Lecture 14 - Illusions Flashcards
What were 5 illusions in the lecture
1.Ames’ Window
2.The moon illusion
3.Lotto’s Cubes
4.Van Lier’s Stars
5.Ugly faces
First three are linked to ‘perceptual constancy’, last 2 are linked to after-effects in general
What is perception?
Scene interpretation, object recognition, face recognition, word recognition
And it happens automatically (once your brain has learnt)
What are illusions?
Illusions are the brain’s interpretation of confusing sensory signals.
What do we need to distinguish between in illusions?
- ‘stimulus’ of the illusion (which we might be misinterpreting in a narrow
sense)
and - ‘what the object would most likely be in real life’ (which our perceptionis
designed to see and nearly always gets right)
Why is perception interpretation?
e.g. it integrates information from edges and colour, and tries to interpret colour
using shadows and shapes appropirately
Example of how perception is complex
e.g. we are not aware that our colour vision is doing all this for us, and because it is
automatic, it’s very hard to get colours exactly right when you try to be an artist
Is perception learned?
Some basic things are innate or learnt very young, but the majority of perception that
we take for granted is learnt in the first few years of life. e.g. We take the ability to
match shapes and colours for granted, but babies take a surprisingly long time to do it
well (they can discriminate between colours from early on, but that’s not the same as perceiving them
as adults do, and being able to match them between objects etc).
What is the moon illusion?
How do we determine perceived size of an object?
The moon looks bigger on the horizon than when it is high in the sky
Perceived size depends on
perceived distance and on
the size of nearby objects
Your brain has learnt that
small stimuli far away are
actually large objects,
while big stimuli close by
are actually small objects.
Moon near horizon - stuff around it informs you it is far, makes it look big. In the sky not much information for your visual system to know it is far away.
What about colour perception?
What is a more complex answer?
Simple answer - light comes into eye, cones in the retina interpret it depending on the length of the wave of light
It starts in the eye but you try to make sense of it in the context of the scene
Perception is based on comparison – locally, with context, with
what you’ve seen before.
Thus, perception is a creative process based on what’s already in
your brains as well as the incoming (sometimes ambiguous)
signals…
Why do you see after effects?
Why? Because an after effect is an
ambiguous signal – the brain does not
always know if to believe it.
And in the real world, faint colours that
represent real objects are normally
bounded by edges, whereas faint
colours that are irrelevant (because
they are due to lighting, shadow,
aftereffects) tend not to be bounded
by edges.
So the brain has learnt to ‘believe’
colours that are bounded by edges.
This is why we don’t see after effects
all the time every day
What are the differences between:
mental imagery
Hallucinaitions
synaethesia
Mental Imagery - Not confused by external reality, experienced by most people but not all (aphantasia)
Hallucinations - often accepted as externally real
Normally considered rare but on a continuum
Synaethesia - Can be vivid and externalised (but understood as real)
Evoked rather than spontaneous (role of learning?)
Key things learned in the lecture about perception
Basic features such as size, shape and colour are perceived (i.e.
automatically interpreted) relative to their context and your
lifetime of experience
Perception is based on comparison – locally, with context, with
what you’ve seen before.
This achieves some computationally complex interpretations
taking into account distance, shapes, lighting and shadows,
gaps in the signal (filling in),
Thus, perception is a creative process based on what’s already in
your brains as well as the incoming (sometimes ambiguous)
signals…
What are some after effects of the visual system
motion, orientation, colour etc
neurons contrast different things, you get a after affect as some cells have more motion in them than another (for motion for example)
