Dr Christopher Taylor New England College of Optometry, US

Dr Taylor is a visual psychophysicist working on emmetropization - the growth of eyeballs & the development of myopia  

Dr Christopher Taylor had some 🍺 at the error bar in episode 5 #rod #cone #astro #psycho #chicken #truth

our discussion

welcome to the error bar. would you like to introduce yourself?

hi i'm Christopher Taylor, from the New England College of Optometry located in Boston's Back Bay. i work chiefly in the field of emmetropization, or how you grow and eyeball to match the power of the optics, so you don't need things like glasses.

and that's what i'm working on now, but i've worked on things such as ageing and vision, and spatial vision, and temporal vision, going on twenty years now, since i was an undergrad. so, i'm really feeling it. makes me feel like i need a drink

what can i get you?

hearing those dulcet British tones makes me really yearn for a real ale

this will do?

this will do, yes, yes, definitely

it's a real coincidence you've dropped in, because we were just talking about colour perception with an astrophysicist, so i think i could have a few questions for you


in a sentence, how do we perceive colour?

well, it starts with a photon, usually more than one photon, that will excite one of two classes of detectors known as photoreceptors either rods and cones - this is way more than a sentence - but, that's the easy bit

the brain is continually taking that physical property of wavelength, and using it to determine not only the relative colours of objects but to take out the colour of the illumination. and the brain's trying to give us the best interpretation of the wavelengths that are coming off an individual surface to give us the colour of an object

in a sentence is really complicated but requires mechanisms all the way up from the retina to the visual cortex. it's a huge matter of interpreting wavelength into even words and language which influences colour perception. colour is simple yet complicated sometime

so this is why it's good to have you here, because i've been speaking to astrophysicists, and for them there are no green stars, or yellow or cyan or purple. and that for me as a psychologist just sounded a bit too neat. so any initial thoughts on the colour of stars?

yeah. that they're confused by the trickery of psychology by using colour names instead of the precision of wavelength, which is understandable. it's a really common error, i think, just to believe that colour is just out there, rather than the brain constructing the world. the whole factoid that there are no green stars is a really neat one, and does say something special and particular about the universe. but it's also an interesting point about how scientific communication can delight, yet mislead at the same time

colour perception is complex. i think we get that. but let's break it down. so, i'm thinking of star gazing and astronomers are going out at night, you know, with a red torch, and then looking at the stars. what happens when it gets dark?

in the first eight minutes the cones will adapt, so that one type of photoreceptor that we see detail with and we see colour chiefly with. and once those are adapted after eight minutes, they aren't sensitive enough to catch light efficiently any more. and then the rods, that have bigger apertures and more sensitive photopigments will start to dominate

at that point, the rods will continually get more and more sensitive. in the central vision, where we tend to look and have our most detailed vision, doesn't contain any rods. so, when going out into the night it's better to look slightly off from any faint star object that you're looking to see

the brain with colour is always doing that trick of comparing the physical input among its sensors. it's a really big trick with cone and colour perception because if you look at the response of any of the different - instead of red, green and blue, let's call them long, medium and short-wavelength cones - they have very wide ranges of responses to different wavelengths of light. so, if you hit a medium or green cone with red light, it's going to respond, and respond quite well. so narrow-band light, aside from some of the shortest wavelengths, will stimulate all the short, medium and [long] wavelength cones

and given that we're talking about adaptation and different times of day and night, the time around dawn and dusk, you can get to light levels where both the rods and cones are active. and this is well known in the film and television industry as 'the golden hour'. but that's when humans can see both the rod vision and the cone vision, so we're functionally tetra - have four receptors - instead of trichromatic. so, everyone often comments on how beautiful things look, but it's probably because of that mere addition of the rods playing a role in perception

you're unweaving the rainbow for me here

oh yeah, and then speaking of rainbow, the whole issue of - there's no purple stars - things like that, if that's said, purple is a mixture of short and long wavelengths. so it's what's known as a non-spectral hue. so, one of those colours that, you can't create if you have a tune-able light, let's say, if you vary the wavelength of it, you can't get purple, you need both red and blue

because a star is like a point of light billions of miles away, could that point just stimulate say a green cone, then a red cone, then a blue one, and you would see - would you see, green and then red and then blue?

so if you hit a green cone with a red light, it will just signal 'i have caught some photons', so people who use systems with mirrors that are flexible called adaptive optics, can do what's known as single cone psychophysics, and stimulate single cones

and having been in one of those machines as a subject, you can stimulate a green cone with red light, and because you're just activating that one cone, there's not that ability to compare the activity across photoreceptors. so my experience of that was of a whitish light. kind of, it really hammers home that point that it's the entire system, not the receptors themselves, that are driving the percept of colour

to get that vivid sensation of green, to get the red cones deactivated with green light, with the green cone activated with green light in what's known as a centre-surround organisation. where one type of cone is being inhibited versus the cone that is being excited. it's that contrast, that comparison, that's really generating that colour signal

the trick with the mirrors is to stablise the light, right? because the eye's always jiggling around and moving, and presumably the optics of everyday life are not very good - there's scratches on your glasses, and everything's wobbling. is that the problem that - or not the problem, maybe it's adaptive that, when things wobble around you actually see a lot better?

yeah, i would say so. the whole system's tuned for things being in constant motion. yeah, and those adaptive optics systems will take into account any imperfections in the eye, the eye shaking about a little bit. those same systems can take pictures of people's photoreceptor arrays. and one thing that came out of that work, done by David Williams at Rochester and Austin Roorda at Berkeley, is how individually variable people's photoreceptor mosaics are

one person can have a retina with many many more medium wavelength cones than long wavelength cones, and another completely the reverse. yet, if you drop them into a behavioural experiment they'll be entirely consistent in what they call red between one another. so that photoreceptor mosaic, somehow despite massive individual differences, and i remember seeing those pictures for the first time, thinking: 'how does the brain even make sense of seeing at all', just is able to wire itself up and extract those signals in a consistent way? obviously there are some behavioural individual differences, but the anatomy, when it's visualised and imaged, is just astoundingly different. the engineers, whoever built those, must have had one too many real ales in those times

i've seen some of those mosaic pictures and yeah, i just thought it was, it couldn't be true. are these unique individuals, how much variability is there between the normal population?

[pffff], i don't have a quantitative number at my fingertips, but those initial images that were published fifteen to twenty years ago - are not atypical in their first small sample of subjects. so, having seen one observer with a green dominated retina and another with a red cone dominated retina. yeah, there are massive individual differences in the cone mediation in the retina

what kind of saves the issue in the end, is if you look at the green and the red cones' responses to wavelengths of light is that they are very close. their sensitivities overlap greatly, and their peak sensitivity is only separated by about thirty nanometres, which is very small. so, perhaps the system can utilise that there's something slightly interchangeable about what they are, and that we're just signalling differences in the spectral output, rather than using those sensor signals to signal wavelength

does the same apply to the rare cases of people with four cone receptors?

yes. those colour anomalous individuals with four - they're pretty rare. and even those individuals who we think of as red-green colour blind, aren't usually missing a photoreceptor type, it's just, instead of that thirty nanometre separation, there's a bit less of a separation between the medium and long wavelength cone

well, there are some extremely rare individuals who do lack the gene for the short wavelength cones, or one of the others. colour anomalous, and individuals lacking a photoreceptor, will have those cones closer together, and those tetrachromats will have what the anomalous individual has, plus the normal versions. so they have those photoreceptors packed in really tightly all around that range of that 530-ish nanometres

whether those tetrachromatic observers, whether that has any functional use, or anything other than something you can observe in the lab, is a question that i don't think is quite resolved, or it's something that's probably not harmful enough for evolution to select out, either in the deleterious case which is generally males, or the tetrachromatic case, which is generally females

i have to mention my friend the chicken, who has a UV cone. so chickens are used in vision research, particularly those questions surrounding colour and eye growth. and they literally, they do have a fourth cone which sees into the UV. this is true for many birds that often used it for sexual selection and mating. and they also have a cone type that has both the green and red photopigment together in one photoreceptor unit that senses luminance overall

so, despite the photopigments being preserved pretty well molecularly across evolution all the way back to the avian branch of the tree, the solutions that nervous systems and organisms have come up with have been really interesting

i feel like you need a philosopher to go along with colour perception research. why is it that massive differences in cone types or cone densities can not lead to any change behaviour?

i guess i would reach for the fact that there are so many pressures on behaving the right way, that if these small differences in anatomy truly mattered they'd be selected out. and you'd probably need a linguist too to distinguish all this, because there's the cross-cultural research where, i believe the Italians have greater distinctions in the short wavelengths, they have two commonly words for blues, or a green-blue and a blue. it's the frequencies at which these words are used vary from language to language

and linguists will analyse, well how many unique colours can folks name. and there are far more than one would predict just based on the anatomy i think

so that brings us right back to the start of this, where you sort of implied that to say that a star is green is almost like a linguistic mistake. it's just a helpful rule-of-thumb for people to communicate with each other about patterns that they observe. is that too philosophical - what am i asking?

yeah, i think so. i think that's worthwhile. colour at once is so fundamental and so common, yet you dig into it, psychophysically, anatomically, or physiologically, we're at once super-sensitive to it, yet we get to this point where we can just name a few shades, and that's our rainbow. it's at once this crucial property that our system is exquisitely sensitive to, but at the same time we categorise and really need to make quick decisions about - is that berry red, is that berry blue?

saying 'there are no green stars' seems striking to say at first, but when you dig into the details about black body radiation and things like that, it's saying something quite different from what we mean when we're talking about colour in a human context

[🎶 "Sloshed," by Dee Yan-Key 🎶]

we can offer absolution for any of your science sins. is there anything you want to own up to?

oh sins. i'm not an expert, but sloth as far as i can tell. sloth in writing up data. that's, err, i don't know if i want to be absolved from that, but maybe you can assign me a few 'hail Marys'?

i think a few bloody Mary's should probably should probably do it

much better, much better. that's the worst one. perhaps it's a virtue in the end, maybe?