S&P Chapter 2

Light ****A narrow band of electromagnetic radiation that can be conceptualized as a wave or a stream of photons.

• Photon: A quantum of visible light (or other forms of electromagnetic radiation) demonstrating both particle and wave properties.

Wave behaviors

• Absorbed: Energy (e.g., light) that is taken up and is not transmitted at all

• Scattered: Energy that is dispersed in an irregular fashion.• When light enters the atmosphere, much of it is absorbed or scattered andnever makes it to the perceiver.

• Reflected: Energy that is redirected when it strikes a surface, usuallyback to its point of origin.

• Transmitted: Energy that is passed on through a surface (when it is

• neither reflected nor absorbed by the surface).

• Refracted: Energy that is altered as it passes into another medium.

Vitreous humor: The transparent fluid that fills the large chamber in the posterior part of the eye.–

Retina: A light-sensitive membrane in the back of the eye that contains rods and cones. The lens focuses an image on the retina, which then sends signals to the brain through the optic nerve.

Fovea is a small depression within the neurosensory retina where visual acuity is the highest.

Optic disc: the raised disk on the retina at the point of entry of the optic nerve, lacking visual receptors and so creating a blind spot.

• Refraction is necessary to focus light rays onto the retina and this is accomplished by the lens.

  • Accommodation: The process in which the lens changes its shape, thus altering its refractive power.–

  • Presbyopia: Literally “old sight”; the age-related loss of accommodation, which makes it difficult to focus on near objects

Rods are responsible for vision at low light levels(scoptic vision), do not mediate color vision and has low spatial acuity

Cones active at higher light levels (photopic vision), are capable of color vision and are responsible for high spatial acuity

Two Mechanisms for Dark and Light Adaption:

Pupillary dilation is when the black center of the eye, or pupil, widens to allow more light into the eye

photopigments are light-sensitive molecules found within the cone and rod cells of the retina

Light: The amount of photopigment available in photoreceptors changes

over time.

• The more light entering the retina, the faster the photopigments are used up, and the fewer photopigments there are to process more light

Dark: The amount of photopigment available in photoreceptors changes

over time.

• The less light entering the retina, the more slowly photopigments are used up, and the more photopigments there are to process what little light is there

Age-related macular degeneration (AMD): A disease associated with aging that affects the macula. AMD gradually destroys sharp central vision.

– Macula: The central part of the retina containing the fovea.

AMD causes central vision loss, resulting in a blind spot in the visual field called a scotoma.

Retinal Information Processing

Contain a chromophore that captures photons and a protein, called an opsin, whose structure determines the wavelength of light to which the photoreceptor responds

• Rods have rhodopsin.•

Cones have three different opsins, which respond to long, medium or short wavelengths.•

Some photoreceptors contain melanopsin and can monitor ambient light levels and influence our sleep/wake cycl.e

Convergence (information in the retina)

• Divergence (disassociation of information)

Once photoactivation starts, photoreceptors become hyperpolarized (negatively charged).

• Changes in photoreceptor activation are communicated to the bipolar cells in the form of graded potentials.

– Graded potentials vary continuously in their amplitudes.

• Bipolar cells synapse with retinal ganglion cells, which fire in an all-or-none fashion (i.e., actionpotential) rather than in graded potentials.

The retina's horizontal pathway: Horizontal cells are specialized retinal cells that run perpendicular to the photoreceptors and contact both photoreceptors and bipolar cells. These cells are responsible for lateral inhibition, which creates the center-surround receptive field structure of retinal ganglion cells.

• The retina's horizontal pathway: Amacrine cells are specialized neurons that synapse horizontally between bipolar cells and retinal ganglion cells. These cells play important roles in contrast enhancement and temporal sensitivity (detecting changes in light patterns over time).

The retina's vertical pathway:

Bipolar cells form synapses with rods, cones, and horizontal cells, then transmit signals to ganglion cells.

• Diffuse bipolar cells receive input from multiple photoreceptors

while midget bipolar cells receive input from a single cone

The retina's vertical pathway: P ganglion cells connect to the parvocellular pathway and receive input from midget bipolar cells.

Small receptive fields, high acuity, work best in high

luminance situations, sustained firing.

The parvocellular pathway specializes in fine visual acuity, color, and shape processing. It offers excellent spatial resolution but poor temporal resolution.

Small receptive fields, high acuity, work best in high

luminance situations, sustained firing.

• Provide information mainly about the contrast in the retinal image

The retina’s vertical pathway:

M ganglion cells: Connect to the magnocellular pathway.• Receive input from diffuse bipolar cells.

Large receptive fields, low acuity, work best in low luminance situations, burst firing.•

Provide information about how an image changes over time78

Magnocellular pathway is involved in motion processing; excellent temporal resolution but poor spatial resolution.

ON-center ganglion cells: Excited by light that falls on their center and inhibited by light that falls in their surround.•

OFF-center ganglion: Inhibited when light falls in their center and excited when light falls in their surroundings 72