EYES!
l.m. of embryonic eye Vitreous humour forming Choroid (pigmented) layer forming conjunctiva cornea Retina forming lens Iris forming
Embryonic eye development Spherical lens retina cornea
External anatomy of the eye Pigmented iris – circular and longitudinal muscles sclerotic Pupil – diameter controlled by iris muscles Curved transparent cornea – responsible for refraction of light
Figure 6.2 Cross section of the vertebrate eye Note how an object in the visual field produces an inverted image on the retina.
Label the following: j a i b h c d g e f
Perception of contours where there really Subjective Contours Subjective Necker Cube Is it really there? Perception of contours where there really are none.
Form Perception & Feature Analysis Bottom-Up Processing Based upon properties of the stimulus (e.g., patterns of light & dark areas Top-Down Processing Based upon higher-order information (e.g., prior knowledge & context) What is this picture?
Figure 6.18 An illustration of lateral inhibition Do you see dark diamonds at the “crossroads”?
Dark & Light Adaptation Adaptation - process by which the eye becomes more or less sensitive to light
Cones and Colour
NO! Colour Vision Do objects possess colour? Is a lemon “yellow”? Light has no colour Is a chili pepper “red”?
Trichromatic Theory of Colour Vision Human eye has 3 types of cone receptors sensitive to different wavelengths of light. Helmholtz 1852 Short Medium Long People see colours because the eye does its own “colour mixing” by varying ratio of cone neural activity
Bleaching Bleaching occurs when you have looked at a red picture too long the red iodopsin has being bleached so when you look at white paper the red iodopsin is temporally out of order.
Transduction Both Rods and Cones contain photopigments (chemicals that release energy when struck by light) 11-cis-retinal is transformed into all-trans-retinal in light conditions this results in hyperpolarization of the photoreceptor the normal message from the photoreceptor is inhibitory… Light inhibits the inhibitory photoreceptors and results in depolarization of bipolar and ganglion cells
Cones Rods Retina Several layers of cells in inner surface of choroid Contains photoreceptors - Rods & Cones Daytime Night Vision High resolution Poor definition Color Black & White Center of retina Periphery of retina Less abundant More abundant Cones Rods
Rods & Cones: Distibution Rod density high away from the center The more sensitive rods (~100_rods-1_neuron map) help track peripheral image motion ~120 million rods in retina Cone density high near the center The 0.3 mm dia fovea has only high density of cones (1_cone-1_neuron map) helps form sharp brilliantly colored images ~6-7 million cones in retina
A rod cell (upper) and a cone cell From which direction would light come?
The Photo-receptors: Rods & Cones Phototopic Chromatic Fast Foveal Rods Scotopic Achromatic Slow Peripheral vision
A rod cell Direction of light
Figure 6.4 Visual path within the eyeball The receptors send their messages to bipolar and horizontal cells, which in turn send messages to the amacrine and ganglion cells. The axons of the ganglion cells loop together to exit the eye at the blind spot. They form the optic nerve, which continues to the brain.
Rods & Cones: Fovea & Blind Spot Fovea a 0.3 mm spot with cone-only distribution: highest acuity and color rendition Blind spot where optic nerve leaves the retina
retina Rod cells B-P Cells G-cells LIGHT
Retinal signal processing Integrator neurons Horizontal cells Bipolar cells Amacrine cells Ganglion cells Cones Cone > Bipolar cell > Ganglion cell Rods Rod > Bipolar cell > Amacrine cell > Ganglion cell
Rods & Cones Photosensitive protein is rhodopsin, membrane protein, that modulates membrane ion conductivity via a biochemical cascade once it absorbs a photon, with the cell getting hyperpolarized as a function of light Different amino-acid sequences in the ‘opsin’ segments of rhodopsin give the different color sensitivities of rods & cones
Bipolar Cells Many Rod cells are connected to one bipolar cell which means that when only one of the Rod cells are activated an impulse is sent to the brain. One Cone cells is connected to one bipolar cell which means that the light needs activate each Cone cell to send an impulse. This is why the Cone cells have a higher acuity and why they cant function in the dark.
Link to brain: Primary pathway Optic nerve Optic chiasm Lateral geniculate body Optic radiation Visual cortex http://www.brother.com/usa/printer/advanced/lcv/light1.html