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Transcript
Computational Vision Lecture 3.1: Human Vision - Colour Hamid Dehghani Office: UG38 Biological Vision • Retinal Processing • Visual Pathway • Colour Rods and Cones • Rods – ~120 m – Extremely sensitive photosensor – Respond to a single photon – Poor spatial resolution as they converge to same neuron within retina • Cones – ~6 m – Active at higher light levels – Higher resolution as Signal processed by several neurons Receptive Field • Receptive field is the area on which light must fall for neuron to be stimulated. • Note difference between centre and periphery of field • As early as 1938, frogs were seen to have different types of ganglion cells • Using Cats, (electrical recordings from ganglion cells) it was seen that receptive field contains a circular centre surrounded by a ring Ganglion cells • Two types: "on-center" and "off-center". – On-center: stimulated when the center of its receptive field is exposed to light, and is inhibited when the surround is exposed to light. – Off-center cells have just the opposite reaction ON/OFF Cells • Note – area of stimulation / inhibition – Rate of signal firing (rebound) Visual Pathway • Vision generated by photoreceptors in the retina • The information leaves the eye by way of the optic nerve • There is a partial crossing of axons at the optic chiasm. • After the chiasm, the axons are called the optic tract. • The optic tract wraps around the midbrain to get to the lateral geniculate nucleus (LGN) • The LGN axons fan out through the deep white matter of the brain and ultimately travel to primary visual cortex, at the back of the brain. Visible Light • Humans perceive electromagnetic radiation with wavelengths 380-760nm (1 nm = 10-9 m) • f = c/λ • • • f = frequency (Hz) λ = wavelength (m) c = speed of light (2.998x108 ms-1) • E = hf • • E = Energy (J) h = Plank’s constant (6.623x1034 Js) Colour • Objects selectively absorb some wavelengths (colours) and reflect others • Human retinas contain three different kinds of cones to provide very elaborate form of vision – Gives ability to distinguish different forms of same objects • Fruits • Camouflage Colour mixing • Many forms of colour vision proposed – Until recently some hard to disapprove • 1802: Proposed that the eye has three different types of receptors, each sensitive to a single hue (Young, a British physicist) – By the fact that any colour can be produced by appropriate mixing of the three primary colours. • This became known as Trichromatic (threecolour) theory. Trichromatic Coding Trichromatic Coding – Explains – Does not account for colour blending: Colour mixing • Many forms of colour vision proposed – Until recently some hard to disapprove • 1930s: Hering (German Physiologist) suggested colour may be represented in visual system as ‘opponent colours’ • Yellow, Blue, Red and Green – Primary colours – Trichromatic theory cannot explain why yellow is a primary colour Opponent Process Coding • Yellow, Blue, Red and Green – Primary colours – Trichromatic theory cannot explain why yellow is a primary colour – Also some colours blend and others do not! • Bluish green, yellowish green, orange (red and yellow), purple (red and blue) OK • Reddish green?? Bluish Yellow?? – Opposite to each other Opponent Process Coding • Neurons respond to pairs of primary colours – Red-Green & Yellow-Blue • Some respond in centre-surround fashion • Response characteristics determined by appropriate ganglion cells connections Opponent Process Coding Reading • Vicki Bruce, Visual Perception, Chapters 1 - 3 • Neil Carlson, Physiology of Behavior, Chapter 3, “Vision”