On 24th February 1987 Oscar Dulhalde was at work at the Los Campanos Observatory in Chile. He was an operator charged with setting up the telescope for the astronomers. He went outside for a coffee break and looked up at the sky. In particular he looked at the Tarantula Nebula which is 168,000 light years from Earth. He noticed something strange. When setting up the telescope, he always used the brightest star in the constellation as a reference . Now there were two bright stars. At the same time Ian Shelton an astronomer at the Observatory was developing a picture of the same part of the sky. He too found an extra star. He went outside to check. The two of them became the first people to see a supernova with the naked eye for nearly five hundred years.

Supernovas are the death of a star. The star implodes and radiates strongly. Johannes Kepler, the famous astronomer, saw the last one with the naked eye in 1604 in Germany. Before that there are ancient records of “visiting stars” especially in China. Seeing a Super Nova with the naked eye shows the sensitivity of the rod sensors in the eyes. Recognizing that something was different in one small part of the sky, shows the power of perception.

Oscars brain did something amazing but at least it was only operating mostly in black and white. The eye has two different types of sensors: those for colour (the cones) and those for black and white (the rods). We have six million colour cones. The balance of the sensors, all one hundred and twenty-four million of them, are the rods. They can only see in monochrome. Cones function better in bright light. Whereas rods are extremely sensitive and can be triggered by a single photon of light.

Even in the womb a baby can sense the difference between light and dark. New-borns can only focus about eight to 12 inches from their face. They see only black, white, and grey. We start our lives in a monochrome world, it is hardly surprising that we are sensitive to brightness. The reaction to bright light can influence things over which we normally have no control. Brightness can change body temperature and heart rate. It will increase the production of cortisol the fight-or-flight hormone. This enhances mental alertness.

It is not only brightness that affects us, colours can stimulate or calm us. One professor went so far as to repeatedly redecorate his classroom. He was trying to understand the impact of colour on his students. Colours with high saturation and brightness produced the highest level of attention. He used yellow, yellow green, and magenta. Blue and green by comparison were calming. They are known to reduce heart rate and blood pressure as well as mental state.

It takes between fifty and sixty photons to trigger a cone. The cones come in three forms. Contrary to popular belief, we do not have separate cones for different colours. All can see most of the visible spectrum of light. They vary only in their peak sensitivities. Each type “peaks” at a different wavelength. These correspond to blue, green and red light. We compare the responses of the different types of cones to the incoming light and infer colour. The process is as much mental as based on the receptors. This is entirely analogous to the olfactory cortex. This can infer the smell of multiple aromatic chemicals from the strength of the bonds to its 450 different types of receptors (see Newsletter #047). Without our brains our sense receptors are useless. This is even before the stage of creating a perception.

Like our smell receptors, we do not have the same number of the different types of cones. For any individual the blue cones dominate. They can account for fifty percent of all cones. That is the low end of the range. Across people it can vary between fifty and seventy percent of all cones. Because of those differences, our individual visual worlds are as different as our smell worlds.

Vision is the dominant sense

Perception will take input from all senses to find a fit with our memories. We can claim to be smelling a flower, but the brain has not turned off the other senses. When we see the rose we are already expecting a certain smell. Perception does not value the different senses equally. Vision is the dominant sense and can “over-ride” the other senses.

Perhaps the most famous example of this is the “rubber hand experiment”. This shows that sight can over-ride touch. A respondent sits at a table and places their left hand on it. They place their right hand behind a screen on the table so they cannot see it. In its place, on the table, is a hand moulded of rubber. It does not even have to be a good representation but it sits where the right hand should be. A cloth over the shoulder hides the fact that the rubber hand is not connected to the body.

The experimenter sits in front of the respondent. They stoke the rubber hand with a soft brush. They stroke the real hand behind the screen the cloth synchronously. In only one minute people come to believe that the rubber hand is their hand. The effect is strong. When the experimenter sticks a pin into the rubber hand the respondent feels it. When the pin is stuck into the real hand (behind the screen) they do not feel it. The rubber hand is then hit with a hammer, the respondents jumps and cries out. When the sense of touch conflicts with sight, sight dominates.   

In another famous study, flavoured clear liquids were coloured with dyes. Respondents were then asked to identify the flavours. Some of the colours “fitted” with their favours. For example, a pink/red liquid and a cherry flavour. Others did not provide a consistent combination. For a significant group of people, when the colour and the taste did not fit, it was the colour that dominated. When given a yellow, cherry flavoured drink, over a quarter of respondents claimed that the drink tasted of lime. They followed what they could see, not what they could taste. All our experience leads us to associate a yellow/green coloured liquid with the taste of lime not cherry (at least in Western cultures). In this case our sense of smell dominates our sense of taste for many people. 

Ageing of Vision

Numerous studies have shown that there is no significant age related deterioration in the retina or the optic nerve. This is in the absence of disease. Those amazing cones and rods continue to function throughout our lives. Instead most of the ageing of the eye comes from mechanical failures. The eyes sink into the socket reducing our ability to look around. The lenses stiffen and yellow. The cornea becomes scratched or distorted. The muscles in the iris fade. Of course diseases of the retina such as macular degeneration and glaucoma, are more common as we age.

Our brains and senses are a single whole. Before the recognition that comes with perception, vast amounts of computing power is needed. Unpicking the strength of the bonds between our smell receptors and multiple organic chemicals is a big task. Inferring colour from the differences in signals from multiple visual sensors is as big a task. Only then can perception combine this input with our memories. All of this has to be done in “real time”.