Physiology exam 2 - Vision

Law of specific nerve energies

the nature of a sensation is determined by the specific nerve pathway activated, not the nature of the stimulus itself. 

Light to brain pathway

Pupil → cornea → focused by the lens → visual/photo receptors → bipolar cells → ganglion cells → optic nerve

fovea

central portion of the retina that allows for acute and detailed vision

• direct line to the brain

fovea receptors

• almost exclusively cones

• attaches to a single bipolar cell and a single ganglion cell known as a midget ganglion cell

periphery of the retina

• primarily rods

• detailed vision is less

  • better with fainter light

Cones

• only attach to one bipolar cell

• best with bright lights and colors

• detailed

• not as abundant (6 million)

rods

• can have multiple rods connect to one ganglion

• good low light vision

• poor detail

• no color

• most abundant (120 million)

photopigments

rods and cones (activate rhodopsin and opsin to start process of light perception)

photopigments process

light strikes the receptor → 11-cis-retinal reacts and is converted into all-trans-retinal → second messengers activated (opsin)

Color Vision

• wavelength

• wavelength between 400-700 nanometers

trichromatic theory/Young-Helmholtz theory

The theory suggests that the human eye has three types of photoreceptor cells, or cones, that are most sensitive to different wavelengths of light: short (blue), medium (green), and long (red). 

opponent-process theory

we perceive colors through three opponent channels: red/green, blue/yellow, and black/white (luminance), where activation of one color in a pair inhibits the other, explaining phenomena like afterimages and color blindness. 

issues with the two main vision theories

color constancy: ability to recognize color despite changes in lighting

retinex theory

the cortex compares information from different parts of the retina to determine the brightness and color for each are

Color Vision Deficiency

• gene responsible on X chromosome

• lack of cone or cone with abnormal properties

  • most common: red/green

Lateral Geniculate Nucleus

• part of the thalamus

• specialized for visual perception

• connects to other parts of the thalamus and occipital cortex

The Receptive Field

Part of the visual field that excites or inhibits a cell in the visual system of the brain

visual field for a receptor

point in space in which light strikes it

3 categories of primate ganglion cells

• parvocellular

• magnocellular

• koniocellular

parvocellular neuron

• located in or near the fovea

• smaller cell bodies and small receptive fields

• highly sensitive to detect color and visual detail

magnocellular neuron

• distributed evenly throughout the retina

• larger cell bodies and visual fields

• highly sensitive to large overall patterns and moving stimuli

koniocellular neuron

• small cell bodies

• found throughout the retina

• several functions, axons terminate in diff. places

Primary Visual Cortex (V1)

receives info from lateral geniculate nucleus and is responsible for the first stage of visual processing

damage to V1

can cause blindsight

Visual Cortex Cells

• feature detectors: neurons whose response indicates the presence of a particular feature/stimuli

• prolonged exposure decreases sensitivity

Stereoscopic Depth Perception

• perceiving distance by comparing inputs from eyes

• ability shaped through experience

retinal disparity

discrepancy between left and right eye sees

astigmatism

blurring of vision for lines in one direction caused by asymmetric curvature of the eyes.

Ventral Stream

• temporal cortex

• “what” path

• identifying objects

dorsal stream

• parietal cortex

• “how” path

  • important for visually guided movements

ventral stream damage

can see where objects are but not identify

dorsal stream damage

can identify objects but not know where they are

visual agnosia

inability to recognize objects despite satisfactory vision that is caused by damage usually to the dorsal stream

prosopagnosia

inability to recognize faces caused by damage to the fusiform gyrus of the inferior temporal cortex

motion perception

involves all four lobes of the cerebral cortex

motion blindness

inability to determine the direction, speed, or movement of an object caused by damage in area MT. Sometimes blind except for the ability to detect direction becuase MT still gets visual input but damage to V1