Lecture 20 - Vision I

March 10th

What type of energy is transduced during hearing?

Mechanical

Which speech/language representations are in the correct order from smallest to largest?

Phonemes, Morphemes, Words, Sentences

Which type of aphasia is traditionally associated with damage to the inferior frontal gyrus?

Broca’s (nonfluent) aphasia

Visible light is a narrow band of electromagnetic radiation…

Wavelength = length between two peaks in a repeated stimulus such as a wave, light, or sound

Photon = quantum of electromagnetic energy in the range of wavelengths we call light

Light is an amalgamation of the two

Why so narrow?

Higher energy past ultraviolet rays, high energy radiation penetrates through your skin. Leaves trails, we don’t see them because they’re too energetic for biological creatures to not be destroyed by them

• Visible light we see covers the most light that reaches the Earths surface from the Sun

• Wavelengths longer than visible light would require much larger cellular structures to capture with the same acuity

• We don’t get many gamma rays because the atmosphere gets rid of it, protected

The structure of the Eye

Cornea = transparent outer layer of the eye, whose curvature is fixed

Pupil = the aperture, formed by the iris, that allows light to enter the eye

Iris = circular structure that provides an opening to form the pupil

Lens = structure that helps the focus an image on the retina

Ciliary muscle = controls the shape of the lens

Accommodation = process of focusing by the ciliary muscles and lens

Retina = the receptive surface inside the eye that contains photoreceptors and other neurons

Cornea

Transparent outer layer of the eye, whose curvature is fixed

• First structure to focus the eye

Pupil

The aperture, formed by the iris, that allows light to enter the eye

• Hole created by the iris, not empty space behind it, over the lens

Iris

Circular structure that provides an opening to form the pupil

• Colored part of the eye, limits amount of light, makes pupil larger/smaller

Lens

Structure that helps the focus an image on the retina

• Key to focus light on to back of the eye, somewhat malleable, near sighted or far sighted lens isn’t as flexible (focus of different distances)

Ciliary muscles

• Controls the shape of the lens

Accommodation:

Process of focusing by the ciliary muscles and lens

Image projected to back of the eye is reversed, light gets bent by the lens. Upside down and mirror reversed

Retina

The respective surface inside the eye that contains photoreceptors and other neurons

Anatomy of the Retina

Layered structure, very awesome, very complex, so many more photoreceptors than bits of information leaving the eye. SO much neural processing during the eye before it gets to the brain.

Pigmented Epithelium = cell layer that reduces scattered light in the eye

Rod and Cone cell layer = photoreceptors, neural cells that respond to light

Pigmented Epithelium

Cell layer that reduces scattered light in the eye

Back most of the eye, absorbs so that light doesn’t create crazy images

Bipolar Cells

Interneurons that receive information from the rods and cones

• Form the middle layer

Horizontal Cells

Contact both the photoreceptors and bipolar cells creating lateral connections

• Also form the middle layer, involves many different photoreceptors

Ganglion Cells

Class of cells in the retina whose axons form the optic nerve

• Proper neurons, have axons that come out of eye and send info back into the brain

Amacrine Cells

Contact both bipolar and ganglion cells and are especially significant in (lateral) inhibitory interactions

Photoreceptor Structure

We have two types of photoreceptor cells - rods and cones

Photoreceptors don’t send action potentials; they have graded potentials that regulate the amount of neurotransmitter (glutamate) they release

The discs in the outer segments contain the photopigment proteins

How is light transduced in rods?

Rods and cones contain photopigment receptor molecules

In rods, the photopigment is called rhodopsin (hit when light it changes shape and creates cascade of chemical reactions)

Cones have similar photopigments

Details about trandsuction in vision

• Even small amounts of light can cause activation of a large amount of molecules - this allows weak signals to be amplified (500 molecules of different protein get release)

• The main effect of all this is to close sodium channels and hyperpolarize the photoreceptor cell (they work in a counterintuitive way, when light enters cell, they close sodium channels)

Light hyperpolarizes the photoreceptor cell

When light enters the cell, it causes the potential to becomes more negative than it already was (hyperpolarization)

This causes it to release less neurotransmitter than it was already releasing

The brighter the light, the greater the hyperpolarization, the less neurotransmitter released

Less glutamate gets released, greater sensitivity to light and more adaptability

Visual System Characteristics

The cascade of processes required to stimulate the visual receptors helps account for three characteristics of the visual system:

1. Its sensitivity, because weak stimuli are amplified to produce physiological effects (one photon blocks millions of Na+ ions)

2. The integration of the stimulus over time, which makes vision relatively slow (compared, for example, with audition) but increases its sensitivity

3. The adaptation of the visual system to a wide range of light intensities

Two Functional Systems of the Retina

Photopic system = high levels of light, sensitivity to color, involves cones

Scotopic system = low levels of light, involves rods

Photopic System

Receptors - Cones

Approximate # of Receptors per eye - 4 million

Photopigments - Three classes of cone opsins; the basis of color vision

Sensitivity - Low; needs relatively strong stimulation; used for day vision

Location in retina - Concentrated in and near fovea; present less densely throughout retina

Receptive field size and visual acuity - Small in fovea, so acuity is high; larger outside fovea

Temporal responses - Relatively rapid

Scotopic system

Receptors - Rods

Approximate # of Receptors per eye - 100 million

Photopigments - Rhodopsin

Sensitivity - High; can be stimulated by weak light intensity; used for night vision

Location in retina - Outside fovea

Receptive field size and visual acuity - Larger, so acuity lower

Temporal responses - Slow

Photoreceptor adaptation

Tendency of rods and cones to adjust their light sensitivity to match the ambient levels of illumination

The visual system is concerned with differences, or changes, in brightness, not the overall level of illumination

This is part of why we can see over such wide ranges of brightness

Distribution of rods and cones

Rods and cones are distributed unequally across the retina

Cones (in red) are concentrated at the fovea, the central portion of the retina

Optic Disc = region on the retina without any receptor cells because that’s where ganglion cell axons and blood vessels exit the eye

Blind Spot = portion of the visual field corresponding to the optic disc

Visual acuity reflects the distribution of cones

Visual Field = the whole area you can see without moving your eyes

Visual Acuity = the sharpness of vision

Saccades = fast movements of the eyes that present various parts of the visual scene to the fovea

Structure of the Fovea

Spreading apart of bipolar, horizontal, amacrine, ganglion, to get to the photoreceptors cells

Perception of Brightness

Sharpness and differentiates things

Lateral Inhibition

When neurons in a region are interconnected, either through their own axons or by means of interneurons (*cough* horizontal cells *cough*) and each neuron tends to inhibit its neighbors

This allows contrasting edges to be emphasized at the level of the retina

Right at the retina, to enhance

Emphasize on differences, no emphasize on sameness, need to see tiny differences