PS231: Vision IV (Receptive fields, central visual pathways, and V1)

Retinal visual information pathway

1. A photon of light is absorbed by opsin proteins in photoreceptor outer segments

2. This causes a decrease in glutamate release between the photorecptor and bipolar cell

3. This light signal is then inverted in ON bipolar cells and passed on to the ON retinal ganglion cells, and then to the brain via the optic nerve in the form of action potentials (opposite in OFF cells)

The first steps in vision

occur in the retina, include phototransduction in photoreceptors and active computations to extract specific features of the visual scene

Light hits the retina

light activates photoreceptors (rods and cones) in the retina

Photoreceptors send signals

the activated photoreceptors send signals to bipolar cells, which then pass the information to ganglion cells (the output neurons of the eye)

Horizontal cells step in

at the same time, horizontal cells, which connect neighboring photoreceptors, detect this activation

Inhibition of neighbors

the horizontal cells release GABA onto nearby photoreceptors that are less activated, this weakens their signal, making the difference between light and dark areas clearer

Lateral inhibition

the phenomenon by which interconnected neurons inhibit their neighbors, producing enhancing contrast at the edges of regions

Staring straight ahead

objects in our left visual hemifield are projected to our right primary visual cortex and vice versa

Medial (nasal) fibers of the optic nerve

cross at the optic chiasm

Lateral (temporal) fibers

stay on the same side

Visual information from each eye

stays segregated in the LGN, input from the left vs. right eye project to different LGN layers, and light information from each eye stays segregated all the way to V1

Photoreceptors

hyperpolarized by light

On center bipolar cells

unusual (sign inverting), glutamate receptor that turns a hyperpolarization (less glutamate release) into a depolarization (more glutamate release) into ON-center ganglion cells

Receptive field

A stimulus (e.g. a portion of visual space) that maximally activates/excites a neuron (increases firing rate)

Receptive fields in the retina and thalamus

simple concentric “spots” or “circles” of light surrounded by darkness (“center-surround”) that are detected by photoreceptors (think of them as individual pixels) coming from specific locations in the visual field

Continue up the visual pathway into V1 and beyond

receptive fields of neurons become increasingly more complex

Retinal ganglion cell RF

center-surround “spots” of light (simple “spot detectors”) coming from specific locations in the visual field

V1 “simple cell RF

bars of light with specific orientations coming from specific locations in the visual field

Visual cortex

made up of 6 main layers (known as a “laminar” structure)

Inputs from the thalamus

arrive into layer IV in V1

Ocular dominance columns

• visual information from each eye is still segregated in layer IV, information from the two eyes begins to converge after layer IV in the superficial layers of V1

• neurons in other layers (as well as beyond V1) will respond to a visual stimulus independent of which eye the stimulus originates from (“binocular”)

• layers 2/3 of V1 is therefore where information from the left and right eye are first integrated