Visual Perception Is a Creative Process

Instructor : Dr. S. Gharibzadeh

Presented By : J. Razjouyan

• Visual perception has often been compared to the operation of a camera.

• Like a camera, retina.

• three-dimensional perception of the world from the two-dimensional

• cognitive function of the visual system– different visual conditions

• the visual system transforms transient light patterns on the retina into a coherent and stable interpretation of a three-dimensional world.

• recognize in a melody is not simply the sequence of particular notes but their interrelationship. – played in different keys will still be recognized as the same

melody because the relationship of the notes remains the same.

• Likewise, different images under a variety of visual conditions, because the relationships between the components of the image are maintained by the brain.

Your perception rapidly alternates between the two figures. The perceptual distinction between figure (or object) and ground is similar to the communication engineer's distinction between signal and noise. As we focus on one signal, other information is relegated to background noise.

An outline drawing, typical of children's drawings, has clearly recognizable objects because edges are powerful cues in the perceptual organization of the visual field.

Illusions • “misreading” of visual information by the brain, also

illustrate how the brain applies certain assumptions about the visual world to the sensory information it receives.

– the brain uses shape as an indicator of size

Illusions

• demonstrate certain organizational mechanisms of visual perception– selection,

– distortion,

– filling in of omissions.

Filling-in

• spatial relationships of objects – we judge the size of an object by comparing it

to its immediate surroundings. – In this comparison we also rely on our

familiarity with objects in the visual field

assumption of occlusion

• The integration of distinctive objects into a coherent visual scene is aided by another central fact of vision: closer structures cover those that are more distant.

Perception on inferencesshadow convex / concave

• You can reverse the depth of these objects by imagining a shift in the light source from the top of the figure to the bottom.

• In this array, once you see one column as convex the other column will appear concave. It is almost impossible to see both rows as simultaneously convex or concave

• The assumption of a single light source may have evolved because our natural environment has only one

source of light, the sun, and we assume that the source of light is always

above.

• Gestalt theorists,

• “What are the basic components of this perception?”

• “How does the brain produce this perception?” – framework

Visual Information Is Processed in Multiple Cortical Areas

• various & unrelated attributes• motion,

• depth,

• form,

• Color

• are all coordinated in a single percept.

• hierarchical neural system

• multiple visual areas

• interacting neural pathways.

• Distributed processing

• photoreceptors

• bipolar cells

• retinal ganglion cells, • the output cells of the retina.

• The axons of ganglion cells of the retina form the optic nerve,

• lateral geniculate nucleus • in the thalamus.

• primary visual cortex • (Brodmann's area 17 or V1, also called the striate

cortex )

32 representations of the retina in the extrastriate areas

• Over 50% of the neocortex of the macaque monkey is devoted to processing visual information,

• while only 11% is somatosensory cortex and

• 3% is auditory cortex.

Different Cortical Areas Make Different Contributions to the Processing of Motion, Depth,

Form, and Color

• How separable is processing of motion from that of form, and either of these from processing of color?

Frames of Reference

• A major task for the brain is to construct three successive frames of reference for visual perception and the control of movement:

• a retinotopic frame of reference,

• a head-centered frame of reference,

• and a body-centered frame of reference.

We refer to this map as a retinotopic map or a retinotopic

frame of reference.

• Visual information leaving the retina is organized into a two-dimensional map of the visual field.

• Each time the eye moves the retinotopic frame of reference moves as well.

• Anything that is anchored to the frame of reference, such as the afterimage produced by a flash of light, moves with it.

• same visual field with respect to the head. In this frame of reference

• anything in the visual field that moves with the head

• remains stable.

• The brain constructs this head-centered frame of reference by combining :– the retinotopic frame of reference – with added information about ;

• the eye position.

• Likewise, a body-centered frame of reference can be constructed by combining;

• information about eye movement

• and head movement with

• information about posture.

• Thus one frame of reference is built upon another.

• clinical observations

– parietal cortex is specialized for spatial representation,

– whereas the temporal cortex is specialized for object recognition.

tests of monkeys with lesions in the posterior parietal or inferior temporal cortex

– Ablation of the posterior parietal cortex altered the monkey's ability to locate objects visually, including the ability to guide hand movements to reach them, but did not affect the ability of the monkey to identify objects. (orientation)

– In contrast, lesions of the inferior temporal cortex impaired the monkey's ability to identify objects when the discriminations required use of color, orientation, pattern, or shape but did not affect the monkey's ability to locate objects in space. (identification)

Table 25-1 The Visual Agnosias

Type Deficit Most probable site of the lesion

Agnosia for form and pattern

   Object agnosiaNaming, using, recognition of

real objectsAreas 18, 20, 21 on left and

corpus callosum

   Agnosia for drawings Recognition of drawn objects Areas 18, 20, 21 on right

   Prosopagnosia Recognition of faces Areas 20, 21 bilaterally

Agnosia for color   Color agnosia Association of colors with objects Area 18 on right

   Color anomia Naming colors Speech zones or connections from areas 18, 37

   Achromtopsia Distinguishing hues Areas 18, 37

Agnosia for depth and movement

   Visuospatial agnosia Stereoscopic vision Areas 18, 37 on right

   Movement agnosia Discerning movement of object Medial-temporal area bilaterally (junction of occipital and

temporal cortex)

Modified from Kolb and Whishaw 1980.

agnosias (loss of knowledge)

Parallel Pathways Convey Information From the Retina to Parietal and Temporal Cortical

Areas