Why white has no color: Receptive fields for color vision
By: Marissa Sumathipala
Is white a color? This age-old question has puzzled scientists, physicists, and philosophers alike for nearly as long as the question of whether color even exists. Nonetheless, certainly all of us have experienced the wonders of color perception. Surprisingly, the dizzying array of hues we perceive in our everyday life arise from combinations of just three types of color receptors in our eyes: red, green, and blue.
When light reflects off an object, such as a bright-red apple, a photon travels at a certain wavelength and enters the retina. There, it triggers a chemical cascade in specialized photoreceptor cells, called cones, that respond to different colored light by relaying an electrical signal to other cells. In the case of the apple, only the red type of cones will send a signal. Eventually, this signal reaches the last step in the retinal pathway: retinal ganglion cells. These cells extend their axons out of the retina and relay the visual information all the way to different brain regions. Along the way, the information is combined and processed to identify features like shape and hue, ultimately allowing us to recognize the object as a delicious red apple.
What about objects that appear white? What we perceive as white light is actually composed of all wavelengths from across the entire visible spectrum. This may seem non-intuitive, but it's the phenomenon that's responsible for rainbows: white light is separated by water droplets into the different colors, producing a colorful spectrum. If white light is composed of the entire color spectrum, why do we perceive it as having no color?
The answer lies in the receptive field of the retinal ganglion cells. A neuron's receptive field is the region of the world where a stimulus can activate the cell. Pioneering neuroscientists Hubel and Wiesel, who made some of the earliest discoveries of how the visual system works, were the one of the first to discover the receptive fields of ganglion cells in mammals.
To do so, they first developed a new method to measure how electrical signals in a single neuron changes with visual stimuli by inserting a tiny electrode into the brain of their model organism, a spider monkey [1]. A small bright spot of light in the center of a cell's receptive field caused it to fire more electrical signals. A donut shaped light stimulus had the opposite effect, making the cell fire less. Lastly, a large spot of light had no effect on the cell's activity.
From this, Hubel and Wiesel concluded that ganglion cells had a concentrically organized receptive field [1] (Figure 1). Stimuli in the center of the receptive field excites the cell, and stimuli around the edges of the receptive field inhibits the cell. But, the earliest of these studies were done on retinal cells that are responsible for our black and white vision. The question of how color vision worked remained unanswered until a few years later. Careful experiments by Hubel, Wiesel, and others systematically characterized the response properties of hundreds of cells in the retina and uncovered a population of ganglion cells that responded to color [2].
Astonishingly, these color receptive ganglion cells had the same concentric receptive field organization as the ones for black and white vision. But instead of being activated by light in the center and inhibited by light on the edges, these cells were activated by red in the center and inhibited by green on edges. In other words, a visual stimulus with red in the center caused maximal activation of the cell, while a stimulus with green on the edges caused maximal inhibition of the cell. A stimulus with red in the center and green on the edges had no effect on the cell [2].
Hubel and Wiesel also found cells with the opposite pattern: they were activated by green in the center and by red on the edges. Later studies by others characterized a similar receptive field pattern but for blue and yellow light [3]. These color receptive ganglion cells can be grouped into four broad categories (Figure 2).
These findings explain why we perceive white as having no color. When white light hits the retina, none of the four ganglion cell types change their firing rate. If we take the red on, green on cell as an example, red light in the center activates the cell, but white light also contains green light. The green light in the edges of the receptive field inhibits the cell. In essence, these two effects compete and cancel each other out. Thus, even though both red and green light is hitting the retina, it has no effect on the cell. The competing activation in the center and inhibition on the edges occurs for all the four cell types, since white light contains every color of light, resulting in perception of no color.
These landmark discoveries transformed our understanding of how vision works. Their findings paved the way for further studies characterizing the increasingly complex receptive fields of neurons in the visual cortex. Though these studies were published over four decades ago, they are a remarkable testament to the elegant principles that guide even the most complex of processes like vision.
References:
1. Hubel, D. H. & Wiesel, T. N. Receptive fields of optic nerve fibres in the spider monkey. J. Physiol. 154, 572–580 (1960).
2. De Monasterio, F. M. & Gouras, P. Functional properties of ganglion cells of the rhesus monkey retina. J. Physiol. 251, 167–195 (1975).
3. Wiesel, T. N. & Hubel, D. H. Spatial and chromatic interactions in the lateral geniculate body of the rhesus monkey. J. Neurophysiol. 29, 1115–1156 (1966)
4. Kandel, E. R., Schwartz, J. H., Jessell, T. M., Siegelbaum, S. A., & Hudspeth, A. J. (Eds.). (2012). Principles of Neural Science, Fifth Edition (5th edition). McGraw-Hill Education / Medical.
Is white a color? This age-old question has puzzled scientists, physicists, and philosophers alike for nearly as long as the question of whether color even exists. Nonetheless, certainly all of us have experienced the wonders of color perception. Surprisingly, the dizzying array of hues we perceive in our everyday life arise from combinations of just three types of color receptors in our eyes: red, green, and blue.
When light reflects off an object, such as a bright-red apple, a photon travels at a certain wavelength and enters the retina. There, it triggers a chemical cascade in specialized photoreceptor cells, called cones, that respond to different colored light by relaying an electrical signal to other cells. In the case of the apple, only the red type of cones will send a signal. Eventually, this signal reaches the last step in the retinal pathway: retinal ganglion cells. These cells extend their axons out of the retina and relay the visual information all the way to different brain regions. Along the way, the information is combined and processed to identify features like shape and hue, ultimately allowing us to recognize the object as a delicious red apple.
What about objects that appear white? What we perceive as white light is actually composed of all wavelengths from across the entire visible spectrum. This may seem non-intuitive, but it's the phenomenon that's responsible for rainbows: white light is separated by water droplets into the different colors, producing a colorful spectrum. If white light is composed of the entire color spectrum, why do we perceive it as having no color?
The answer lies in the receptive field of the retinal ganglion cells. A neuron's receptive field is the region of the world where a stimulus can activate the cell. Pioneering neuroscientists Hubel and Wiesel, who made some of the earliest discoveries of how the visual system works, were the one of the first to discover the receptive fields of ganglion cells in mammals.
To do so, they first developed a new method to measure how electrical signals in a single neuron changes with visual stimuli by inserting a tiny electrode into the brain of their model organism, a spider monkey [1]. A small bright spot of light in the center of a cell's receptive field caused it to fire more electrical signals. A donut shaped light stimulus had the opposite effect, making the cell fire less. Lastly, a large spot of light had no effect on the cell's activity.
From this, Hubel and Wiesel concluded that ganglion cells had a concentrically organized receptive field [1] (Figure 1). Stimuli in the center of the receptive field excites the cell, and stimuli around the edges of the receptive field inhibits the cell. But, the earliest of these studies were done on retinal cells that are responsible for our black and white vision. The question of how color vision worked remained unanswered until a few years later. Careful experiments by Hubel, Wiesel, and others systematically characterized the response properties of hundreds of cells in the retina and uncovered a population of ganglion cells that responded to color [2].
Astonishingly, these color receptive ganglion cells had the same concentric receptive field organization as the ones for black and white vision. But instead of being activated by light in the center and inhibited by light on the edges, these cells were activated by red in the center and inhibited by green on edges. In other words, a visual stimulus with red in the center caused maximal activation of the cell, while a stimulus with green on the edges caused maximal inhibition of the cell. A stimulus with red in the center and green on the edges had no effect on the cell [2].
Hubel and Wiesel also found cells with the opposite pattern: they were activated by green in the center and by red on the edges. Later studies by others characterized a similar receptive field pattern but for blue and yellow light [3]. These color receptive ganglion cells can be grouped into four broad categories (Figure 2).
These findings explain why we perceive white as having no color. When white light hits the retina, none of the four ganglion cell types change their firing rate. If we take the red on, green on cell as an example, red light in the center activates the cell, but white light also contains green light. The green light in the edges of the receptive field inhibits the cell. In essence, these two effects compete and cancel each other out. Thus, even though both red and green light is hitting the retina, it has no effect on the cell. The competing activation in the center and inhibition on the edges occurs for all the four cell types, since white light contains every color of light, resulting in perception of no color.
These landmark discoveries transformed our understanding of how vision works. Their findings paved the way for further studies characterizing the increasingly complex receptive fields of neurons in the visual cortex. Though these studies were published over four decades ago, they are a remarkable testament to the elegant principles that guide even the most complex of processes like vision.
References:
1. Hubel, D. H. & Wiesel, T. N. Receptive fields of optic nerve fibres in the spider monkey. J. Physiol. 154, 572–580 (1960).
2. De Monasterio, F. M. & Gouras, P. Functional properties of ganglion cells of the rhesus monkey retina. J. Physiol. 251, 167–195 (1975).
3. Wiesel, T. N. & Hubel, D. H. Spatial and chromatic interactions in the lateral geniculate body of the rhesus monkey. J. Neurophysiol. 29, 1115–1156 (1966)
4. Kandel, E. R., Schwartz, J. H., Jessell, T. M., Siegelbaum, S. A., & Hudspeth, A. J. (Eds.). (2012). Principles of Neural Science, Fifth Edition (5th edition). McGraw-Hill Education / Medical.
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