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Sensation & Perception lectures 1-7
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This document contains lecture notes of chapter 1 till 7 of the course Sensation & Perception at Utrecht University.
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SENSATION & PERCEPTION LECTURESLECTURE 1 INTRODUCTION
Most of our brain is involved in processing sensory information. Interactions between physical forces in the
world and our sensory organs produce neural activity that carries information about the world. This analysis is
often very complex, so a lot of the brain’s information processing power is involved.
Senses:
- Vision – sight – eyes – retina
- Hearing – audition – ears – cochlea
- Taste – gustation – tongue – taste buds
- Smell – olfaction – nose – olfactory epithelium
- Touch – somatosensation – skin etc. – many
- Balance – vestibular – vestibular labyrinths
You can separate true senses from perceptions we have like our “sense of time”. We will not study this.
Furthermore, our sense of touch is not a single sense at all. There are many distinct sensory receptors involved,
each producing distinct sensations.
In our other senses, a single type of sensory receptor’s activity can be analyzed in multiple ways, so that there
are multiple parts within each sense. These multiple analyses of the same input should not be considered
different senses. They involve different brain areas and are often studies separately.
Vision is the primary sense in humans. Almost every action we make is guided by vision. Vision is also
particularly easy to study. It is 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.
Neural computations: how individual neurons are processing information to help us understand the world. This
involves a more technical way of thinking than almost any area of psychology.
Dualism: humans have mind or spirit is a separate entity from the physical body. Monism is the idea that the
mind is an aspect of the body, held in the brain and nervous system. Likewise our body is reflected in our mind,
as our nervous system runs through our body from the toes to the top of the head. So monism takes the view
that the mind and body are manifestation of the same physical thing.
In this course we work from the principle that our perception 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 behavior. / The subjective
experience of the outside world.
Sensation and perception are:
- Dependent on physical properties of the world
- Limited by the physical properties of our sensors
Sensation and perception have evolved to help us survive and reproduce so they are:
, - Optimized for useful representations of the environment
o Our brains optimize perception to understand useful aspects of the environment. Therefore,
perception is often inaccurate.
- Influenced by interpretation: context and experience
o Perception is often inaccurate because it is strongly influenced by experience and
expectations.
- Dependent on limited resources of attention and awareness.
o Perception is sometimes inaccurate because our sensory organs receive more input than we
can process. Therefore, we use attention to focus on our limited resources on the input that
seems important for our goals.
If we see sensation and perception as: “A translation of the physical environment into a pattern of neural
activity that can be used by our brain to guide behaviour” Then to study sensation and perception
experimentally, we will first have to change the physical environment to change what we perceive: 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.
So to study perception experimentally:
- Change the physical environment
- Measure the resulting behavior
- Measure the resulting change in neural activity
How to study perception:
- Psychological approach:
o Quantitative measurements of behavior resulting from perception
o Psychophysics: the scientific study of the relation between stimulus and sensation
o The just noticeable difference (JND)
Psychophysics is best understood as testing whether subjects perceive a difference
between two stimuli.
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.
o Weber-Fechner law
Describes the relationship between a physical intensity and its perceived intensity
Logarithmic relationship: detectable difference increases with average stimulus
intensity.
The probability that we perceive the difference increases as the difference increases. Most psychophysics rely
on a 2-alternative forces 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. Normally we care about the 75% threshold, because this is the
middle of the psychometric function, its steepest point. This point is most accurate because there is the largest
change in perception for the smallest stimulus change.
How to determine a perceptual threshold (efficiently): put simply, we make the next trial harder after a correct
answer and easier after an incorrect answer.
- Biological approach
, o What are perception’s neural substrates?
o Correlate a neural activity measure with a change in the presented stimulus or the animal’s
behavior
o Neural activity is either:
Spiking activity (action potentials)
Must be measured directly from the neuron
Invasive recordings inside the brain of an animal or human
Synaptic activity (synaptic potentials)
Several measures at different scales and resolutions
Smallest: local field potential (LFP). We normally analyze this complex wave
by summarizing it as the sum of responses at different frequencies.
Metabolic activity (oxygen & glucose consumption)
Theta oscillation are associated with transitions from sleep to waking.
Alpha oscillations are associated with inhibition of neural activity.
Gamma oscillations are associated with increases in neural activity.
Oscillations arise from interactions between excitatory and inhibitory neural population. At oscillation peaks,
excitatory population activity is highest, including spike rate. This also affects perception. Threshold stimuli
presented at an oscillation peak are more likely to be perceived than those presented at a dip.
EEG records the field potential from the scalp, so is non-invasive. Because of this, it only captures very large
changes in synchronized activity, and with poor spatial resolution.
- Advantages:
o Cheap, high temporal resolution, moves with the subject, silent
- Disadvantages:
o Poor spatial resolution, poor signal-to-noise ratio, only senses activity near the scalp (cortex),
slow to set up.
Functional MRI:
- Advantages:
o High spatial resolution, straightforward analysis, safe and non-invasive, easy access
- Disadvantages:
o Indirect measure of neural activity, low signal to noise ratios, awkward environment, poor
temporal resolution, expensive
- Because we are mostly made from water, our tissues contain a lot of hydrogen atoms. In these atoms,
the electron moving around the proton acts like a tiny magnet. Normally, the orientation of these
atoms is random, but a large magnetic field can align them all in one direction. Adding a smaller
magnetic field briefly in another direction (input RF) changes this atom spin direction. When this is
removed, the atom goes back to its original orientation. This change releases energy (exit RF) that we
measure. The amount of energy released at each location determines that location’s intensity in the
image
- So the energy they release will depend on the amount of water they contain.
fMRI images the brains activity on an effect of deoxygenated hemoglobin, the body’s oxygen carrier, on the T2
MRI image. Oxyhemoglobin concentration increases due to increased blood flow. BOLD signal. There is an
early small dip, but than a far larger increase.
There are two types of neural activity measured here: neural firing (MUA) and synaptic activity (LFP) BOLD
responses are a little more correlated with LFP than MUA- but LFP and MUA are typically correlated This tells us
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