The Seeing Brain - Visual Perception - Lecture 2 PDF

The Seeing Brain LGN, V1 & Extrastriate Areas Visual Perception Lecture 2  The Seeing Brain: Topics covered in Lecture 2 01 The Lateral Geniculate Nucleus [LGN]: (1) Position & (2) Anatomy 02 LGN: (1) ‘Bottom-Up...

The Seeing Brain LGN, V1 & Extrastriate Areas Visual Perception Lecture 2  The Seeing Brain: Topics covered in Lecture 2 01 The Lateral Geniculate Nucleus [LGN]: (1) Position & (2) Anatomy 02 LGN: (1) ‘Bottom-Up’ Inputs, & (2) ‘Top-Down’ Inputs 03 Retinotopic Organization: (1) From Retina -> LGN -> V1, (2) Visual Representation in the V1 04 Striate Cortex [V1]: (1) Anatomy & (2) V1 Fx Org: (a) Discovery, (b) V1 cells, (c) V1 in action 05 Extrastriate Areas: V4 - Colour perception, V5 - Movement perception 2  Lateral Geniculate Nucleus (LGN): Position LHS of the Thalamus L L L L R R R L 3 LGN: Anatomy - The different layers of the LGN (in each hemisphere) receive different kinds of visual information from different types of ganglion cells in the retina. - Layers 1 & 2 in the LGNs in the LHS & RHS receive visual information, regarding motion, from Magnocellular Ganglion Cells (M-Type Cells). - Layers 3, 4, 5 & 6 in the LGNs receive visual information, regarding colour & fine- grained detail, from the Parvocellular K6 6 Ganglion Cells (P-Type Cells). Koniocellular K5 5 Parvocellular layers: Colour & fine-grained - The more recently discovered Layers K4 4 detail Koniocellular layers exist between the M- K3 3 type cell and P-type cell layers of the LGN. K2 These K-type cells show less functional K1 2 Magnocellular layers: specificity. 1 Motion detection 4  Left Hemisphere LGN LGN: ‘Bottom-Up’ Inputs Right Hemisphere LGN K6 06 K5 -The LGN in the left hemisphere receives K4 05 visual information about the Right Visual 04 K3 Field from the right side of both retina's 03 K2 (i.e. from the the Right Nasal Retina & 02 K1 01 Left Temporal Retina). -The LGN in the right hemisphere receives visual information about the Left Visual Field from the left side of both retinas (i.e. from the Right Temporal Retina & Left Nasal Retina). -Note: Each layer of the LGN on the LHS LGN Left and RHS, receives input from only one Temporal Retina eye. Right Left Nasal Nasal Retina -Remember: LGN layers 1 & 2: M-Type Cell Right Retina Left Temporal Input, LGN Layers 3-6: P-Type Cell Input Retina Visual Field Right Visual 5 Field LGN: ‘Top-Down’ Inputs - We mentioned in the previous slide that ‘Bottom-Up’ (sensory) visual information is fed to the layers of the LGN from the retinas of both eyes. - However, most of the information being sent to the LGN (≈ 95%) comes from other parts of the cortex in a ‘Top-Down’ fashion. - i.e. From the Superior colliculus, Pretectum, Thalamic reticular nuclei, V1 (feedback loops). - These inputs from other cortical areas are also ‘segregated’, meaning that different regions of cortical cells send input to different layers of the LGN. - This ‘Top-Down’ information from the cortex may modulate (amplify or dampen) the representation of information coming from the retinas of both eyes. 6  Retinotopic Organization: Retina -> LGN -> Striate Cortex - Visual information from the M-Type Cells and P-Type Cells in the retina project to specific layers of the LGN and are then projected to specific layers of the Striate cortex (V1). Retina - In other words, the Striate cortex is ‘Retinotopically organized’ - Retina -> LGN -> Striate Cortex LGN - NB: The first major area to receive information from the LGN is the Striate Cortex (V1). Striate Cortex - The connections from the LGN to the V1 (via the Visual/Optic Radiations) are one of the most important ‘outputs’ – responsible for conscious vision (Geniculostriate pathway). 7  Retinotopic Organization: Visual Representation in the V1 - The right half of the visual world is - The layout of visual information in V1 represented in the left visual cortex and parallels that found on the retina. That the left half of the visual world is is, points that are close in space on the represented in the right visual cortex. retina are also close in space in V1. 8  The Striate Cortex (V1): Anatomy - The Striate Cortex (V1) is located in and around the Calcarine fissure in the occipital lobe. - The V1 is segregated into 6 layers, based on the relative density of neurons (NB. Several layers have sub-layers). - Projections from the Magnocellular (M-Type) and Parvocellular (P-Type) cell layers of the LGN are projected to layer 4 (IV) of the Striate Cortex (V1). 9  V1 Functional Organisation: Discovery David Hubel (left) and Torsten Wiesel (right) recorded electrical activity from individual neurons in the brains of cats. They used a slide projector to show different light patterns to the cats and noted that specific patterns stimulated activity in particular parts of the brain. Hubel & Wiesel won the the Nobel Prize for Medicine in 1981 for their breakthrough discoveries regarding the visual system and visual processing. 10  V1 Functional Organisation: Simple Cells - The receptive field of a Simple cell is bar-shaped with an excitatory centre and an inhibitory surround. - The response of simple cells are derived from a combination of responses from different cells with Centre-surround properties (e.g. LGN cells). - The cell will only fire is the bar of light is oriented in a particular way. - Different simple cells respond to static bars of light angled in different directions. 11  V1 Functional Organisation: Complex Cells - Simple cell responses are combined to form Complex cells and are also responsive to specific line orientations. - Complex cells differ from simple cells in a few significant ways: 1. They have larger receptive fields. 2. They are less selective about the location of the bar of light, because they do not have an excitatory centre or an inhibitory surround. 3. They show a preference for lines moving in particular directions: - For example: some complex cells are responsive to lines towards the L or R. - i.e. Complex cells are sensitive to motion, particularly motion in a specific direction. 12  V1 Functional Organisation: Hypercomplex Cells - Hypercomplex cells are built from the responses of several Complex cells. - Hypercomplex cells are also selective for orientation but they also prefer lines of particular lengths. - This is because they have strong inhibitory areas at one end of their receptive fields (end-stopped). - Therefore, if a pattern of light extends beyond a certain point, the receptive field will not respond. 13 V1 in Action: Contextual Modulation & Figure Ground Segregation Contextual Modulation Figure Ground Segregation Refers to the influence of a surround The process of segmenting a visual pattern on either the perception of, or display into objects versus background the neural responses to a target pattern surfaces. 14 Extrastriate Areas -Extrastriate cortical regions refer to regions adjacent to the striate cortex, also involved in aspects of visual processing. -These areas are specialized for processing specific visual attributes, e.g. colour [V4], motion [V5/MT]. V5 -The receptive fields for these V3 V2 extrastriate visual areas become V4 V1 increasingly broader and less V1 Striate Cortex coherently organized in space V2 Prestriate Cortex (i.e. not strictly retinotopically V3 Visual Area 3 organized like the striate cortex) V4 Visual Area 4 V5 Visual Area 5 (MT) 15 Visual Area 4 (V4) - Area V4 is specialized for processing colour (ventral surface of brain). - Colour constancy V5 - Another key function of the V4 is that it V3 V2 maintains relatively constant colour perception V4 V1 despite varying light conditions. - Cells in this region respond to different wavelengths: - i.e. Cells respond to different colours. Shorter wavelengths = blue, longer wavelengths = red. - Damage to the V4 can result in: - Achromatopsia: The inability to perceive colour. This is different from colour blindness, which is a deficiency of the photoreceptors in the retina (bilateral damage to the V4s of both hemispheres) - Hemiachromatopsia: The inability to perceive colour in one half of the visual field (damage to the V4 in one hemisphere). 16 Visual Area 5 (V5/MT) - Visual area V5 is also called the middle-temporal area (MT) - Cells in this region respond to V5 patterns that move in particular V3 V2 directions and at specific speeds. V4 V1 - Different cells in this area have different speed and directional preferences. - Damage to the V5/MT results in Akinetopsia: - The inability to detect the movement of objects in the visual world. - Patients with this disorder report viewing the world as a series of still frames where objects suddenly appear and then disappear. 17  Key terms ? Lateral geniculate nucleus (LGN) Position ? Anatomy (layers) Inputs (‘top-down’ & ‘bottom-up’) ? Retinotopic organization QUESTIONS? Retina -> LGN -> Striate cortex (V1) ? Striate cortex (V1) Anatomy (layers) ? Striate Cortex (V1) cells build in complexity: o centre surround -> simple -> complex -> hypercomplex Contextual modulation Figure-ground segregation Extrastriate areas V4 – Colour perception (colour constancy) V5 – Movement perception 18  References Georgiev, D. (2002). Photons do collapse in the retina not in the brain cortex: Evidence from visual illusions. NeuroQuantology, 9, 206-230. Gilbert, C. D., & Li, W. (2013). Top-down influences on visual processing. Nature Reviews Neuroscience, 14(5), 350- 363. Gheorghiu, E. (2017). Dynamics of contextual modulation of perceived shape in human vision. Scientific Reports, 7(1), 1-15. Li, B., Todo, Y., & Tang, Z. (2022). Artificial Visual System for Orientation Detection Based on Hubel–Wiesel Model. Brain Sciences, 12(4), 470-491. Paulun, L., Wendt, A., & Kasabov, N. (2018). A retinotopic spiking neural network system for accurate recognition of moving objects using NeuCube and dynamic vision sensors. Frontiers in Computational Neuroscience, 12, 42-73. Vidyasagar, T. R. (2013). Reading into neuronal oscillations in the visual system: Implications for developmental dyslexia. Frontiers in Human Neuroscience, 7, 811. Ward, J. (2020). The seeing brain. In The student’s guide to cognitive neuroscience (4th ed., pp. 143-173). Psychology Press. 19