Chapter 2 - Light to Neural Signals

Wave

Oscillation that travels through medium by transferring energy from one article / point to another without causing permanent displacement of medium

Photons

Quantum of visible light / other form of electromagnetic radiation demonstrating particle and wave properties

Hue

Perceptual attribute of colors that enables them to be classed as similar to red, green, blue, or something in between

Transmitted

To convey something from one place or thing to another

• Light moving through a glass window

Refracted

• Alter course of wave of energy that passes into something from another medium

  • Like light entering water vs air

  • Light being focused by a lens

Eye

Have array of neurons and light-sensitive photoreceptor cells that transduce light into neural signals (action potentials)

Optic Nerve

Axons from later layer of neurons in retina emerge from eyeball through optic disk to form this

• Carries visual signals to rest of brain

Optic Chiasm

Neurons from two eyes cross here

Lateral Geniculate Nucleus (LGN)

Primary relay station for info flowing from eye to brain

• Axons leaving eye finally synapse with new neurons

• Axons coming in are reorganized and info is sent off to primary visual cortex

Superior Colliculus

Involved in eye movements

• Mostly evolutionarily stagnant

Optic Radiation

• Axons that synapse in LGN fan out here on way to primary visual cortex

• Used for stereoscopic depth perception

Striate / Primary Visual Cortex

• Neurons from LGN synapse here then branch out, communicating to every other part of cortex

• 40% directly involved in processing visual information

Focusing structures in the eye

Where light is transmitted and refracted

The Retina

Where light is absorbed and transduced into neural energy

Visible Light

Tiny portion of electromagnetic spectrum

Cornea

Transparent “window” into eyeball

Aqueous Humor

Watery fluid in anterior chamber of eye

Lens

Structure inside the eye that enables the changing of focus

Pupil

Dark, circular opening at center of iris in the eyes where light enters eye

Iris

• Colored part of eye

• Regulates light entering the eye by expanding and contracting the pupil

Vitreous Humor

Transparent fluid that fills chamber in posterior part of eye

Accommodation

Process where eye changes its focus

• Lens gets fatter as gaze is directed toward nearest objects

Focal Distance

Distance between lens (or mirror) and the viewed object

Presbyopia

• “Old Sight”

• Age-related loss of accommodation

• Makes it difficult to focus on near objects

Cataracts

Opacity of the crystalline lens

Emmetropia

• Condition where there is no refractive error

• Refractive power of eye is perfectly matched to length of eyeball

Refractive Errors

• Common disorder where image of world is not clearly focused on retina

• Include myopia, hyperopia, astigmatism, and presbyopia

Myopia

• Nearsightedness

• Light entering eye is focused in front of retina and distance objects cannot be seen sharply

Hyperopia

• Farsightedness

• Light entering eye is focused behind retina and accommodation is required to see near objects clearly

Astigmatism

Visual defect caused by unequal curving of 1+ refractive surfaces of eye, usually cornea

Transduced

• Converting one form of energy to another

  • Light —> neural electrical energy

  • Mechanical movement —> neural electrical energy

• Neurons use electrical signals in their communication

Fundus

Back layer of retina

• What eye doctor sees through ophthalmoscope

Photoreceptors

Light-sensitive receptor in the retina

Fovea

• Small pit located near center of macula containing highest conc. of cones NO rods

• Portion of retina that produces highest visual acuity

• Serves as the point of fixation

Macula

• Pigmented region near center of retina

• In vestibular system:

  • Specialized detectors of linear acceleration and gravity found in each otolith organ

Rods

• Photoreceptor specialized for night vision

• Absent in fovea

• Common in periphery

Cones

Photoreceptor specialized for daylight vision, find visual acuity, and color

• Only photoreceptor in fovea

Eccentricity

• Distance between retinal image and fovea

• Angular distance from fovea (region of highest visual acuity)

Duplex

Consists of rods and cones, which operate under different conditions

Visual Angle

Angle that an object subtends at the eye (retina)

Photoreceptor Type

• Fovea

  • Mostly cones

• Periphery

  • Mostly rods

Bipolar Cell Type

• Fovea

  • Midget

• Periphery

  • Diffuse

Convergence

• Fovea

  • Low

• Periphery

  • High

Receptive-Field Size

• Fovea

  • Small

• Periphery

  • Large

Acuity (Detail)

• Fovea

  • High

• Periphery

  • Low

Light Sensitivity

• Fovea

  • Low

• Periphery

  • High

Macula

In retina, area under fovea and has slightly yellowish appearance

Blind Spot

Area of retina where optic nerve leaves the eye and blood vessels enter the eye

Receptive Field

Region on retina where visual stim influence neuron’s firing rate

Age-Related Macular Degeneration

Aging that affects macula

• Destroys sharp central vision, making it hard to read, drive, and recognize faces

• Two forms: wet and dry

Retinitis Pigmentosa

• Progressive degeneration of retina that affects night vision and peripheral vision

• Runs in families and can be cased by defects in different genes

Signals

Generated by photons absorbed from the star

Noise

Generated by spontaneous electrical events

Scotopic

Rod conditions

Photopic

Cone conditions

Rhodopsin

Single photopigments in rods

• Capture quanta with particular wavelength

Horizontal Cells

Cells responsible for lateral inhibition in the retina

Retinal Ganglion Cells

Cells forming the optic nerve that leaves the eye

On-Center Ganglion Cell

Type of cell that best responds to bright circle surrounded by a ring of darkness

Firing Rate of Cell Doesn’t Change

What happens if a spot of light covered both ON and OFF regions of retinal ganglion cell’s receptive field

Outer Segment

Part of a photoreceptor that contains photopigment molecules

Inner Segment

Part of photoreceptor that lies between the outer and the cell nucleus

Synaptic Terminal

Location where axons terminate at the synapse for transmission of information by release of a chemical transmitter

Chromophore

Light-catching part of visual pigments of the retina

Rhodopsin

Visual pigment found in rods

Melanopsin

Photopigment that is sensitive to ambient light

Hyperpolarization

Change in membrane potential such that inner membrane surface becomes more negative than the outer membrane surface

Graded Potentials

Electrical potential that can vary continuously in amplitude

Horizontal Cells

Specialized retinal cell that contacts both photoreceptor and bipolar cells

Lateral Inhibition

Antagonistic neural interaction between adjacent regions of the retina

Amacrine Cells

Retinal cell found in the inner nuclear layer that makes synaptic contacts with bipolar cells, ganglion cells, and other amacrine cells

Bipolar Cell

Retinal cell that synapses with either rods or cones and with horizontal cells and the passes the signals on to ganglion cells

Diffuse Bipolar Cell

Retinal cell whose processes are spread out to receive input from multiple cones

Visual Acuity

Measure of the finest detail that can be resolved by the eyes

Midget Bipolar Cells

Small cell in the central retina that receives input from a single cone

ON Bipolar Cell

Cell that depolarizes in response to an increase in light captured by the cones

OFF Bipolar Cell

Cell that hyperpolarizes in response to an increase in light captured by the cones

Ganglion Cells

Retinal cell that receives visual information from photoreceptors via intermediate neuron types and transmits information to the brain and midbrain

P Ganglion Cells

Receives excitatory input from single midget bipolar cells in central retina and feeds the parvocellular layer of the lateral geniculate nucleus

M Ganglion Cells

Resemble a little umbrella that receives excitatory input from diffuse bipolar cells and feeds magnocellular layer of LGN

Koniocellular Cells

Neuron located between magnocellular and parvocellular layers of LGN

ON-Center Cell

Cell that increases firing in response to an increase in light intensity in its receptive-field center

OFF-Center cells

Cell that increases firing in response to a decrease in light intensity in its receptive-field center