Vision September 12 2014 Primary Visual Pathways Neural signals from the retina travel to the lateral geniculate nucleus in the thalamus and then to primary visual cortex in the occipital lobe Structure of the Eye Cornea and lens focus the image on the retina Iris controls how much light enters the eye The visual system works over 1011 orders of magnitude only 107 for the auditory system Eye muscles control where gaze is directed Structure of the Retina Receptors are rods and cones located at the back of retina There are about 100 million rods and 4 million cones at the front of retina The output from the retina comes from 1 million retinal ganglion cells Color Vision Cones are sensitive to different colors of light red green blue which is the basis for the trichromatic theory of color vision Rods most sensitive to blue green primarily detect whether light is present or not The Visual System is Backward The image is upside down and reversed on the retina The receptors are at the back of the retina behind the ganglion cells and other neurons Light turns off hyperpolarizes the rods and cones Before A Photon Strikes In dark cGMP keeps the Na channels open A Photon Strikes A photon spits rhodopsin into retinal opsin The opsin changes cGMP to 5 GMP which then can not keep the Na channels open depolarized The Na channels close and the rod or cone hyperpolarizes and stops releasing neurotransmitter Center Surround Receptive Fields Two Types of Retinal Ganglion Cells X retinal ganglion cells Receive input from cones Sensitive to color red green blue Y retinal ganglion cells Receive input from rods Sensitive to black and white Both types of retinal ganglion cells have center surround receptive fields Receptive Fields for Retinal Ganglion Cells Y retinal ganglion cells respond to light dark differences input from rods in periphery X retinal ganglion cells respond to color differences input from cones in fovea Pointillism Distribution of Rods and Cones Cones are mainly located in the fovea Rods are found mainly in the periphery Problem for vision is acuity vs sensitivity Acuity to see details Sensitivity to see faint stimuli at night Acuity vs Sensitivity Photopic system In the fovea cones Low convergence High acuity Good daylight vision Scotopic system In the periphery rods High convergence Low acuity Good night vision Photopic System Light cones color vision X retinal ganglion cell 80 of neurons trichromatic theory fovea center surround high acuity receptive field low sensitivity daylight Scotopic System Light rods color blind periphery Y retinal ganglion cell 20 of cells center surround receptive field low acuity high sensitivity night vision Visual Pathway Retina projects directly to the thalamus lateral geniculate nucleus From thalamus to primary visual cortex in occipital lobe Visual Fields Are Separated Right visual field Pathway from retina to thalamus crossed at optic chiasm Only some of the fibers cross Left thalamus and visual cortex respond to light in the right visual field etc L L R R Optic chiasm L R Right visual field Thalamus