the eye

fovea

• center of the retina

• only cone cells (no rods) which are less sensitive to light

• high visual acuity

optic disc

• blind spot

• exit of the optic nerve and vessels

retina: direct pathway

• photoreceptors

• bipolar cells

• ganglion cells (send axon projections to the forebrain)

retina: lateral connections

horizontal and amacrine cells

which cells are directly light sensitive?

photoreceptors

which cell type provides output from the retina?

ganglion cells

which cell type can fire action potentials?

ganglion cells

organization of the retina

• ganglion cell layer

• inner nuclear layer (bipolar cells, amacrine and horizontal cells)

• outer nuclear layer (photoreceptors)

photoreceptors

• converts light into neutral signals (change in Vm)

• rods and cones

• contains photopigment proteins that sense light

rods

• used during night time or low light levels

• contains rhodopsin (photopigment)

• 1000x more sensitive to light 

• can’t see low/dark colors

cones

• used during day time or high light levels

• L, M, and S opsins

L opsins

views mainly red

M opsins

views mainly green

S opsins

views mainly blue

trichromacy

3 cones

dichromacy

2 cones

monochromacy

1 cone

colorblindness

• most individuals are anomalous trichromats 

• shifts of cones among the wavelengths makes it harder to distinguish between colors

why does the fovea have high visual acuity?

there are no cells in the inner layer meaning less scattering and there is less averaging

peripheral retina

• more rods than cones

• more sensitive to light

what is phototransduction?

light energy causing changes in membrane potential (Vm)

are photoreceptors typical neurons?

no!

photoreceptors without light..

• depolarize at -30 mV

• cGMP gated channels (permeable to Na+) are opened

• dark Na+ current in the dark

photoreceptors with light..

• hyperpolarize at -60 mV

• light reduces cGMP to GMP

can photoreceptors release neurotransmitters?

yes, but they decrease with light

phototransduction in rods

• rhodopsin is activated with light

• G protein (transducin) is activated and activates phosphodiesterase (PDE)

• PDE reduces cGMP to GMP 

• Decrease in cGMP causes a cGMP gated Na+ channels to close causing hyperpolarization

photobleaching

• when light hits rhodopsin (receptor), 11-cis retinal ligand (inactive form) undergoes conformational changed to all-trans retinal (active form)

rhodopsin activation requires…

• 11-cis retinal bound to receptor

• light

what is the retinoid cycle?

• the process of restoring retinal to a form capable of signaling photon capture

• all-trans retinal is transported to the pigment epithelium to be converted back to 11-cis retinal

dark adaptation

• the transition from all-cone daytime vision to all rod nighttime vision

• regeneration of unbleached rhodopsin (opsin and 11 cis retinal)

do rods function during daytime?

no, almost all rods are photobleached and cannot respond to light due to saturation

regeneration of unbleached rhodopsin

• retinoid cycle needs to occur

• process is slow usually minutes to an hour

• light sensitive is higher compared to day time

phototransduction in cones

• similar to rods except of the type of opsins

• less sensitive to light and more energy is required to photobleaching

what is the receptive field?

the area within which a stimulus changes the avidity of neuron (changes in AP firing on Vm)

where in the visual system are receptive fields found?

retina

bipolar cell receptive field

• no dark currents

• 2 types of bipolar cells that respond to light in the receptive field center: ON bipolar and OFF bipolar cells

ON (center) bipolar cells

• cell is depolarized in response to light in the receptive field center

• less glutamate is released from photoreceptors which leads to less mGluR6 receptor activation

• light in the receptive field surround causes hyperpolarization due to horizontal cells counteracting the light in the center

OFF (center) bipolar cells

• cell is hyperpolarized in response to light in the center

• less glutamate is released from photoreceptors which leads to less activation of AMPA and kainate receptors

• cell is depolarized in the receptive field surround when activated with light

ganglion cell receptive field

• ON and OFF center ganglion cells with ligand-gated glutamate channels

• the response to stimulation of the center is cancelled by stimulation of the surround

• responds to differences in illumination that occur within their receptive field

ON center ganglion cells

• input from ON bipolar cells via excitatory synapse

• depolarized by light in the center

• hyperpolarized by light in the surround

• allows us to see edge with high light in center (light increment)

OFF center ganglion cells

• input from OFF bipolar cells via excitatory synapse

• hyperpolarized by light in the center

• depolarized by light in the surround

• allows us to see edge with low light in center (dark increment)

why do we need both ON and OFF center ganglion cells?

to allow us to effectively perceive light and dark increments which is contrast

the visual field

• binocular visual field

• visual hemifields

• partial decussation

• retina divided into nasal and temporal retina by vertical line passing through the fovea

visual hemifield

• left visual hemifield is viewed by right hemisphere

• right visual hemifield is viewed by left hemisphere

partial decussation

only fibers from the nasal retina cross

targets of the optic tract

• lateral geniculate nucleus (LGN) in thalamus to the primary visual cortex (90%)

• (10%) midbrain: superior colliculus and pretectum

• hypothalamus

superior colliculus

orients the eyes to stimuli (visual grasp)

pretectum

reflect control of the pupil and lens

hypothalamus

controls circadian rhythm

lateral geniculate nucleus 

• found in dorsal thalamus

• major target of optic tract

is input from the two eyes kept separate?

yes

primary visual cortex (striate cortex)

• contains 6 layers

• layer 4C (=IVC) receives most of LGN inputs

ocular dominance columns

stripes of neurons in the V1 that respond preferentially to input from one eye or the other

layer 4C

• each neuron receives input from only one eye

• project to layer 4B and 3

layer 4B and 3

• integrates information from both eyes

• response is still dominated by one eye though they are binocular

binocularity receptive fields

most neurons in layer 2 and layer 3 in the striate cortex are binocular

binocular disparity

• difference in the location of a feature between the right eye’s and left eye’s image

• basis for the sensation of depth

orientation selectivity

• neurons in V1 present the greatest response to a bar with a particular orientation

• not a spot of light in the receptive field center

• analysis of object shape

direction selectivity

• subset of neurons in V1 show direction-selective response to a moving stimulus

• analysis of object motion

extrastriate areas

V2, V3, V4, and MT (V5)

dorsal stream

• towards parietal lobe

• processing of object motion

ventral stream

• toward temporal lobe

• process stimuli other than motion, such as color and form

• critical for facial recognition