Light and Electromagnetic Radiation

Light and Electromagnetic Radiation

• Light is a form of electromagnetic radiation and becomes visible due to energy produced by vibrations of electrically charged material.
• Light's dual theory explains it behaves as both a wave and stream of particles.
• Light energy is packaged into photons, traveling through space in a wave-like manner.
• Light propagates like a wave.
• Light is absorbed like a particle of energy.

Visible Light and Wave Properties

• Visible light represents a small portion of the electromagnetic spectrum.
• Wavelength (λ) refers to the different wavelengths which produce color perceptions.
• Frequency equates to number of waves per second and is proportional to energy content.
• Amplitude is the difference between the baseline and the peak/trough of a light wave.

Optics and Light Ray Interactions

• Optics is the study of light rays and their interactions.
• Light rays travel in straight lines until they interact with objects/matter.

Reflection

• Light rays bouncing off a surface, most of what we see is reflected light

Absorption

• Light energy is transferred to a particle or surface.
• Black objects absorb all wavelengths.
• Certain compounds only absorb a limited range of wavelengths.

Transmission

• Light passes through a surface without being reflected or absorbed.

Refraction

• Light bends as it travels from one medium to another due to speed differences.

Scattering (Diffraction)

• Light is dispersed in an irregular fashion.

Structure of the Eye

• The sclera continues as a transparent structure containing sensory endings and is refractive.
• The cornea is related to the ciliary muscle and Zonules of Zinn.
• The pupil admits light into the eye.
• The iris controls the amount of light entering the eye.
• The aqueous humor is a transparent fluid.
• The lens is elastic and refractive.
• The vitreous humor is transparent and refractive.
• The retina contains photoreceptors that absorbs light and relays signals.
• The fovea focuses incoming light to form an image.
• The optic disc transmits visual information to the brain with bundled axons.
• The optic nerve transmits visual information to the brain.

Optical Properties of the Eye

• The schematic eye contains the cornea and lens which perform most of the refraction for focusing light on the retina.
• The emmetropic eye is optically normal, and the refractive power of its optical elements matches eyeball length.
• Parallel light rays come from objects positioned from "optical infinity". Objects are more than 6m/20ft away.

Focusing on Near Objects

• For closer/near objects, the refractive power of the cornea and lens isn't enough to converge divergent rays onto the retina.
• Results in blurry image to form.
• Accommodation allows the lens to change its curvature in response to changing object distances.
• For most young people, the limit of accommodation is approximately 10 cm, referring to the near point.

Accommodation and Lens Power

• With accommodation, the lens power can vary.
• Focal distance (f) describes he distance from refractive surface to the point where parallel light rays converge, which is measured in meters (m).
• Diopter (D) refers to a unit measuring the optical power of a lens.
• Lens power (P) is equal to 1/f.
• The distance between the cornea and the retina's focal point = 2.4 cm
• Refractive power of the cornea = ~42 diopters.
• "Thicker" or more curved lenses have more refractive power.

Retinal Images

• Extended objects, not point sources, are usually viewed.
• The retinal image is formed by light rays passing through a nodal point near the lens's back surface.

Refractive Errors

• Hyperopia (farsightedness), occurs when the focal point is behind the retina, the lens lacks sufficient refractive power, or the eyeball is too short
• Myopia (nearsightedness), occurs when the focal point is in front of the retina because the lens cannot flatten enough

Corrective Lenses

• Convex lenses correct hyperopia.
• Concave lenses correct myopia.

Presbyopia

• Presbyopia is age-related hardening of the lens reduces the elasticity of the capsule encircling it that affects everyone.
• Can be treated with convex or bifocal lenses.

The Retina and Phototransduction

• Transduction occurs in photoreceptors in the outermost layer of the retina and these photoreceptors absorb light.
• Two types of cones and rods create human duplex retinas.

Retinal Organization

• The retina is a layered sheet of neurons.
• The outermost layer comprises the photoreceptors.
• Photoreceptor activity stimulates neurons in the intermediate layer, which connect with ganglion cells in the innermost layer
• Photoreceptors are positioned at the back, sending signals to intermediate and ganglion cells.
• Ganglion cell axons leave the eyeball.

Photoreceptor Properties

• Rods have a cylindrical outer segment and Cones have a conical outer segment.
• Rods number around 90 million, while cones number 4-5 million
• Rods are located in the periphery, but Cones are mostly in the fovea.
• Rod has one photopigment, while Cones contain three photopigments (for color vision).
• Rods are specialized for night vision and Cones are specialized for day vision
• Outer segments contain photosensitive GPCR's.
• Inner segments connect the outer segment to a synapse.
• Synaptic terminals display changes in intracellular Ca2+.

Distribution of Rods and Cones

• Cone density is highest in the fovea and they get larger and sparser away, which are specialized for detailed vision
• Fovea is directly behind pupil, subtending a visual angle of ~1 degree
• Rod density is highest in the periphery.
• Fovea, lacking rods, makes us effectively blind in the central 1 degree of visual field under dim illumination, also considered the optic disk.

Retinal Ganglion Cells

• Retinal ganglion cell axons leave the eye at the optic disk.

Light Adaptation: Scotopic, Mesopic, Photopic

• In scotopic (dim) conditions, rods are more efficient than cones at converting photon absorption to neural signals and are active at low light levels.
• In mesopic (intermediate) lighting, both cones and rods are active. Rods are only active at low levels, above photopigment cannot be activated any more.
• In photopic (brighter) conditions, only cones are active and contain mechanisms to prevent bleaching at high light intensities.

Photopigment Regeneration and Visual Acuity

• Bleaching occurs when photopigment absorbs light and cannot regenerate anymore. Therefore, cones adapt to very high brightness levels and do not become bleached when exposed to different illuminations.
• Rods are very sensitive but can get overwhelmed by moderate light levels.
• Cones are less sensitive but have a broader operating range.
• Lots of photopigment presents itself in dim light.
• A photopigment must regenerate after getting bleached (used up) in which molecules must regenerate so not all photons are captured. It takes time, but eventually is regenerated and can reabsorb.
• Pupil size also affects light levels.
• Pupil diameter changes in response to light.
• As the pupil dilates in dim light and contracts in bright light, this influences brightness.
• Ganglion cells respond best to contrast, not diffuse light. These also detect differences in light intensity between centre and receptive field surround.

Dark Adaptation

• A dark adaptation experiment determines the absolute threshold for a small light at various intervals after bleaching (exposure to bright light)
• It takes about 5 minutes for cones to reach their maximum sensitivity.
• By 20-25 minutes the rods have entered their maximum sensitivity
• After about 5 minutes, rods take over to achieve better sensitivity in dim lighting, but recover much more slowly.

Visual Pigments and Phototransduction

• Photoreceptors contain visual pigment composed in the inner segment and stored in its outer segment and each contains a protein and chromophore.
• Protein's structure determines the wavelengths of light a pigment molecule absorbs
• The chromophore is retinal, which absorbs light.
• The first event in phototransduction is the capture of light photons by retinal.
• Cis-retinal converts to trans-retinal upon light absorption, triggering a signalling cascade to cause light perception.

Rhodopsin and Vision

• The amount of light absorbed by a photoreceptor hinges on the intensity/wavelength of the incoming light and can be measures using spectrophotometry by what isn't absorbed.
• Rhodopsin is derived from animal retina and measures light that is not absorbed.
• Absorption spectrum and spectral sensitivity of rod vision is best with 500 nm of bluish green light.
• Human sensitivity to light in dim conditions peaks at 500 nm.

Cone Opsins and Color Vision

• There are 3 types of cone opsins and it all depends on each absorption spectrum.
• Opsin part determines which wavelength each cone can absorb.
• The brain combines inputs from 3 types of cones to create coloured vision.
• Sensitivity function is a combination of multiple cone opsins.

Composition of Cone Photopigments

• Cone photopigments are not distributed equally: 5 - 10% contain short wavelength-sensitive cones and other are equal parts L and M cones
• Photochromatic interval (X): difference between just seeing a light and being able to tell its colour
• Purkinje shift (Y): difference in perceived brightness of objects due to spectral shift
• Photopic sensitivity (reliant on colour) is higher only at very long wavelengths expect in cases of nyctalopia (night blindness)

The Dark Current

• Rhodopsin is inactive in the dark where a molecule called cyclic GMP (cGMP) binds to sodium and calcium ion channels to keep them open.
• The dark current describes the flow of cations into outer segment.
• Potassium leaves the cell through potassium leak channels in the inner segment.
• The Na+/K+ pump maintains concentrations of Na+ and K+ inside and outside of the cell.
• By contrast, the membrane potential of a photoreceptor in the dark is ~40 mV and it is much more depolarized.
• Darkness leads to glutamate release because the neurotransmitter is being constantly released from receptor terminals.

Phototransduction

• Light absorption by retinal changes the rhodopsin conformation. Then, active rhodopsin activates a G-protein called transducin.
• The G-protein activates an emzyme called PDE that breaks down cGMP which ultimately closes cGMP-gated channels

Neural Activity in Photoreceptors

• Because cGMP-gated ion channels are closed, the membrane potential becomes more negative (hyperpolarized).
• This reduces positive charges and generates receptor potentials in every system and leads to depolarization.
• Less glutamate is released due to hyperpolarization of the cell, since depolarization allows Ca2+ channels to open and vesicles to fuse.
• Photoreceptors, bipolar and ganglion cells create a vertical (radial) pathway in the retina

Bipolar and Retinal Ganglion Cells (RGCs)

• Bipolar cells synapse with either rods or cones and pass signals onto retinal ganglion cells (RGCs).
• RGCs are the only neurons whose axons leave the eye that generate action potentials and are part of parallel signals

Periphery and Fovea

• In the periphery, a high degree of signal convergence occurs (100 million photoreceptors vs. 1.25 million ganglion cells.
• In the fovea, a few photoreceptors converge upon a single midget bipolar cell with a roughly 1:1 ratio
• Convergence of signals onto diffuse bipolar cells pool information from photoreceptors. The the more distance from the periphery, the more convergence is needed
• There is lower visual acuity in the periphery because detail is lost during convergence

Visual Processing across the Retina

• Foveal vision allows us to see find details because retinal neurons excluding cones are shifted to allow light to reach the cones without impediment. Additionally, Cones are tightly packed and are connected to bipolar cells in a 1:1 ratio for high spatial sampling
• OFF bipolar cells gets hyperpolarized by light like photoreceptors, while ON bipolar cells are depolarized. The type of cells determines the polarity of the signal.
• Photoreceptors release glutamate, and bipolar cell responses are explained due to presence of different glutamate receptors on cell dendrites.
• Donut shape represents receptive fields of bipolar cells with light polarization and inhibation.
• P-ganglion cells receive input from the midget bipolar cells and go to the parvocellular LGN layers with small receptive fields
• Conversely, M-ganglion cells receive input from diffuse bipolar cells and transmits to the magno cellular LGN layers

Retinal Ganglion Cell Behaviour

• Stephen Kuffler studied RGC response in living cats where Microelectrodes records electrical activity and the signals are amplified onto some monitor.
• All RGC's have certain levels of spontaneous activity and can increase or decrease from light- the retina detects changes in light. So the Field of Retina (or visual field) causes either Inhilation/Exhilation.
• There are Concentric Zones with an Excitatory and Inhibitory balance to maintain a rough circle or antagonism.
• ON-center ganglions have increased or reduced firing rates when light is shined on the center.

Center-Surround Antagonism in RGCs

• Inhibit the Effect of Lighting Center. When light fills entirely, there is no change in fire.
• Antagonism involves the interaction between the center of an RGC's receptive field and a surround region, known as lateralization inhibitation.
• Acts as a Spot filter to detect Contrasts by Detecting the Intensity differences in its center.

Center-Surround Effects

• The lightness of an object will be altered under different conditions.
• Lightness Constancy has the Ambient Illumination, where the object and its surrounding is constat

Mach Bands

• This is caused as the Center and Surround have the same Brightness but firing at different points. The surround will have more light then the center

From Spots to Stripes: Visual Acuity

• Although neural signals must be processed by the brain to create the perceptual experience of seeing, many aspects of visual information is already found within the retina
• Photoreceptors converge onto bipolar cells to signal spatial detail and low light with tradeoffs between resolution and sensitivity
• To measure doctors use the Snellen chart (decreasing in difficulty over several errors) with varying letter sizes for stroke size

Measuring Visual Acuity

• Scientists can measure the angle of the object and its resolution to determine vision. At Viewing Distance, there is an estimated 0.017 for Spatial

Resolution Limits

• Spacial Vision is determined by Photo receptors in our retina. More receptors allow us to pick up Spacial gratings for better precision with cycles per degree

Object Acuity

• Number of repeats per degree
• Variable Freq. Alters Level's of Amplitude Spacial

Frequency Experiment

• Gabor Patch is used to find minimum Amplitude that's needed to measure Spacial threshold. The data of this measures and Repeats Frequency and plotted to Sensitivity

Human Contrast Function CSF

• 1% of Contrast on grey cycles show sensitivity
• Need a certain Limit to sample Space of Width to find range and spacing.

Ganglion Cells Tuned

• Acts as Space Filter - Specific Receptive. Field Size
• Depends on Phase (phase between the grille and position). The path for processing image and the brain cortex cortex

Vision to Brain

• Retina to to laterus (thalamus)
• The Retinas are split into nasal and temporal. Gangion axons travel into Optic Chiasm and Split - cross the Optic Nerve
• Hemisphere (visa versa)

LGN Relay

• Organizational input relay; 2 Layers - Small Cells - P Ganglian- MAGNO cells Each layer is a 6-Structure (2 layer) - Respond (Light & Spots). More connection from lg to forward from retina Maps are Input from (one eye). ObectRight is Visual - layers 4 and 6 - Ordered Map M-ganglion is larger and Faster

Eye Structure

• Ganglion project with parallel Stationary Object
• Super colliculus receives Auditory Input. The Colliculus Integrates
• controls movements that are fovea oriented. Iris adjustment lens

Striate Cortex

• Located in the Ocuipal Lobe. (layer6) Differentiate by layers: Coordinate info with with others, Main Fiber terminate with new horns
• Axons that Communicate

V1 Visual Brain Map

• Mapped with Regions Adjacient Vision. Distorted Visual
• Distorted visual Cortex (fovea Magnification) From lesions: damage to area of observation. Then brain flow shows blankness of the Cortex. Fovial Degree for periphery - degrees in foxation

Cortical Magnification

• Dence Array Photoreceptors and High Revolutions due to the structure or cortex or V1 regions - to fovereate objects with heads/eyes
• Peripheral Vision

Impairment

• Cannot identify - jumbled and surrounded by objects Hubel - Torsten Wiesel; Receptive fields recorded the cortex with a screen

Vision Domincance

• Neuron's Intermingle in Layer 4. (Binocular Stimulatation) Neuron clasity
• Histogram response depends on the right and left eyes

Visual Image Depth

• Binolcularity with more depth gives stimulus Ability

Selectivity

• Elongate Along Bar lines V1's- Spatial Frequency
• Organization Preference for orientation by area A neurons firing depends on its stimulus of the lines

Colour

• Cells in the parvocellular pathway provide information about red-green light
• Cells in the koniocellular pathway provide information about blue-yellow light
• Pathway lead to Blob region.

Cell Class

• Cortical neuron of Inhibition with complex cells insensitive to receptor

Column and Dominance

• Orientation Change In Area - Horizontal

Object Recognition

• Neurons Response Shows Pathway - Border cells and illusionary Contours - Stimull set and v4 (complex attributes)

Recognition

• Neuron has high - select - for shape. Rules operate a committee model. The visual system uses principles such "honor physics" or just looks in different ways

Form Signal

• Signal can go through process of re-entran, a form of highlow - can be canibalistic to make

Visual Object Theory

• Low can be the visual field vs know or feel or "what and what not" theory

Kantiza Figure

• Contour in visual has made guess due to the world around it, such as occlusion

Committee and Rule

• Rules operate like a commitee on its way of high knowledge.

Visual Search Task vs Bayes

• Avoid interpretation- with specificity. See is to seek consensus from bias.
• Bayesian - mathematical model - world and state.

Object Recognition

• Proffressive cells sensitive, in other location's sensitive cortex and stimuli, they can make form basis

Functional Brain

With Imaging (fRMI) show identity

• Activity in scan
• Categorial with activity

Neural Networks

• Recpgnizing and Building objects has led to building blocks and solving for problems of what you can identify and all of their shapes and textures - not only touch.

Computer - Al

The process that has brought computers to be more advanced with processing to look and extract information of neurons based on cells, a system - can determine like Cow

Faces vs The Field

• Faces have high Inversion vs Inverting and are easier to detect with familiarization of details - don't jumble unlike other objects with contrast