9 Visual system

electromagnetic spectrum

400 nm (blue) —> 700 nm (red)

higher energy —> lower energy

UV—> 400-700 nm —> red

reflection: bouncing of light rays

absorption: transfer light energy —> particle/surface

refraction: bending of light rays

• light strikes object = reflection/absorption

• visual perception is based on light coming from

  • reflected

  • luminous objects (neon lights)

• images are formed by refraction

Ophthalmoscopic Appearance of the Eye

= ophthalmoscopic view through pupil

pale circular region where

• retinal blood vessels enter the eye

• optic nerve fibers exit toward the brain

  
  

blind spot = devoid of photoreceptors

—> light falling on this area cannot be sensed

= central vision, no large vessels

—> see most clearly

= thin retina, key anatomical reference point

• above = superior

• under = inferior

• left = nasal

• right = temporal

focal distance [m] = distance from

• cornea

• point where light runs parallel

refractive power = 1/focal distance = diopters

e.g. cornea 42 diopters —> 0,024 meter = focal distance

—> The greater the curvature of the cornea, the shorter the focal length

Far accomodation

• ciliary muscle relaxed

• zonule fibers stretched

• flat lens

Near accomodation

• ciliary muscle contracted

• zonule fibers limp

• fat lens

= connections between the retina and neurons in the brain stem

• oculomotor nerve —> controls muscles of the iris

• bright light = pupils constrict

• dim light = pupils dilate

—> consensual (shining light in one eye, both of them are adapted)

Hyperopia = farsightedness

• eyeball is too short

• light rays focused behind the retina

• corrected by convex lens

Myopia = nearsightedness

• eyeball is too long

• light rays focused in front of the retina

• corrected by concave lens

= basic system of retinal information processing

rods

• dark light (more disks)

• Rhodopsine high

• not in Fovea (central Retina)

• no colours

cones = daylight

• bright light (less disks)

• no Rhodopsine

• central and peripheral retina (lots in Fovea)

• colours: opsine

  • blue (short wavelength)

  • green

  • red (long wavelength)

part of central retina = location where we see the best

• lots of cones

• no rods

consists of

• Retinal = small molecule derived from vitamin A

  • in dark inactive

  • conformational change when absorbing light

  • activating Opsin

• Opsin = G protein receptor

light bleaches Rhodopsin

—> activates G-Protein transducin —> effector enzyme phosphodiesterase PDE

—> breaks down cGMP —> cGMP gated sodium channels close —> hyperpolarisation (-60 mV)

CAVE!: cAMP olfactory

no activation of G-protein transducin

• open ion channel

• inward sodium current

• depolaristion -30mV

light bleaches Opsin

—> activates G-Protein transducin —> effector enzyme phosphodiesterase PDE

—> breaks down cGMP —> cGMP gated sodium channels close —> hyperpolarisation (-60 mV)

• S-Opsin

• M-Opsin

• L-Opsin

Young-Helmholtz trichromacy theory

CAVE: red + green on X-chromosome, blue on chromosome 7

"getting used to the dark" —> minutes to nearly an hour

• Pupil Dilation

• Rhodopsin Regeneration

• Functional Circuitry Changes

  • information conduction of more rods to ganglion cells

5 to 10 minutes

• changing calcium levels within the cones

• influences cGMP

• transformations in the outer plexiform layer

  • photoreceptor membrane potential = electrical

  • glutamate release = chemical

  • bipolar/horizontal cells membrane potential = electrical

• repeats until

• change in AP firing frequency ganglion cells

= group of photoreceptors that send inputs to 1 bipolar cell

= first stage of retinal processing

• direct input from receptive field center

• indirect input from receptive field surround via horizontal cells

• on-cells

  • light in the center of their receptive field: depolarizes the cell

  • light in the surround: hyperpolarises the cell (turn off)

• off-cells

  • light in the center of their receptive field: hyperpolarises the cell

  • light in the surround: depolarises the cell (turn on)

—> antagonistic

—> passed on to the ganglion cells

—> seeing edges and boundaries of objects

CAVE: in bipolar/ganglion cells depolarization = activation again (usual AP building)

M-type

• larger, more extensive dendrites

• larger receptive fields

• higher conduction velocity

• project exclusively to the magnocellular layers (layers 1 and 2) of the LGN

  —> analysis of object motion

P-type

• more numerous

• color-opponent cells

• project exclusively to the parvocellular layers (layers 3 through 6)

  —> colour perception, high-resolution vision

p-types

• e.g. red on, green off cell

  red light activates, green light inactivates

bipolar, horizontal and amakrine cells filter visual information

• side-to-side connections that allow neighboring neurons to inhibit one another

• cells "ignore" the uniform middle of the wall and fire most intensely only at the edges and boundaries, where the light levels change

optic nerve —> optic chiasm —> optic tract

optic chiasm: partial decussation

• right visual hemifield is processed in the left optic tract

• left visual hemifield is processed in the right optic tract

• nasal hemisphere decussate, temporal hemisphere don´t

= Lateral Geniculate Nucleus (Thalamus)

= primary gateway to visual cortex

right LGN

• axons from the right (ipsilateral) eye synapse on 2, 3, 5

• axons from the left (contralateral) eye synapse on 1, 4, 6

left LGN

• axons from the left (ipsilateral) eye synapse on 2, 3, 5

• axons from the right (contralateral) eye synapse on 1, 4, 6

Ganglion types

• M type: magnocellular layers 1, 2

• Parvocellular layers (3, 4, 5, 6)

• nonM–nonP ganglion cells (ventral to each principle layer)

  • colour analysis

Magnocellular LGN cells

• Large, monocular

• transient response

Parvocellular LGN cells

• Small,monocular

• sustained response

• Primary visual cortex

  80% of excitatory synapses

• Brain Stem :

  related to alertness and attentiveness

  • These inputs can modulate the magnitude of LGN responses

  • what we see is influenced by how we feel (e.g., being startled in a dark room)

occipital lobe

• primary visual cortex (17) = Striate cortex

• secondary visual cortex (18)

• visual association cortex (19)

lots of sensory inputs —> large layer IV

= topographic organization of the visual system in which

• neighboring cells in the retina transmit information to neighboring locations in target structures (LGN, striate cortex)

• Magnocellular LGN neurons

  —> IVCα

• Parvocellular LGN neurons

  —> IVCβ

• Koniocellular LGN axons

  —> bypasses IV to make synapses in layers II and III

II, III, IVB

—> other cortical areas

V

—> superior colliculus (Midbrain)

—> Pons

VI

—> LGN

2×2 mm block of the striate cortex (kleinste funktionelle Einheit)

• Ocular Dominance Columns

  • compares informations from left and right eye

• Orientation Columns

• blobs

  • colour processing

e.g. roter Bleistift

Für einen einzigen Punkt auf dem Bleistift muss das Gehirn gleichzeitig wissen:

• Welches Auge liefert die Information?

• Welche Orientierung hat die Kante dort?

• Welche Farbe hat dieser Punkt?

—> outside layer IVC of the striate cortex

—> essential for the brain's analysis of object shape and motion

simple cells

• on/off cells

• responds to a specific orientation at a specific position

complex cells

• only one receptive field

• responds to a specific orientation, regardless of the exact position within its field of detection

• Dorsal Stream

  —> parietal lobe

  • visual motion + visual control of action

  • referred to as the "where" or "how" pathway

• Ventral Stream:

  —> temporal lobe

  • object recognition incl. perception of shape and color

  • referred to as the "what" pathway