Very comprehensive notes made using lecture recordings. For reference, I got an A1 on this module using these notes to study. They contain all information found on Powerpoint slides, explained and complemented by lecturers comments, sometimes even word for word, for clarity and accuracy. Not excess...
Vision dominates the human experience and that is reflected in our brains: around 25% of
the human cortex is involved at some level with the processing and understanding of visual
information (we have a visually-dominated brain). Vision is the most studied and best
understood sense. Vision beautifully illustrates some key concepts relevant to understanding
perception in other sensory modalities and also in thinking about the organization of complex
cognitive processes more generally.
Key concepts useful to thinking about human psychology and cognitive abilities in geeral:
-Evolutionary utility: Perception is not designed to give us some access to objective reality
out there. We can only experience reality using whatever sensory operators the process of
evolution through natural selection actin on our ancestors has given us. We’re designed to
experience reality in certain ways. Our sensory and perceptual systems can’t represent <nd
objective reality, they have evolved to reconstruct reality in certain ways (not necesarrily
ways that are most truthful), but in a way that is biologically useful to us (help us survive and
reproduce). In order to survive, we need to be fast in reachin decisions about what wer’e
looking at, so uite often evolution has traded off accuracy against speed. Our perceptual
systems will be happy with an approximate answer that is useful most of the time and gets
us there earlier, than a more accurate but slower answer. We experience the world in certain
ways because we’re designed to experience it in those ways, not because the world is
actually like that. Sesnsory qualiites that we exoerience like redness, sweetnes… are not out
there in the world, they’re a product of how wer’re designed to experience certain aspects.
e.g. the property of colour is an experience that certain animals have when the encounter
stimulation to certain wavelenghts, but there is no blueness out there in the world, only
different wavelenghts. e.g. The molecule of sugar cannot be sweet, sweetness is a property
of the taste sensation that wer’re designed to have whne ur tongue encounters that
molecule. We’re designed to experience sweetness as pleasurable because sugar is an
energy-dense molecule that provides our body with the energy we need to survive and
bevause it’s a scarce resource, we’re designed to find it irresistable so we eat as much as
we can while it’s available. tHIS IS not s fitting in an industrial society where sugar is so
widely available.
Foundations of experimental psychology:
Psychophysics was founded by Fechner; he developed a research program that hed
develop in order to try and build a bridge between the private subjective inner world that
psychology had been RELEGATED to and the objecive measurable outer world of objective
phenomena. it marks the beginning of makig psycholgy into a science, first method to study
mental phenomena objectively . Psychophysics: scientific method for investigation of
relationships between psysical stimuli and psychological experience. He developed a
number of methods to do so and by so initiated experimental psychology. He took a
depenedent variable (sensation) AND OBSERVED how thatchanged when we manipulated
independt objective variablez in the world. How we can manipulate our sensations by
changing the input/physical stimuli.
-Determining sensitivity:
method of constant stimuli, observer is shown a Ramge of constant stimuli of fixed intensity
, Perception (Lectures 10-15)
in random order manyy many times. Then, you can determine the absolute/detection
threshold. Sigmoid function: grdual change/transition between being able to perceive the
stimuli and not being able to. Where do we say they become sensitive to the stimulus?
Fechner decided that the detection/absolute threshold was at the 50% responsind pint
(observer as likely to see it as to not see it). The absolute threshold is diferened as the
minimum stimulus that can be reliably discriminated from not sitmulus at all; how sensitive a
person is in terms of detection of stuff out there in the world.
Absolute detection is a specal case of detecting differences between different levels of
intensirty.
Difference threshold: how good are we at telling the difference between a standrad stimulus
of a given intensity and a range of constant comarison stimuli of more/less or same intensity.
You can plot out the percentage of traisl when the person says that the comparison stimuli is
more than the standard and again you get a sigmoid function. At 50% is the point that you
can’t tell the difference between the comprison and the stanrdard. The difference threshold
or Just noticeable difference is the minimum difference in stimulus intensity necessary to tell
two stimuli apart. This JND is not the same for telling that something is bigger than the
standrad for tellling that it is smaller (JND is smaller for this one). JND changes depending
upon which bit of the stimulus intensity rage you’re in.
If you’ve got a lot of something, you need a bigger difference to be able to tell the difference.
This is Weber - Fechner Law: the change in intensity that you need divided by the standard
density is a constant. JND is a constant proportion!! of a standard stimulus intensity.
In other words: in ot¡rder to get the sme change in sensation (to tell the difference) you need
progressively bigger changes in the actual stimulaiton in the world. Its a logarithmic funciton.
Perceptual decisions are inherently prone to a degree of uncertainty, because humans are
not passive perceivers but active decision-makers that adopt strategies; how can
psychophysics deal with this?
Signal Detection Theory SDT.
In detection experiments, the task is to discriminate the presence of a stimulus from
background noise.
An observer’s performance on a detection task is affected by both their (sensory) sensitivity
and their (criterion) bias. SDT is an experimental method that allows us to measure
sensitivity and criterior separately by taking accounts of hits and false alarms. It is a good
model of perceptual decision and applies to any decision made in the presence of
uncertainty. Your evaluation of the costs and benefits of the different possible outcomes
affects your criterion, as well as your prior beliefs about the likelihood of different states of
the world.
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