Chapter 2-Light Energy to Neural Signals Flashcards
Light
Form of electromagnetic radiation that can be described as a wave of photons
Light waves
Colorless but provide color info to the visual system
-Light Waves vary in
–Wavelength
–Amplitude
Parts of the eye
-Cornea and sclera
-Aqueous and Vitreous Humors
-Crystaline Lens
-Pupil
-Iris
-retina
-Optic Nerve (Cranial Nerve II)
Refraction
Focuses light onto the retina
Retina
Contains light sensitive photoreceptors
-Fovea
-Periphery
-Optic disc
-Where light energy is transduced into neural energy
-Done through rods and cones
Retina contains
-Bipolar cells
-Ganglion cells
Rods v. Cones
Rods-Dark vision
Cones-Light vision
Types of cones
S-cones- Short wavelengths
M-cones- Medium wavelengths
L-cones- Long wavelengths
Duplex Retina
Division of labor between rods and cones which is shown through dark adaptation
Dark adaptation- Absolute luminance threshold decreases, flattens, then decreases again
Photoactivation
Capture of photons by the photoreceptors
How it works:
1. Ions on light wave
2. On outer segment there are opsin proteins
–Rhodopsin (Rods)
–Melanopsin (Cones)
3.On the proteins there are retinals that branch off
4. Particle of light goes in and strikes a retinal which causes it to detach
5. This stimulates an opsin protein
6. If enough break off it causes an action potential
-Rods can be triggered by one photon
Bipolar cells
Consolidate info from the photoreceptors and send signals to the ganglion neurons
Two types of Bipolar cells:
- Diffuse- Found in the periphery of visual field (Rods)
- Midget- Found in fovea (Center of visual field) (Cones)
Ganglion Cells
-Ganglion axons form the optic nerve
-Last layer before visual info is sent to the brain
-Two types:
–P-ganglions- Parvocellular pathway (Monitors fovea)
(Cones to Midget Bipolars to P-Ganglions)
–M-ganglions- Magnocellular pathway (Monitors Periphery)
(Rods to Diffuse Bipolars to M-Ganglion)
-P-Ganglion has small receptive field while M-Ganglion has much wider receptive fields
Center-surround Receptive Fields
-In each ganglion there is an on and off area
-Neuron is excited if light stimulates ON area
-Neuron is inhibited if light stimulates OFF area
-This phenomenon is called lateral inhibition (Stimulation of receptors at one level can suppress stimulation of receptors at the same time)
Visual Pathway
- Eye
- Photoreceptors
- Bipolars
- Retinal ganglions
- Lateral Geniculate Nucleus
- Striate Cortex
- Extrastriate Cortex
- Beyond Occipital Lobe
Optic chiasm and visual fields
-Optic Nerve crosses at optic chiasm
-Visual fields cross over and are processed on the contralateral hemisphere
-Right visual field —> Left hemisphere
-Left visual field —> Right hemisphere
Lateral Geniculate Nucleus
-First synapse of the optic nerve
-One LGN in each hemisphere
- Each has six layers in a retinotopic layout
-Magnocellular layers process the periphery
-Parvocellular layers process the fovea
-Visual field in received upside down and mirrored in the brain
-Cortical magnification of the fovea relative to the periphery
Primary Visual Cortex(V1, Striate Cortex)
-First area of the cerebral cortex to process visual information
-Retinotopic layout and cortical magnification are maintained
-Elongated and striped receptive fields
V1 Neurons
-Receptive fields are linearly arranged on retina
-Respond best to straight lines of light
-V1 neurons detect straight edges
Types of V1 Neurons
- Simple
-Small receptive fields - Complex
-Larger
-Sensitive to motion in receptive field - End stopping
-Increase firing rate based on how much the line takes up the receptive field
-Size sensitive to a point
-Size perception
Orientation tuning curves
Show V1’s have a “preferred” orientation, but will respond to other orientations
Shows the firing rate of a single neuron to various orientations
Selective Adaptation
Reduction in neural activity to specific stimuli
Tilt Aftereffect
-Human example of selective adaptation
-Illusion of tilt produced by adapting to a pattern of a given orientation
