Transcript Visual Attention - Memorial University of Newfoundland

Chapter 7 - Visual
Attention
Cognitive overview of the problem
of attention
What is Attention?
“Everybody knows what attention is. It is taking
possession by the mind, in clear and vivid form,
of one out of what seem several simultaneously
possible objects or trains of thought.
Focalization, concentration of consciousness are
of its essence. It implies withdrawal from some
things in order to deal effectively with others ,
and is a condition which has a real opposite in
the confused, dazed scatterbrain state … “
William James (1890)
What is Attention?

‘Taking possession of the mind’
– Controlling the focus of attention; intentionality; “Please pay
attention …”

‘one out of what seem several simultaneously possible
objects’
– Our apparent inability to attend to multiple things at once

‘It implies withdrawal from some things in order to deal
effectively with others’
– Attending has a cost
– Attending is a limited capacity process

‘has a real opposite in the confused, dazed scatterbrain
state’
– Attending is the glue that keeps perception together
Attention: Selecting some stimuli
over others for further processing

Why select? Why not process all visual input?
– Limited processing capacity

Limited processing capacity of what?
– Response systems: Two hands, one mouth, etc.
– Input: One pair of foveas
– Use of distributed representations: When more than
one pattern is activated simultaneously, interference
or cross-talk may occur.
Coping with the problem of
interference and cross talk

Reducing the degree of distributedness at
higher levels of visual representation
 Evidence of sparse population codes (e.g., IT face
representations)
 Binding attributes via synchronized oscillations

Selective Attention: Focusing processing
on a selected portion of the scene
 Reduces information overload
Selective Attention
Selective attention: Limits the processing
to one portion of the scene at a time.
 Stimulus selection could be based on:

– location in a (retinotopic map)
– object representation
Spatial-based Selection


Posner’s Spatial Cuing
Procedure.
- See fixation cross
- Brightness change
- Respond to target onset
Fastest response to a
target that occurs on the
cued side (valid) and
slowest when target
occurs on the non-cued
side (invalid).
Object-based Selection

Task
–
–
–
–
–

Told what dimension(s) to report
Fixation point
See target (~79 ms)
Pattern mask
Report
Target: Two overlapping objects, each with 2
dimensions:
– Box: Height; Gap
– Line: Texture; Orientation


Equally accurate reporting one or two dimensions
from the same object
More accurate reporting two dimensions from the
same object than one dimension from each of two
objects. (Vecera & Farah, 1994, E1: 86% > 80%)
After Duncan (1980)
Visual Attention: The Glue that Binds

We normally experience our complex environments as a coherent
world of integrated objects.

Remember: Sensory information (i.e., shape, color, motion) arrives
in parallel and is processed by different pathways in brain.
To create useful mental representations of objects, we collect their
features, bind them into the correct temporal and spatial bundles,
and interpret these bundles to specify their real world origins.
This process depends upon focused attention.



If focused attention fails, binding may fail and we perceive illusory
conjunctions (i.e., upon seeing two cars (red Focus and black Echo,
may falsely report a red Echo).
Feature Integration Theory:
Treisman (1998)
Limited Visual Input

Although we seem to have a detailed,
highly veridical and continuous mental
representation of visual world about us,
visual input is limited.
– Visual acuity is good only for foveal focus.
– Eyes movement from one fixation point to
another while viewing a scene.
 Saccades: 3-5 saccades/sec, 40 – 50 msec each.
 Fixations: ~300 msec, during which we pick up
visual information.
Eye Movements:
Saccades and Fixations
Change Blindness
Failure to detect supraliminal changes in a visual
scene. (i.e., editing discontinuities in movies).
 Flicker technique: In your demonstration,
changes involved: color, size, location,
presence/absence of objects in scene.

Winter 2006 data (Psych 3450)
Accuracy
Central Interest
Marginal Interest
96%
84%
Latency
5.5 sec
13.5 sec
Two Empirical Issues to be Addressed
from the Neuroscience Perspective
1)
How does attention affect visual
information processing at different
stages?
2)
What neural systems control the
allocation of attention?