most simply, in the ‘Method of constant stimuli’, we test every possible difference over a wide
range. Plotting the choice made as a function of the difference. This reveals the ‘psychometric
function’, a sigmoid (s-shaped) curve.
Normally we care about the 75% threshold, because this is the middle of the psychometric function,
its steepest point. This steepest point is most accurate to measure because there is the largest
change in perception for the smallest stimulus change. For some research questions, we also
quantify the slope of the function: how quickly we change from missing the difference to perceiving
it.
BUT Building a whole psychometric function takes a lot of measurement, particularly if we are only
interested in one point, the threshold. Adaptive staircases include some kind of ongoing estimate of
the threshold, and focus the measurements on that stimulus difference
How do we determine the perceptual threshold?
Estimate parameter of interest (threshold) without determining entire psychometric function. This is
widely used, fast and efficient. The change stimulus difference intensity depending on pattern of
previous responses. We make the next trial harder after a correct answer and easier after an
incorrect answer. This aims to make the next trial maximally informative about threshold.
Biological approach → What are perception’s neural substrates (i.e., sensory receptors and brain
processing)? Correlate a neural activity measure with a change in the presented stimulus or the
animal’s behaviour.
‘Neural activity’ is either, These are all closely related to each other:
- Spiking activity (neurons firing and releasing action potentials)
Spiking activity is often seen as the gold standard of neural activity. Spikes or neurons firing are very
small changes, so they must be measured directly from the neuron. These are invasive recordings
inside the brain of an animal or human. These recordings have extremely small scales and this is a
very precise measurement. It is ethically difficult to make invasive recordings in humans. Typically
very small coverage: experimenters have to decide where to look and cut a hole in the skull over the
relevant area.
- Synaptic activity (neurotransmitter release, synaptic potentials)
Several measures at different scales and resolutions. The simplest to explain is the local field
potential (LFP). If we place an electrode between neurons and amplify the signal we see a small
spiking activity. On top of this small spiking signal, we see a much larger signal arising from
synchronized synaptic activity of all the nearby neurons, the local field potential. The LFP is a
complex wave form of large, synchronized increases and decreases of synaptic potential. We
normally analyze this complex wave by summarizing it as the sum of responses at different
frequencies.
Hertz (Hz) is the number of wave cycles per second. Each of these frequencies is associated with a
specific process, though the evidence base for these associations is often questionable. Typically this
comes from a correlation between a particular oscillation frequency and a particular behavioural or
perceptual state.
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