A Vision of the Brain by Semir Zeki

Author:Semir Zeki
Language: fra
Format: mobi, epub
Published: 2020-08-05T10:48:58.086099+00:00

R

G

В

Response

©

R vs G

G vs R

□

B vs Y

(a)

Plate 12 (a) The opponent inputs from the three cone types to the ganglion cells of the retina. Long-wave (R) cones are opposed by middle-wave (G) cones in the so-called red-green channel. Short-wave (B) cones are opposed by the long plus middle-wave cones in the so-called blue-yellow channel. These are the colour pairings that have been described as the ones that cannot live with each other and cannot live without each other. If you stare at any one of the spots and then look at a neutral white or grey patch, the opponent colour will appear.

Plate 13 (a) The receptive field organization of a wavelength selective opponent cell in the lateral geniculate nucleus. The cell is excited by long-wave (red) light in the centre and by middle-wave (green) light in the surround, (b) The receptive field organization of a double opponent cell. The cell is excited by long-wave (red) light and inhibited by middle-wave (green) light in the centre and is excited by middle-wave and inhibited by long-wave light in the surround. Other double-opponent cells have the reverse arrangement.

3-7°

Plate 14 Plate 15

Plate 14 [left] An experiment to show that the corpus callosum (arrow), linking the two hemispheres, is essential for assigning the correct colour to a surface, when the surface is presented to one hemi-field and the rest of the multicoloured display to the other hemi-field. If the two are separated by 3.7°, the normal human brain can use the information coming from one hemi-field to generate the colour of the patch presented to the other hemi-field. If the corpus callosum is cut or absent, the colour generating interactions cannot occur. Since V4 is the first visual area which has wavelength selective cells and callosal connections extending beyond the representation of the 1° strip of the vertical meridian, it follows that V4 is the first possible area in which such colour generating interactions can occur. (Redrawn from Land, E. et al. (1983). Nature 303, 616-618.)

Plate 15 {right) (a) The responses of a narrow-band, wavelength selective cell (inset shows its wavelength sensitivity profile) in area VI to different areas of a Mondrian. When each area was put in the cell's receptive field, it was made to reflect the same amount of long-, middle- and short-wave light. Clearly, the cell is not interested in the colour of the area in its receptive field but only in the wavelength composition of the light reflected from it. (b) The responses of a short-wave (blue) ON, long-wave (red) OFF cell in area VI to different areas of a Mondrian. Inset shows the wavelength selectivity profile of the cell. The cell could be made to give an ON or an OFF response to an area of any colour, by simply changing the wavelength composition of the light reflected from it, without changing the colour.

Plate 16 The responses of a double-opponent cell in VI to rectangles of different colour in a display. The cell was excited by long-wave light and inhibited

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