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Light & Eyes. Vision begins when light comes into the eye Light: Focused by the cornea and the lens onto the retina (which is a thin layer of neural tissue at the back of the eye; contains photoreceptors) Photoreceptors transduce light into neural signals and pass their signals on to the brain.
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Light & Eyes • Vision begins when light comes into the eye • Light: • Focused by the cornea and the lens onto the retina (which is a thin layer of neural tissue at the back of the eye; contains photoreceptors) • Photoreceptors transduce light into neural signals and pass their signals on to the brain
Light • What kind of light do we see? • Electromagnetic radiation in the range between about 400 and 700 nanometers • Though we see only a small portion of the spectrum, 83% of the light present in our atmosphere is in this range…we’re making good use of information available around us!
Light is changed by objects in its path • Light is reflected • Light is absorbed • Light rays are bent or refracted • Light is diffracted
What makes a good eye? • Different creatures have different eyes because the eyes have adapted to the needs and environment of their owners • Prey usually have eyes on the sides of their head so they can see behind them; predators have eyes in front • Rabbits have eyes spaced so far apart that they almost have 360 degree vision • Marine animals also have eyes far apart on the sides of their heads - e.g. whales have a huge blindspot right in front, so they can't see a boat coming straight at them • Hawks and other birds have amazing visual acuity (as much as 8 times that of humans)
Anatomy of the human eye: Outside • Sclera: white part • Pupil: hole • Iris: like the aperture on a camera - comes in several fashionable colors. The iris has to be dark enough to not reflect all the light incident on the eye (this is why albinos have such poor vision)
Anatomy of the human eye: Inside • Cornea: thin, transparent covering of eye ball • Lens: adjustable focus for near/far (more on this later) • Fovea: part of retina corresponding to central part of visual field. Fovea is latin for "pit", and it is actually shaped like a pit. • Optic disc: part of the retina corresponding to blindspot • Optic nerve: made up of ganglion cell axons that exit through the optic disc • Anterior chamber/aqueous humor: fluid filled region in front of the lens • Posterior chamber/vitreous humor: fluid (jelly-like) filled region behind the lens
Eye Problems? • Glaucoma: the draining of the aqueous humor is blocked and pressure is built up inside the eye which impinges on the blood vessels and the optic nerve. If caught early, it can be treated by medication or by surgery • Cataract: a clouded lens which, if serious, can be removed and replaced surgically • Macular Degeneration: a disease of the retina where the very center of the visual field (the fovea and close surround) is damaged (a devastating disease because you can't see anything that you try to look at - you can only see periphery, which also makes it impossible to read)
Optics The retina is pink, but when we look into someone's eye, it looks black - why is that? You can't see the light reflecting straight off the retina because your head is blocking the light that would come in…so we need an ophthalmoscope to see the inside of an eye The back of the eye of cats, raccoons and many nocturnal animals has a reflective coating, which is why their eyes shine at night. The reflective coating is useful for detecting as much light as possible in low-light conditions. Why do you get "red-eye" in photos taken with a flash?
Visual Angle • Visual angle is a measure of the amount of the visual field that an object takes up • Visual angle is not a measure of object size because: • objects of the same size, but at different distances from the eye take up different visual angles • objects of different sizes at a different distances from the eye can take up the same visual angle
Accommodation • The process of adjusting the lens in your eye for different viewing distances • Almost 70% of the optical power of our eyes is accomplished by the cornea…but the cornea can't adjust, so it's not good for focusing on objects at different distances
The focusing power of the lens can be adjusted • The lens has muscles attached to it that change its shape and focusing power • For viewing distant objects we relax the muscles; When the muscles are relaxed the lens has little curvature (is flat) • For looking at nearby objects, we adjust the shape of the lens by pulling the muscles taught so the lens becomes more round
The lens changes shape to focus on objects at different depths
Rods and Cones • Conveniently, rods are rod-shaped and cones are cone-shaped • Rods are good for low-light black and white vision • Cones only work in good light and can detect color • Three types of cones are sensitive to different wavelengths of light: S-cones to short wavelength light, M-cones to medium wavelength light, and L-cones to long wavelength light
Retinal distribution of photoreceptors: fovea = all cones; periphery = predominantly rods
Transduction • Visual transduction is the process by which light energy is converted to neural energy (or electrochemical signals) so that it can be interpreted by the brain • When a photon is absorbed by a molecule of rhodopsin, it changes the chemical state of the photopigment • The two parts of the molecule split. • This change in state is called the isomerization of the photopigment • The isomerization sets off a biochemical chain reaction that eventually leads to an electrical current flowing across the membrane
Once an electrical response is initiated in the photoreceptors, this signal is relayed to the bipolar cells, which in turn relay it to ganglion cells, and thus to the brain • Absorbing even a single photon of light in a rod is enough to evoke a regular/reliable photocurrent • In fact, Hecht, Schlar and Perrine demonstrated in the 40's that human subjects can reliably detect single photons
Photoreceptors pass their signal onto bipolar cells and then to ganglion cells
Vials of rhodopsin in different bleached states. (Each vial contains a solution of rhodopsin that has been exposed to a different amount of light. The rhodopsin pigment changes color when exposed to light. This is called bleaching.)
The rods all have the same photopigment, rhodopsin. But there are three different types of cones in the human retina, each with a slightly different photopigment. In the butterfly you can actually see the cone photopigments by just shining light into the eye. In the human retina, the photoreceptors are way too small to make a picture like this.
Light/Dark Adaptation • Between night and day, light intensity can change dramatically (by a factor of forty billion) • By adjusting the pupil size, we can reduce the change in light intensity on the retina, but the number is still really high • So how can we see in such different light conditions? • Our eyes accomplish this feat by switching off between using rods and using cones
Rods are good for detecting single photons, while cones never saturate even at high light levels--together they can cover the full range of light intensities • Switching between rods and cones can take a while • Light adaptation is the second most important function of the retina • Important point: the brain is not interested in absolute intensities, but rather in intensities relative to the ambient light level
This dark adaptation curve shows the switch from cones to rods (open circles indicate when the violet test color was no longer noticeable)
