Ch.6 Pt.1 Flashcards
Wavelength
The property of light that allows us to perceive color
What 2 things can light be thought of as?
- Particles of energy (photons)
2. Waves of electromagnetic radiation
Intensity
The property of light that allows us to perceive brightness
Humans see light between _____ to ____ nanometers
380- 760
What does light enters the eye through?
The pupil
Sensitivity
The ability to see when light is dim (pupil dilates)
- decrease in depth of focus
Pupil size changes in response to what? Between what two things does it allow a dynamic compromise for?
Changes in illumination
- pupil adjustment allows for a dynamic compromise between SENSITIVITY and ACUITY
Acuity
The ability to see details (pupil constricts)
-Increase of in depth focus
What does the Lens and Ciliary Muscles do?
- Focuses light onto the Retina
- Alter the shape of the lens as needed
Describe the process of Accommodation to do with the Lens
The process of adjusting the lens to bring images into focus. Problem with accommodation are usually fixed by wearing glasses
What’s a common disease of the lens?
Cataracts (eyes looks foggy)
- develops as a result of again, metabolic disorders, trauma or heredity
Normal vision occurs when?
When light is focused directly on the Retina rather than in front or behind it
Nearsightedness occurs when?
Visual image is focused in front of the Retina
Farsightedness occurs when?
Visual image is focussed behind the Retina
EYE POSITION AND BINOCULAR DISPARITY
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Convergence
Eye must turn slightly inward when objects are close (greater when objects are close together. They provide brain with 3D image and distance information)
Binocular Disparity
The differences between the images on the 2 Retinas (greater when objects are close together. They provide brain with 3D image and distance information)
The Retina
- Inside out
- Light passes through several cell layers before reaching its receptors
Verticle Pathway of the Retina?
Receptors > bipolar cells > retinal ganglion cells
Lateral communication of the Retina
- Horizontal Cells: interactions between receptors (rods/cones)
- Amacrine Cells: interaction between bipolar cells
FIND OUT MORE
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What are the 2 ways Retinal Neurons communicate?
- Chemically via synapse
2. Electrically via gap junctions
What are the 2 challenged with this “inside out” organization?
- Light is distorted while passing through these cells before they refract forward of the back of the eye
- The axons of the Retinal Ganglions Cells (the optic nerve) need to leave the eye.. Therefore there needs to be a gap in receptors - this is the Blind Spot
Solution to the distortion of light as it travels through the layers of the retina is?
The Fovea: high acuity area at the centre of the retina
- thinning of the ganglion cell layer reduced distortion due to cells between the pupil and the retina
What is the spot called where information exits the eye?
The blind spot
- the visual system uses information from cells around the blind spot for “completion”, filling in the blind spot
What is the Duplexitry Theory of Vision?
Cones and Rods mediate different kinds of vision. Differing in 2 ways: Through Convergence and Divergence
Cones
Photopic (daytime) vision
- High acuity, colour information, used in good lighting
- Low convergence
- More Cones situated in the middle - only cones at the fovea
Rods
Scotopic (nighttime vision)
- High - sensitivity, allowing for low-acuity vision in dim light, but lack detail and colour information
- High convergence
- More rods laterally
SPECTRAL SENSITIVITY
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Why can Wavelength also influence our perception of brightness?
Because our visual system isn’t equally sensitive to all wavelengths of light
Photopic (cone) and Scotopic (rod)
There are different spectral sensitivity curves for these 2 forms of vision
Purkinje Effect
- During the day photopic vision is predominant - the sensitivity of these rods is maximal in the yellow range
- During the night scotopic vision is predominant - the sensitivity of these cones is maximal in the blue green range
Despite the distribution of cones, crisp colour vision isn’t limited to the centre of our visual field. Why?
- We continually scan the world with small and quick eye movements: saccades
- These bits of information are then integrated
- The visual system responds to change. Making these eye movements critical for our visual processing. If the image projected to the retina remains stable in space objects start ‘disappearing’
What is Transduction?
Conversion of one form of energy to another
What is Visual Transduction?
Conversion of light to neural signals by visual receptors
What do Pigments do?
Absorb light
What is Rhodopsin?
A photopigment that absorbs light maximally at ~ 500 nm.
- Presence of rhodopsin in cones explains their spectral sensitivity distribution
- Pigment found in rods
A G-protein-linked receptor that responds to light rather than to neurotransmitters ( Interacts with Na+ channels
- In the dark (constitutive release): Na+ channels remain partially open (partial depolarization), releasing glutamate
- When Light Strike (inhibitory): ‘Breaks down’ cGMP that is holding the Na+ channels open
- Na+ channels close
- Rods become hyperbolized
- Glutamte release is reduced
Visual Field In the Retina
- The left visual field falls upon the nasal retina of the left eye, and the temporal retina of the right eye
- Only nasal retinal ganglion cells of each eye send their axons across the midline, forming the OPTIC CHIASM
What projects where?
The right visual projects tot he left hemisphere and the left visual project to the right hemisphere
Name 2 significant of the Retinal-geniculate-striate pathways
- Sends signals from the retina to primary visual cortex
2. 90% of retinal ganglion axons makes up this projection
What does the nasal hemiretina of each eye do?
It bifurcates at the optic chasm then projects to LGN
What does the temporal hemiretina of each eye do?
Remains ipsilateral then projects to LGN
Where do most LGN neurones that project to primary visual cortex (V1, striate cortex) terminate?
In the lower part of cortical layer IV (4)
Lateral Geniculate Nucleus
- It is composed of 6 layers
- 3 from the contralateral portion of the respective visual field
- And 3 from the ipsilateral respective visual field
Retinotopic Organization
Literally the LGN is like a map of the reina
- Information received at adjacent portions of the retina remains adjacent in the LGN the optic radiation and the striate cortex (reintopic)
More cortex is devoted to ares of what?
High acuity - like the disproportionate representation of sensitive body parts somatosensory cortex
What percent of the primary visual cortex is dedicated to the fovea?
25%
The LGN is divided into what 2 streams?
- The Dorsal 4 layers which are called the P layers
- The Ventral 2 layers which are called the M layers
The P Layers
The cells in these 4 layers (3-6) are very small - they are called parvocellular
The M layers
The cells in these 2 layers (1-2) are large hence the cells there are termed magnocellular
What does M Layer stand for?
- Magnocellular Layer (1-2)
- Large cells
- Large receptive fields
- Responsive to movement
- Primarily innervated by RODS
What does the P Layer stand for?
- Parvocellualr Layer (3-6)
- Small Cells
- Small Receptive Fields
- Responsive to colour, fine details, and stationary objects
- Primarily innervated by cones
