Lecture 10: The Visual System PDF

The Visual System Announcements Next Tuesday (10/8): Extra review session during class time with Nilu Study guide and extra practice questions provided Attendance will be recorded Chapter 10 (Vision) Vision What is vision? What is light? Range of responsiveness  Photoreceptors...

The Visual System Announcements Next Tuesday (10/8): Extra review session during class time with Nilu Study guide and extra practice questions provided Attendance will be recorded Chapter 10 (Vision) Vision What is vision? What is light? Range of responsiveness  Photoreceptors Rods: scotopic vision - Low light levels - More sensitive to light - 90 million in retina, concentrated in periphery Cones: photopic vision - High light levels - High visual acuity - 4.5 million in retina, concentrated in fovea (inside macula)  Photoreceptors Rods: scotopic vision - Low light levels - More sensitive to light - 90 million in retina, concentrated in periphery Cones: photopic vision - High light levels - High visual acuity - 4.5 million in retina, concentrated in fovea (inside macula) Human Dog Human Garden Snail Visual field Visual fields correspond to portions of the retina Visual Pathway  Summary: the visual pathway Photoreceptors Anatomy of the eye 1. Fibrous tunic: Sclera: white outer coating of eye Cornea: transparent; covers iris and pupil 2. Vascular tunic: Iris: colored tissue around the pupil Ciliary body: Choroid: 3. Retina: Pigmented layer: tight layer of cells between choroid and photoreceptors Neural layer: contains retinal cells  Summary: the visual pathway Photoreceptors Retinal ganglion cells Organization of the retina Cell layers of the retina Photoreceptors Photopic system (cones) Three opsins: Scotopic system (rods) S-cone opsin: blue Rhodopsin M-cone opsin: green Convergence: many rods synapse on a single bipolar L-cone opsin: red cell Less convergence: some ganglion cells Slow receive information from only a single cone Fast Lateral inhibition: horizontal cells Lateral inhibition: horizontal cells Retinal ganglion cells Beta cells (Parvocellular) Central retina Input mainly from cones Good for color, detail Alpha cells (Magnocellular) Located in peripheral retina Input mainly from rods Good for motion and contrast detection  Summary: the visual pathway Photoreceptors Retinal ganglion cells Optic nerve Optic chiasm Optic tract  Summary: the visual pathway Receptor Photoreceptors Thalamus Retinal ganglion cells Optic nerve Optic chiasm Optic tract Superior colliculus Lateral geniculate nucleus The lateral geniculate nuclei LGN Primary visual cortex (occipital lobe)  Summary: the visual pathway Receptor Photoreceptors Thalamus Retinal ganglion cells Optic nerve Cortex Optic chiasm Optic tract Lateral geniculate nucleus Inferior visual field (space): Superior Retina Dorsomedial LGN Direct route to Cuneus (superior bank of calcarine sulcus) Superior visual field (space): Inferior Retina Ventrolateral LGN Indirect route to Lingual gyrus (inferior bank of calcarine sulcus)  Summary: the visual pathway Receptor Photoreceptors Thalamus Retinal ganglion cells Optic nerve Cortex Optic chiasm Optic tract Lateral geniculate nucleus Visual Processing  Two streams hypothesis Dorsal stream: where is it? Parietal lobe Spatial location Ventral stream: what is it? Temporal lobe Object identification and recognition (memory) Summary: the visual pathway Dorsal stream: mirror neurons Color perception Trichromatic hypothesis Three colors: blue-sensitive, green-sensitive, red-sensitive Labeled lines Opponent-process hypothesis Four hues and three opposed pairs White and black Blue and yellow Green and red Support for trichromatic hypothesis Each cone responds to a specific range of wavelengths Each cone has one of three types of pigments Problem: we can see more than three colors Support for opponent-process hypothesis An object of any color stimulates at least two types of cones We can see a range of colors with high acuity Implication: cones are not color receptors Form (rods) and color (cones) are created in the cortex, not at the level of the receptor Rods are not form specific: they respond to single points of light Cones are not color specific: they respond to ranges of wavelengths of light The dress that broke the internet Practice questions Describe a situation in which you might be relying on scotopic vision. Explain how light in this scenario would be sensed and processed from the photoreceptors to the cortex. Describe a situation in which you might be relying on photopic vision. Explain how light in this scenario would be sensed and processed from the photoreceptors to the cortex. Look at each ”stop” of the visual pathway for one side of the brain (right or left). Draw a picture showing what portion of the visual field and which eye are involved.