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 inferences shadow  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 TypeDeficitMost probable site of the lesion Agnosia for form and pattern Object agnosia Naming, using, recognition of real objects Areas 18, 20, 21 on left and corpus callosum Agnosia for drawingsRecognition of drawn objectsAreas 18, 20, 21 on right ProsopagnosiaRecognition of facesAreas 20, 21 bilaterally Agnosia for color Color agnosiaAssociation of colors with objectsArea 18 on right Color anomiaNaming colorsSpeech zones or connections from areas 18, 37 AchromtopsiaDistinguishing huesAreas 18, 37 Agnosia for depth and movement Visuospatial agnosiaStereoscopic visionAreas 18, 37 on right Movement agnosiaDiscerning movement of objectMedial-temporal area bilaterally (junction of occipital and temporal cortex) Modified from Kolb and Whishaw agnosias (loss of knowledge)

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