Senses and their organs
Within every organ, there is something that captures the information.
This differs from e.g sense of time, because this doesn’t have a physical capturer, they come
from the internal state of the body.
A 6th sense: sense of Balance (vestibular)
organ: vestibular labyrinths
Touch
- Light touch: good for feeling texture
- Deep touch: pressure, not really for feeling textures
- Stretch
- Heat
- Cold
- Pain
Vision
Different parts of vision rely on different systems
It all starts in the eye, then they are seperated through different pathways
This is also the case with hearing
,Analyses through vision:
- Object recognition
- Form and color
- Space and motion
- Location distance depth
Because of this: a lot of the brain is devoted to processing vision compared to other senses.
Vision is easier to understand cause you can control experiments better
- Inputs are easy to create
We can follow the pattern of neural activity
Top neurons dont care about the orientation of the light bar hitting their receptive fields
But if activated together, they activate the bottom neuron, which then gets new
information: orientation
,Properties of sensation and perception
Dualism
Body and spirit are separate things
Monism
mind = aspect of body
Body is represented in the mind, because the nervous system runs through the brain
Perception is a manifestation of the patterns of neuron firing in the brain, so, monism.
- 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
- perception is a subjective conscious experience of the outside world
So in short: Sensation & Perception = reflection of interactions between sensory organ and
physical properties of the world
- dependent on physical properties of the world
- limited by physical properties of our sensors
- optimized for useful representations of the environment
- influenced by interpretation: context and experience
- dependent on limited resources of attention and awareness
influence by context and experience
faces never inwards
Optimized for useful representation:
Same colored blocks look different color based on light surroundings.
Limited attention and awareness:
Vliegtuigmotor soldaten Plaatje
Limitations of the sensors
- limted colorspectrum: 400-700nm wavelength
,Experimental methods
How to study perception
- change physical environment
- measure resulting behavior
- measure resulting change in neural activity
Psychological approach
- quantitative measurements of behavior resulting from perception
- Psychophysics: “the scientific study of the relation between stimulus (physic) and
sensation (psycho)”
Just noticeable difference (JND)
if brightness is low, difference is perceived accurately
If brightness is high, you need a bigger change to perceive the change.
To change the perceptual intensity by one, you need to double the stimulus intensity
(logarithmic) (Fechner’s Law)
,How do we measure the just noticeable difference?
Determining a perceptual threshold
Present a stimulus, then give two alternatives and ask whether option A or B is brighter.
50% guess chance, that’s why function starts at 50% correct
At the 75% level, the function is steepest
At this point, you see large differences in percentage correct when difference in stimuli
increases
We measure the JND here, because it’s the place where we can most accurately determine
the difference between stimuli that people can detect
Because we want as much data in this area, we could do trials, and after every trial, make a
new trial that’s harder when the answer was right, and easier when the answer was wrong.
This way we design a trial where most points are plotted in the steepest part
This is how we estimate the Parameter of interest (threshold)
you basically estimate the threshold where a subject can
detect the difference.
,Instead of behavior, we can also measure the resulting change of neural activity
Biological approach
- What are perception’s neural substrates
- Correlate neural activity with change in stimulus or animals behavior
Neural activity is either
- Action potentials
- Synaptic potentials
- Metabolic activity (oxygen & glucose consumption e.g, fmri)
- These are all closely related
Measuring spiking activity (action potentials)
- Invasive recordings
Several measures at different scales and resolutions
- Smallest: Local Field Potential
This results in waveforms consisting of different Hz speeds
Different oscillation speeds are associated to different neural processes
Theta: transitions from sleep to waking
Alpha: inhibition of neural activity
Gamma: increases of neural activity
,Spiking oscillation
So we see an oscillation: increase followed by decrease followed by another increase of
neural activity
At oscillation peaks we can see a lot of neural spiking
At throughs we see a decrease in neural spiking
So, spiking activity and local field potential activity are related
This influences perception as well, because when a stimulus is presented at the time of a
peak in the oscillation, is it easier perceived as when presented at the time of a through.
EEG records LFP from the scalp, over larger areas of the brain
- Poor spatial res (centimeters)
- Poor signal to noise ratio (lots of measurements are needed so eliminate noise)
- Only senses activity near the scalp (so really only cortices, no deeper structures)
- Slow to set up
- High temporal res
- Cheap
- Silent, good for auditory perception
- Moves with the subject (good for children)
,fMRI
- Indirect measure of neural activity
- Low signal to noise ratio
- Awkward environment, strong magnetic field, very loud, cramped
- Poor temporal resolution
- High spatial resolution
- Straightforward interpretation
- Safe and non-invasive
- Easy access to equipment (most hospitals in developed countries have these)
MRI
- You can combine EEG with MRI to get good temporal and spatial res, this can also be
combined with eye movement trackers
How MRI works:
We have a lot of hydrogen atoms in our body (water)
Normally, orientation of these atoms is random
With magnetic field, these become aligned
With another, disrupting magnetic field (low frequency electromagnetic pulse) it is pushed
out of alignment
When this pulse is released, the atoms realign again, and this creates energy that can be
measured
How fMRI works:
Neural activity consumes oxygen
Deoxygenated hemoglobin leads to a signal loss
Blood response follows neural activity
But this blood flow overcompensates, so more oxygenated blood flows into it. This reduces
the signal loss from deoxyhemoglobin, making the fMRI signal increase
So we don’t see the decrease of oxyhemoglobin resulting the neural activity
We see the increase of oxyhemoglobin when the blood flow overcompensates to the area in
which neural activity took place -> it reduces signal loss, and creates a BOLD signal (blood
oxygenation level dependent)
,Red number = signal, so at 99 you see signal loss due to increase in deoxyhemoglobin
At 102 you see increase due to overcompensation of oxyhemoglobin
RMS: LFP
Here you see that BOLD signal is more closely related to LFP than to MUA
So it suggests that BOLD follows synaptic activity, rather than spiking activity
When neurons fire, neurotransmitter glutamate increases, this widens the blood vessels
GABA narrows the blood vessels
This widening of the vessels with help of glutamate happens so it readies up for a change in
metabolic action (oxygen usage) to compensate for that, before it even happens.
, Another way to measure perception is to change neural activity, and measure behavior
When this is done carefully, it can conclusively demonstrate that the certain neural activity
is necessary for perception: a kind of causal link instead of just a correlation
- Lesion studies
TMS
Artificial ‘Lesions” with transcranial magnetic stimulation
- Temporarily inhibits activity in the target area
