PSY106 Fall 2024 Lecture 9 PDF

WEEK 6, NOV 4TH – 10TH Two in-person lectures (Mon & Wed) Read Week 6 Readings on Canvas Complete Quiz 5 on Canvas by Sunday at 11:59pm Available at 5pm on Wednesday Optional: Attend a ULA session Optional: Participate in Week 6 Discussion by Sunday at 11:59pm Today’s Topics: 9A: R...

WEEK 6, NOV 4TH – 10TH Two in-person lectures (Mon & Wed) Read Week 6 Readings on Canvas Complete Quiz 5 on Canvas by Sunday at 11:59pm Available at 5pm on Wednesday Optional: Attend a ULA session Optional: Participate in Week 6 Discussion by Sunday at 11:59pm Today’s Topics: 9A: Retinal Circuit 9B: Optic Nerve 9C: Visual Cortex 9D: Other Visual Pathways DR. SCUDDER PSY106 LECTURE 9 TAKEAWAYS By the end of this lecture, you should be able to: Explain how ON and OFF bipolar cells differ from each other Describe the path that retinal axons take through the optic chiasm in order to reach the LGN Differentiate between the optic nerve and the optic tract and the organization of each Differentiate between the dorsal and ventral streams of visual processing Provide one explanation for the phenomenon of blindsight Trace the flow of visual information through three distinct pathways from the retina INTRODUCTION TO BIOPSYCHOLOGY FALL 2024 LECTURE 9A: RETINAL CIRCUIT DR. SCUDDER PSY106 GOALS OF THIS SECTION Describe how rods and cones affect bipolar neurons Describe how bipolar neurons affect retinal ganglion cells RETINAL PATHWAYS We just learned that light affects the amount of glutamate released by photoreceptors (rods & cones) This glutamate is directly affecting a set of neurons called bipolar cells BIPOLAR CELLS All bipolar cells are neurons that release glutamate (glutamatergic), and volume of glutamate release is tied to membrane potential (no action potentials here either – similar to photoreceptors) More depolarized = more glutamate released Dendrite(s) These neurons collect information from rods and Cell Body cones and relay it to retinal Axon ganglion cells Some bipolar cells collect information from many photoreceptors, others just connect to a one or two GLUTAMATE RECEPTORS The dendrites of bipolar cells have glutamate receptors, which respond to the glutamate released by rods & cones Elsewhere in the nervous system, glutamate is almost always an activator –it binds to ionotropic receptors (like AMPA receptors) and depolarizes the postsynaptic cell Here, some bipolar cells (called “ON bipolar cells”) have a special kind of metabotropic glutamate receptor causes glutamate to act as an inhibitor rather than an activator Glutamate release from rods & cones leads to the hyperpolarization (suppression) of these bipolar cells, causing them to release less glutamate “ON” BIPOLAR NEURONS When light is present, rods and cones will be inhibited and will release less glutamate, which will allow ON bipolar cells to return to their depolarized membrane potential and release their own glutamate But, there are bipolar cells with the opposite response! “OFF” BIPOLAR NEURONS When light is present, rods and cones will be inhibited and will release less glutamate, which leads to less activation of OFF bipolar cells, causing them to release less glutamate (light turns them OFF) BIPOLAR CELLS ON cells: bipolar cells that depolarize when light is present – light turns them ON, because glutamate turns them OFF Have metabotropic glutamate receptors at their synapses that cause hyperpolarization (suppression) of these cells, so darkness- induced glutamate release from rods and cones will suppress these cells OFF cells: bipolar cells that are quiet when light is present, and depolarize when it is absent – light turns them OFF, because glutamate turns them ON Have ionotropic glutamate receptors (including AMPA receptors) at their synapses, so glutamate release from rods and cones (which happens in darkness) will depolarize these cells BIPOLAR CELLS Sarah Freeman WHY SO COMPLICATED?! By having some bipolar neurons activated by light and some activated by darkness, this cell layer provides very valuable information about the pattern of light waves striking the eye OFF and ON bipolar cells will be arranged in meaningful patterns, and this patterned information will feed into the retinal ganglion cells that carry information into the brain There are actually two other sets of neurons (amacrine and horizontal cells) that signal between bipolar neurons (mostly using GABA), but we won’t get into that here RETINAL GANGLION CELLS Bipolar cells release glutamate onto the final layer of cells in the retina: retinal ganglion cells (RGCs) RGC’s have ionotropic glutamate receptors (like AMPA receptors), so they are depolarized by this glutamate RGC’s are more “normal” neurons, as they fire action potentials and have a long axon that projects far away These cells are constantly spiking, but the rate of firing (how rapidly they are generating action potentials) is determined by how much glutamate is being released from bipolar cells EXITING THE EYE This bundle of axons from RGC’s is called the optic nerve Action potentials in various patterns and rates are constantly propagating down this bundle, bringing this information into the brain  RETINAL CIRCUITRY RGCs fire more or fewer APs depending on input from bipolar cells Light enters the eye ON bipolar OFF bipolar and must pass cells are more cells are more through a few cell depolarized hyperpolarized layers before hitting when light is when light is photoreceptors present present (except right at the (release more (release less fovea) glutamate) glutamate) (rods and cones) Photoreceptors are hyperpolarized by light (release less glutamate) INTRODUCTION TO BIOPSYCHOLOGY FALL 2024 LECTURE 9B: OPTIC NERVE DR. SCUDDER PSY106 GOALS OF THIS SECTION Describe how signals from the retina exit the eye and make their way to the thalamus and other regions of the brain Explore the organization of the lateral geniculate nucleus of the thalamus EXITING THE EYE Retina includes a layer of retinal ganglion cells (RGCs) that are spiking at various frequencies and patterns Most of the axons from these neurons travel from the eye to a brain region called the thalamus This bundle of axons from RGCs is initially called the optic nerve Goal of this circuit: get all of the information about the right side of the visual scene over to the left visual cortex, and get all of the info about the left side of the scene over to right visual cortex Left visual hemifield Right visual hemifield  Goal: get all of the visual information from the left visual hemifield to the right visual cortex, and all of the visual information from the right visual hemifield to left visual cortex Nope! But why not?! Left Right eye eye Can we send all info from the left eye to right visual cortex and all info from the right eye to the left visual cortex? Left Right hemisphere hemisphere What the left eye sees What the right eye sees Binocular Zone: both eyes see this Left Right eye eye Left Right Left Right hemisphere hemisphere hemisphere hemisphere  The nasal (medial) The temporal (lateral) retina of the left eye retina of the left eye and the temporal and the nasal (medial) (lateral) retina of the retina of the right eye right eye can see the can see the right visual left visual hemifield hemifield Both sets of retinal Both sets of retinal neurons will send neurons will send axons to the right axons to the left hemisphere hemisphere Left Right hemisphere hemisphere VISUAL FIELDS Bundle of axons from the retina partially decussates (crosses to the contralateral side) at the optic chiasm – information from the right visual hemifield ends up sent to the left side of the brain, and information from the left hemifield ends up on the right OPTIC TRACT After the optic chiasm (where some of the axons cross over to the opposite side), this bundle of axons is known as the optic tract Left optic tract only has information about the right visual hemifield (but includes a mixture of info from both eyes); right optic tract only has information about the left visual hemifield The axons in the optic tract synapse onto neurons in a brain region called the thalamus – specifically, the lateral geniculate nucleus (LGN) of the thalamus LGN sorts visual information and passes it to primary visual cortex LATERAL GENICULATE NUCLEUS (LGN) LATERAL GENICULATE NUCLEUS (LGN) The different “stripes” or layers of the LGN have neurons that are responsible for processing different aspects of a visual stimulus For example, some layers process qualities of an object such as color and texture, while others process the location and movement of an object Some of this segregation begins in the retina, and various characteristics continue to be processed in parallel LAYERS OF THE LGN Because the bundle of axons that contacts these neurons is a mix of axons from both eyes, the LGN also helps keep that information straight Information from each eye stays segregated in the thalamus – each layer is either gathering information from the ipsilateral eye or the contralateral eye LAYERS OF THE LGN Layers 1 & 2: “where” information Key principle throughout Layers 3 - 6: “what” information visual system: Information is sorted out Layers 1, 4, 6: and processed in parallel Input from contralateral eye Layers 2, 3, 5: Input from ipsilateral eye You don’t need to memorize the layers of the LGN, just know that this region separates out information based on the eye that it came from and whether the information pertains to the location (“where”) or the qualities of the stimulus (“what”) INTRODUCTION TO BIOPSYCHOLOGY FALL 2024 LECTURE 9C: VISUAL CORTEX DR. SCUDDER PSY106 GOALS OF THIS SECTION Describe the organization of the primary visual cortex Explore the dorsal and ventral streams of visual processing PRIMARY VISUAL CORTEX The neurons in the LGN send their axons to basically just one region: primary visual cortex, located at the back of the brain This region is also called striate cortex The organization of the outside world (the visual scene) is still maintained here – you can see what we call a retinotopic map in nearly any of the several cortical areas that process vision RETINOTOPIC MAPS Take your time to really understand how this image of a woman gets represented in the brain – note the left-right and up-down reversals, and the fact that the two visual hemifields (1-5 and 5-9) end up separated in the LGN and cortex PRIMARY VISUAL CORTEX The neurons in this region are highly responsive to features in the visual field, but different neurons will care about different things Some respond to a very specific orientation of a line – they will fire only when the visual stimulus includes a line of that orientation Others are more complex, and need both a particular orientation and movement in a particular direction  VISUAL CORTEX NEURONS Hubel & Wiesel placed electrodes into the primary visual cortex of cats, allowing them to listen to the action potentials of these cells Neurons in this area spike in response to specific visual stimuli on the screen – a cell will typically have a preferred orientation and location Every tiny “pop” that you hear is a spike Technique called “in vivo electrophysiology” EXTRASTRIATE CORTEX Neurons in primary visual cortex send axons to neurons in nearby regions of cortex called extrastriate cortex, also called higher-order visual cortex Even more excitatory projections (glutamate) The retinotopic map of the world is passed to all of these other areas, but each area is specialized to analyze a different feature of the world Basic qualities like color and movement, but some “higher- order” visual areas care about particular categories of objects, such as faces PARALLEL PROCESSING Dorsal pathway = “where” (motion of stimuli) Extends into the parietal lobe Ventral pathway = “what” (qualities of stimuli such as color and texture) Extends into the temporal lobe If you want to eliminate incoming visual information about the woman’s raised hand (numbers 8-9), what part of visual cortex would you damage? A) Lateral part of right visual cortex B) Medial part of right visual cortex C) Lateral part of left visual cortex D) Medial part of left visual cortex https://forms.office.com/r/a91EBDJa6E  You can literally trace the number 9 through the visual system and see where it ends up Or, consider the fact that her hand is in the right visual hemifield, and thus must end up on the left side of visual cortex Right visual Medial vs Lateral: cortex Medial is closer to the midline Left visual (line between hemispheres), cortex lateral is towards sides of brain If you want to eliminate incoming visual information about the woman’s raised hand (numbers 8-9), what part of visual cortex would you damage? A) Lateral part of right visual cortex B) Medial part of right visual cortex C) Lateral part of left visual cortex D) Medial part of left visual cortex VISUAL PERCEPTION Conscious visual perception is the result of this series of events: 1. Distinct patterns of light (different wavelengths) strike neurons in the retina called photoreceptors 2. These neurons signal to another layer of neurons in the retina, which then excite one final layer of neurons in the retina (retinal ganglion cells), which have axons that exit the eye and enter the brain as the optic tract 3. APs course down those axons, which contact neurons in the thalamus and excite them 4. Thalamic neurons spike, and those APs propagate down axons that connect to the primary visual cortex 5. Neurons in primary visual cortex spike, exciting neurons in “higher-order” visual cortex  VISUAL PATHWAYS The eye (retina) Photoreceptors (rods & cones) Bipolar Cells ? Retinal Let’s briefly explore two Ganglion Cells other pathways that use Retina -> Thalamus = visual information Thalamus “retinofugal” ? Lateral Geniculate Nucleus The pathway from retina -> LGN -> visual cortex is primarily supporting Cortex conscious visual perception – are there other pathways? Primary Visual Extrastriate Cortex Cortex INTRODUCTION TO BIOPSYCHOLOGY FALL 2024 LECTURE 9D: OTHER VISUAL PATHWAYS DR. SCUDDER PSY106 GOALS OF THIS SECTION Define blindsight and explore the basis of this phenomenon Describe the pathway that sends information to the hypothalamus BLINDSIGHT Some individuals are “cortically blind” – damage to the back of the brain (primary visual cortex) have left them unable to consciously perceive visual stimuli Hold a basketball up in front of them and ask them what it is, and they wouldn’t even be able to tell you that you’re holding something up Some of these “blind” individuals can still react to an incoming object, or avoid obstacles when walking – we call this blindsight BLINDSIGHT He wouldn’t be able to describe the obstacles in front of him, but has no problem moving around them – what could explain this? ANOTHER ROAD FOR RGC AXONS About 10% of RGCs send their axons to a different brain region after reaching the optic chiasm – the superior colliculus (also called optic tectum in some animals), located in the midbrain This retinotectal pathway allows for rapid and somewhat automatic (non-conscious) movements of our eyes, head, and limbs If the retina -> LGN -> cortex pathway is damaged, retina -> superior colliculus pathway will be intact and can allow for some basic visually guided movements Mystery #2 = Solved! MYSTERY #2 A man has been blind from birth – if you show him a picture of a ball, he would not even be able to tell that you are holding up a picture, much less tell you what the picture is of. However, you toss a ball at his face, and he grabs it from the air. Was he lying about being blind? How could he do this? This man likely has damage to his visual cortex, and thus cannot consciously perceive any visual information. However, his retinotectal pathway is intact, allowing him to react to some types of visual stimuli VISUAL PATHWAYS The eye (retina) Photoreceptors (rods & cones) Bipolar One final visual Cells pathway! Retinal Ganglion Cells Retina -> Thalamus = “retinofugal” Thalamus Tectum (Midbrain) Lateral Superior Geniculate Colliculus Nucleus Retina -> Superior Cortex Colliculus = “retinotectal” Primary Visual Extrastriate Cortex Cortex CIRCADIAN RHYTHMS Many biological functions (and our wakefulness) vary throughout the course of a 24-hour day These fluctuations are known as circadian rhythms External light is used to “tune” these rhythms and keep them in sync with daytime vs nighttime Light information is passed from the retina to the suprachiasmatic nucleus (SCN) in the hypothalamus  VISUAL PATHWAYS The eye (retina) Retina -> Hypothalamus= Photoreceptors This pathway supports “retinohypothalamic” (rods & cones) conscious visual perception, but Hypothalamus our brain also does some visual Bipolar processing that we’re not Suprachiasmatic Cells consciously aware of nucleus (SCN) Retinal Ganglion Cells Retina -> Thalamus = Thalamus Tectum (Midbrain) “retinofugal” Lateral Superior Geniculate Colliculus Nucleus Retina -> Superior Cortex Colliculus = “retinotectal” Primary Visual Extrastriate Cortex Cortex LECTURE 9 TAKEAWAYS By the end of this lecture, you should be able to: Explain how ON and OFF bipolar cells differ from each other Describe the path that retinal axons take through the optic chiasm in order to reach the LGN Differentiate between the optic nerve and the optic tract and the organization of each Differentiate between the dorsal and ventral streams of visual processing Provide one explanation for the phenomenon of blindsight Trace the flow of visual information through three distinct pathways from the retina  KEY CONCEPTS Bipolar neurons react Information is passed from differently to the presence photoreceptors to bipolar cells of light based upon their to retinal ganglion cells, which expression of glutamate send axons out of the retina as receptors the optic nerve Visual information is processed in the dorsal RGC axons get reorganized and ventral streams, at the optic chiasms to allow allowing us to build a mental picture of the Some visual information is for the segregation of visual world around us sent to the midbrain and hemifields to the hypothalamus WEEK 6, NOV 4TH – 10TH Two in-person lectures (Mon & Wed) Read Week 6 Readings on Canvas Complete Quiz 5 on Canvas by Sunday at 11:59pm Available at 5pm on Wednesday Optional: Attend a ULA session Optional: Participate in Week 6 Discussion by Sunday at 11:59pm