Lecture 6: Visual System PDF

Lecture 6: Visual System How we see Required readings: Chapter 6 1  Learning Objectives Describe the structure of the human eye Explain how the pupil and the lens can affect the image that falls on the retina. Explain why some vertebrates hav...

Lecture 6: Visual System How we see Required readings: Chapter 6 1  Learning Objectives Describe the structure of the human eye Explain how the pupil and the lens can affect the image that falls on the retina. Explain why some vertebrates have one eye on each side of their head, whereas other vertebrates have their eyes mounted side-by- side on the front of their heads. Explain the importance of binocular disparity.  review here If there is no light, there is no vision Light: waves of electromagnetic energy Wavelengths visible to humans Two properties of light: Wavelength (colour) Intensity (brightness) Figure 6.2 The electromagnetic spectrum and the colors that had been associated with wavelengths visible to humans. Pupil and Lens Light enters the eye through the pupil (hole in the iris) and reaches the retina pupil is the hole in the centre of the retina The amount of light reaching the retina is regulated by the irises irises controls the pupil, which controls the pupil size Pupil size is regulated by iris (which give your eye its colour) Pupil size is compromise between sensitivity and acuity Sensitivity and acuity Sensitivity: the ability to detect the presence of a dimly lit object “he is sensitive to dimly lit lights” Acuity: the ability to see the details of the object. “ Aaron pays attention to detail” Pupil size is compromise between sensitivity and acuity dimly lit object vs details The Human Eye Figure 6.4 Pupil and Lens Lens Focuses incoming light on the retina Focus is called accommodation Ciliary muscles adjust lenses to focus visual images When focused on something near, lens is cylindrical When focused on something far away, lens is flattened - main purpose is to focus light this is called accommodation - ciliary muscles is attached to the lens ( so think focus) - Closed-up curved ( cylindrical), Far away flattened refers to the shapes of the lens The Human Eye Figure 6.4  Eye Position Eye placement Predators−eyes in front Prey−eyes on side Predators have eyes in the front, giving them binocular vision for focusing on prey and judging distance. Prey have eyes on the sides, providing wide peripheral vision to detect predators from a distance and escape danger. Eye Position and Binocular Disparity Most mammals have two eyes on the front of their heads Most of what is seen is seen through both eyes Eyes see things from a slightly different perspective Difference in the two retinal images: binocular disparity Greater for closer things Helps create depth perception (3D perception) If you hold your finger out at arm’s length and then look at it alternately with your left eye only and then your right eye only, the image of your finger relative to the world behind it will shift somewhat. This is binocular disparity. Binocular disparity - great for close things and create depth perception The Human Eye Figure 6.4  Learning Objectives Describe the structure of the retina and name the cell types that make up the retina. Describe the duplexity theory of vision and explain the differences between the scotopic and photopic systems. Describe the types of involuntary fixational eye movements and explain what happens when all eye movements are blocked. Describe the process of visual transduction. Structure of the Retina Five layers Receptor layer (farthest from the light) Horizontal cell layer Bipolar layer Amacrine cell layer Retinal ganglion cell layer Rita always buys headphones ready Incoming light must pass through four layers before reaching receptors. Five Layer of the Mammalian Retina Figure 6.5. Structure of the Retina Five layers Receptor layer Horizontal cell layer Bipolar layer Amacrine cell layer Retinal ganglion cell layer Fovea is the center of attention ( vision ) In center of your field of vision High-acuity vision Optic disk or optic nerve Blind spot Completion (filling in) The Fovea Figure 6.6 A section of the retina. Blind spot and completion (activity to try)  Cones and Rods Red = cones Figure 6.5. Blue = rods. Cone and Rod Vision Duplexity theory Rods and cones mediate different types of vision Two visual systems: Photopic cone-mediated, lighted conditions Scotopic rod-mediated, dim light Figure 6.5.  Photopic system – low sensitivity Convergence Figure 6.8 Scotopic system – high sensitivity Convergence of cones or rods on a retinal ganglion cell. Low degree of convergence in cone-fed pathways  Photopic system – low sensitivity Convergence Figure 6.8 Scotopic system – high sensitivity Convergence of cones or rods on a retinal ganglion cell. Low degree of convergence in cone-fed pathways High degree of convergence in rod-fed pathways. Distribution of Cones and Rods over the Human Retina Figure 6.9 The number of cones and rods per square millimeter as a function of distance from the center of the fovea. Next to the temples Close to the nose Cone and Rod Vision Photopic Scotopic Cones Rods Light vision Dark/dim vision High acuity Low acuity Low sensitivity with few receptors High sensitivity with many receptors Fovea Periphery Low convergence cone fed High convergence * this would be high acuity because we * this would be high senstivity because it would need light to see an objects details refers to dimly lit objects * fovea is detailed vision make sense to be * high convergence is rod fed associated with photopic system Eye Movement Eye Movement Fixations Saccades: rapid movements between fixations Stabilized retinal images disappear Eye Movement Fixations Saccades: rapid movements between fixations Stabilized retinal images disappear Temporal integration why we see the things we do Sum of the inputs in such detail Explains why images are detailed, colored, and wide-angled Explains why things don’t disappear when we blink Eye Movement Most visual neurons respond to change so if you block the change (artificially stabilized) then images start to disappear. Make sure to try the “Check it out activity” in the textbook:  Visual Transduction Transduction is the conversion of energy. Visual transduction is conversion of light to neural signals by the visual receptors. Transduction by rods is well understood. Visual transduction is conversion of light—> neural signals ——> visual receptors Visual Transduction by Rods In the dark, rods are depolarized, release glutamate (neurotransmitter) When exposed to light, rhodopsin (found in rods) is bleached → separates into retinal and opsin rhodopsin separated into retinal and opsin Bleaching hyperpolarizes the rods → glutamate release is reduced (neural signal) Rods transmit signals through inhibition Inhibitory Response of Rods to Light Figure 6.12 In the dark: rods are depolarized R Glutamate inhibits polar cells Inhibitory Response of Rods to Light Figure 6.12 When light bleaches rhodopsin molecules, the rods’ sodium channels close; as a In the light: rods are result, the rods become hyperpolarized and release less hyperpolarized glutamate. Rods transmit signals through the neural system via inhibition. Polar cells are activated Learning Objectives Describe the components and layout of the retina-geniculate-striate system. In the context of the retina-geniculate-striate system, explain what is meant by retinotopic. From Retina to Visual Cortex: Retina-Geniculate-Striate System Visual information travels from: Retina: from rods and cones Geniculate: located in thalamus (relay nucleus) Primary visual cortex: also called striate cortex or V1 (occipital lobe) Retina-Geniculate-Striate There are two hemiretina System * Temporal * Nasal optic chiasm is higher than the optic tract Figure 6.13 Neural projections from the retinas through the lateral geniculate nuclei to the left and right primary visual cortex (striate cortex). thalamus The colors indicate the flow of information from various parts of the receptive fields of each eye to various parts of the visual system. (striate cortex) From Retina to Visual Cortex: Retina-Geniculate-Striate System Visual information travels from: Retina: from rods and cones Geniculate: located in thalamus (relay nucleus) Primary visual cortex: also called striate cortex (occipital lobe): Pathway Nasal hemiretinas decussate at optic chiasm Temporal hemiretinas stay ipsilateral Retinotopic Organization The surface of the visual cortex is a map of the retina Study in the 1970s: Stimulated visual cortex of blind participants Patients reported pattern of light corresponded to electrode placement If the stimulation was in the shape of a cross, participants would report “seeing” a glowing image of that shape. Learning Objectives start here Define the term receptive field Describe the work by David Hubel and Torsten Wiesel to map the receptive fields of visual system neurons. Describe the receptive fields of an on-center cell and an off-center cell Describe how views about the receptive fields have recently changed. Receptive Field of a Sensory Cell Defined as the stimulus region and the features that excite or inhibit the sensory cell. Receptive fields in the different levels of the visual system: Retinal ganglion cells Lateral geniculate nucleus neurons Lower layer IV of striate cortex Primary visual cortex Receptive Fields of the Retina and LGN Similar receptive fields in these areas: Retinal ganglion cells Lateral geniculate nucleus (LGN) neurons Characteristics: Smaller in the foveal area Circular Receptive Fields of the Retina and LGN Two types of receptive fields On-centre: burst of firing when light is turned on in the centre of the field and inhibition when light shines in the surrounding area on -centre- detects bright light off- centre - detects dark light Think about a light switch  Receptive Fields of the Retina-Geniculate-Striate System Figure 6.15 Receptive Fields of the Retina-Geniculate-Striate System then whats the difference ? Two types of receptive fields On-centre: burst of firing when light is turned on in the centre of the field and inhibited when light shines in the surrounding area Off-centre: cells are activated when the receptive field is dark and surrounding area is illuminated. Cells are inhibited when the light shines in the centre  Receptive Fields of the Retina-Geniculate-Striate System Figure 6.15  Receptive Fields of the Retina-Geniculate-Striate System Figure 6.15 Cells respond to amount of light but also the contrast of light and dark falling on the centre of the receptive field vs surrounding regions  Receptive Fields of Primary Visual Cortex Neurons Simple striate cells Respond best to bars or edges in a particular location and orientation Complex striate cells Respond best to straight lines of particular orientation complex is actually simple because that the ones that responds the best in a straight line Changing Concept of the Characteristics of Visual Receptive Fields So regina is wears a uniform the main concept was oreintataion, motion and direction of motion Receptive fields are more complex than originally thought. Retinal ganglion cells with receptive fields that are selective to: (1) uniform illumination, (2) orientation, (3) motion, and (4) direction of motion. Lateral geniculate cells have receptive fields that are sensitive to: (1) orientation, (2) motion, and (3) direction of motion. Changing Concept of Visual Receptive Fields: Contextual Influences Visual responses to natural scenes Assumptions of initial studies don’t hold Contextual influences shape properties of the receptive field: Timing, location, amount of light but also particular actions or emotional states.  Learning Objectives Describe the three class of visual cortex and identify their locations in the brain. Describe the areas of secondary visual cortex and association cortex involved in vision. Explain the difference between the dorsal and ventral streams and the functions that have been attributed to each stream. Describe the phenomenon of prosopagnosia and akinetopsia and discuss the associated theoretical issues. Three Different Classes of Visual Cortex Primary visual cortex Located in occipital lobe Receives most inputs from visual relay nuclei of thalamus Secondary visual cortex Located in the prestriate cortex (surrounds primary visual cortex) Receives input from primary visual cortex  Visual association cortex Visual Areas of Cortex Secondary visual cortex Visual association cortex Figure 6.20 Three Different Classes of Visual Cortex primary starts out at the occipital lobe Primary visual cortex Located in occipital lobe Receives most inputs from visual relay nuclei of thalamus Secondary visual cortex Located in the prestriate cortex (surrounds primary visual cortex) Receives input from primary visual cortex Visual association cortex Two areas: Inferotemporal cortex Posterior parietal cortex Receives input from secondary visual cortex and secondary areas of other sensory systems  Visual association cortex Visual Areas of Cortex Secondary visual cortex Visual association cortex Figure 6.20 Functional Areas of Secondary and Association Visual Cortex Portions of secondary and association cortex Areas specialized for particular type of visual analysis PET and fMRI have helped with identification of various areas in humans Dorsal and Ventral Streams Two anatomically & functionally Posterior distinct pathways parietal cortex WHERE Inferotemporal Figure 6.24 cortex WHAT Prosopagnosia https://www.cbc.ca/natureofthings/episodes/in-your-face Agnosia: failure to recognize Prosopagnosia: inability to recognize faces May not be specific to faces Difficulty distinguishing between visually similar members of stimuli Associated with damage to fusiform face area Area between the occipital and temporal lobes Akinetopsia Deficiency in the ability to see smooth movement Can be triggered by high doses of antidepressants Result of damage to the medial temporal area (MT) Patients with damage akinetopsia tend to have unilateral or bilateral damage to MT Activity in the MT increases when humans view movement (fMRI) Blocking activity of the MT with transcranial magnetic stimulation (TMS) produces motion blindness Electrical stimulation of the MT induces visual perception of motion