1. Contrast copy.docx

CRITICAL READING: CORNELL NOTES Contrast Name: Date: 26 July 2023 Section: Lecture 1 Period: Questions/Main Ideas/Vocabulary Notes/Answers/Definitions/Examples/Sentences Light Electromagnetic energy. Wavelength: Distance between two peaks (a) or throughs (b). Amplitude: Distance to p...

CRITICAL READING: CORNELL NOTES Contrast Name: Date: 26 July 2023 Section: Lecture 1 Period: Questions/Main Ideas/Vocabulary Notes/Answers/Definitions/Examples/Sentences Light Electromagnetic energy. Wavelength: Distance between two peaks (a) or throughs (b). Amplitude: Distance to peak of wave (c), corresponds to intensity. The human eye is sensitive to light waves of about 400 – 700nm. The Eye The Retina Photoreceptors Rods (~120 million) Low light conditions (scotopic) Black, white and grey Away from the centre of retina Cones (~8 million) High light conditions (photopic) Colour (blue, green and red) Fovea is exclusively cones Ganglion cell layer (~1 million) Bipolar cells combine graded potentials, transmit to ganglion cells Ganglion axons form the optic nerve Carries signals to the visual cortex Distribution & Spacing of Photoreceptors Photoreceptor width can be 1 micrometre. The fovea consists exclusively of cones. Outside the fovea, rods are much more common than cones. Yet, the average spacing across the retina is very consistent. Information About A Stimulus is Transmitted From One Part of the Brain to Another Via Neurons Action potentials are the electrical messages that neurons send each other. 90% of retinal ganglion cells (RGN) project to the lateral geniculate nucleus (LGN). V1 Primary visual cortex. Receives all visual input. Begins processing of colour, motion and shape. Cells in this area have the smallest receptive field. Contrast Contrast is critical to human vision. Processed in multiple areas: Retina LGN Visual cortex Is defined as the relative difference in luminance levels over a given area. Surface reflectance Michelson contrast: (Lmax – Lmin) / (Lmax + Lmin) Difference calculations required depending on the spatial structure of the scene. Luminance Measure of energy emitted or reflected by a light source. Measured in candelas per square metre of surface (CD/M2). Huge variation throughout the day: A white piece of paper Moonlight: 0.2cd/m2 Room light: 100cd/m2 Sunlight: 40,000 (+) cd/m2 Michelson Contrast Example Contrast value varies from 0.0 – 1.0. Coding Contrast Cells respond to contrast but not much to absolute illumination. Michelson contrast quantifies the difference in luminance across surface. This difference comes from discontinuity between light and dark areas the edges. Humans Aren’t Equally Sensitive to All Contrasts Can result from other eye conditions, such as: Age-related macular degeneration Glaucoma cataracts Human Contrast Sensitivity Function Pelli-Robson Contrast Sensitivity Chart Unlike the Snellen visual acuity chart, the letters remain the same size. But change in contrast 6 letters per line 3 letters per level Stop once patient fails to read all 3 letters in a group Further down = higher CS Contrast & Driving Owsley et. Al (2001) Recruited 274 older drivers with cataracts, and 103 without. Measured visual acuity and contrast sensitivity (using Pelli-Robson chart). DV: car crashes in previous 5 years. Results: Drivers who had crashed were 8 times more likely to have serious CS deficit than those who hadn’t. CS deficit predicts performance deficits better than VA. D’Elia & Newstead (2023) “Daytime running lights” Reduce the risk of being involved in a multi-vehicle accident by 8.8%. During the day, this was 7.6%. But a 20.3% reduction was found at dawn/dusk. Why might DRL be particularly effective at dawn and dusk? Lower contrast conditions. Receptive Fields The area of a stimulus field in which presentation of a stimulus causes a change in the firing rate of a given sensory neuron. The retina contains more than 20 types of ganglion cells (GC). Midget GC (70 – 80%) High level lights Smaller receptive field Parasol GC (~10%) Low level lights Larger receptive field Less resolution Centre-Surround Receptive Fields (Antagonistic) On-Centre Ganglion Cell No light = noise Light on centre increases Covered in light = reduced firing Light on surround decreases. Lateral inhibition Spatial opponency We also have off-centre RGCs Completely opposite Linear Receptive Field Light falls on the retina coding of this information leads to excitation or inhibition of neuronal responses. The linear receptive field applies a set of weights to this image processing. The weighted sum of excitation and inhibition for a receptive field is linear and this is used to predict cellular activity to a particular image. Contrast: RGC and LGN have linear receptive fields, though non-linear processing occurs in V1. Human Contrast Sensitivity Function Contrast sensitivity is influenced by the size and arrangement pf the receptive fields. Left: the light covers the centre and surround, causing a muted response. Middle: the light covers the centre and the dark bars cover the surround, leading to peak response. Right: the light and dark bars cover the centre and surround, leading to a smaller response (and then, eventually, no response). Edge Detection The arrangement of Centre-Surround (both on/off centre) RGC allow for edge detection. Edges have sharp changes in luminance/contrast Little response from RGC when the receptive field is covered entirely by dark or light (because of inhibition) Most active when part of the receptive field is covered. Image C offers a representation of edge detection Grey: no/little response White: large on-centre response Black: large off-centre response