Week 9: Vision Flashcards
What is light? What is visible to humans?
▪ Electromagnetic radiation, defined by its frequency (or wavelength measured in nanometers, nm) and amplitude
▪ Spectrum between 380 and 760 nm is visible to humans ▪ Light rays (made of photons, photons activate receptors) emitted by a luminous object can be reflected, absorbed and/or refracted
Light travels at 300,000 km/hr
Structures of the eye
▪ Enters the eye through the cornea (clear dome)
▪ Progresses through the pupil (opening in the centre of iris) ▪ Is bent by the lens (cilia muscles contract/ relax) image reversed and inverted) ▪ Continues through the vitreous humour (clear gel) ▪ Projected onto the retina
What is the process of focussing called?
A process of accommodation
Accommodation
▪ Lens shape changes to focus image/ lens on the retina
○ Focus on objects on range of distances ○ Controlled by ciliary muscles ○ Contract = zonules loosen = see shorter distances Relax = zonules contract = see longer distances
Presbyopia
as we age, the lens become stiffer = harder for lens to change shape = why as we get older vision is more likely to worsen
Layers of the retina
▪ Photoreceptors
▪ Bipolar cells
▪ Ganglion cells
▪ Horizontal and amacrine cells
Photoreceptors
convert light energy into neural activity (photopigment in cells - lamella - break down under photon energy = hyperpolarisation of rods & cones as channels close = depolarises the bipolar cells)
(more on rods and cones later)
Bipolar cells
Transmit information to ganglion cells (then send info to ganglion cells)
Ganglion cells
integrate information and send APs to brain (occipital lobe through the thamalus)
Horizontal and amacrine cells
lateral neurites influence cells close by (communicate between bipolar cells and ganglion cells, improve integration of info)
Photoreceptors: Rods: Where?
▪ 92 million (a lot more than cones)
▪ Found mainly in the retinal periphery (not the fovea)
Rods: Function? Colour? Focus?
▪ Very sensitive to light (identify light and dark)
▪ Monochromatic information (not sensitive to colour)
▪ Poor acuity (not as focussed on sharpness of objects - why peripheral is less clear)
Photoreceptors: Cones: Where?
- 4.6 million
- found mainly in the fovea
Cones: Function? Colour? Focus?
▪ Less sensitive (to light)
▪ Provide information about hue (colour/ wavelength) ▪ High acuity (very focussed on shape, sharpness of image) (There are different types of cones, sensitive to different wavelengths/ colours)
Why is there a blind spot in vision?
Optic disk - all the axons from ganglion cells here (form the optic nerve) so does not have rods & cones = blind spot
Transduction
▪ A process that converts an external stimulus to an internal stimulus
▪ Transduction of light energy into changes in membrane potential
What causes hyperpolarisation of photoreceptors?
○ Hyperpolarisation of photoreceptors because photons cause a breakdown of photopigment, so sodium & calcium channels close
= Depolarisation of bipolar cells
Receptive field
▪ The area of the visual space in which a stimulus must be presented to change the activity of a neuron
▪ Size of the neuron’s receptive field determines its acuity (smaller is better) and sensitivity (larger is better)
Receptive field size of Fovea?
Small as few photoreceptors converge on ganglion cell (parvocellular ganglion cells)
Small receptive field = sharper image, more details
(not as many cones in fovea - the centre of retina)