copyright © 2009 allyn & bacon how we see chapter 6: the visual system

Copyright © 2009 Allyn & Bacon

How We See

Chapter 6: The Visual System


What Do We See?• Somehow a distorted and upside-down 2-D

retinal image is transformed into the 3-D world we perceive


Light Enters the Eye• No species can see in the dark, but some are

capable of seeing when there is little light• Light can be thought of as – Particles of energy (photons)– Waves of electromagnetic radiation

• Humans see light between 380-760 nanometers


Figure 6.2

The electromagnetic spectrum: colors and wavelengths visible to humans


Light Enters the Eye (continued)

• Wavelength – perception of color• Intensity – perception of brightness• Light enters the eye through the pupil, whose

size changes in response to changes in illumination

• Sensitivity – the ability to see when light is dim

• Acuity – the ability to see details


Light Enters the Eye (continued)

• Lens – focuses light on the retina• Ciliary muscles alter the shape of the lens as

needed • Accommodation – the process of adjusting

the lens to bring images into focus


A diagram of the human eye


Eye Position and Binocular Disparity

• Convergence – eyes must turn slightly inward when objects are close

• Binocular disparity – difference between the images on the two retinas

• Both are greater when objects are close – provides brain with a 3-D image and distance information


The Retina and Translation of Light into Neural Signals• The retina is in a sense “inside-out”

– Light passes through several cell layers before reaching its receptors

• Vertical pathway – receptors > bipolar cells > retinal ganglion cells

• Lateral communication–Horizontal cells–Amacrine cells


The Retina

• Blind spot: no receptors where information exits the eye– The visual system uses information from cells around

the blind spot for “completion,” filling in the blind spot

• Fovea: high acuity area at center of retina– Thinning of the ganglion cell layer reduces distortion due

to cells between the pupil and the retina


Cone and Rod Vision

• Duplexity theory of vision – cones and rod mediate different kinds of vision– Cones – photopic (daytime) vision• High-acuity color information in good lighting

– Rods – scotopic (nighttime) vision• High-sensitivity, allowing for low-acuity vision in dim

light, but lacks detail and color information


Cone and Rod Vision (continued)

• More convergence in rod system, increasing sensitivity while decreasing acuity

• Only cones are found at the fovea

Distribution of rods and cones


Human photopic

and scotopic spectral

sensitivity curves


Visual Transduction: The Conversion of Light to Neural

Signals• Transduction – conversion of one form of energy to another

• Visual transduction – conversion of light to neural signals by visual receptors


From Retina to Primary Visual Cortex

The Retinal-Geniculate-Striate Pathways• ~90% of axons of retinal ganglion cells • The left hemiretina of each eye (right visual field)

connects to the right lateral geniculate nucleus (LGN); the right hemiretina (left visual field) connects to the left LGN

• Most LGN neurons that project to primary visual cortex (V1, striate cortex) terminate in the lower part of cortical layer IV


The retina-geniculate-

striatesystem

From Retina toPrimary Visual Cortex


Retinotopic Organization

• Information received at adjacent portions of the retina remains adjacent in the striate cortex

• More cortex is devoted to areas of high acuity – like the disproportionate representation of sensitive body parts in somatosensory cortex

• About 25% of primary visual cortex is dedicated to input from the fovea


The M and P Channels

• Magnocellular layers (M layers)– Big cell bodies, bottom two layers of LGN– Particularly responsive to movement– Input primarily from rods

• Parvocellular layers (P layers)– Small cell bodies, top four layers of LGN– Color, detail, and still or slow objects– Input primarily from cones


The M and P Channels (continued)

• Project to slightly different areas in lower layer IV in striate cortex, M neurons just above the P neurons

• Project to different parts of visual cortex beyond V1


Receptive Fields of Visual Neurons

• The area of the visual field within which it is possible for a visual stimulus to influence the firing of a given neuron

• Hubel and Wiesel looked at receptive fields in cat retinal ganglion, LGN, and lower layer IV of striate cortex


Receptive Fields: Neurons of the Retina-Geniculate-Striate System

• Similarities seen at all three levels: – Receptive fields of foveal areas are smaller than

those in the periphery– Neurons’ receptive fields are circular in shape– Neurons are monocular– Many neurons at each level had receptive fields

with excitatory and inhibitory area


Receptive Fields

• Many cells have receptive fields with a center-surround organization: excitatory and inhibitory regions separated by a circular boundary

• Some cells are “on-center” and some are “off-center”


Receptive Fields in Striate Cortex

• In lower layer IV of the striate cortex, neurons with circular receptive fields (as in retinal ganglion cells and LGN) are rare

• Most neurons in V1 are either– Simple – receptive fields are rectangular with “on”

and “off” regions, or– Complex – also rectangular, larger receptive fields,

respond best to a particular stimulus anywhere in its receptive field


Columnar Organization of V1

• Cells with simpler receptive fields send information on to cells with more complex receptive fields

• Functional vertical columns exist such that all cells in a column have the same receptive field and ocular dominance

• Retinotopic organization-like map of retina


Cortical Mechanisms of Vision and Conscious Awareness

• Flow of visual information:– Thalamic relay neurons, to– 1˚ visual cortex (striate), to– 2˚ visual cortex (prestriate), to– Visual association cortex

• As visual information flows through hierarchy, receptive fields– become larger– respond to more complex and

specific stimuli

Visual areas of the human cerebral cortex


Damage to Primary Visual Cortex

• Scotomas– Areas of blindness in contralateral visual field

due to damage to primary visual cortex– Detected by perimetry test

• Completion– Patients may be unaware of scotoma – missing

details supplied by “completion”


Damage to Primary Visual Cortex (continued)

• Blindsight– Response to visual stimuli without conscious

awareness of “seeing”– Possible explanations of blindsight• Islands of functional cells within scotoma• Direct connections between subcortical structures and

secondary visual cortex, not available to conscious awareness


Functional Areas of Second and Association Visual Cortex

• Neurons in each area respond to different visual cues, such as color, movement, or shape

• Lesions of each area results in specific deficits• Anatomically distinct• Retinotopically organized


Dorsal and Ventral Streams

• Dorsal stream: pathway from primary visual cortex to dorsal prestriate cortex to posterior parietal cortex– The “where” pathway (location and movement), or– Pathway for control of behavior (e.g. reaching)

• Ventral stream: pathway from primary visual cortex to ventral prestriate cortex to inferotemporal cortex– The “what” pathway (color and shape), or– Pathway for conscious perception of objects


Prosopagnosia

• Inability to distinguish among faces• Most prosopagnosic’s recognition deficits are not

limited to faces• Often associated with damage to the ventral

stream• Prosopagnosics have different skin conductance

responses to familiar faces compared to unfamiliar faces, even though they reported not recognizing any of the faces


Retinal Diseases

• Macular Degeneration-destruction of photoreceptors– Wet (blood vessels) and Dry (drusen)


Retinitis Pigmentosa• Progressive degeneration of photoreceptors


Prostethic Retina

• Bioelectronic implant• Images collected by camera hidden in

glassesdata sent to the unharmed retinal cells then onto optic nerve

• 60 pixels (distinguish btwn light and dark)• Artifical Retina Project Video

copyright © 2009 allyn & bacon how we see chapter 6: the visual system

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