5.1Sensory Systems: Vision
Sensory Systems: Vision
Introduction
Lecture focuses on sensory systems, starting with vision.
Covered in chapter 5 of the Colette textbook.
The five senses: sight, touch, hearing, taste, and smell.
Today's lecture: vision.
Tomorrow's lecture: hearing (audition) and touch.
Taste and smell will not be covered in this subject.
Objectives
Explore sensory system models.
Understand how information enters the central nervous system.
Understand the pathway of information within the central nervous system.
Examine the overarching model of sensory information flow.
Understand the function of components within the models.
Visual processing through the sensory system model.
Simple Model of a Sensory System
Organization features: hierarchical, functionally homogeneous, serial, and bottom-up.
Levels: receptor level, thalamus, primary sensory cortex, secondary sensory cortex, association cortices.
Hierarchical: different levels within the sensory system.
Functionally homogeneous: each level works towards understanding and processing sensory information.
Serial: information flows through each step in one pathway.
Bottom-up: information flows from receptors (bottom) to association cortex (highest level).
Neuroanatomical complexity increases through the hierarchy.
More complex functions are performed by more complex neuroanatomical structures.
Sensation is less complex than perception.
Sensation vs. Perception
Sensation: simple detection of stimuli using receptors, thalamus, and primary sensory cortex.
Example: hearing a sound without knowing what it is or where it's coming from.
Basic sensory process, often automatic.
Perception: complex process of integrating, recognizing, and interpreting sensations using the secondary sensory cortex and association cortex.
Example: recognizing a knock at the door and identifying it as the delivery person because you're expecting a parcel.
Sensation involves transduction of external energy into neural responses.
Transduction: converting one signal or stimulus into another.
Different cells are specialized for different stimuli (e.g., light, hearing).
Signals are converted into action potentials for processing by the central nervous system.
Perception involves higher-order systems.
Current Sensory System Model
More complex than the simple model.
Still hierarchical but functionally segregated with parallel pathways.
Information flows between structures within the same level and between different levels simultaneously.
Allows for complex processing and interaction with other brain systems.
Visual information processing interacts with memory, emotional processing, and reasoning.
Vision
Processing of light entering the eyes.
Light is essential for transduction.
Properties of light: wavelength and intensity.
Wavelength variations -> perceptual variation in color.
Intensity variations -> perceptual variation in brightness.
The Eye
Cornea: protects the eye.
Iris: regulates light input.
Pupil: light enters; gauges sensitivity and acuity.
Lens: focuses light on the retina.
Ciliary muscles: adjust the lens for focusing.
Binocular disparity: each eye sees a slightly different perspective.
The brain integrates information from both eyes to construct a three-dimensional perception.
Binocular disparity gives us depth perception.
Depth perception cues in the absence of binocular disparity: motion, color, perception, distance fog.
Retina
Contains multiple types of cells: ganglion cells, amacrine cells, bipolar cells, horizontal cells, rod, and cone receptors.
Light travels through these cells to reach rod and cone receptors.
Exiting axons merge to form the optic nerve; Creates a blind spot.
Fovea: high-acuity vision, located roughly in the center of the field of vision.
Fewer cells blocking the light, so light is less distorted, and thus high acuity is obtained.
Optic disc: where axons of retinal ganglion cells exit the eye, creating a blind spot.
Brain uses completion to fill in the blind spot using surrounding visual information.
Rods and Cones
Receptors for vision.
Rods: used in poor lighting (scotopic vision), high sensitivity, less acuity.
Cones: used in good lighting (photopic vision), high acuity, associated with color processing.
Duplexity theory: species active during the day have only cone retinas, while species active at night have only rod retinas.
Cones have low convergence onto bipolar and retinal ganglion cells.
Rods have high convergence onto bipolar and retinal ganglion cells, which helps differentiate vision even if it's dim.
Transduction: light is absorbed by rhodopsin in rods, bleaching it from pink to white and hyperpolarizing the receptors.
Visual Pathway
Retina -> thalamus -> primary visual cortex.
Retina geniculate striate pathway.
Axons from nasal hemiretinas become contralateral at the optic chiasm.
Information reaches the thalamus via the lateral geniculate nucleus.
It's sent on to the primary visual cortex, located in the occipital lobe.
The surface of the primary visual cortex is a map of the retina.
The retinotopic layout allows accurate representation of the external world internally.
M and P pathways: two pathways from the retina to the primary visual cortex.
M pathway (magnocellular): large cells, movement information.
P pathway (parvocellular): small cells, color, and fine pattern details.
Primary visual cortex (V1) receives information from the lateral geniculate nucleus.
Responsible for early visual processing, processing the physical properties of the stimulus.
Damage to the Primary Visual Cortex
Scotoma: area of blindness in the visual field. Brain uses completion to fill the gap.
Quadrantanopia: loss of one full quadrant of the visual area.
Hemianopia: loss of half of the visual field.
Blindsight: loss of both primary visual cortices, individuals are cortically blind but can still perform visually mediated tasks.
Signals don't have to go through the visual cortex, but will go through the secondary visual cortex
Further processing
Information goes from receptors to thalamus to primary visual cortex to secondary visual cortex
Secondary visual cortex is referred to as V2 and it's found in 2 areas. Prestriate cortex as well as the inferotemporal cortex.
The Dorsal stream is for information going dorsally (up) from the parietal cortex. Known as the how or where stream. Damage to this can cause difficulty grabbing objects, but the person can still see what the object is.
The Ventral stream is for information going ventrally (forward) through the inferotemporal cortex. Known as the what stream. damage to this ventral oathway means that the person can grab an object, but they will have difficulty identifying what the object is.
The posterior parietal cortex is known as the association cortex, and thus connects the visual, auditory, and somatosensory processes. This lets the individual know where their body is in relation to other objects.
Sensory Systems: Vision
Introduction
Lecture focuses on sensory systems, starting with vision.
Covered in chapter 5 of the Colette textbook.
The five senses: sight, touch, hearing, taste, and smell.
Today's lecture: vision.
Tomorrow's lecture: hearing (audition) and touch.
Taste and smell will not be covered in this subject.
Objectives
Explore sensory system models.
Understand how information enters the central nervous system.
Understand the pathway of information within the central nervous system.
Examine the overarching model of sensory information flow.
Understand the function of components within the models.
Visual processing through the sensory system model.
Simple Model of a Sensory System
Organization features: hierarchical, functionally homogeneous, serial, and bottom-up.
Levels: receptor level, thalamus, primary sensory cortex, secondary sensory cortex, association cortices.
Hierarchical: different levels within the sensory system, where lower levels perform basic processing and higher levels integrate and interpret information.
Functionally homogeneous: each level works towards understanding and processing sensory information, with specialized cells and circuits dedicated to specific aspects of sensory input.
Serial: information flows through each step in one pathway, from the receptors to the association cortex, in a sequential manner.
Bottom-up: information flows from receptors (bottom) to association cortex (highest level). This flow is driven by the external stimulus.
Neuroanatomical complexity increases through the hierarchy. The association cortex has the most complex structure.
More complex functions are performed by more complex neuroanatomical structures.
Sensation is less complex than perception.
Sensation vs. Perception
Sensation: simple detection of stimuli using receptors, thalamus, and primary sensory cortex. The process involves transduction of external energy into neural signals.
Example: hearing a sound without knowing what it is or where it's coming from. This is mere detection without interpretation.
Basic sensory process, often automatic.
Perception: complex process of integrating, recognizing, and interpreting sensations using the secondary sensory cortex and association cortex. It involves higher-order cognitive processing.
Example: recognizing a knock at the door and identifying it as the delivery person because you're expecting a parcel. This includes recognizing the sound, associating it with past experiences, and making a judgment.
Sensation involves transduction of external energy into neural responses.
Transduction: converting one signal or stimulus into another. Specialized receptor cells perform this function.
Different cells are specialized for different stimuli (e.g., light, hearing). For example, photoreceptors in the eye are specialized for light, while hair cells in the ear are specialized for sound.
Signals are converted into action potentials for processing by the central nervous system. This conversion allows the brain to process the sensory information.
Perception involves higher-order systems, such as memory, emotion, and reasoning, to interpret sensory inputs.
Current Sensory System Model
More complex than the simple model. It addresses the limitations of the simple model by incorporating parallel processing and feedback loops.
Still hierarchical but functionally segregated with parallel pathways. This allows for different aspects of sensory information to be processed simultaneously.
Information flows between structures within the same level and between different levels simultaneously. This bidirectional flow allows for feedback and refinement of sensory processing.
Allows for complex processing and interaction with other brain systems. This integration enables more sophisticated responses to sensory stimuli.
Visual information processing interacts with memory, emotional processing, and reasoning. This interaction shapes our perception and understanding of the world.
Vision
Processing of light entering the eyes. The visual system transforms light energy into neural signals that the brain can interpret.
Light is essential for transduction. Photoreceptors in the retina are responsible for transducing light into electrical signals.
Properties of light: wavelength and intensity. These properties determine the color and brightness of what we see.
Wavelength variations -> perceptual variation in color. Different wavelengths correspond to different colors.
Intensity variations -> perceptual variation in brightness. Higher intensity corresponds to brighter light.
The Eye
Cornea: protects the eye and helps to focus light.
Iris: regulates light input by controlling the size of the pupil.
Pupil: light enters; gauges sensitivity and acuity. The pupil dilates in low light to increase sensitivity and constricts in bright light to increase acuity.
Lens: focuses light on the retina. It is flexible and can change shape to focus on objects at different distances.
Ciliary muscles: adjust the lens for focusing. These muscles contract or relax to change the shape of the lens.
Binocular disparity: each eye sees a slightly different perspective. This difference is crucial for depth perception.
The brain integrates information from both eyes to construct a three-dimensional perception. This integration occurs in the visual cortex.
Binocular disparity gives us depth perception. It allows us to judge the distance of objects.
Depth perception cues in the absence of binocular disparity: motion, color, perception, distance fog. These monocular cues can provide depth information even with only one eye.
Retina
Contains multiple types of cells: ganglion cells, amacrine cells, bipolar cells, horizontal cells, rod, and cone receptors. These cells work together to process visual information.
Light travels through these cells to reach rod and cone receptors. The photoreceptors transduce light into electrical signals.
Exiting axons merge to form the optic nerve; Creates a blind spot where there are no photoreceptors.
Fovea: high-acuity vision, located roughly in the center of the field of vision. The fovea contains a high density of cones.
Fewer cells blocking the light, so light is less distorted, and thus high acuity is obtained. This arrangement minimizes the distortion of incoming light.
Optic disc: where axons of retinal ganglion cells exit the eye, creating a blind spot. This area lacks photoreceptors.
Brain uses completion to fill in the blind spot using surrounding visual information. This process, also known as filling-in, allows us to perceive a continuous visual field.
Rods and Cones
Receptors for vision. These photoreceptors convert light into electrical signals.
Rods: used in poor lighting (scotopic vision), high sensitivity, less acuity. Rods are more sensitive to light and are responsible for night vision.
Cones: used in good lighting (photopic vision), high acuity, associated with color processing. Cones require more light to be activated and are responsible for color vision and fine details.
Duplexity theory: species active during the day have only cone retinas, while species active at night have only rod retinas. This theory explains the differences in visual systems based on the activity patterns of different species.
Cones have low convergence onto bipolar and retinal ganglion cells. This low convergence results in high acuity.
Rods have high convergence onto bipolar and retinal ganglion cells, which helps differentiate vision even if it's dim. This high convergence increases sensitivity in low light conditions.
Transduction: light is absorbed by rhodopsin in rods, bleaching it from pink to white and hyperpolarizing the receptors. This process initiates the neural signal that travels to the brain.
Visual Pathway
Retina -> thalamus -> primary visual cortex. This is the main pathway for visual information to reach the brain.
Retina geniculate striate pathway. This pathway includes the lateral geniculate nucleus (LGN) in the thalamus and the striate cortex (V1) in the occipital lobe.
Axons from nasal hemiretinas become contralateral at the optic chiasm. This crossover allows information from the left visual field to be processed in the right hemisphere and vice versa.
Information reaches the thalamus via the lateral geniculate nucleus. The LGN acts as a relay station for visual information.
It's sent on to the primary visual cortex, located in the occipital lobe. The primary visual cortex (V1) is the first cortical area to receive visual input.
The surface of the primary visual cortex is a map of the retina. This retinotopic map preserves the spatial relationships of the visual field.
The retinotopic layout allows accurate representation of the external world internally. This representation is crucial for visual perception.
M and P pathways: two pathways from the retina to the primary visual cortex.
M pathway (magnocellular): large cells, movement information. This pathway is sensitive to motion and spatial information.
P pathway (parvocellular): small cells, color, and fine pattern details. This pathway is sensitive to color and fine details.
Primary visual cortex (V1) receives information from the lateral geniculate nucleus.
Responsible for early visual processing, processing the physical properties of the stimulus, such as orientation, spatial frequency, and color.
Damage to the Primary Visual Cortex
Scotoma: area of blindness in the visual field. The location and size of the scotoma depend on the location and extent of the damage in the visual cortex.
Quadrantanopia: loss of one full quadrant of the visual area. This condition results from damage to one quadrant of the visual cortex.
Hemianopia: loss of half of the visual field. This condition results from damage to one half of the visual cortex.
Blindsight: loss of both primary visual cortices, individuals are cortically blind but can still perform visually mediated tasks. This phenomenon suggests that there are alternative visual pathways that bypass the primary visual cortex.
Signals don't have to go through the visual cortex, but will go through the secondary visual cortex
Further processing
Information goes from receptors to thalamus to primary visual cortex to secondary visual cortex
Secondary visual cortex is referred to as V2 and it's found in 2 areas. Prestriate cortex as well as the inferotemporal cortex. V2 refines the information received from V1.
The Dorsal stream is for information going dorsally (up) from the parietal cortex. Known as the how or where stream. It is involved in spatial processing and action.
Damage to this can cause difficulty grabbing objects, but the person can still see what the object is. This condition is known as optic ataxia.The Ventral stream is for information going ventrally (forward) through the inferotemporal cortex. Known as the what stream. It is involved in object recognition.
damage to this ventral oathway means that the person can grab an object, but they will have difficulty identifying what the object is. This condition is known as visual agnosia.The posterior parietal cortex is known as the association cortex, and thus connects the visual, auditory, and somatosensory processes. This integration of sensory information is crucial for spatial awareness and navigation.
This lets the individual know where their body is in relation to other objects. This awareness is essential for interacting with the environment.