Dr. Mohanad R.Alwan

Light: The Stimulus for VisionWe see light or electromagnetic energy which comes to us:

Directly from a light producing object (a light bulb)Indirectly from a light source reflecting off an object

Light waves come in different lengths The small range of ‘visible light’ is due to properties of visual receptors, not properties of the light itself.

Inter-species differences in light range detection

Stimulus

• The eye is sensitive to a narrow band of wavelength

• The human visual system is sensitive to

• wavelengths from 400 to 700 nanometers

(10-9 meter)

The Electromagnetic Spectrum

The EyeCornea: Transparent tissue covering the front of the eye. Does not have any blood vessels; does have nerves.

Iris: Circular band of muscles that controls the size of the pupil. The pigmentation of the iris gives "color" to the eye. Lens: Transparent tissue that bends light passing through the eye. To focus light, the lens can change shape by bending.

Pupil: Hole in the center of the eye where light passes through.

Sclera: Protect coating around the posterior five-sixths of the eyeball

Retina: Layer of tissue on the back portion of the eye that contains cells responsive to light (photoreceptors)

The EyeThe lens projects an inverted image onto the retina and the brain adjusts this inversion so we see the world in its correct orientation.

The cornea and lens bend or refract light rays as they enter the eye, in order to focus images on the retina.

The eye can change the extent to which rays are bent and thus can focus images of objects by varying curvature of the lens.

•Through the ciliary muscle •This ciliary muscle is smooth or involuntary muscle.

Light passes through the cornea, pupil, and lens on its way to the retina PHOTORECEPTORS, which absorb and then convert it into electrical potentials that carry information to the brain.

The Photoreceptors

The retina performs dual function involving

the two different receptors :

1. rods

2. cones

The Photoreceptors

A sensory receptor is a specialized neuron thatA sensory receptor is a specialized neuron thatdetects a specific physical stimulus.detects a specific physical stimulus.

*Note: *Note: Do not confuse a sensory receptor withDo not confuse a sensory receptor withthe protein molecules that function asthe protein molecules that function asneurotransmitter receptors in the membranesneurotransmitter receptors in the membranesof neurons.of neurons.

Most sensory receptors do not have axons. TheirMost sensory receptors do not have axons. Theircell bodies synapse on dendrites or cell bodies ofcell bodies synapse on dendrites or cell bodies ofneurons.neurons.

The Photoreceptors

The photoreceptors process the light energy. There are two types of photoreceptors: "rods" and "cones."

•Rods are sensitive enough to respond to a single photon, but functioning together they are optimized for seeing in poor light.•Cones are optimized for responding to fine detail and color; they need a lot more light and work best in broad daylight.

Inside the human eye, there are eighteen times more rods than cones. These are arranged in such a way as to produce the best possible combination of night and day vision.

The Photoreceptors: located in the back of the retina

When light hits the retina it first processed in the photoreceptors located in the back of the retina.

PHOTORECEPTORS BIPOLAR CELLS GANGION CELLS

The ganglion cells’ axons join together to form the optic nerve, which exits through the back of the eye at the optic disk.

The Photoreceptors: RODS120 million rods distributed over most of the retina except near the fovea Rods are connected in groups; there are far fewer optic nerves going to the brain than rods

Rod vision detects edges and motion very wellRod pigment is bleached by light and is less effective in bright light; rods take about 20 - 30 minutes of ‘dark adaptation’ before they are most efficient

Rods

• 120 millions

• Absent in fovea centralis

• Increase number towards periphery

• Lower threshold

• Night vision (scotopic vision)

• Adapts slowly

• The photochemical substance is rhodopsin

The Photoreceptors: CONESThere is a concentration in fovea, region about 1.5 mm in diameter. Most acute vision limited to foveola, covering ~0.4 mm No rods at all in central part of fovea Color vision is provided by 3 types of cone with different colored light absorptions: red, green, and blue cones.

Cones

• 6 millions • Concentrated centrally • Found in fovea centralis • Higher threshold • Daylight vision(photopic vision)• Adapts faster• Concerned with color vision• The photochemical substance is iodopsin

•

The Photoreceptors: PHOTOTRANSDUCTION

Rods and cones contain photopigments- chemicals that release energy when struck by light.

Photopigments consist of 11-cis-retinal (derivative of Vitamin A) which is bound to opsin (a protein).

Light converts 11-cis-retinal to all-trans-retinal, which ultimately activates 2nd messenger systems that work to close NA+ channels, hyperpolarizing the receptor.

•More light = hyperpolarization.

Photoreceptors input to retinal bipolar cells, which input to retinal Photoreceptors input to retinal bipolar cells, which input to retinal ganglion cells.ganglion cells.

Photoreceptors and bipolar cells do not produce Photoreceptors and bipolar cells do not produce action potentialsaction potentials. They . They produce produce graded potentials.graded potentials.

Retinal ganglion cells produce action potentials.Retinal ganglion cells produce action potentials.

light

The Photoreceptors: Phototransduction

Glutamate hyperpolarizes some bipolar cells, and depolarizes other Glutamate hyperpolarizes some bipolar cells, and depolarizes other bipolar cells.bipolar cells.

At rest, (i.e. in the dark) photoreceptors continuously release neurotransmitter (glutamate).

.

...

.

.

.

..

in the dark

Some bipolar cells provide hyperpolarizing input to ganglion cells, and Some bipolar cells provide hyperpolarizing input to ganglion cells, and some bipolar cells provide depolarizing input.some bipolar cells provide depolarizing input.

The Photoreceptors: Phototransduction

Dark and Light Adaptation

Periphery of the retina sensitive to dam lightDark adaptation: 60% accomplished in the first 5

minutes (12%/min) and completed → 20 minutes (2.66%/min).

Light adaptation: loss of sensitive attained through dark adaptation occurs upon re-exposure of the eyes to the light.

Less time to loss dark adaptation than to acquired it

Change Taking Place During Dark Adaptation

• Re-syntheis of rhodopsin

• Dilatation of the pupil

• Purkinje shift• Change in sensitive to different wave length

• Blind to red

• Increase sensitive to the blue-green

• Change over from cone vision to rod vision

Change involved in light adaptation

• Bleaching of rhodopsin

• Constriction of the pupil

• Purkinje shift to the yellow side

• Change over from rod vision to cone vision

close your right eye. With your left eye, look at the red circle. Slowly move your head closer to the image. At a certain distance, the blue line will not look broken!!

THE OPTIC DISK: YOUR BLIND SPOT

The Receptive Field

• The receptive field of a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron and it usually have a "hot spot"

• Receptive fields have been identified for neurons of the auditory system, the somatosensory system, and the visual system.

Processing in the Retinal Ganglion Cell

Receptive field is the area of visual space to which a given cell responds Ganglion cell receptive fields are circular.

•Ganglion cell receptive fields have a concentric antagonistic surround.

•on-center cells mostly excited by light falling on the center of the receptive field•off-center cell mostly excited by light in the surround.

Each ganglion cell ONLY responds to the presence or absence of light in its receptive fieldOR

The receptive field of a retinal ganglion cell is The receptive field of a retinal ganglion cell is roughly circular with 2 concentric regions.roughly circular with 2 concentric regions.

center

periphery

Receptive Fields

ON-cells respond with anON-cells respond with an excitatory burst when lightexcitatory burst when light shines on the center of theirshines on the center of their receptive field.receptive field.

light

light

time

Action potentials

Receptive Fields

> ON-cells are inhibited when lightON-cells are inhibited when light shines on the periphery of theirshines on the periphery of their receptive field.receptive field.

light

light

time

Action potentials

Receptive Fields

> ON-cells respond with fairlyON-cells respond with fairly constant activity when lightconstant activity when light shines on their entireshines on their entire receptive field.receptive field.

light

light

time

Action potentials

Receptive Fields

> OFF-cells are inhibited when lightOFF-cells are inhibited when light shines on the center of theirshines on the center of their receptive field.receptive field.

light

light

time

Action potentials

Receptive Fields

> OFF-cells respond with anOFF-cells respond with an excitatory burst when lightexcitatory burst when light shines on the periphery ofshines on the periphery of their receptive field.their receptive field.

light

time

light

Action potentials

Receptive Fields

ON-cells are particularlyON-cells are particularly important for distinguishingimportant for distinguishing objects that are brighter thanobjects that are brighter than their background.their background.

+-

-+

OFF-cells are particularlyOFF-cells are particularly important for distinguishingimportant for distinguishing objects that are dimmer thanobjects that are dimmer than their backgroundtheir background..

Peripheral vision is all ON-cells. This makes sense givenPeripheral vision is all ON-cells. This makes sense given that rods mediate night vision, and we are better able tothat rods mediate night vision, and we are better able to see light spots against the dark background.see light spots against the dark background.

Receptive Fields

Retinal bipolar cells have receptive fields composed of the inputs from photoreceptors and horizontal cells.

Retinal ganglion cells have receptive fields composed of the inputs from retinal bipolar cells.

Dorsal lateral geniculate cells have receptive fields composed of the inputs from retinal ganglion cells...

Receptive Fields

photoreceptorsphotoreceptorshorizontal cellshorizontal cells

retinal bipolar cellsretinal bipolar cells}}

retinalretinalbipolar cellsbipolar cells

retinal ganglion cellsretinal ganglion cells}}

retinalretinalganglion cellsganglion cells

lateral geniculate cellslateral geniculate cells}}

laterallateralgeniculate cellsgeniculate cells

primary visual cortex cellsprimary visual cortex cells}}

and so on...and so on...

Receptive Fields

Retinal Receptive Fields

Receptive field structure in bipolar cells

Light

Retinal Receptive Fields

© Stephen E. Palmer, 2002

Receptive field structure in bipolar cells

Receptors

Bipolar Cell

A. WIRING DIAGRAM

HorizontalCells

Direct excitatory component (D)

B. RECEPTIVE FIELD PROFILES

LIGHT

Direct Path

Indirect Path

Indirectinhibitory

component (I)

D + I

Retinal Receptive Fields

© Stephen E. Palmer, 2002