a small central pit located in the macula region of the retina; composed of closely packed cones
macula
the retinal area responsible for fixed central vision during good light
retina
sensory part of the eye; transforms light into electrical impulses; found at the the inner surface of the eye
optic nerve
transmits visual information from the retina to the brain
optic disk
location in which retinal ganglion fibers exit the eye to form the optic nerve; no photoreceptors exist at this point; causes the blind spot
foveola
found in the center of the fovea; contains only cones and no rods
photoreceptors
specialized neurons that are capable of phototransduction; includes rods and cones
pigment epithelium
layer of cells found just outside the retina; responsible for maintaining retinal environment
rods
photoreceptors; first-order neurons; detect low levels of light (scotopic); low temporal resolution; exist in greater number than cones
cones
photoreceptors; first-order neurons; functional in high levels of light (photopic); high temporal resolution
structure of rods
short cone-shaped outer segment; inner segment; synaptic terminal
structure of cones
long cylindrical outer segment; inner segment; synaptic terminal
rod and cone systems
rod systems are highly convergent - many rods synapse on a single bipolar cell; cone systems are not convergent - one cone sypapses on one bipolar cell
rhodopsin
visual pigment that absorbs light for rods; composed of opsin and 11 -cis retinal molecules; captured light causes a conformational change to metarhodopsin
phototransduction process
light activates rhodopsin to metarhodopsin; metarhodopsin activates transducin (a G-protein); transducin activates cGMP phosphodiesterase; cGMP levels are reduced; the photoreceptor cell is hyperpolarized
rod currents in dark and light
cGMP is high in the dark; cells are depolarized and produce current in the dark; light closes potassium channels and prevents potassium from leaving the cell; light hyperpolarizes cells
calcium-dependent adaptation
light opens calcium channels; influx of calcium reduces cGMP; reduction in cGMP helps to shut down channels in bright light
retinal ganglion receptive fields
have center and antagonistic surround receptive fields
on-center retinal ganglion cells
cell is activated when the cell is stimulated in the center and deactivated when the cell is stimulated in the surround; good at recognizing increases in light
off-center retinal ganglion cells
cell is deactivated when the cell is stimulated in the center and activated when the cell is stimulated in the surround; good at recognizing decreases in light
on-center bipolar cells
hyperpolarized by glutamate; are depolarized following illumination; have metabotropic receptors
off-center bipolar cells
depolarized by glutamate; are hyperpolarized following illumination; have ionotropic receptors
projection to hemiretinas
right visual field is projected onto the left hemiretinas and vice versa; nasal hemiretinas cross at the optic chiasm while temporal hemiretinas follow an ipsilateral path
subcortical targets of retinal ganglion cells
90% project -> LGN of thalamus -> occipital lobe; others project to hypothalamus and tectum (among other areas)
pupillary reflex
pretectal neurons proceed to bilateral Edinger-Westphal nuclei -> production of bilateral pupil constriction
direct response
bright light shined in one eye will provoke the pupil of that eye to constrict
consensual response
bright light shined in one eye will provoke the pupil of the contralateral eye to constrict
lateral geniculate nucleus
found with the thalamus; important relay center; 6-layered structure; project via optic radiations to the occipital lobe
cells of the LGN
have on- and off-center receptive fields; are magnocellular (layers 1 & 2) or parvocellular (layers 3-6)
retinotopy
LGN and cortex are retinotopically organized; retina is topographically represented
monocular vision
results from damage to one optic nerve; complete loss of vision in one eye
binocular hemianopsia
results from splitting of the optic chiasm; bilateral loss of temporal vision
homonymous hemanopsia
results from damage to one optic chiasm; bilateral loss of vision in one half of the visual field
quadrantanopsia
results from damage to the optic projections; bilateral loss of vision in one-quarter of the visual field
macular sparing
foveal vision often is not lost when cortical visual damage occurs; may be spared due to extensive cortical representation
cortical organization of visual information
projections from the LGN enter in layer 4C of the cortex; ocular dominace columns are formed; L/R eye separation is maintained
receptive field of visual cortical cells
cells in the primary visual cortex are maximally receptive to specifically oriented bars of light
organization in the primary visual cortex
there is a columnar organization of orientation selective cells in the visual cortex
blobs
found in layers 2 and 3 of the primary visual cortex; respond selectively to color
arrangement of ocular dominance columns
cells with preferences for specific orientations of light are arranged in columns that show preference for input from one eye
hypercolumns
hypercolumns are groups of columns that possess a complete representation of visual input -- all orientations & blobs & both ocular dominance columns are represented
color agnosia
damage between the occipital and temporal lobe may lead to an inability to recognize color
motion agnosia
damage in the middle temporal region may lead to an inability to assess motion
prosopagnosia
damage to the inferior temporal area may lead to an inability to recognize faces
2 pathways for visual processing
spatial vision pathway and object recognition pathway
spatial vision pathway
also called the dorsal pathway; involves projections from V1 -> V2 -> middle temporal region -> parietal lobe; involved in motion detection
object recognition pathway
also called the ventral pathway; involves projections from V1 -> V2 -> V4 -> temporal lobe; involved in identification of fine detail and color
magnoceullular pathway
M RGCs ->layers 1 & 2 of LGN -> layer 4-alpha of cortex; involved in analysis of gross features and movement
parvocellular pathway
P RGCs -> layers 3-6 of LGN -> layer 4-beta of cortex; involved in analysis of color and fine detail
koniocellular pathway
K RGCs -> areas in between LGN layers; analyze color information; primarily involved in ventral pathway
M cells
M RGCs and LGN cells = large; extensively branched; large receptive fields; respond optimally to large objects; high tempora resolution
P cells
P RGCs and LGN cells = small; small receptive fields; high spatial resolution
monocular clues for depth perception
familiar size; occlusion; linear perspective; size perspective; illumination; motion parallax
binocular disparity
differences in visual input between eyes helps brain to construct 3-D image (depth perception)
reflectance function
shows the reflectance/absorbance of different objects for light of different wavelengths
types of cones
S (blue) cones - most sensitive to short wavelengths; M (green) cones - most sensitive to middle wavelengths; L (red) cones - most sensitive to long wavelengths