2 - Vision

Light + Optics

• Electromagnetic radiation exists in waves, from long radio waves to short X rays.

• Light is a type of electromagnetic radiation with a wavelength of between 400 and 700 nanometers. (among other types of electromagnetic radiation: radio waves, x-rays, etc)

        -wavelengths a little shorter than 400 nanometers = ultraviolet

        -wavelengths a little longer than 700 nanometers = infrared

• Eyes are special organs that you use to detect this radiation.

• Different wavelengths are perceived as different colours

Wavelengths of Light

• We can use a prism to split white light into different wavelengths due to refraction.

**white light is all of the colours combined — ALL wavelengths

**darkness/black is no light — due to receiving NO wavelengths

• The colours that we perceive are reflections of those wavelengths off objects.

• We perceive grass as green because it absorbs all wavelengths of light except green. The green light is reflected by the grass, enters our eyes and is absorbed by our green cone cells

Colour vision

• The ancestor of birds, reptiles, mammals and most fish had four kinds of cone cells to see different colours: red, green, blue and ultraviolet. These creatures are called tetrachromats.

• Mammals lost two of these four kinds of cones, becoming dichromats with difficulty differentiating between red and green but better night vision.

• Humans and some other mammals re-developed the ability to see reds and oranges, making us trichromats with three kinds of cone cells

        **Remember this — I AM a trichromat

FUN FACT — How does an orange tiger hide in a green environment?

        Tigers mostly need to hide from dichromats — as the orange colour

        of a dichromat is picked up in the same red-green cone as green would

Phototransduction

• Phototransduction is the process by which light energy produces graded receptor potentials.

• Photoreceptors are modified neurons that have their photoreceptive ends inserted into the pigmented layer of the retina.

• Photoreceptors contain visual pigment (photopigment) molecules that change shape as they absorb light

Photopigments, rods and cones

Photopigments are embedded in stacks of discs in rod and cone cells.

Photoreceptors are vulnerable to damage. They can be destroyed by intense light.

Vision is maintained because the outer segment is renewed every 24 hours as the tip of the rod or cone fragments off and is phagocytized

Rods + Cones

• A retina has ~92 million rods and ~6 million cones.

Rods:

contain one type of pigment, so vision is perceived in only one colour. Rods are 100 times more sensitive to light than cones. Up to 50 rods converge on a single bipolar interneuron, increasing sensitivity but making images fuzzy and indistinct.

*Work best in low light

Cones:

Each cone has one of three pigments, which detect different colours (red, green or blue) but need more light to activate. Each cone gets its own connection to the optic nerve, so the visual image is sharper

Rods and cones send signals to bipolar neurons

Bipolar neurons send signals to ganglion cells.

Ganglion cell axons form the optic nerve.

Visual pigments

• A photopigment is made by combining a light- absorbing molecule, retinal, with one of four different four opsin proteins, which determine what colours of light are absorbed by that pigment.

    *retinal is a modified form of vitamin A — making vitamin A sort of beneficial to your       sight (only affects vision if you're very deficient)

• Rods only contain one kind of opsin, rhodopsin. (RODopsin)

Cone opsins absorb light within a given range of wavelengths, so they are named after the colours they absorb (blue, green, and red).

• Exposure to light causes pigment breakdown (bleaching) as the retinal and opsin separate

Changing resting membrane potential

• Neurons use changes in membrane potential as communication signals.

• Changes in membrane potential from sensory or neurotransmitter receptors can produce graded potentials, brief changes in polarization near the dendrites of the neuron.

• If the graded potential depolarizes the cell enough to cross the action potential threshold, voltage- gated ion channels will open, producing an action potential that travels along the neuronal axon

• Graded potentials are temporary local changes in membrane potential. They can be depolarizations or hyperpolarizations.

• Depolarization, when the inside of the membrane becomes less negatively charged, makes it easier to produce an action potential.

• Hyperpolarization, when the inside of the membrane becomes more negatively charged, makes it more difficult to produce an action potential

Action potentials

• Graded potentials are “graded” because the magnitude of the stimulus corresponds to the magnitude of the change in membrane potential.

• A critical minimum, or threshold, depolarization is the amount of depolarization required to activate voltage-gated ion channels to allow Na+ influx.

• Action potentials are all-or-none phenomena: they either happen completely, in the case of a threshold stimulus, or not at all, in the event of a subthreshold stimulus

Membrane potentials + special senses

• This model of graded potentials depolarization
leading to action potential signalling is found
throughout the sensory systems.

• The rods and cones of the eye seem to be an
exception to this rule. They depolarize in the dark
and hyperpolarize in the light.

• Now that we know how receptor potentials
normally work, let’s look at photoreceptors in the
eye and how they behave differently

Photoreceptors in the dark

In the dark, photoreceptor cells are slightly depolarized (ion channels are open).

As long as the photoreceptor is depolarized, it will release an inhibitory neurotransmitter to the bipolar cell. While the bipolar cell is inhibited by the photoreceptor, it cannot stimulate the ganglion cell to fire.

VS

Photoreceptors in the light

When light bleaches the pigment, it hyperpolarizes the photoreceptor.

The hyperpolarized photoreceptor stops releasing inhibitory neurotransmitter to the bipolar cell.

The bipolar cell, no longer being inhibited, can now depolarize and release an excitatory neurotransmitter to the ganglion cell, sending an action potential along the optic nerve

LIGHT + DARK ADAPTATION

Light adaptation occurs when we move from darkness into bright light.

• Initially, both rods and cones are strongly stimulated. Large amounts of photopigments are broken down (bleached) instantaneously, producing glare. Pupils constrict to reduce the amount of light reaching the retina, allowing us to functionally adapt to bright light very quickly.

• Over 5-10 minutes, visual acuity improves as retinal sensitivity decreases, particularly for rods

Dark adaptation occurs when we go from a bright area into a dark one.

• Cones cannot function in low-intensity light. Bright light has recently bleached rod pigments, so they are turned off and require time to reactivate. Pupils dilate to maximize light reaching the retina.

• Rhodopsin accumulates in the dark, so retinal sensitivity starts to increase, significantly improving after 5 minutes and reaching peak sensitivity after about 20-30 minutes

Visual perception

Retinal ganglion cells merge in the back of the eyeball to become the optic nerve, which crosses at the optic chiasma to become the optic tracts connected to the thalamus.

From there, optic radiations project to the primary visual cortex in the occipital lobes

Visual processing

• Information is simplified and condensed by retinal ganglia that look for particular things, such as cells that look for edges (sharp changes in colour and brightness).

• The lateral geniculate nucleus of the thalamus integrates visual information to emphasize cone vision and to begin processing depth perception.

• The primary visual cortex maps retinal information onto the occipital lobe for further processing for contrast, colour, shape and movement

From the primary visual cortex in the occipital lobe, the “two-streams” hypothesis proposes how the accessory visual areas of our brain work with our visual information:
• A ventral “what” stream goes to the temporal lobes for memory and the limbic system for emotions.
• A dorsal “where/how” stream goes to the occipital and parietal lobes to let you recognize what you’re looking at and how you can interact with it

Depth perception

Depth perception is created when the visual fields of each eye, which differ slightly, overlap.

The visual cortex fuses these slightly different images into a three- dimensional perception

Your brain relies on many other cues in order to perceive depth

Figure out:
• Describe the events that convert light into a neural signal.

    Photo transduction is the process by which light energy produces graded potentials inside photoreceptor cells — photo transaction is generally defined as just what's happening in that one cell, the photoreceptor of the rod or the cone.

• Compare and contrast the roles of rods and cones in vision.

    These are modified neurons, they've got their photo receptive ends inserted into a pigmented layer of the retina.