NEU 101: Vision II
NEU 101: Vision II
Welcome
Learning Outcomes
Students are expected to:
Identify how the three different types of cones contribute to color perception (trichromacy).
Trichromacy is the theory that the human retina contains three types of cone photoreceptors, each maximally sensitive to different wavelengths of light: short (blue), medium (green), and long (red) wavelengths. Color perception arises from the brain's interpretation of the differential activity across these three cone types.
Understand the pathways (described humorously as "crissy-crossy uppy-downy") through which light enters the eyeball and is processed by the primary visual cortex.
The visual pathway involves light hitting photoreceptors in the retina, activating bipolar cells, then ganglion cells. The axons of ganglion cells form the optic nerve. These nerves partially cross at the optic chiasm (the "crissy-crossy") ensuring that the left visual field projects to the right hemisphere and the right visual field projects to the left hemisphere. From here, signals primarily go to the lateral geniculate nucleus (LGN) in the thalamus, then via optic radiations to the primary visual cortex (V1) in the occipital lobe.
Recognize definitions and anatomy diagrams of:
Lateral geniculate nucleus (LGN): A principal relay center in the thalamus for visual information from the retina to the primary visual cortex. It acts as a processing station, organizing visual input by eye, visual field, and feature detection (e.g., motion, form).
Optic nerve: A bundle of over a million ganglion cell axons that transmit visual information from the retina to the brain.
Optic chiasm: The point where the optic nerves from both eyes converge and partially decussate (cross over), allowing visual information from each visual field to be processed by the contralateral cerebral hemisphere.
Identify the "horizontal" layers within a retinal circuit.
Within the retina, horizontal cells and amacrine cells form 'horizontal' or 'lateral' circuits. Horizontal cells mediate lateral inhibition between photoreceptors and bipolar cells, enhancing contrast. Amacrine cells integrate signals between bipolar and ganglion cells, contributing to motion detection and other complex visual processes.
Explain the role of convergence in striking a balance between acuity and sensitivity in rod and cone visual systems.
Convergence refers to multiple photoreceptors synapsing onto a single ganglion cell. High convergence (many rods to one ganglion cell) leads to high sensitivity (ability to detect dim light) but low acuity (poor detail resolution). Low convergence (fewer cones to one ganglion cell, especially 1:1 in the fovea) results in low sensitivity but high acuity (sharp detail and color vision).
Distinguish between the properties of parvocellular and magnocellular ganglion cells.
Parvocellular (P-cells): Characterized by small receptive fields, slow response to stimuli, and are primarily responsible for detailed color and form perception. They project to the parvocellular layers of the LGN.
Magnocellular (M-cells): Characterized by large receptive fields, fast transient response to stimuli, and are crucial for detecting motion and depth. They project to the magnocellular layers of the LGN.
Identify the specific layers of LGN responsible for processing visual information from each eyeball and the corresponding visual field sides.
The human LGN has six layers. Layers 1 and 2 are magnocellular layers, while layers 3-6 are parvocellular layers. Within these layers, cells respond exclusively to input from one eye: layers 1, 4, 6 receive input from the contralateral (opposite side) eye, and layers 2, 3, 5 receive input from the ipsilateral (same side) eye. The left LGN processes information from the right visual field, and the right LGN processes information from the left visual field.
Understand the concept of receptive fields.
A receptive field is the specific region of the sensory surface (in vision, the retina) that, when stimulated, causes a change in the firing rate of a particular neuron in the visual system. It defines the area from which a neuron receives its input and thus the stimulus properties it responds to.
Visual Field Representation
Explanation of how light hits the retina from both the left and right visual fields.
Light from the left visual field strikes the nasal retina of the left eye and the temporal retina of the right eye. Similarly, light from the right visual field strikes the nasal retina of the right eye and the temporal retina of the left eye. Due to the partial crossing at the optic chiasm, all information from the left visual field is sent to the right cerebral hemisphere, and all information from the right visual field is sent to the left cerebral hemisphere.
Diagram Depictions:
Light pathway from left and right eyes.
Graded Potentials
Definition: Graded potentials are not discrete (all-or-nothing action potentials); they exist on a continuum of more-or-less, meaning their amplitude varies with the strength of the stimulus. They are localized, decremental (lose strength over distance), and can summate both spatially and temporally. They are essential for sensory transduction, allowing for a nuanced response to varying stimulus intensities.
All types of sensory receptor cells generate graded potentials:
Visual (Vision)
Auditory (Hearing)
Tactile (Touch)
Olfactory (Smell)
Gustatory (Taste)
Neural Circuits
Common neuron representation discussed.
Acknowledgment of complexity in drawing detailed neural circuits. Simplification of representations is common for clarity.
Neural Circuit Types:
Vertical Circuits: These are the direct, afferent pathways that convey information from photoreceptors, through bipolar cells, to ganglion cells, and then towards the brain. They are primarily responsible for transmitting visual signals from the retina to higher brain centers.
Horizontal/Lateral Circuits: These circuits, mediated by horizontal and amacrine cells, run perpendicular to the main vertical pathway. They modify the main pathways by providing inhibitory or excitatory signals to neighboring neurons, playing crucial roles in processes like lateral inhibition (enhancing contrast) and motion detection.
Rod and Cone Receptors
Structure of rod and cone receptors described.
Both rods and cones have an outer segment containing photopigments (e.g., rhodopsin in rods), an inner segment containing metabolic machinery (nucleus, mitochondria), and a synaptic terminal for transmitting signals to bipolar and horizontal cells.
Pathway of light involving neuronal components including:
Horizontal Cells: Receive input from photoreceptors and inhibit neighboring bipolar cells, contributing to lateral inhibition and contrast enhancement.
Bipolar Cells: Receive input from photoreceptors and horizontal cells, and transmit graded potentials to ganglion cells and amacrine cells.
Amacrine Cells: Diverse interneurons that modify the output from bipolar cells before it reaches ganglion cells, involved in transient responses, directional selectivity, and other complex retinal processing.
Ganglion Cells: The final output neurons of the retina, which integrate signals from bipolar and amacrine cells. Their axons form the optic nerve and transmit all-or-nothing action potentials to the brain.
Neural Convergence
Definition of Neural Convergence: Occurs when multiple presynaptic neurons synapse onto a single postsynaptic neuron, summing their inputs.
Sensitivity and Ganglion Cell Firing Threshold:
Example scenario with a ganglion cell requiring units of excitation to fire. Convergence can boost excitement towards this threshold. If one photoreceptor provides units of excitation, photoreceptors converging on one ganglion cell could collectively provide the units needed to trigger a response, making the system highly sensitive to even weak light stimuli.
Illustrative example shows if individual ganglion cells cannot exceed an excitation threshold from single photoreceptors, they require convergence to reach the firing threshold. This mechanism is especially prominent in the rod system, enhancing its sensitivity to dim light.
Discussion Question: Considering differences in convergence between rods and cones:
Which system provides better visual acuity?
Correct answer: Cones (due to less convergence, often a 1:1 ratio in the fovea, allowing better detail resolution and spatial precision).
Convergence and Acuity
Comparisons of light stimuli:
Close vs. far dots of light:
Rod system: Due to high convergence, light from two closely spaced dots may fall onto different rods that converge onto the same ganglion cell. This spatial summation makes it difficult for the rod system to distinguish details or separate stimuli, leading to lower visual acuity.
Cone system: With low convergence (sometimes 1:1), light from two closely spaced dots will activate separate cones, which then activate separate ganglion cells. This allows the cone system to accurately differentiate between separate stimuli, providing high visual acuity.
Focus Test
Exercise asking students to focus on an "X" and count identifiable letters to the left, testing visual acuity and the impact of convergence on peripheral vision.
Rod and Cone Distribution
Illustration of perception versus actual sight, highlighting the distinctions between rods and cones in visual processing.
Rods are most numerous in the periphery of the retina and are responsible for scotopic vision (vision in low light), detecting motion, and peripheral vision. Cones are concentrated in the fovea (the central part of the retina) and are responsible for photopic vision (vision in bright light), color vision, and high-acuity tasks. The fovea is almost exclusively cones, enabling our sharpest vision, while the optic disc (blind spot) has no photoreceptors.
Summary of Functionality: up to Big Picture: Sensory receptors (rods and cones) are key in shaping our visual experiences by processing light via the lenses. The distribution and specialized functions of rods and cones across the retina lead to distinct visual capabilities—high sensitivity and broad field in the periphery versus high acuity and color in the fovea—ultimately leading to optic nerve signal transmission and complex brain processing.
Receptive Fields
Definition: The receptive field is the specific region in the retina that influences a neuron’s electrical activity (firing rate, membrane potential) in the visual system.
Hubel & Wiesel (1961) define it as: “The retinal region over which a cell in the visual system can be influenced by light.” Their pioneering work mapped receptive fields and discovered specific response properties of neurons in the visual cortex.
Components: Receptive fields encapsulate all sensory receptors (photoreceptors) that, directly or indirectly through intermediate neurons (bipolar, horizontal, amacrine cells), impact a particular neuron's firing, regardless of their physical distance within the neural circuit.
Shape of Receptive Fields:
Theoretically can take various shapes based on the complex connectivity between neurons and receptors (example: heart-shaped for certain cells). In the retina and LGN, receptive fields are typically circular with a center-surround organization. In the visual cortex, they become more complex, often responding to oriented lines or edges.
Center-Surround Receptive Fields
Involvement of lateral inhibition in receptive field circuitry:
Fields can be entirely excitatory, entirely inhibitory, or a combination, most notably exhibiting center-surround antagonism. This antagonistic organization means that light in the center of the field has an opposite effect on cell firing compared to light in the surrounding area.
Diagrams illustrating responses of neurons to light presented on parts of the receptive field are discussed:
ON-center/OFF-surround cells: Are excited by light in the center of their receptive field and inhibited by light in the surround. Diffuse light over both center and surround often produces a weak response due to cancellation.
OFF-center/ON-surround cells: Are inhibited by light in the center of their receptive field and excited by light in the surround.
This antagonistic organization, largely mediated by lateral inhibition, makes these cells highly effective at detecting contrasts and edges in the visual field, rather than uniform illumination, which is crucial for object recognition.
Receptive Field Overlap
Exploration of how different receptive fields can overlap, whereby a specific retinal area could serve both as a center for one ganglion cell and as a surround for another ganglion cell's receptive field.
This overlap is essential for providing a continuous and comprehensive representation of the visual scene, ensuring no gaps in perception. It also allows for the precise localization of visual stimuli and supports complex processing like stereopsis (depth perception), where the brain integrates slightly disparate information from the overlapping receptive fields of both eyes.