Chapter 5: Space and Form
Form & Pattern Perception:
Humans are second to none in processing visual form and pattern
information. Our ability to see patterns and details is unparallel on earth, and
yet is still not fully understood. In this chapter we will examine what we know
about human form and pattern visual perception.
What is a form?
Form perception: the ability to segment the spatial attributes (size and
shape) of single entity from the other entities and from background.
Separating figure from ground. Form perception involves two subcomponents:
1) Detection: the ability to determine that an object is present in the visual
environment. A lightness change or contour is perceived in the environment,
cues to this may be the lightness change itself, or movement, or depth cues,
or some combo.
2) Discrimination: the ability distinguish one object from another. This is a
more refined process in which specific spatial attributes of the object are
used not just to determine the presence of the object but to segregate
different classes of items – faces vs. trees; squares from circles, etc.
3) Identification: Recognizing and labeling a particular item (grandma’s face).
Defining Form
Form as contour change: Luminance or reflectance level changes,
digitized or quantified.
Structuralist Approach
1) Structuralist approach: the final perceived form is combination of
more elemental or primitive components. Visual system combines
elemental components to produce forms.
Structuralists used analytic introspection as primary methodology for
uncovering the elemental components. Subjects trained to ignore
more subjective aspects of stimulus and only report "objective
sensations."
Problems:
1) the catalogue of elemental components grew to unmanageable
numbers (over 40,000)
2) little consistency was found across observers as to what an
elemental component was.
Lesson learned: Question of what are basic units of perception was
valid, but method was wrong. More modern approach stresses
physiological and experimental strategies.
Analyzing spatial scale
Begins with assumption that forms and patterns exist at different levels of detail or
scale, and each of these scales must be processed by visual system.
Scales:
1) largest scale: separation of form from background
2) intermediate scale: segregation of form from other similar forms
3) smallest scale: separating various details on single form.
Hybrid images
Images with contrasting information at high and low spatial scales. Used to
“sensitize” visual system to particular spatial scale (women: low; man:
high). Shows how visual system is “tuned” to different scales by meaning.
If meaning at high level – biased to high; if meaning at low level – biased
to low (Shyns & Oliva study, 1997).
Multi-channel approach to form perception: Campbell &
Robson
At each spatial scale four parameters
defining forms:
1) Spatial orientation: the angle of the
form relative to the perceiver.
2) Contrast: the degree of intensity
difference between the form and its
background
3) Spatial frequency: the number of
cycles from highest to lowest
lightness per visual angle.
4) Spatial phase: placement of the
frequency cycle relative to some
reference point.
Multi-channel approach to form perception: Campbell & Robson
Form processing: Lens transfer functions
Left axis: contrast
Horizontal axis: spatial frequency
Lesson: as spatial frequency increases greater contrast is needed to faithfully transfer image
Contrast Sensitivity Functions: “window” of visibility for forms
•
Contrast Sensitivity Functions (CFS): a plot of the minimum contrast necessary for
a grating of varying spatial frequency to be discriminated from a homogenous
background. Humans are most sensitive (ie require least degree of contrast for
detection) for spatial frequencies of around 5 cycles per degree of visual angle.
Increased contrast is necessary for spatial frequencies higher or lower.
Contrast Sensitivity Functions: “window” of visibility for forms
Scotopic vs. photopic
systems
Humans vs. other species
Infants vs. adults
CSF: different channels
• Overall
human CSF
is actually
aggregate
of
separate,
more
narrowly
tuned
channels.
Contrast normalization
Local contrast
homogeneity used as
“cue” for normalizing of
responses among visual
channels processing that
region. Thus, spatial
frequencies and
orientations tend to be
perceived as similar for
local feature.
Normalization contributes
to metamers – physically
different signals that
evoke same response in
visual system and are thus
perceived the same.
Selective adaptation: altering perception by adapting or
fatiguing certain spatial frequency channels
Indicates that
perception results
from aggregation of
responses from
differently tuned
frequency channels.
Mid-spatial frequency
(right gratings) result
from combination of
low response in
high/low channels
and larger response
in mid channel.
:
Texture discrimination pop out effects
Repetitive
homogenous
pattern establishes
background texture,
figure contrasts
with background.
Orientation,
contrast or spatial
frequency change
defines border,
texture change or
figure.
Gestalt principles of pattern/form recognition
Gestalt approach: visual
system segmented
the visual stimuli into
meaningful "wholes."
Visual system had
natural tendency
toward "Pragnanz" or
simple organized
perception, while
visual stimuli had
"forces of attraction"
which organized
scenes.