02_Light & Eyes.pdf

Light & Eyes Light Light: part of the electromagnetic spectrum that human can perceive Perceptual dimension of brightness = intensity of light Perceptual dimension of colour = dominant wavelength of light ELECTROMAGNETIC SPECTRUM LIGHT INTERACTIONS When light waves strike an object, they...

Light & Eyes Light Light: part of the electromagnetic spectrum that human can perceive Perceptual dimension of brightness = intensity of light Perceptual dimension of colour = dominant wavelength of light ELECTROMAGNETIC SPECTRUM LIGHT INTERACTIONS When light waves strike an object, they can be: Absorbed: When light waves are absorbed by a material, their energy is taken in and converted into heat or another form of energy Example: Black clothing absorbs sunlight, making it warmer than white clothing Scattered: Light waves are deflected in multiple directions as they pass through a medium or encounter small particles Example: The blue colour of the sky is due to the scattering of sunlight by not travelling in a straight line, with blue light (short waves) bounce off more than other colours. Reflected: Light waves bounce off a surface at the same angle as they hit it, returning to the original medium Example: the light waves will fall upon an object, some waves will be absorbed and others will be reflected; the reflected waves give its colour Transmitted: Light passes through a material without being absorbed or reflected Example: Sunlight transmitting through a clear window pane, or coloured windows preferentially transmitting a specific light wave Refracted: Light waves change direction when passing from one medium to another, due to a change in speed Example: A straw appears bent when placed in a glass of water because of the refraction of light at the water surface The Eye Field of view is limited ◦ But can be controlled using the 3 pairs of muscles ◦ Inferior/Superior rectus (Up & Down) ◦ Medial/lateral rectus (Left & Right) ◦ Inferior/Superior oblique (rotation of eyeball) ANATOMY Sclera: white of the eye; forms a tough, protective, coating Cornea: transparent membrane at the from of the eye and is where light enters Iris: coloured part of the eye and is a muscle that controls the size of the pupil Pupil: opening in the middle of the iris which controls the amount of light entering the eye - bright light = contracts - Dim light = fully relaxed/dilated Lens: controls how much light is refracted/bent, focusing the image on the retina - controls how light bents to focus on something close or far away Retina: where the photoreceptor cells responsible for sensory transduction are located FOCUS - 80% of refraction is done by cornea & aqueous liquid - 20% is done by the lens Accommodation of the lens: result of the ciliary muscles around the lens tightening, causing the lens to thicken Causes increased bending of the light allowing focus on near objects Thin = ciliary muscles are relaxed = focus on far objects Thick = ciliary muscles are contracted = focus on near objects REFRACTIVE PROBLEMS Emmetropia: normal refraction; focus is on retina 1. Myopia: inability to see far objects; focus is in front of the retina 1. Hyperopia: inability to see close objects; focus in behind the retina AGING As we age, the lens become less elastic Presbyopia: Closest point we can focus on (near point) becomes further away Eyes that see light Using an opthalmoscope (fundus camera), we can look at the back of the eye (fundus) Retina 3 main nuclear layers 1. Ganglion cell layer 2. Inner nuclear layer 3. Outer nuclear layer Photoreceptors (rods & cones) 2 synaptic layers 1. Inner synaptic layer 2. Outer synaptic layer PHOTORECEPTORS Photoreceptors are in the outermost layer of the retina Light passes through layers of other neurons before reaching photoreceptors Where transduction occurs Rods: allow to see in low-light conditions Very sensitive No contribution to colour vision Cone: provide high-acuity colour vision in bright (daylight) conditions Less sensitive Types of cones: Short wavelength/blue cones Medium wavelength/green cones Long wavelength/ red cones ◦ Concentrated at the center of the retina (fovea) Spectral Sensitivity 1. S-cones (443 nm wavelength) 2. Rods (500 nm) 3. M-cones (543 nm) 4. L-cones (574 nm) Sensory transduction Photopigments in the rods and cones convert light to a neural code 1. Photopigment absorbs a photon 2. Photopigment changes shape 3. Change in the photoreceptor membrane potential 4. Above change modulates the firing of action potentials Distribution of photoreceptors High [ ] of cones / no rods at fovea Higher [ ] of rods away from fovea Lower [ ] of cones away from fovea Optic disk/blindspot: no photoreceptors because that is where the optic nerve is Diseases of the retina RETINIS PIGMENTOSA: rare hereditary disease which affect rods at first, resulting in night-blindness Also attacks foveal cones in severe cases causing loss of vision MACULAR DEGENERATION: disease that destroys the macula, central area of the retina that includes the fovea, creating a blindspot on the retina Most common in older people Retina Neurons in retina do more than just capture light Neural processing and interpretation of our visual world begins with arrange and connections between neurons in the retina Dynamic range: ability to see well in bright sunshine and dimly-lit spaces - size of the pupil controls how much light enters the eye - Bright light: photopigments get used up faster than they regenerate = fewer able to process light - Dim light: photopigments get used up slower than they regenerate = more left to respond to photons NEURONS IN THE RETINA 1. Rodes & Cones 2. Horizontal cells 3. Bipolar cells 4. Amacrine cells 5. Ganglion cells  THE HORIZONTAL PATHWAY Horizontal cells allow lateral connections between rods or cones - responsible for lateral inhibition Amacrine cells allow lateral connections between bipolar cells or ganglion cells - involved in contrast enhancement and temporal sensitivity THE VERTICAL PATHWAY - TRANSMIT INFO TO CNS 1. Rods and Cones connect to the Ganglion cells through the Bipolar cells 2. Bipolar cells receive input from 1+ cones or many rods & horizontal cells passes it on to the ganglion cells - Diffuse Bipolar cells = receive input from multiple photoreceptors - Midget Bipolar cells = receive input from single cones in the fovea - All Synapse onto ganglion cells 3. The axons of Ganglion cells form the optic nerve - P ganglion/midget ganglion cells receive input from midget bipolar cells and connect to the parvocellular/small cell pathway - Involved in visual acuity, colour & shape perception - Good spatial/poor temporal resolution - M ganglion/parasol ganglion cells receive input from diffuse bipolar cells and connect to the magnocellular/large cell pathway - Involved in motion perception - Good temporal/poor spatial resolution Receptive elds CONVERGENCE OF PHOTORECEPTORS More convergence of rods than cones ◦ ~ 50 rods synapse onto each bipolar cell 1 single rod is more sensitive to light than a cone = Greater sensitivity = less spatial resolution ◦ ~ 6 cones synapse onto each bipolar cell In fovea, each cone synapse onto a single midget bipolar cell = Greater acuity fi RECEPTIVE FIELDS Receptive field [of a neuron]: area on the retina for which stimuli affect that neuron's firing rate Each ganglion cell axon in the optic nerve will respond to a specific location on the retina ◦ Midget: small receptive fields, high acuity Work best in high luminance situations Sustained firing Information about the contrast ◦ Parasol: large receptive fields, low acuity Works best in low luminance situations Burst firing Information about change over time of an image LATERAL INHIBITION Lateral inhibition: Light falling on photoreceptors surrounding the one in the middle will inhibit its response leads to annular/ring-shaped receptive fields Arrangement for an "on-center" ganglion cell "ON-center" ganglion cell ◦ Excited by light falling on the center of the receptive field ◦ Inhibited by light falling in surrounding areas ◦ Sensitive to size of illumination = higher response at a specific size and smaller response if smaller/bigger "OFF-center" ganglion cell ◦ Inhibited by light falling on the center of the receptive field ◦ Excited by light falling on the surrounding area WHY each ganglion cell will respond best to spots of specific size and respond less to spots too big or too small - retinal ganglion cells are most sensitive to differences in light intensity - Helps emphasize object boundaries Retinal Illusions Mach bands: perceived darker or brighter illusory lines at the ends of gradients Explained by lateral inhibition While the strips have constant intensity, lateral inhibition causes the to appear to have a gradient Simultaneous contrast There is a gradient, same each time Perception of brightness is influenced by the background Areas of the same intensity will appear: ◦ Lighter against a darker surrounding  ◦ Receptors stimulated by bright surrounding area send large amount of inhibition to cells in centre ◦ Darker against a lighter surrounding ◦ Receptors stimulated by darker surrounding area send small amount of inhibition to cells in the centre Visual acuity Visual acuity (resolution): smallest spatial detail that our visual system can resolve Depends on different factors: ◦ Optical: Focus, Clarity of optics ◦ Sensorineural: receptive field size (incl. convergence, density of photoreceptors) Use distance to quantify visual acuity ◦ 20/20 vision means that when standing 20 feet away, you can read the line on the Snellen chart that person can read from 20 feet ◦ 10/20: need to be 10 feet away to read what average person can read at 20 feet ◦ 20 feet = angle is 1 arc min (1/60 degree) SPATIAL FREQUENCY Spatial frequency: number of cycles per degree of visual angle Fourier analysis most basic type of wave = sinusoidal wave More complicated waves as different sine waves are added together Contrast sensitivity Spatial acuity depends on the contrast of the visual image, and its frequency The lower contrast (higher y-values) = the narrower the range of frequencies we can see Size of ganglion cell receptive fields influences frequency selectivity Centre has to correspond well