Hoorcolleges Sensation and Perception
Hoorcollege 1 – Introduction
In this course, we will study how humans perceive our environment, and how this results from the
brain analysing the information from our senses. For psychology, sensation and perception is
important because it is how we see and understand the world around us. This affects how we behave
in response to our environment, and how we understand and interact with everything in our world.
Introducing: The Senses
This is also a major topic for neuroscience and
artificial intelligence because most of our brain
is involved in processing sensory information. So
there is a lot we need to investigate. Interactions
between physical properties of the world and
our sensory organs produce neural activity that
carries information about the world But then we
need to analyse the patterns in these physical
forces and neural responses to understand what
is happening around us. This analysis is often very complex, so a lot of the brain’s information
processing power is involved.
Introducing: The Senses
− So, what are our senses? We all know that we have five
senses: sight, sound, taste, smell and touch. Each of these has a
sensory organ associated with it, the eyes, ears, tongue, nose or
skin.
− Within these sensory organs, there are specific parts or
sensory receptors that interact with something physical in the
outside world. -This separates these ‘true’ senses from perceptions
we have like our ‘sense’ of time, or feelings like our ‘sense’ of familiarity, etc. These are not
true senses, although they are things we feel, because they do not come from a particular
sensory organ interacting with something physical in the world. Therefore, we will not study
these other feelings.
− There are other feelings and sensations that result from the internal state of the body: thirst,
hunger, fullness, heart rate, blushing. We will also not study these.
− However, there is a sixth sense that is often missed but does have a sensory organ
interacting with external physical forces in the world. This is our sense of balance, or
vestibular sense. This relies on interactions of our vestibular labyrinths of the inner ear with
gravity and inertia. We will study this later.
Furthermore, our sense of touch is not a single sense at all. Although much of our sense of touch
relies on the skin, there are many distinct sensory receptors involved, each producing distinct
sensations. We use light touch to sense what a material is made of or to place our body precisely in
relation to an object. This feels quite different to deeper pressure, which does not give these fine
details. We also feel stretch, a different sensation that can come from our muscles as well as our
skin. It is fair to say that these are all types of touch, although they feel different. We also have a
sense of heat, again quite different from light touch and pressure. And a sense of cold, which feels
quite different from heat. Extreme heat or cold, or damage to the body gives yet another sense: pain.
There are several distinct types of pain: burning, itching, and throbbing pain feel very different. The
,main thing that unites these very different sensations is that they mainly rely on one sensory organ,
the skin, and similar processing in the brain.
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. But they involve different brain areas and are often studied
separately.
Introducing: The Senses
So within vision, all of these diferent aspects begin in the same
place, the eye, and share early processing stages But then the are
separated into diferent functions performed by diferent pathways,
so we will study these diferent parts separately. Similarly, hearing all
starts with the ear, but then has separate analyses for the sounds
location and frequency structure.
Why so much vision?
• Vision is the primary sense in humans
o Almost every action we make is guided by vision
• We do a lot of analysis of vision
o Object recognition
▪ Form & colour
o Space and motion
▪ Location, motion, distance and depth
As a result, a huge proportion of our brain is devoted to visual processing. Conversely, only small
specific brain areas are involved in human touch processing and auditory processing. Very small
areas are involved in taste, smell and vestibular processing.
Why so much vision?
Furthermore,
→ Vision is particularly easy to study. It is very easy to give any visual input we want to the
brain. Particularly with modern computers, we can make any image easily. But we can’t, for
example, produce complex patterns of touch sensations, smells or emotions so easily.
→ We can see exactly where the eye is looking, so we know exactly how the retina is being
stimulated. We can easily test which information in this input the observer is seeing, and
what they are not sensitive to.
→ We can even take measurements from the visual cortex, even at very high resolution.
This allows us to understand how the brain is responding and processing information. So in vision, it
is particularly 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. As a result, we understand vision in far
more detail that the other senses. And because we do so much with vision, there is a lot to
understand.
Neural Computations
In the early stages of vision, it is possible to examine the inputs and
responses of neurons in so much detail that we can follow how a
pattern of activity in one set of neuron is analyzed by the next neuron
to give a new type of response. In other words, we can see how
individual neurons are processing information to help us understand
the world.
,This field, neural computation and its basis in neural physiology, involves a more technical way of
thinking than almost any area of psychology, though it is probably more comfortable for those
studying AI. We think this is very important to understand because it reveals how the brain is actually
doing things: not just which area is involved, but what is actually happening in the brain to allow us
to understand our world.
Dualism or Monism?
In philosophy and religion, it has often been considered humans have mind or spirit is a separate
entity from the physical body. This concept is called dualism.
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 manifestations of
the same physical thing. Our perception is part of our mind, and in this course we work from the
fundamental principle that our perception is a manifestation of the activity of the neurons in our
brain and nervous system.
Sensation & Perception
• Sensation = A translation of the external physical environment into a pattern of neural
activity (by a sensory organ)
• Perception = An analysis of this neural activity to understand the environment and guide
behavior
Or: The subjective conscious experience of the outside world.
As we discussed at the start of the class, all of our senses have something in common, allowing us to
define what sensation means. This definition is in terms of the activity of neurons in the brain, so
from the viewpoint of monism. It’s not always clear how perception is different from this. Perception
is used more broadly, and means different things in different contexts. In the context of neural
activity, it can be seen as an analysis of neural activity from the senses to understand the outside
world. But perception is perhaps best understood in terms of our conscious experience of the world,
making it harder to define. Perhaps we could say it is our subjective conscious experience of the
outside world.
Sensation & Perception
• Sensation and perception reflect interactions between our sensory organs and physical
properties of the world, so they are:
o Dependent on physical properties of the world
o Limited by the physical properties of our sensors
We may feel that we perceive the world as it truly is. But in fact we don’t perceive the world
accurately at all. Understanding perception often reveals why we perceive things as we do. AT END:
So let’s look at some examples of how these properties influence how we perceive the world.
Dependence on physical properties of the world
Visible light has a short wavelength and does not reflect from most surfaces, so it does not travel
around corners However, sound has a longer wavelength and reflects more easily, so we can
generally hear around corners fairly well.
, Limitations of our physical properties of our sensors
The human eye is only sensitive to a short range
of wavelengths of light, part of a much larger
electromagnetic spectrum.
Using our limited spectrum, these flowers have a dark center and
the same colour across the whole petal. However, honey bees can
see ultraviolet wavelengths which humans can’t, changing the
appearance of the flower. This is very useful for the honey bee and
the flower because the large dark center can be seen from much
further away, allowing the bee to feed and the flower to be pollinated more easily.
Sensation & Perception
• Sensation and perception have evolved to help us survive and reproduce, so they are:
o Optimized for useful representations of the environment
o Influenced on context and experience on interpretation
o Dependent on limited resources of attention and awareness
AT END: So let’s look at some examples of how these properties influence how we perceive the
world.
Optimization for useful representations of the environment
Humans optimise perception to understand useful aspects of the
environment. As a result of this, perception is often inaccurate. If I ask you
which of these two squares is darker, you will probably say the upper
square. But in fact, both are the exact same shade. Perhaps the best way to
understand this is to think that we don’t care about the colour reaching our eyes: it is more useful to
know the colour of the surface we are looking at. We can see that the lower square appears to be in
shadow, so is probably actually a lighter colour in the real world, even though it is not lighter in this
picture.
Influence of context and experience on interpretation
Perception is often inaccurate because it is strongly influenced by experience and expectations. The
face on this card is moving when we turn the card or move the camera. It’s often moving in some
strange ways that don’t seem to follow how it should: it seems to follow the camera as the card
moves. Can anyone explain what is happening here?
One thing that you probably didn’t consider is that the face is concave. This becomes very clear when
we see the back of the card. You don’t consider this because in our experience of the the real world,
faces are never concave. Even when you know what is happening, it’s hard to see the face as
concave.
