5 - Vision.docx

BMS 143: Vision Visible light 400 and 700 nm Photoreceptors in the eye are sensitive to wavelengths Wavelength amplitude determines color intensity All wavelengths – white light Absence of wavelength – black Visual field Field of view that can be seen without moving head or eyes Light waves diverge...

BMS 143: Vision Visible light 400 and 700 nm Photoreceptors in the eye are sensitive to wavelengths Wavelength amplitude determines color intensity All wavelengths – white light Absence of wavelength – black Visual field Field of view that can be seen without moving head or eyes Light waves diverge in all directions Light ray = Forward movement Colour Perceived colour is the wavelength that is reflected by objects All other wavelengths are absorbed Accommodation Adjustment of lens strength Strength dependant on lens shape Ciliary muscles (of the ciliary body) Relaxed ciliary muscles → Suspensory ligaments pull → Lens flattens (weak lens) Clear for far objects (20ft) Sympathetic control Contracted ciliary muscles → Suspensory ligaments relax → Lens rounded (strong lens) Clear for close objects Parasympathetic control Muscles need to be relaxed occasionally Focal point Light must be concentrated on the fovea for the image to be clear Visual acuity Determined by using the Snellen eye chart 20/20 is determined to be normal vision What you see / Normal people see Legally blind = best corrected ≤ 20/200 Nearsightedness (Myopia) Far items are not clear Focal point of eye behind where it should be Corrected with a concave lens Eyeball too long Lens is too strong Lens doesn’t relax enough to be flat enough Farsightedness (hyperopia) Close items are not clear Focal point of eye in front of where it should be Corrected with a convex lens Eyeball too short Lens is too weak Lens doesn’t tighten enough to round Common with older people Optic chiasm Medial side crosses over Lateral side remains Optic tract Nerves leaving the optic chiasm Lateral half + Opposite medial half Visual pathways Unilateral optic nerve lesion Peripheral vision knocked out for the affected eye Optic chiasma lesion Peripheral vision loss Vision loss depends on which optic tract is damaged Depth perception Ability to distinguish the relative distance of objects Information from the visual field is delivered to each half of the cortex simultaneously Binocular field of vision Each eye views an object from a slightly different vantage point The image from each eye is different (disparate images) Brain uses disparity between images to estimate distance Only some depth perception with one eye Ex: relative size, interposition, linear perspective, lighting, shading, … Phototransduction Converting light stimuli into electrical signals In the retina Photoreceptor cells Rods and cones Bipolar cells Ganglion cells Lateral inhibition Feedback signals between photoreceptors Only important signals are sent to the brain Involved horizontal and acronine cells Horizontal cells Modify photoreceptor cell signals Synapse rods, cones, and bipolar cells Amacrine cells Modify photoreceptor cell signals Synapse bipolar cells and ganglion cells Receptor cells Receptor cells Rods = 120 million cells Cones = 6 million cells Optic nerve Ganglion cells = 1 million cells Convergence > 100 million receptor cells to 1 million neurons Signal produced is the average stimuli Receptor cell may not have its own label line to the brain Each receptor would have its own neuron (in a perfect world) Visual acuity Highest in the fovea Cones highest at the fovea Rods increases towards the periphery Ratio of receptors to ganglion More ganglion = Better image Signal transduction Light activates photopigments Receptor potential Action potential generated in ganglion cells Information is transmitted to the brain for visual processing Optic disc Convergence of all ganglions Causes a blink spot No photo receptors Image filled in by the brain Photoreceptors Outer segment Houses discs of photopigments Inner segment Contains metabolic machinery of the cell Synaptic terminal Stores and releases neurotransmitters to bipolar cells Rods vs Cones Cones Need more light to activate Colour vision Low sensitivity Rods Low light vision Shades of grey High sensitivity Photopigment Rhodopsin is found in rod membrane Opsin Protein Retinal Vitamin A derivative 11-cis-retinal is inactive All trans-retinal is active Cones Red L-type Long-wavelength-sensitive Green M-type Middle-wavelength-sensitive Blue S-type Short-wavelength-sensitive Receptive field response pattern Emphasizes differences in relative brightness Helps define contours of images No information on absolute brightness Colour vision Colour depends on the stimulation ratio Red/Green/Blue White = all wavelengths Black = absence of light Opponent-process theory Cones become saturated Activates opposing colour Blue-Yellow + Green-Red + Black-White Colour blindness Protanopia Red can’t be seen 4 types Tritanopia Blue-Yellow can’t be seen 2 types Light that enters the eye Regulated by the size of the pupil Contraction of pupillary sphincter Restricts light Myosis = Smaller pupils Parasympathetic control Dilation of pupillary dilator Increases light Mydriasis = Larger pupils Sympathetic control Light sensitivity Depends on light-responsive photopigment quantity in rods and cones Eyes adapt to light intensity by adjusting the amount of photoreceptors Intense light (sunlight) breaks down photopigments Decreases photoreceptor sensitivity Absence of light (dark) allows photopigments to regenerate Increased photoreceptor sensitivity Dark adaptation Bright sunlight into darkened surroundings Photopigments are increasing Light adaptation Dark to bight light Little contrast between lighter and darker parts Entire image appears bleached