Vision

Optic disc

• the blind spot is located at the optic disc, it is a blind spot because there are no receptors there to receive light

• this is where blood vessels enter the eye and the axons that form the optic nerve leave the eye

sclera

• the white of the eye

cornea

• eye’s clear outer covering

iris/opens and closing for what?

colored portion that opens and closes to let more or less light through the hole, the pupil

Retina

• The light-sensitive tissue lining the back of the eye that turns light rays into electrical signals

Fovea

• the region in the center of the retina where vision is the sharpest, greatest density of photoreceptors specialized for color (cones)

• rod free

• center of macula

Macula Lutea

• high acuity

• yellowish pigment, filters uv light

• site of macular degeneration (blurred vision in center of visual field

neurons in the retinal circuit

• photoreceptors - rods and cones

• Interneurons - horizontal, bipolar, and amacrine cells

• projection neurons - ganglion cells

Interneurons

• only make connections in the retina

• light “ON” and “OFF” signals - not just letting you know there is light, just letting you know about relativity

• relative light differences between spots (contrast - edges, differences etc.)

projection neuron: ganglion cells

• generate action potentials and send information to the brain

• axons form the optic nerve

photoreceptors: Rods and Cones

• change light intensity to graded receptor potentials

• do NOT produce APs

• hyperpolarizes when light intensity increases (inverse)

• depolarize when light intensity decreases

• release glutamate at synapses with bipolar cells

process by which photoreceptors release neurotransmitter (outer segment)

• cGMP activated Na/Ca channels that depolarize the photoreceptor, these receptors open and close in response to changing levels of cGMP

• light causes cyclic GMP to decrease

process by which photoreceptors release neurotransmitter (inner segment)

• potasium channels that hyperpolarize the photoreceptor - leak channels that are always open

process by which photoreceptors release neurotransmitter (synapse)

• graded glutamate release depending on the net change in potential between the inner and outer segments

• the more depolarized the receptor the more calcium channels open and the more NT is released

In the dark, what is the effect on cGMP concentration and what is the net effect on glutamate release?

In the dark, cGMP concentration is high and active, causing the membrane to be depolarized and photoreceptors to continuously release glutamate

In the light, what is the effect on cGMP concentration and what is the net effect on glutamate release?

In the light, cyclic cGMP concentration is lower, leading to not as many cation channels being open. This causes a hyperpolarization of the membrane and glutamate release is reduced

Rods

• more numerous then cones

• sensitive to low levels of light (dim light)

Cones

• specialized for color and high visual acuity

• highly responsive to bright light

distribution of rods and cones over retina

• cones are very concentrated in center (fovea) however rods are generally more concentrated over the rest of the retina

Convergence of rods and cones

• cones are low convergence and high acuity

• rods are high convergence low acuity

high convergene ex - 5 rods to 2 bipolar cells to one ganglion

low convergence ex - one cone for one bipolar for one ganglion

transmission of visual stimulus out of retina

• axons of retinal ganglion cells form the optic nerve

• via geniculostriate system

• optic nerve has axons from only one eye, optic chiasm contains axons from each eye

Lateral geniculate nucleus

• in thalamus

striate cortex

• primary visual cortex (V1) located in occipital lobe

visual pathway

• retina —> optic nerve —> optic chiasm —> thalamus —> visual cortex

Optic Chiasm

• Nerve fibers from the inner half of each retina (the nasal fibers) cross to the opposite side of the brain

• ipsilateral —> same side

• contralateral —> opposite side

Optic Tract

• paired bundle of nerve fibers in the brain that carries visual signals from the optic chiasm to the lateral geniculate nucleus of the thalamus

• now carries information from both ipsilateral and contralateral neurons to provide information to the opposite hemisphere then the visual field

Optic Radiations

pathway that carries information from the lateral geniculate cortex of the thalamus to the primary vision cortex in the occipital lobe

Lateral Geniculate Nucleus (LGN) organization

• inputs from each eye are separated in layers

• the bottom 2 layers are from magnocellular inputs (rod fed circuits)

• the top 4 layers are from parvocellular inputs (cone fed circuits)

• info from contralateral side goes into layers 1,4,6

• info from ipsilateral side goes 2,3,5

damage to mangnocellular layers impairs what type of perception?

• motion perception (rode fed circuits)

damage to parvocellular layers impairs what type of perception?

• visual acuity and color perception

• cone fed circuits

Ocular dominance columns and orientation columns

• ocular dominance is the strips of neurons that preferentially respond to input from a particular eye

ex) layers 1,4,6 respond to contralateral side, that is input from the left eye in the right LGN

• orientation columns in that within these columns running top to bottom are neurons with line orientation bias. every neuron in the same column has the same orientation bias, adjacent neurons are slighlty different. Over all of the columns, there is an array of 180 degrees of orientation of lines

Receptive fields of visual neurons as you get further from the receptor

• RF gets larger

• RF of many retinal ganglion cells combine to form the RF of a single LGN cell

• the RF of many LGN cells combine to form the RF of a single V1 cell

Receptive field properties of visual neurons

• there are cells w/ on center and off center receptive fields

• for an on center cell, light striking the center will produce an excitatory response

• when it strikes the surround of an on center cell it will be inhibitory

• vice-versa

Where/how pathway

• dorsal steam —> parietal lobe

• object location and how to do something

What? pathway

• ventral stream—> temportal lobe

• object recognition and identification

injury to the what pathway

• agnosia —> not knowing

• visual form, color, and face

injury to the how pathway

• optic ataxia

• deficit in the visual control of reaching and grasping

• can still recognize objects normally

blindsight

• condition where individuals with damage to the primary visual cortex (V1) are cortically blind yet can react to visual objects

• reveals that the human brain can process complex visual information and guide actions without any conscious awareness