Lecture 10 - Vision (Part 2) PDF

Introduction to Behavioral Neuroscience PSYC 211 Lecture 10 of 24 – Vision (part 2) (rest of chapter 6 in the textbook) Professor Jonathan Britt Questions? Concerns? Please write to [email protected]  QUIZ 1 & MIDTERM 1 GRADES Th...

Introduction to Behavioral Neuroscience PSYC 211 Lecture 10 of 24 – Vision (part 2) (rest of chapter 6 in the textbook) Professor Jonathan Britt Questions? Concerns? Please write to [email protected]  QUIZ 1 & MIDTERM 1 GRADES The Sunday morning before Midterm 1… 50% of the class had already gotten 100% on Quiz 1 (at least 21/25). The average midterm 1 grade for these students was 80%. 25% of the class had not yet opened Quiz 1. The average midterm 1 grade for these students was 64%. Quiz 2 will open Wednesday night, October 9th. It covers lectures 9-11. You can take this quiz as many times as want. Only your highest grade is recorded. It will remain open throughout the semester (until the final exam). Remember there is also 2% extra credit available (see details on MyCourses).  VISUAL PROCESSING Back of eye Light Lateral geniculate nucleus (LGN) of the thalamus  RECEPTIVE FIELDS The receptive field of a neuron is a description of the (external) stimuli that activate it. For a neuron involved in visual processing, its receptive field is where light must be in visual space and what properties it must have to change the activity of the cell. It is in an area of visual space relative to a fixation point (where the animal is looking). To identify the receptive field of a cell involved in visual processing, we record the cell’s activity as the animal maintains focus on one spot on a computer screen (a fixation point). We then systematically shine light in different areas of the monitor (e.g., relative to the fixation point) to determine where in visual space the presence of light influences the activity of the cell. Once we find where the receptive field is, we determine if the cell responds differently to different colors or patterns of light in that location. While you are looking at this spot When I find the spot where the presence of this circle influences (the fixation point), I will move the the activity of the recorded neuron, we say this is the location of red circle all around the screen. the neuron’s receptive field, defined relative to the fixation point. Next, we assess how different colors or patterns of light in this location influence the cell’s activity.  PHOTORECEPTOR CELLS Photoreceptor cells, like most cells in the retina, do not have action potentials. They release glutamate in a graded fashion dependent on their membrane potential: the more depolarized they are, the more glutamate they release. But their responses to light are opposite of what you might expect. In complete darkness, photoreceptor cells sit at -40 mV. This is their resting membrane potential. At rest, photoreceptor cells continuously release glutamate. When activated by light, photoreceptor cells hyperpolarize to -70 mV and stop releasing glutamate.  THE DARK CURRENT Photoreceptor cells express an uncommon “leak” sodium ion channel that sits open at baseline (in the dark). The influx of sodium ions through these ion channels (the dark current) causes photoreceptor cells to sit at -40 mV, where they continuously release glutamate. When an opsin protein absorbs light, it launches an intracellular g-protein signaling cascade that closes the open sodium ion channels. The closing of these ion channels causes the membrane to hyperpolarize to -70 mV, at which point the photoreceptor cell stops releasing glutamate. All photoreceptor cells work like this. All the opsin proteins responsible for our conscious perception of vision are inhibitory metabotropic receptors. When activated by light, they cause membrane hyperpolarization, which stops the photoreceptor cell from releasing glutamate.  BIPOLAR CELLS Dark Light Dark Bipolar cells also do not have action potentials. Like photoreceptor cells, they release glutamate in a graded manner dependent on their membrane potential. There are two types of bipolar cells: OFF bipolar cells and ON bipolar cells. OFF bipolar cells express normal excitatory ionotropic glutamate receptors, so their activity patterns follow that the photoreceptor cells that connect to them.  In the dark, when photoreceptors are depolarized (-40mV) and releasing glutamate, OFF bipolar cells will also be depolarized and releasing glutamate.  In the presence of light, when photoreceptors are hyperpolarized (-70mV) and not releasing glutamate, OFF bipolar cells will also be hyperpolarized and not releasing glutamate. ON bipolar cells have the opposite pattern of activity because they only express inhibitory (metabotropic) glutamate receptors. So, in the dark, when photoreceptor cells are releasing glutamate, ON bipolar cells will be hyperpolarized and not releasing glutamate. In the light, ON bipolar cells depolarize and release glutamate. And finally, retinal ganglion cells (RGCs) are typical neurons. They have normal action potentials and express normal excitatory ionotropic glutamate receptors.  HORIZONTAL CELLS Bright Dim Bright light light light Horizontal cells interconnect neighboring photoreceptors cells. They regulate the amount of glutamate that is released from photoreceptors cells based on the activity of their neighbors. In the figure on the right, the center photoreceptor cell senses dim light, which should slightly hyperpolarize it. However, neighboring photoreceptor cells are activated by bright light, which makes them relatively more hyperpolarized (more light = a stronger response). Horizontal cells compare the activity of neighboring photoreceptor cells. They recognize that the center photoreceptor cell is getting less light than its neighbors, and they accentuate this difference by counteracting the small light-induced hyperpolarization in the dimly lit cell. Thus, horizontal cells depolarize the “axon terminals” of photoreceptor cells according to how brightly lit the neighboring photoreceptor cells are. (Like photoreceptor and bipolar cells, horizontal cells do not have action potentials. They release glutamate in a graded manner dependent on their membrane potential.)  BIPOLAR CELLS Membrane potential Membrane potential and neurotransmitter release and neurotransmitter release of an ON bipolar cell of an OFF bipolar cell more neurotransmitter more neurotransmitter release release Response when Response when upstream upstream photoreceptor photoreceptor cell is in cell detects light -45 mV -45 mV darkness, but neighboring cells are brightly lit Resting state -60 mV Resting state -60 mV (in darkness) (in darkness) Response when upstream -75 mV Response when -75 mV photoreceptor cell is in upstream photoreceptor darkness, but neighboring cell detects light cells are brightly lit less neurotransmitter less neurotransmitter release release  BIPOLAR CELL RECEPTIVE FIELDS The influence of horizontal cells creates a “center-surround” organization in the receptive fields of bipolar cells. ON bipolar cell response to light Normal response Exaggerated to light response to darkness (OFF bipolar cells respond in the opposite manner.)  RETINAL GANGLION CELL RECEPTIVE FIELDS Third cell in the pathway: Retinal ganglion cells have action potentials and a baseline firing rate in the dark. They inherit their receptive fields from bipolar cells, so they also have a “center-surround” organization and are classified as ON or OFF cells. ON retinal ganglion cells increase their rate of spiking when light is in the center of their receptive field. They decrease their rate of spiking when light is brighter in the surround area of the receptive field. OFF retinal ganglion cells show the opposite pattern. They decrease their rate of spiking when light is in the center of the receptive field and increase their rate of spiking when light is in the surround area. When light fills the entire receptive field, the response is a smaller version of the center illumination response.  RETINAL GANGLION CELL RECEPTIVE FIELDS Retinal ganglion cells in the fovea process color information. They integrate information from many bipolar cells and have these types of receptive fields: SUMMARY OF RECEPTIVE FIELDS IN THE RETINA The first cell in the pathway – Photoreceptor cells: Rod cells and cone cells are sensitive to different wavelengths of light, but they all hyperpolarize and release less glutamate when the appropriate wavelength of light is in their receptive field. Thus, their receptive fields are generally quite simple, defined by a location in space and a wavelength of light. Second cell in the pathway – Bipolar cells: The receptive field of a bipolar cell is the sum of the receptive fields of the cells they receive input from. They receive synaptic input photoreceptor cells directly and indirectly (via horizontal cells), so their receptive fields are larger than the receptive field of an individual photoreceptor cell. The receptive fields of bipolar cells are defined by a location in space, a wavelength of light, and whether they exhibit ON or OFF responses to light in the center of their receptive field. Bipolar cells in the fovea receive direct synaptic input from only one photoreceptor cell, so the center of their receptive fields are the same size as one photoreceptor cell). Bipolar cells outside the fovea receive direct synaptic inputs from many photoreceptor cells, so the center of their receptive fields are quite large (the sum of the receptive fields of all the photoreceptor cells that connect to them). Third cell in the pathway – Retinal ganglion cells: These cells have normal action potentials, and they send information out of the retina. Their receptive fields are similar to that of the bipolar cells (but in the fovea they also show color opponency).  RECEPTIVE FIELDS IN THALAMUS AND V1 Retinal ganglion cells (RGCs) transmit visual information from the retina to the thalamus (the lateral geniculate nucleus). Thalamic neurons relay the information to primary visual cortex (area V1). The receptive fields of thalamic neurons are similar to that of the retinal ganglion cells. The receptive fields of neurons in V1 are the sum of many LGN neurons. Thalamus Cerebral cortex Simple cells in V1 are sensitive to lines of light, and their receptive fields are typically organized in a center-surround fashion. RECEPTIVE FIELDS IN PRIMARY VISUAL CORTEX Primary visual cortex (also known as V1 or striate cortex) Neurons in V1 spike when there is a line of light in a particular orientation in their receptive field. On the right are recordings from one V1 neuron as different patterns of light are displayed in the cell’s receptive field. The cell is most responsive to a vertical line of light in the center of its receptive field. Some V1 neurons respond best to vertical lines, some to horizontal lines, and some to lines oriented somewhere in between. CORTICAL COLUMNS IN PRIMARY VISUAL CORTEX Every spot in your visual field is rigorously analyzed by a cortical column in V1. All neurons within a cortical column analyze the same area of visual space. Together, they analyze the orientation of light in the associated receptive field. The location of sharp transitions in the contrast/color of light reveals borders, edges, and corners. Neurons downstream of V1 put all this information together to identify whole objects and their position in space.  VISUAL ASSOCIATION CORTEX >25% of the cerebral cortex is dedicated to processing visual information. All of the occipital lobe that is not primary visual cortex is considered visual association cortex. Visual association cortex extends into the parietal and temporal lobes, forming respectively the dorsal and ventral streams of visual information processing. Different areas of visual association cortex are sensitive to different features of the visual environment (e.g., particular shapes, locations, movements, and colors). Striate cortex is synonymous with primary visual cortex (Area V1) Extrastriate cortex is synonymous with visual association cortex (areas V2, V3, V4, etc.) ‘WHAT’ AND ‘WHERE’ VISUAL STREAMS The dorsal stream of visual information starts in primary visual cortex and ends in posterior parietal lobe. It is involved in identifying spatial location. It encodes where objects are, if they are moving, and how you should move to interact with them or avoid them. The ventral stream starts in primary visual cortex and ends in inferior temporal lobe. It is involved in identifying form (shape). It encodes what the object is and its color.  DEPTH PERCEPTION Requires the dorsal stream “where” pathway in the parietal lobe. Monocular vision: Some V1 neurons respond to visual input from just one eye. Binocular vision: Most V1 neurons respond to visual input from both eyes. Depth perception: There are many monocular cues that can be used to estimate depth, such as relative size, amount of detail, relative movement as we move our eyes, etc. These are the cues we use to appreciate depth when looking at a photograph or TV screen (any flat, 2-dimensional image). Only one eye is required to perceive depth with monocular cues. Stereopsis: The perception of depth that emerges from the fusion of two slightly different projections of an image on the two retinas. The difference between the images from the two eyes is called retinal disparity. It results from the horizontal separation of the two eyes. It improves the precision of depth perception, especially for moving objects. Two eyes are helpful when playing sports, but also (to some extent) when pouring a glass of water.  AGNOSIA An agnosia is a deficit (problem) in the ability to recognize or comprehend certain sensory information, like specific features of objects, people, sounds, shapes, or smells, although the specific sense is not defective nor is there any significant memory loss. An agnosia relates to a problem in some sensory association cortex (typically in a single sensory modality) - not to problems that relate to the sensory neurons themselves or to the primary sensory areas. – For example, being blind or deaf is not considered to be an agnosia. Blindness can result from damage anywhere between the eye and primary visual cortex. – Visual agnosia is caused by damage to visual association cortex (cortical visual areas that are downstream of primary visual cortex). Akinetopsia – a deficit in the ability to perceive movement – is a type of visual agnosia caused by damage to the dorsal visual stream in the parietal lobe of the cerebral cortex  VISUAL AGNOSIA RELATED TO THE VENTRAL STREAM Cerebral achromatopsia – In contrast to regular achromatopsia, which is complete color blindness due to defective cone opsin signaling, cerebral achromatopsia is a visual agnosia caused by damage to the cerebral cortex in the ventral visual stream. People with cerebral achromatopsia deny having any perception of color. They say everything looks dull or drab, and that it is all just “shades of grey”. (People born with regular achromatopsia don’t say those things, because they have no conception of color.) Prosopagnosia – Failure to recognize particular people by sight of their faces; a visual agnosia caused by damage to the fusiform gyrus (fusiform face area) in the ventral visual stream.