CHAPTER 3: PERCEPTION
Illusion of clarity – an example:
o A grid placed over a blocky image makes the image appear clearer than the same
image without the grid. But a grid placed on a high-resolution image decreases
Visual Agnosia: The eyes are fine but unable to recognize what is seen.
o But visual recognition is not always completely impaired.
o Impaired recognition only applies to vision. Patients can identify objects through
other sense like touch.
Associative Agnosia: A form of visual agnosia. Patients can‟t recognize objects even
though they can reproduce the objects by drawing them.
Time Spaces: The perceptual experience of time units such as days of the week or
months as occupying spatial locations outside of the body.
o This experience is automatic, goes with them wherever they go, and cannot be
Perception: Processing sensory information such that it produces a meaningful
understanding of the information.
Stimulus: An entity in the external environment that can be perceived by an observer.
3.2 Perception as a Function of the Environment
Theory of Ecological Optics – Gibson:
o Perception involves directly absorbing the visual information present in the
Ambient Optical Array (AOA) (Gibson):
o All the visual information that is present at a particular point of view.
o Gibson said that every different viewing point has a unique pattern of light that
enters the eyes.
o Gibson thought that perception was accomplished mostly by the sensory organs.
Texture Gradients: Gradual changes in the pattern of a surface that is normally assumed
to be uniform, which provides information about the surface characteristics such as
whether the surface is receding or curved.
Topological Breakage: The discontinuity created by the intersection of two texture
o A useful indicator for the edges of objects.
Scatter-reflection: The degree to which light scatters when reflected from a surface.
o How light scatters from objects tells us a lot about the nature of the object‟s
surface. Gibson thinks that the problem with classical theories of perception is that they relied on
assumptions of a fixed, monocular perspective. But, with illusions as an example, the
moment you allow an observer to move about, the illusion vanishes.
Transformation: The change of optical information hitting the eye when the observer
Optic Flow Field: The movement of objects or the observer through the environment
produces changes in what is seen.
o When you look out the front windshield while driving, the objects straight in front
of you appear stationary but those at the edge move faster.
3.3 Pattern Recognition
Differs from Gibson‟s theories in 2 ways:
o Theories of pattern recognition do not consider the complex array of light
information reflecting from all surfaces and objects.
o They focus on how it is that we build internal representation of objects during the
process of recognition.
Pattern Recognition: The ability to recognize an event as an instance of a particular
category of event.
Percept: Meaningful interpretation of sensory information.
Recognizing a configuration involves contact between the emerging percept and memory.
Memory Trace: The trace that an experience leaves in the brain.
To recognize a letter “a,” your emerging perception of “a” must make contact with the
memory trace of “a.”
o This process is called the Hoffding function: When an experience makes contact
with a memory trace, resulting in recognition.
Template: A model against which a stimulus is compared to determine whether it
belongs to a particular category.
Prototype: A model that possesses all the typical characteristics of its class.
Template matching theory: Stimulus is compared to template and if they match the
stimulus belongs to that category.
o Problem: How to specify a template can match not only patterns that are identical
to it, but also patterns that are similar enough to it.
Multiple-trace memory model - Hintzman: Traces of each individual experience are
recorded in memory. No matter how often a particular kind of event is experienced, a
memory trace of the event is recorded each time.
o This approach distinguishes between primary and secondary memory:
Primary memory: what we experience at any point in time. Secondary memory: all the memory traces created out of all the
experiences we‟ve had.
Primary memory → probe → secondary memory → activated memory trace → echo
back to primary memory → echo leaves memory trace in secondary memory
Experiment with prototypical patterns showed that:
o Prototypical patterns were quite well classified, even though they had never been
o Sometimes participants falsely recognized the prototype as a pattern they had seen
before, even though they had previously only seen distortions of the prototype.
Hintzman‟s explanations for these results:
o Memory traces of the set of distorted patterns produce an echo which contains
what the different distortions have in common and the prototype is recognized.
Feature Detection Theory – Selfridge:
o Detecting patterns on the basis of their individual features.
Pandemonium: A model of pattern recognition consisting of three levels: data (e.g.
image and feature) cognitive demons, and decision demons!
o Data level: features such as size, color, shape…
o Cognitive demons: Feature detectors that decide whether the stimulus matches its
The more similar the pattern in the image is to the one that the demon is
looking for, the louder this demon shouts.
o Decision demon: selects the cognitive demon that is shouting the loudest and this
choice constitutes the pattern that is recognized.
Black letters on a white background have more contrast energy than grey letters on
o To identify a word that is flashed on a screen for 200 milliseconds, greater
contrast energy is needed when there are more letters in the word.
o Letters with low contrast energy are weak signals that the visual system may
Squelching: Nervous system inhibits processing of unclear features.
Recognition by Components
Recognition by Components (RBC) - Biederman: A model of perception based on
subdividing objects into a basic set of geometric shapes.
Geons: The basic geometric shapes that compromise objects.
o There are 36 geons. The theory says that once objects are reduced to their constituent geons, then these geons
are compared with existing geon configurations stored in memory.
Object recognition should be a function of the number of geons available to perception.
When people were shown 2/3 geons, accuracy of recognition was 80%.
Recognition improved with the addition of more geons.
More complex objects were recognized more efficiently than simpler ones.
Recoverability: The degree to which geons can be made out in a degraded image of an
o Obscuring geons by making them unrecoverable reduces object recognition.
3.4 Context & Knowledge
Bottom-up = Data-Driven Processing: When perception results from the combination
of individual pieces of sensory information.
o Not enough to explain illusions.
Top-Down = User-Driven Processing: When perception is driven