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Human Visual Perception
Image Processing
+Todays Lecture
Theories of Perception (psychological vs enviromental)
A little bit about the stimulus for vision
The human eye as the visual organ
Enviromental theory (Bottom up approach)
Single cell levels (neurophysiology)
Behavioural (psychophysics)
Psychological Theory (Top down approach)
+What is Perception?
Perceptionis the
translation of the physical world
into a pattern of neural activity
that can be used by the brain to guide behaviour
- Puts us in contact with the world,
- Shapes our knowledge, &
- Improves chances of survival
+Theories of Perception
J.J.Gibsons psychological theory
suggests that perception is determined
by information from the environment
Richard Gregorys constructivist theory
suggests that perception is determined
by psychological processes.
+Sensory Overload
What information do we have in our environment?
- Electromagnetic radiation
- Sound
- Changes in air pressure & temperature
- Changes in chemical composition in the air
What would aperfectsystem do with all these information?
What would arealsystem do?
+Understanding Stimulus
What is the stimulus for vision?
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+The Human Eye:
Understanding receptors
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+The Retina
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Neurophysiology of vision
+Retinal Ganglion Cells
Do not absorb light but receive input from other
retinalcells
Output is in form of generating action potentials or neural
impulses (brief electrical discharges)
We can record changes in electrical activity occurr ing near
isolated neurone (single cell recording) and monitor the
number of action potentials occurring in response to
different visual stimuli
+Single Cell Recording
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+ +Perceptual Consequences of
Centre/Surround Antagonism
Hermann Grids
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Physics of perception
+Linear System Analysis,
Fourier Analysis, or
Using Grating Targets to Investigate Vision
Originated in striking mathematical
discovery by French physicist, Jean
Fourier in 1822
Demonstrated that a periodic waveform of
any complexity could be broken down
(analysed) into linear sum of sine waves of
specified spatial frequencies, amplitudes
and phases
+Fourier Transform
Useful because it provides a way to quantify
simply any complex waveform
Any complex waveform can be specified by
combinations of a limited number of elemental
components
Widely used in auditiondealing with
oscillations of sound pressure over time
Any complex visual scene will produce variations
in luminance across space: these can be broken
down into sine wave components
+ Fourier Analysis
A square wave
can be
constructed from
a series of sine
waves of
appropriate
phases and
amplitudes
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+ A visual scene can be decomposed into
component parts of different spatial scale andreconstructed
+Psychological Theory of
Vision
+Gestalt
Light hitting the retina creates a rapidly changing, complex
sensory image, but we perceive the world as relatively
simple and stable
Instead of perceiving different unconnected features (lines,
textures, shapes) our mind combines these features into
perceptual objects
Gestalt theoryour brains automatically organise sensory
information, grouping some features and separating others in
a predictable way
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+ +Visual Illusions
Visual illusions occur when there is adiscrepancy between what
we perceive
and what we can measure objectively
E.g. Rubin vase & protruding mask
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