From Colour to Perception to Neuroscience Arne Valberg Norwegian University of Science and Technology Section of Biophysics and Medical Physics Trondheim, Norway
The nature of colours (Leonardo da Vinci about 1500) ”There are six simple colours…. White is the first among the simple colours, yellow the second, green the third, blue the fourth, red the fifth, and black the sixth” Leonardo da Vinci: Traktat von der Malerei. Jena, Diederichs 1925, p.85.
Colour perception The hue circle
” In the eye there are three types of nerve fibres. Stimulation of the first one excites the sensation of red, stimulation of thre second the sensation of green, stimulation of the third the sensation of violet”and he continues”…(the essence of Young’s hypothesis is) that the sensations of colour are imagined as composed of three mutually and completely independent processes in th neural substrate.” Hermann von Helmholtz, 1860/1911 Young-Helmholtz’ view on colour vision
Distribution of cone receptors.(no rods and no S-cones in the central fovea). λmax= 570 nm (yellow), 545 nm (yellow-green), and 445 nm (violet) Figur fra Valberg, 2005
jj James Clerk Maxwell (1831 – 1879) and the colour top
Maxwell on colour vision • ”All vision is colour vision, for it is only by obesrving differences of colour that we distinguish the forms of objects. I include differences of brightness or shade among differences of colour.” ”… it is not necessary to specify any given colours as typical for these primary sensations. … any of three colours might have been chosen, provided that white resulted from their combination in proper proportions” • James Clerk Maxwell (1856)
The visual spectrum (380 to 780 nm) CIE chromaticity diagram Symmetric hue circle
Registration of nerve pulses from a nerve cell with a microelectrode [imp/s]
PC-cells respond to wavelength distributions MC-cells respond to contours
Threshold- and suprathreshold responses of cone-opponent cells and how to simulate them Data base: Single cell recordings from the LGN of macaque monkeys (done at Max Planck Institute for biophysical Chemistry, Göttingen (D)) . Collaborators: Barry B. Lee, Thorstein Seim, and Jo Tryti.
Spectral threshold contrast sensitivity of 4 opponent cells and one non-opponent MC-cell that resemble human psychophysics
How do we arrive at a model for colour scaling?A possible (theoretical) combination of the different cone opponent cell responses along orthogonal axes.(response to the white adapting stimulus subtracted)
OSA-UCS (Y = 30%)colour scalig represented by a theoretical combination of opponent cell responses
How does the brain do it? (Top down)
Visual areas of the cortex Orientation, eye dominance Movement and direction of movement (MC) Depth, 3D colour Contour (PC, KC)
Colour perception The hue circle
The modelling of opponent cell responses (retina and LGN) Opponent combination (difference) of cone signals, for instance VM and VL: • NM–L= (AMVM – ALVL )+ Nofor a ’M-L’ cell (I- and D-cells) • NL–M = (BLVL – BMVM )+ No for a ’L-M’ cell (I- and D-cells) • N: parvocellular cell (PC-cell) response in impulses/s • V: receptor signals (hyperpolarization of cones) • A and B: weighting constants (AM>AL for I-cells and AM<AL for D-cells, etc.) • No: cell activity without stimulation
Summary ● Perception. Opponent unique colours red-green, yellow-blue, white-black. Mechanisms unknown. ● The spectrum ● Colour matches. Photoreceptors L, M, and S (retina). ● Opponent ganglion cells: ’L-M’, ’M-L’, ’M-S’, ’S-L’. Incr. & Decr. cells (retina, LGN). ● Simulation of cell responses (retina, LGN)/model (V1?). Scaling. ● ”Bottom up” (retina, LGN) versus ”top down” (V4, V2...). ● Unique colours and surround effects (simultaneous contrast, adaptation, ”colour constancy”) unexplained.
The two main roads • Leonardo da Vinci (ca. 1500) • Isaac Newton (ca. 1670) • Thomas Young and George Palmer (1770 – 1800) • James Clerk Maxwell (ca. 1860) • Hermann von Helmholtz (ca. 1860) • Hermann Grassmann (1850) • Ewald Hering (ca. 1870) • Wilhelm Ostwald (1920) • Erwin Schrödinger (1925) • Dorothea Jameson and Leo M. Hurvich (1955) • Thorsten Wiesel, David Hubel, Russell deValois (1960) • Edwin Land (1960-1985)
23 problems in colour vision How did colour vision evolve? What is the neural code for colour? How is colour organized in the cortex? How are colour differences scaled? How to explain the aesthetics of colour? How does the eye adapt to light and colour? Which are the cone inputs to cone-opponent cells? What is the significance of colour? How do colour qualia come into being? How do colour opponent neurons interact? How many colour vision deficiencies are there? How are cone-opponent cells spatially organized? What is the status of elementary (opponent) colours? Which are the most important non-linearity in colour vision? What are the neural correlates for white and black? What is the genetic basis of colour vision deficiencies? How can colour constancy and adaptation be explained? How does neural activity correlate with colour perception? How many (shifted) visual pigments can a photoreceptor have? How many dimensions do colour perception have (modes of appearance)? How is the relation between receptor specificity and pigments established? How do ganglion cells pick receptors for receptive field centre and surround? Is a receptor identity (as L or M type) established solely by its pigment (or cant here be an L type with an M pigment?)
Area V1: Eye dominance and orientation sensitive columns The knots (corresponding to the circles in fig.A) are colour sensitive
Perception of light and colour. Model stages and important parameters. Stimulus → sensory response → neural representation →perception Stimulus: Spectral distribution of interference filters, Intensity, Size, Spatial contrast. 5 log units of intensity Sensory response: Receptors (L,M,S), Light absorption and non- linearity, Standing potentials, Adaptation Neural representations: Firing frequency, Receptive field,Increment- and Decrement opponent cells (ON & OFF), Adaptation (retina and LGN) Perceptual properties: Elementary colours (Y, R, B, G) Hue, Colour strength, Relative lightness (black, grey, white), Brightness. Object colours and light colours (reflecting and emitting sources of colour), Colour scaling
”Sense perception is the beginning of all research, and the truth of theoretical thought is given exclusively by its relation to the sum total of those experiences.” Albert Einstein, 1950 ”Nothing can exist in conscioussness without having first passed the senses.” Aristoteles