vision - orange coast collegeocconline.occ.cccd.edu/online/lattore/ch06 slideshow.pdf · 35...

Copyright (c) Allyn & Bacon 20041

Chapter 6Vision

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Vision

Sensory receptor:• A specialized neuron that detects a particular

category of physical events.


The StimulusPerceived color of light is determined by:

Hue:Determined by wavelength

Saturation:Determined by the purity of the light wave

Brightness:Determined by the intensity of the electromagnetic radiation


Anatomy of the Visual SystemThe Eyes

Orbits:Bony pockets in the front of the skull

Sclera:The white tissue of the eye

Conjunctiva:Mucus membranes that line the eyelid and protect the eye



Cornea:Transparent outer covering of the eye that admits light

Pupil:Adjustable opening in the iris that regulates the amount of light that enters the eye

Iris:Pigmented ring of muscles situated behind the cornea



Lens:Consists of a series of transparent, onion-like

layers. Its shape can be changed by contraction of ciliary muscles.

Accommodation:Changes in the thickness of the lens, accomplished by the ciliary muscles, that focus images of near or distant objects on the retina


Anatomy of the Visual SystemPhotoreceptors

Retina:The neural tissue and photoreceptive cells

located on the inner surface of the posteriorportion of the eye.

Rod:Photoreceptor cells of the retina, sensitive the light of low intensity.

Cone:Photoreceptor cells of the retina; maximally sensitive to one of three different wavelengths of light and hence encodes color vision.



Fovea:Area of retina that mediates the most acute vision. Contains only color-sensitive cones.

Optic Disk:Location on retina where fibers of ganglion cells exit the eye. Responsible for the blind spot.



Bipolar cell:A bipolar neuron located in the middle layer of the retina, conveying information from the photoreceptors to the ganglion cells.

Ganglion cell:A neuron that receives visual information from bipolar cells, its axons give rise to the optic nerve.



Lamella:A layer of membrane containing photopigments; found in the rods and cones.

Photopigment:A protein dye bonded to retinal, a substance derived from vitamin A; responsible for the transduction of visual information.

Opsin:A class of protein that, together with retinal, constitutes the photopigments



Retinal:A chemical synthesized from vitamin A, joins with an opsin to form a photopigment

Rhodopsin:A particular opsin found in rods


Anatomy of the Visual SystemConnections between Eye and Brain

Dorsal lateral geniculate nucleus:Cell bodies within the lateral geniculate body of the thalamus; receives inputs from the retina and projects to the primary visual cortex.

Magnocellular layer:One the inner two layers in the dorsal lateral geniculate nucleus; transmits information necessary for the perception of form, movement, depth, and small differences in brightness to the primary visual cortex.


Anatomy of the Visual SystemConnections between Eye and Brain

Parvocellular layer:One of the four outer layers of neurons in the dorsal lateral geniculate nucleus; transmits information necessary for perception of color and fine details to the primary visual cortex.


Coding of Visual Information in the RetinaCoding of Light and Dark

Striate cortex:The primary visual cortex.

Optic chiasm:A cross-shaped connection between the optic nerves, located below the base of the brain, just anterior to the pituitary gland.


Coding of Visual Information in the RetinaCoding of Light and Dark

Calcarine fissure:Horizontal fissure on the inner surface of the posterior cerebral cortex; the location of the primary visual cortex.


Analysis of Visual Information:Role of the Striate Cortex

Anatomy of the Striate cortex

David Hubel and Torsten Wiesel

1960’s at Harvard University

Discovered that neurons in the visual cortex did not simply respond to light; they selectively responded to specific features of the visual world.


Analysis of Visual Information:Role of the Striate Cortex

Modular Organization of the Striate Cortex:

Striate cortex:The striate cortex is divided into approximately 2500 modules, each approximately 0.5 X 0.7 mm and containing approximately 150,000 neurons.

Ocular dominance:The extent to which a particular neuron receives more input from one eye than from the other.


Analysis of Visual Information:Role of the Visual Association Cortex

Extrastriate cortex:A region of the visual association cortex; receives fibers from the striate cortex and from the superior colliculi and projects to the inferior temporal cortex.

Regions respond to particular features of visual information such as orientation, movement, spatial frequency, retinal disparity, or color.



Perception of Color

Achromatopsia:Inability to discriminate among different hues; caused by damage to the visual association cortex.

Inferior temporal cortex:In primates the highest level of the ventral stream of the visual association cortex; located on the inferior portion of the temporal lobe.



Analysis of FormAgnosia:

Inability to perceive or identify an object by means of a particular sensory modality.

Visual agnosia:Deficits in visual perception in the absence of blindness; caused by brain damage.

Aperceptive visual agnosia:Failure to perceive objects by their shape, even

though visual acuity is relatively normal.



Analysis of Form

Prosopagnosia:Failure to recognize particular people by the sight of their faces.

Associative visual agnosia:Inability to identify objects that are perceived visually, even though the form of the perceived object can be drawn or matched with similar objects.



Perception of Spatial Location

Balint’s syndrome:A syndrome caused by bilateral damage to the parieto-occipital region; includes optic ataxia, ocular apraxia, and simultanagnosia.

Optic ataxia:Difficulty in reaching for objects under visual guidance.



Perception of Spatial Location

Ocular apraxia:Difficulty in visual scanning.

Simultanagnosia:Difficulty in perceiving more than one object at a time.

vision - orange coast collegeocconline.occ.cccd.edu/online/lattore/ch06 slideshow.pdf · 35...

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