Perception

The process of organizing and interpreting

information, enabling us to recognize
meaningful objects and events.
Five Functions of Perception
1) Attention is used to make decisions about which incoming information is to be
further processed and which is to be discarded

2) The system must be able to determine where objects of interests are in the
external environment

3) The perceptual system must be able to determine which objects are out there
in the environment

4) The system must be able to abstract the critical features of a recognized

object

5) The perceptual system must maintain certain inherent features of objects

Attention
The study of attention concerns primarily the cognitive resources and their limitations. At
any given time people have only a certain amount of mental energy to devote to all the possible
tasks and all the incoming information confronting them. Attention is sometimes synonymously used
with mental concentration.

Does people’s concentration level change with practice ?

Selective Attention
The term selective attention refers to the fact that we usually focus out attention on one
or a few tasks or events rather than on many. We mentally focus our resources implies that we shut
out (or atleast process less information from) other competing tasks. As attention researcher Hal
Pashle puts it
at any given moment [people’s] awareness encompasses only a tiny proportion
of the stimuli impringing on their sensory systems
Eye Movements
The process of seeing starts with visual
scanning in the form of fixations, which
are – brief periods during which the eyes
are relatively stationary, separated by
saccades, which are – quick jumps of the
eye from one place to the next.

The points on which the eyes fixate are

not random, but rather are the areas of
scene that contains the most information.
Experiments to verify this stated by
showing scenes of a farmyard with either
a tractor / octopus in the middle. It was
found that eye fixations were directed
earlier and more frequently to the un-
usual object (tractor).
Divided Attention Without Eye Movements

Humans can also selectively attend to some visual stimulus without moving their
eyes. In experiments that demonstrate this, observers have to detect when an
object occurs. On each trial, the person stares at a blank field and then sees a
brief cue, which is a directing stimulus such as a small arrow that directs the
subject to attend wither to the left or to the right. An object is then presented
either in a location indicated by the cue or the opposite location. The interval
between the cue and the object is too brief for observers to move their eyes, yet
then can detect the object faster in the cued location than when it occurs than
elsewhere.
Localization

Localization of information in the external environment is essential for solving

the following problems
1) Navigating our way through the ever cluttered environment
2) Grasping an object

Localization is achieved by first separating the objects from one another and the
separating each from the background

Separation of Objects

Figure and Background – the most elementary form of perceptual organization

is that in a stimulus with two or more distinct regions, we usually see part of it
as a figure and the rest as background.

The regions seen as figure contains the objects of interest, which appear more
solid than the ground and appear in front of it. The ground is the region that
appears to be behind the figure.
Figure Ground Relationship

Our first perceptual

decision is what is the
image is the figure and
what is the background.
 Gestalt Psychology
• Gestalt psychologists focused on
how we GROUP objects together.
• We innately look at things in groups
and not as isolated elements.
• Proximity (group objects that are
close together as being part of
same group)
• Similarity (objects similar in
appearance are perceived as being
part of same group)
• Continuity (objects that form a
continuous form are perceived as
same group)
• Closure (like top-down
processing…we fill gaps in if we can
recognize it)
Perceiving Distance
Perceiving distance requires depth cues, different kinds of visual information
that, logically or mathematically, provide information about some objects depth.

Monocular Cues

1) Relative Size – If an image contains an array of similar objects that differ in size, the
viewer intercepts the smaller objects as being farther away

2) Interposition – If an object is positioned so that it obstructs the view of the other, the
viewer perceives the overlapping object as being nearer

3) Relative Height – Among similar objects, those that appear closer to the horizon are
perceived as being farther away

4) Perspective – When parallel lines in a scene appear to converge in the image, they are
perceived as vanishing in the distance

5) Shading & Shadows – the configuration if shading and shadows provides information
about an objects depth
Interposition
Perceiving Distance
Perceiving distance requires depth cues, different kinds of visual information that, logically
or mathematically, provide information about some objects depth

Depth Cues
• Eleanor Gibson and her Visual
Cliff Experiment.
• If you are old enough to
crawl, you are old enough to
see depth perception.
• We see depth by using two
cues that researchers have
put in two categories:
• Monocular Cues
• Binocular Cues
Monocular Cues

1) Relative Size – If an image contains an array of similar objects that

differ in size, the viewer intercepts the smaller objects as being farther
away

2) Interposition – If an object is positioned so that it obstructs the view

of the other, the viewer perceives the overlapping object as being
nearer

3) Relative Height – Among similar objects, those that appear closer to

the horizon are perceived as being farther away

4) Perspective – When parallel lines in a scene appear to converge in the

image, they are perceived as vanishing in the distance

5) Shading & Shadows – the configuration if shading and shadows

provides information about an objects depth
 Interposition
Relative Size

prespective Shading and Shadow

Binocular Cues
• We need both of our
eyes to use these cues.
• Retinal Disparity (as an
object comes closer to
us, the differences in
images between our
eyes becomes greater.
• Convergence (as an
object comes closer our
eyes have to come
together to keep
focused on the object).
 Perceived Motion
• Stroboscopic effect
(flip book effect)
• Phi phenomenon
• Autokinetic Effect
(if people stare at a
white spotlight in a
dark room, it
appears to move.)
Recognition

Recognizing an object entails several sub-problems –

a) Acquire fundamental or primitive bits and pieces of information from the

environment and assemble them properly

b) Figure out what the objects actually are

Fundamental information assembly starts with the binding problem: how activity in
different parts of the brain, corresponding to different primitives such as color and
shape, are combined into coherent perception of an object.

The Binding Problem: Pre-attentive and Attentive Process

from the visual world is pre-attentively encoded along separate dimensions
Feature Integration Theory - states that information – (shape & color are encoded
separately) – and then integrated in a subsequent attentive processing stage. The
theory was proposed by Anne Treisman.
the general idea is that in a first pre-attentive stage primitive features
qualities such as shape & color are perceived and in the second attentive stage
focused attention properly “glues” the feature to form integrated wholes
Figuring out what an object is –
This problem has shape of an object, playing a major role in the identification
process. The process is a two part system where

a) In early stages the perceptual system uses information on the retina, particularly
variation in intensity to describe the object in terms of primitive components such
as lines, edges and angles.

b) In the later stages, the system compares this description to those of various
categories of objects stored in visual memory and select the best match.

Relations Among Feature

There is more to a description of a shape than just its features: The relations among
features must also be specified.
Later stage of Recognition Network
Models

Simple Networks – Most research on the matching stage

has used simple patterns, specifically handwritten or
printed letters or words. The basic idea is that letters are
described in terms of certain features, and the knowledge
about what features goes with what letter is contained in
a network of connections.

Networks with Feedback – A letter is easier to perceive

when it is presented as a part of a word than when it is
presented alone. This finding has led to the certain
features in the Simple Network Model.

a) A level of word is added to the simple network and

along with its excitatory and inhibitory connection that
go now from letters to words

b) New addition of excitatory connections that go from

words down to letters are now added. These
connections easily explain the word superiority effect.
Recognizing natural objects and Top-Down Processing
Features of Natural Objects – it has been suggested that features of objects
include a number of geometric forms, such as arcs, cylinders, cones, blocks
and wedges. These features referred to as “(Geometric Ions) geons” were
identified by Biederman (1987). There are 36 geons that explain almost all
shapes in the physical world.
The Importance of Context – Top down processing are driven by a person’s knowledge,
experience, attention and expectation. These processes in addition to the geons (bottom
up processing) make up for most of the perception of complex stimuli in our physical
environment.
Bottom-Up Processing
• Also called feature
analysis.
• We use the features
on the object itself
to build a
perception.
• Takes longer that
top-down but is more
accurate.
 Top-Down Processing
• We perceive by filling
the gaps in what we
sense.
• I _ant ch_co_ate ic_
cr_am.
• Based on our
experiences and
schemas.
• If you see many old men
in glasses, you are more
apt to process a picture
of an old man (even when
you may be in error).
Abstraction

Abstraction – is the process of reducing the vast amount of information that comes in
from the physical world through our senses to a more manageable set of categories
Perceptual Constancies

A remarkable ability of the perceptual systems, is to maintain, Constancy, which refers to the
brain’s ability to maintain a perception of the underlying physical characteristics of an
object, such as shape, size or color, even when the sensory manifestation of these objects
change drastically.

Color and Brightness constancy

Color constancy – is the ability of the visual system to perceive the reflectance characteristics
– an inherent property of an object – no matter what the source wavelength.

Brightness constancy – refers to the fact that the perceived lightness of a particular object
changes very little, if at all, even when the intensity of the source changes dramatically.

Color and brightness constancy depends on the relations among the intensities of the light
reflected from the different objects
Shape Constancy – refers to the ability of the perceptual system to main shape of objects in
the external environment

Size Constancy – is the ability of the perceptual system to maintain the objects perceived
size relatively constant no matter how far away it is
Constancy
• Objects change in
our eyes constantly
as we or they
move….but we are
able to maintain
content perception
• Shape Constancy
• Size Constancy
• Brightness
Constancy