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samenvatting senation and perception deeltt 1
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Sensation and perceptionCOLLEGE 1
Vision is the primary sense in humans. 1/3 of the brain helps with vision.
Easy to study we can make visual inputs easily, we can see where the eye is looking, we
can test what a person can perceive, the visual cortex is easy to access because it is in the
back of the brain.
Dualism= mind and body are separate
Monism= mind Is an aspect of the body
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, subjective conscious experience of 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
- Limited by the physical properties of our sensors
Sensation and perception have evolved to help us survive and reproduce, so they are:
- Optimised for useful representations of the environment
- Influenced by interpretation: context and experience
- Dependent on limited resources of attention and awareness
How to study perception:
Change environment and look at behaviour (psy) or neural activity (bio).
Psychological approach:
Psychophysics= the scientific study of the relation between stimulus and sensation (weber
and Fechner).
Just noticeable difference= the change in the stimulus that we can perceive.
Weber-fechner psychophysical law= describes the relationship between a physical intensity
and its perceived intensity. Logarithmic relationship.
Weber fraction= ratio between this predicts the noticeable difference at a range of
intensities.
2-alternative forced choice: you give 2 options and ask wich is correct and incorrect or yes or
no.
Threshold= the middle of the range. Here if you change the stimulus the reaction will be
maximal. 75% large respons.
,The psychometric function= Plot detection probability against extent of Difference.
How can you find the threshold?
1. Method of constant stimuli= a lot of trails, where there is an extent of differences
randomized from one trial to the next. So, 1 trial you have a difference of 15 and trial
2 95.
Efficiently estimate parameter of interest (threshold) without determining entire
psychometric function. Change stimulus difference intensity depending on pattern of
previous responses aims to make next trial as informative as possible about threshold.
(Gaat sneller, is makkelijker).
Biological approach=
Neural activity:
1. Spiking activity= action potentials. Very small changes so they must be measured
directly from the neuron. Invasive recording inside the brain of an animal or human.
+ Is the clearest/ finest measure of neural activity.
-Ethically difficult to put holes in heads
-you have to choose where to put the electron.
2. Synaptic activity= synaptic potentials. Several measures at different scales ad
resolutions. Smallest measure local field potential
We have arousal brain waves (gamma and alpha) those waves come from exhibition and
inhibition. Through negative feedback there is a state of mind in wich we are more exhibited
or more inhibited. This results in fasces that we are more likely to perceive information.
Local field potential is high (exhibition) more likely to perceive information.
3. Metabolic activity= oxygen & glucose consumption
Electro-encephalography (EEG):
+ cheap, high temporal resolution, moves with the subject, silent.
-poor spatial resolution (signal not specific only if it comes from sides/back/front), poor
signal-to-noise ratio (you need lots of trials), only senses activity near the scalp (cortex not
deeper in the brain), slow to set up.
Functional MRI:
+ high spatial resolution (millimeters of neurons), straightforward analysis/interpretation, safe
and non-invasive, easy access.
-indirect measure of neural activity, low signal to noise ratios, awkward environment, loud,
expensive, resource-intensive (uses a lot of electricity and water).
Mri measures with water, proton, blood and magnet.
The amount of energy that is released is caused by the amount of water in the tissue.
White matter less water, grey matter more water
Fmri how much oxygen is bound to the blood?
oxygenated blood no signal loss
deoxygenated blood signal loss
1. Deoxyhemoglobin affect T2.
2. Blood response follows neural activity.
, 3. Step 2 overcompensates.
Effect 2 does not compensate accurately for effect 1.
So, oxyhemoglobin concentration increases due to increased blood flow. Does not
decrease due to oxygen use.
BOLD signal= blood oxygenation level dependent. 10 sec after neural activity.
After neural activity there is a small decrease in oxygen in the blood (in a certain part) but
after that the body sends a lot of oxygen to that part. That is wat measured.
Change neural activity and measure change in animals’ behaviour.
- When carefully done this conclusively demonstrates the neural activity is necessary
for the perception.
- Measuring neural activity only show that neural activity is correlated with perception
not that is it necessary.
1. Lesion studies, rera bilateral damage to MT complex influences motion
2. Transcranial magnetic stimulation (TMS)= large changing magnetic field to disrupt
the electrical activity in a specific part of the brain.
COLLEGE 2
Photoreceptors:
- 2 types of photoreceptors: cones and rods (kegeltjes en staafjes)
- Humans have 3 types of cones and rod Blue con= short wave, green cone=
medium wave, red cons= long wave.
- There are more rods than cones.
- There are more long wave cones than short wave cones.
- You need more than one photoreceptor type to distinguish whether it is more light or
different wavelength. Different waves necessary to distinguish colors.
- Rods more sensitive than cones.
- Rods have longer integration ‘window’. Photons reaching a rod add up over time, so
fewer are necessary for a respons.
- Photoreceptors are in the back.
- The dip in the fovea: wiring and cell bodys are moved to the side so photoreceptors
can pick up more light and less noice. Also, in this dip there are more
photoreceptors this is why we see sharp when we look straight ahead.
- Fovea= only cones. If you move more to the side, there soon
will be more rods than cones.
- The further you go to the sides the bigger the photoreceptors
get= larger area where they can catch a photo but less sharp
(like pixels).
- Cross membranes protein in outer segment of photoreceptors.
- Photoreceptors work in the opposite way (normal:
depolarization opening iron channels) = in the dark release
neurotransmitters. When light is catch on a protein the
sodium channel closes. Photoreceptor is hyperpolarized and
will release less neurotransmitter. (Light results in rhodopsin to
be broken down and remade).
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