7. Visual System I

retino-geniculo-cortical pathway

between thalamus and primary visual cortex

• visaul orienting

• eye reflexes

• pupil size regulation

• nasal RGC cross at optic chiasm

• eye segregated representations in LGN

eye - optic nerve - optic chiasma - optic tract - LGN (lateral geniculate nucleus) - visual cortex

1. light enter through the pupil

2. light is focused through the lens

3. light is projected as images onto the reitina (upside down)

4. photoreceptors in retina convert spatio-temporal pattern of photons to electrical signals

5. retina circuits process visual information

6. retina send information to the rest of the brain via optiv nerve

5

outer segments

photosensitive molecules on intracellular disks, more numerous, more sensitive

• only black/white

• night

photosensitive molecules on stack of membranes continuous with plasma membrane, high scuity, color vision

• active for daylight

• 3 types of colour

• most is fovea

• humans have 3 types

• scotopic

• mesopic

• photopic ( > 3cd/m^2)

1. darkness: cGMP-gated sodium in photoreceptors open (depolarization)

2. light: leads to conformational change in rhodopsin & activation of G protein

3. reduction of intracellular cGMP

4. cGMP gated sodium channels close -> photoreceptor hyperpolazies

region from which a stimulus can activate a neuron

1. ON-center RGCs – activated by light in the center of their receptive field.

2. OFF-center RGCs – activated by light in the surround but not the center.

   -> Both enhance contrast and edge detection. (not simply respond to light but analyze spatial patterns)

Center-sign bipolar cells respond to light in the center of their receptive field:

• ON-bipolar cells: activated (depolarized) by light in the center, sign inversion between phtprecepors and ON bipolars

• OFF-bipolar cells: activated (depolarized) by dark in the center, follow sign of photoreceptors

• → They preserve the "sign" of the stimulus (light increase or decrease) when relaying to ganglion cells.

• Center response is determined by type of glutamate receptor in bipolar cells

• parallel processing starts with bipolar cells

The LGN (Lateral Geniculate Nucleus) is a part of the thalamus that acts as a relay and processing station between the retina and the primary visual cortex (V1).

Key functions:

• Relays visual input from retinal ganglion cells (RGCs) to V1

• Separates signals from the two eyes (binocular organization)

• Filters and modulates signals based on:

  • Attention

  • Arousal (brainstem input)

  • Cortical feedback

• Increases informational precision compared to raw retinal output

• Contains layered structure, often with magnocellular, parvocellular, and koniocellular pathwaysobligatory relay station before cortex

dLGN = dorsal part of LGN, transmission of visual stimulus

final bottleneck for information flow before cortex

1. Simple: Orientation + position-specific, clear ON/OFF zones

2. Complex: Orientation + motion-sensitive, no fixed ON/OFF zones

They found that V1 neurons respond to edges, orientation, and motion.Identified simple & complex cells and discovered columnar organization in V1.

color, form, fine detail -> object recognition

motion, depth -> spatial location