SENSATION AND PERCEPTION
, LECTURE I-Introduction
1-What is perception?
A basic experience of the world around us and how we do take this information.
It seems to be effortless (we take it for granted), complete (but it is full of holes and mistakes) and external (we are picking up
physical signals from the outside, but it is our brain, internal, who inferences and gives meaning to this information), but it is
in fact none of these things.
What our brain is actually doing while perceiving is impressive and pretty far from effortless.
Everything we see, feel, hear, taste, and smell is a product of the biological machinery inside your brain.
2-What do we sense?
The main five senses, pain, inner sensations like hunger, position, etc.
But we actually sense 4 elemental building blocks: light (electric magnetic radiation around us), chemicals (limited range of
them), mechanical forces (sense of hearing comes from mechanical forces in the world) and temperature, where all the others
come from.
We get these 4 inputs from the physical world, a selective set of the aspects of the physical world that exist and that we sense
too. So, there is a relationship between the input from the physical world and the impact of that input on our psychological
processes, a mapping between the physical and the psychological that follows some rules. This set of rules is named
psychophysics, so we are mapping the psychological onto the physical. You can change elements on the physical environment
and look at the psychological changes and determine what those rules are. They help us construct the range of sensations we
have from the basic elemental building blocks. And that’s the main focus of perception: understand those rules that get us from
those physical signals to range of sensations we experience around the world.
3-What´s the relationship between the input from the physical world
and the actual world it is out there?
Because all we have is these physical signals, we have no way to know what is in the
world out there beyond our perception. This is a philosophical debate that goes from Plato
and his Cave Myth to the empiricism and the idea that the way we perceive the world is
very similar to the way the world is (we develop in the world so our perceptions of it must
be based on it) and the rationalism and the idea that we are limited by our biology so we
can get distortions of what we actually perceive and what is actually out there.
4-How do we get from the elemental building blocks to the huge range of sensations we have?
Three steps:
1-Sample physical information: we have these physical signals out there in the environment and sensory organs that help us
pick that information up to sample it.
2-Integrate and encode it in the brain.
3-Interpret and use it to actually make a representation of the world.
,5-Sensory transduction
The process of taking the physical signal and turning it into a neural signal.
Humans have 4 types of receptors:
• Photoreceptors: sense light, they are in our eyes (and some other parts of our bodies but we focus on the ones that respond
to physical information)
• Mechanoreceptors: sense mechanical pressure, they are in our skin, tendons, muscles, digestive track and other parts of
our body to give us information about the mechanical pressures applied inside and outside of our bodies. We also have
them in our ears to give us a sense of balance and motion and are also receptive to isolating waves of air pressure and
sound, so the sense of hearing comes from mechanoreceptors.
• Chemoreceptors: sense molecules, they are in the digestive track but also in nose, mouth to give us these different senses.
• Thermoreceptors: sense temperature, they are in the skin, mouth, eyes, etc.
6-Photorecetors
They sense light, but what is light? Visible light is electromagnetic radiation emitted by the sun, lightbulbs, etc which exist on
spectrum (related to the wavelength). The wavelength our photoreceptors are sensitive to is 380-760nm, called visible light.
A prism can break visible light to show the spectrum of colours, but we need to be careful no to say the wavelengths are the
colours because the colours that we experience are interpretations of these different wavelengths of light.
We are sensitive to electromagnetic radiation and use such a big part of the brain to process it because light energy is reflected
and absorbed by surfaces around us. This changes the properties of light and make it contain information about surfaces. Our
brain extracts surface information from light, which is fundamental to interact.
The structures in the front of the eye have the function to get the right amount of light into the eye and focus it on the back
where the retina is with the photoreceptors. Eyes in different species have evolved different according to their needs, for
instance, owls as nocturnal animals have a much higher sensitivity to small amounts of light. Humans are the only species that
have white sclera (in other mammals matches the colour of the iris), it is supposed to be because as social species it is helpful
for us in terms of knowing where other humans are looking.
The light comes in through the cornea and the hole in the middle of the iris and the lens focusses the light so it hits the back of
the eye where the retina is and then there is a bunch of cells at the very back of the eye that light comes in and find the
photoreceptors there and sensory transduction happens, converting the light waves into signals and sending back to the ganglion
cells and to the brain through the optic nerve.
, LECTURE II- Sensory transduction: light, the eye and the retina
1-How the photo receptors work
These are the photoreceptors, rod and cones (because they are kind of conical) are the two types of
photoreceptor cells we have at the back of the eye.
The light is coming in from the right and
going in to the left. So, the light comes
into the eye, photons come in and interact
with the photoreceptor cells. There is a
photopigment molecule at the very back
of the photoreceptor cells and when the photon reaches these
molecule, these molecules splits into 2 pieces, this changes
the biochemist of the cell causing it to generate and action potential which then sends it up through the sequence of cells to the
retina ganglion cell and then on up to the brain. The split of the photopigments activates the photoreceptor cell and this is the
moment of transduction from light wave to neural impulse. But there is an important detail here with important consequences
for perception: once the photopigment molecule has split it takes some time to get ready to be used again, so there is this process
of regeneration the photoreceptor to make it ready to be used again that takes a little bit of time, this refractory period has a
consequence for our perception, it is called photopigment depletion.
2-Consequences of photopigment depletion
• Dark adaptation: when turn the light of in a room, everything seems to be black initially, you cannot see anything and
after a little bit you can see more and more. Pupil expansion is only part of this, it is also the case that photopigment
molecules or as you are in the dark for a long time there is an entire rare of photoreceptor ready to be fired because they
aren’t being bombarded with light energy and thus have all the photopigments they need. So, it just takes one little photon
for your neurons to fire because they are not depleted, neurons become more sensitive because they have lots of
photopigments available.
• Troxler fading: focussing your eyes without moving them on the middle of the picture, the ping ring is falling on the same
part of your retina and as those neurons are firing signalling the presence of the ring, they are
going to get contiredo, and if you keep staring at it you will probably find its blink-side of
existence (it disappears), and if you want to make it reappear by moving your eyes, by moving
your ayes you are moving that ring onto fresh photoreceptors that have a higher sensitivity
because they haven’t been depleted, and this is called Troxler fading. This is not only due to the
photopigment molecules in the back of the eye, there is also another principle caller neural
adaptation, which states that brain responses to change so when signals stay the same for a long period of time your
neurons stop firing and when neurons stop firing those things do not exist for you perceptually, so it still exists on the
world as a physical signal but because it is a sustained signal your brain stops responding to it and thus you can no longer
perceive it.
We mentioned its functions is to allow the right amount of light to get into the eye, so this photopigment depletion is one of
the reasons why we have this mechanism. That is why the pupil widen or dilates when it is dark so it can take lots of photons
and light information, which in important to see better in the dark but it is also important that pupil constricts under very bright
conditions because you are just as blind when your photopigments are depleted, you cannot differentiate the signals anymore.
So, the pupil constricts and widens to control the amount of light coming in to optimize sensitivity of the photoreceptors for
the light conditions. Photoreceptors are at the back of the eye rather than forward so that photopigments can be really
replenished (the choroid or blood supply is bringing fresh photopigment molecules to the photoreceptors so they can rapidly
replenish their supply and be as sensitivity as they possible can be, being in the front would complicate that process).
3-Types of photoreceptors
This is a division of labour between acuity and sensitivity. It
seems to be hard for probably most of the systems, or at least
the envious system is to be both highly acuity and highly
sensitive, so, the way the visuals system solves that problem
is to split it into 2 different subsystems working for the visual
system.
Cones have high response and recovery, but that means they
miss signals sometimes, so they are good in acuity but bad in
