Psychology 224: Neuroscience Lecture Notes PDF

Psychology 224: Neuroscience Sensory Systems: Vision Reading: Carlson, Chapter 6  Announcement Exam #3 Exam 3 will be on 11/7, not 11/4. The Visual System  Retina Anatomy Posterior vitreous detachment (PVD): A common condition that occurs when the gel- like vitreous sepa...

Psychology 224: Neuroscience Sensory Systems: Vision Reading: Carlson, Chapter 6  Announcement Exam #3 Exam 3 will be on 11/7, not 11/4. The Visual System  Retina Anatomy Posterior vitreous detachment (PVD): A common condition that occurs when the gel- like vitreous separates from the retina at the back of the eye Symptoms: Floaters, flashes of light, less acute vision  Retina Anatomy There are the 3 major types of cells in the retina forming different layers: – 1) Photoreceptors – the cones and rods – 2) Bipolar cells – neurons that synapse with the photoreceptors – 3) Ganglion cells – neurons that synapse with the bipolar cells and whose axons give rise to the optic nerve  Carries visual information into the brain LIGHT Optic Nerve  Anatomy of the Photoreceptor Photoreceptors contain an outer segment and inner segment The outer segment contains several hundred lamellae Rod lamellae Outer segment Inner segment  Photopigments: First Step of Visual Perception The lamellae contain photopigments The photopigment consists of 2 parts: – opsin (a protein) – retinal (a lipid synthesized from vitamin A) Exposure to light splits the photopigment into its 2 parts, producing the receptor potential: hyperpolarization Hyperpolarization decreases release of neurotransmitter  LIGHT *Depolarized in darkness* INFORMATION FLOW  Neural Circuitry in the Retina Photoreceptors and bipolar cells do not produce action potentials – depolarization increases neurotransmitter release – hyperpolarization decreases neurotransmitter release Photoreceptors are depolarized in the dark and are continuously releasing neurotransmitter When the neurotransmitter binds to the bipolar cells, it leads to  LIGHT *The signal is sent from ganglion cells *Depolarized in darkness* via action potentials to the brain INFORMATION FLOW  Beyond the Retina Ganglion cell axons form the optic nerve and leave the retina The optic nerve crosses and forms the optic chiasm at the base of the brain At the optic chiasm, the axons from the cells of the inner halves of each retina (the nose side) cross to the opposite (contralateral) side of the brain The axons from the outer halves of each retina do not cross. They stay on the ipsilateral side. – Some information from the right visual field thus ends up on the left side of the brain and vice versa Optic chiasm After the Optic Chiasm… Afterthe optic chiasm, axons of the retinal ganglion cells travel to the lateral geniculate nucleus of the thalamus (LGN) – the first synapse LGN neurons send their axons through a pathway known as the optic radiations to the primary visual cortex Information Coding In the Visual System: The Retina  Foveal vs. Peripheral Acuity In the periphery of the retina, many photoreceptors converge on a single ganglion cell. So there is convergence of information from a large area of the retina. This leads to less visual acuity in the periphery than the fovea  Types of Ganglion Cells There are 3 types of ganglion cell in the retina; we are concerned with 2 of these: – 1) “ON” cells – respond with an excitatory burst when the retina is illuminated – 2) “OFF” cells – respond with an excitatory burst when the light is turned off These 2 types of cells are thus able to encode changes in illumination as you move around and look at things  Receptive Fields The receptive field of a neuron in the visual system is the part of the visual field that affects the neuron’s firing rate The location of a receptive field depends on the location of the photoreceptors that provide it with visual information. If a neuron receives information from photoreceptors located in the fovea, its receptive field will be at the fixation point.  Receptive Fields The receptive field of a neuron in the visual system is the part of the visual field that affects the neuron’s firing rate The receptive fields of ganglion cells have center-surround organization Each region (center and surround) responds to light in opposite ways https://www.google.com/search? sca_esv=577959321&rlz=1C5CHFA_enUS974US974&sxsr f=AM9HkKkRCReXIIXXNgCIlxeTLi2ZMwsyHw:1698707459 748&q=center+surround+retinal+ganglion+cells&tbm=v id&source=lnms&sa=X&ved=2ahUKEwjbq9y18p6CAxWZ v4kEHWO_BxMQ0pQJegQIDRAB&biw=1674&bih=1088&d pr=2#fpstate=ive&vld=cid:cd41c1de,vid:XOCCZfQPwTk, st:0  Advantages of ON-OFF System & Center- Surround Affects our ability to encode changes in light: – Increased brightness is encoded by ON-cells – Decreased brightness is encoded by OFF-cells Center-surround allows for objects to have enhanced contrast We perceive the right edge of each square to be lighter