7 Properties of light and perception In general:Wavelength – color (hue) perceptionIntensity – brightness perceptionSaturation – purity perception
8 Light enters the eye through the pupil size of the pupil is regulated by the irisThe lens focuses lighton the retinaNote: that theretinal imageis upside down.
9 Pupil sizeAdjusted in response to changes in illumination, which is a tradeoff between:Sensitivity – ability to detect the presence of dimly lit objectsAcuity – ability to see the details of objectsWhen illumination is high, pupils are constricted allowing a greater depth of focus of the image falling on the retinaWhen illumination is low, pupils dilate in response to low activation of receptors allowing more light to enter the eye but sacrificing acuity and depth of focus
11 AccomodationProcess of adjusting the configuration of the lens to bring images into focus on the retinaFocus on a near objectciliary muscles contractputting less tension on the ligamentsallowing the lens to take its natural cylindrical shapethus increasing its ability to refract (bend) lightFocus on a distant objectCiliary muscles relaxIncreasing tension on the ligamentsflattens the lensthus decreasing its ability to refract (bend) light
12 Binocular disparityThe difference in the positions of the same image on the two retinasIs greater for close objects (eyes must converge or turn slightly inward)The degree of binocular disparity enables the visual system to construct 3-D perception from two 2-D retinal images
13 The retina Composed of 5 layers of neurons Receptors (photoreceptors) 1 rod3 conesHorizontal cells (2 subtypes)Bipolar cells (10 subtypes)Amacrine cells (25-30 subtypes)Ganglion cells (10-15 subtypes)
15 The cellular structure of the retina Appears to be inside-outLight passes through the 4 cell layers before reaching the receptorsAfter receptor activation, signals are transmitted back out to the ganglion cells whose axons project across inside surface of the retina, gathering at the optic disk where the optic nerve begins as the ganglion cell axons leave the eye.
16 Two visual problems result from the inside out arrangement: Incoming light is distorted as it passes through the cell layersThere is a blind spot (no receptors or cells) at the optic disk where the axons gather to exit the eye
17 SolutionsThe fovea is an area (0.33 cm diameter) in the center of the retina where there is a thinning of the retinal ganglion cell layer.Less distortion of lightSpecialized for high-acuity vision (seeing details)Completion – the visual system uses information from receptors around the blind spot to fill in the gap in the retinal image.
18 photopic and scotopic vision The two systems are “wired” differentlyCones – low degree of convergence (a single ganglion cell receives signals from a few cones).Rods – high degree of convergence (a single ganglion cell receives signals from hundreds of rods).
20 photopic and scotopic vision Cones are concentrated in the fovea, which contains no rods.Rods are concentrated 20 degrees from the fovea and in the nasal hemiretina (retina half of both eyes near the nose).
21 Spectral sensitivity curve In general, more intense light appears brighter. However, wavelength also has an effect on the perception of brightness.A graph of the relative brightness of lights of the same intensity but at different wavelengths is called a spectral sensitivity curve (see Pinel p. 138).
22 Spectral sensitivity curves There are two spectral sensitivity curves.The photopic spectral sensitivity curve has a peak brightness at 555 nm (yellow-green)The scotopic spectral sensitivity curve has a peak brightness at 507 nm (green-blue)The Purkinje effect – walking through his garden, Purkinje noticed that his yellow and red flowers were brighter than the blues ones just before dusk; just a few minutes later the trend was reversed (blue flowers appeared as brighter greys).
23 Transduction - Conversion of one form of energy to another. Visual transduction – conversion of light to neural signals.Rhodopsin – the red pigment in rods becomes bleached when exposed to light.It is a G-protein-linked receptor that responds to light.
25 Light activation of rods: Light bleaches rhodopsin molecules.cGMP is broken down, closing sodium channelsSodium ions cannot enter the rod resulting in hyperpolarization.Glutamate release is reducedTransduction of light by rods demonstrates that signals can be transmitted through neural systems by inhibition.
26 signal transduction Light bleaches rhodopsin Opsin separates from retinal moleculeTransducin (G-protein) activatedα-subunit breaks away and activates PDE6PDE breaks down cGMPWith cGMP broken, the Na+ channel closes
27 From retina to primary visual cortex Pathway: retina lateral geniculate nucleus (LGN) primary visual cortex~90% of axons of retinal ganglion cells make up this pathwayLGN channelsParvocellular (P layers) run through top 4 layers of LGN – responsive to color and fine detail (input from cones)Magnocellular (M layers) run through bottom 2 layers of LGN – responsive to movement (input from rods)Most LGN neurons that project to primary visual cortex (V1, striate cortex) terminate in the lower part of cortical layer IV
28 Is retinotopic – each level is organized like a map of the retina Temporal hemiretinadoes not crossNasal hemiretinacrossesRight visual fields ofBoth eyesLeft visual fields ofBoth eyesBottom of visual fieldto dorsal cortexTop of visual fieldto ventral cortexNote that 25% of V1 is dedicated to fovea
29 Seeing Edges A visual edge is “nothing” Where two different areas of an image meet.A perception of contrast between two adjacent areas of the visual field
31 Lateral inhibition of ommatidia receptors in lateral eye of horseshoe crab A, B & C fire at the same rate (same high level of light stimulation and same degree of lateral inhibition)D fires more (same high level of stimulation from light but less lateral inhibition from E)F, G & H fire at the same rate (dim light)E, fires less because of greater inhibition from D.
32 SaccadesThe eye continually scans the visual field and makes a series of brief fixations (3/sec) connected by quick eye movements called saccades.The fixations are integrated to produce greater color and detail than the restricted foveal region can produce if it remained stationarystabilized retinal images, projected from a contact lens that moves with the eye; image disappears in a few seconds.
33 Brief fixations associated with saccades while a person views different pictures Making visual saccades to items of interest is a function of the superior colliculus