Copyright © 2009 Allyn & Bacon

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 nanometers Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

Cone and Rod Vision (continued)
Distribution of rods and cones More convergence in rod system, increasing sensitivity while decreasing acuity Only cones are found at the fovea Copyright © 2009 Allyn & Bacon

Human photopic and scotopic spectral sensitivity curves
Copyright © 2009 Allyn & Bacon

Eye Movement We continually scan the world with small and quick eye movements – saccades These bits of information are then integrated Stabilize retinal image – see nothing Visual system responds to change Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

From Retina to Primary Visual Cortex
The retina-geniculate-striate system Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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” Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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” Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

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 Copyright © 2009 Allyn & Bacon

Prostethic Retina Bioelectronic implant Images collected by camera hidden in glassesdata 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

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