Receptive Fields and Visual Pathway - Part II PDF

Receptive fields and the visual pathway (Part II) Dr Tony Redmond PhD MCOptom FHEA Reader in Vision Science School of Optometry & Vision Sciences Cardiff University, UK The visual pathway The Lateral Geniculate Nucleus (LGN) Superior colliculus Retina (LE) Geniculocortical pathway Optic chiasm LGN (...

Receptive fields and the visual pathway (Part II) Dr Tony Redmond PhD MCOptom FHEA Reader in Vision Science School of Optometry & Vision Sciences Cardiff University, UK The visual pathway The Lateral Geniculate Nucleus (LGN) Superior colliculus Retina (LE) Geniculocortical pathway Optic chiasm LGN (Thalamus) Optic tract 90% Retina (RE) 10% Optic nerve Retinotectal pathway 10% Geniculocortical pathway Superior colliculus Superior colliculus is involved in eye movement control Retinotectal pathway The Lateral Geniculate Nucleus (LGN) The LGN is located in the thalamus, in the forebrain. Contains 6 layers. 6 5 4 3 2 1 Layers 1-2: magnocellular layer Layers 3-6: Parvocellular layer Between layers: Koniocellular cells Layers 2, 3, 5: ipsilateral eye Layers 1, 4, 6: contralateral eye The Lateral Geniculate Nucleus (LGN) First extra-retinal synapse in visual pathway. Centre-surround receptive fields. Retinotopic map in each layer, aligned. Relays information to higher cortical centres e.g. striate cortex. Receives feedback from visual cortex and other cortical and subcortical areas Regulates signal e.g. slows firing rate to prevent over-stimulation of visual cortex. The Striate Cortex Aka: Primary Visual Cortex Aka: V1 Aka: Brodmann Area 17 Located in the occipital lobe of the brain. The Striate Cortex Brodmann Area 17 = Primary Visual Cortex The Striate Cortex Note entry of: Magnocellular fibres Parvocellular fibres Koniocellular fibres http://www.webvision.med.utah.edu/imageswv/cortex.jpeg The Striate Cortex 6 Brodmann layers. Numerous subdivisions. Fibres from M- and Players of LGN enter layer IVc. (magno, IVcα; parvo, IVcβ) Fibres from K- cells of LGN enter layer III. Fibres are then sent to other layers of striate cortex, and to extrastriate regions. Introduce cytochrome oxidase into V1…. Areas of staining = BLOBS. Areas of no staining = INTERBLOBS. The Striate Cortex: Receptive Fields Hubel & Wiesel (1959, 1962, 1968): single cell recording using microelectrodes; they discovered 3 types of cortical neurone: Simple cortical cells Complex cortical cells End-stopped cells (‘hypercomplex’) All respond best to bar-like stimuli with specific orientation i.e. are ORIENTATION SPECIFIC http://bit.ly/hubelwiesel Receptive Fields: Simple Cortical Cells Excitatory and inhibitory regions arranged side by side. + + Edge detector Bar detector Strongest response to a bar of light aligned along length of receptive field. Size of response diminishes when orientation of bar is altered. No response when perpendicular to receptive field. Different cells tuned to different orientations. Response rate + - - 40 20 0 20 Stimulus orientation 40 Receptive Fields: Complex Cortical Cells Not sensitive to stationary stimuli Respond best to correctly orientated bar of light moving across receptive field. Sensitive to: Orientation of stimulus. Direction of movement. Speed of movement Receptive Fields: Complex Cortical Cells Receptive Fields: End-stopped Cells Have inhibitory regions at the end of the receptive field, so sensitive to length of stimulus. Like complex cells, also sensitive to movement. Respond best to moving corners. + - + - Cell Receptive Field Characteristics Centre-surround receptive field. Responds best to spots of Retinal Ganglion Cell certain size, but also responds to other stimuli. LGN Simple Cortical Complex Cortical Centre-surround receptive fields, similar to RGCs. Excitatory and inhibitory areas arranged side-by-side. Responds best to bars of particular orientation. Responds best to movement of correctly orientated bar in usually sensitive to direction of movement and speed. Responds to corners, angles, or bars of particular length (usually End-Stopped Cortical sensitive to direction and speed). Organisation of the Striate Cortex (V1) Hubel & Wiesel (1962, 1965, 1968, 1974), inserted electrodes perpendicularly or obliquely through cortex. Measured responses of cells along electrode track Found columnar structure. Cells within a column have same functional properties: Location Perpendicular electrode track Orientation preference Oblique track Ocular dominance Cortex Location columns Cells within a location column have receptive fields from same part of the retina. Cells in neighbouring columns have receptive fields from neighbouring parts of the retina. Retinotopic map of retina in striate cortex (as in LGN). Fovea = 0.01% of retinal area but 8% of striate cortex!!! Cortical Magnification Cortical Magnification Foveal representation in cortex is magnified compared to representation of the periphery Due to combination of: Greater density of ganglion cells in central retina. Each ganglion cell in central retina has greater representation in cortex than ganglion cell from periphery (Azzopardi & Cowey, 1993). Orientation columns Cells within an orientation column are sensitive to stimuli of the same orientation. Adjacent columns → slightly different preferred orientation. Oblique track through cortex → orderly progression of preferred orientation. 1mm → move through 180 deg. 1mm Oblique electrode track Perpendicular electrode track Ocular Dominance Columns (ODCs) Cells in layers IVcα and IVcβ receive monocular input from LGN. Cells in other layers receive binocular input. Most respond better to one eye = Ocular Dominance. Cells in same column → same ocular dominance In 1mm oblique section → gradual shift from monocular RE eye, to monocular LE eye. Ocular Dominance Columns (ODCs) 1mm RL RL RL RL RL RL RL RL LR L R LR LR L R LR LR L R LR L R LR LR L L L L Perpendicular electrode track R R R R Oblique electrode track Ocular Dominance Columns (ODCs) The effect of monocular deprivation The visual cortex is plastic during development (i.e. it’s structure is shaped by visual experience). When one eye is occluded (long-term) during development, the seeing eye becomes dominant and has a greater representation in the cortex. Ocular dominance columns for the seeing eye are greater than those for the occluded eye Adapted from: Hubel DH, Wiesel TN, LeVay S. Plasticity of ocular dominance columns in monkey cortex. Philos Trans R Soc Lond B Biol Sci. 278(961):377-409 Ocular Dominance Columns (ODCs) The effect of monocular deprivation Optic radiations from non-deprived eye = Greater axonal arbor complexity Optic radiations from deprived eye = Reduced axonal arbor complexity Antonini A & Stryker MP. Rapid remodelling of axonal arbors in the visual cortex. Science. 1993;260(5115):1819-1821 1 is the magic number… Location columns are 1mm wide Cells sensitive to all possible orientations found within 1mm In 1mm, ocular dominance shifts from one eye to the other. RL RL R RL RL LR LR L LR 1mm Hypercolumns 1mm block of cortex → Processing module serving particular area of retina. Known as a Hypercolumn. Each hypercolumn contains blobs and interblobs. Blobs → chromatic (colour coded) information from red-green and blue-yellow opponent pathways. Interblobs → cells specific to orientation and achromatic contrast. http://www.webvision.med.utah.edu/imageswv/cortex.jpeg To the Striate Cortex and beyond… Organisation of pathways Remember our two major pathways from retina to visual cortex Magnocellular: Low-contrast, flicker, movement, spatial location of objects, achromatic, poor spatial resolution Parvocellular: Chromatic, high contrast, high spatial resolution Streaming of information into pathways continues into the striate cortex and beyond… Visual areas of the brain https://afterimagia.pl/vision/ Extrastriate area V2 Receives bulk of input from striate cortex. CO staining → thin stripes, thick stripes, interstripes. Thick stripes: Magno information. Sensitive to disparity, orientation & movement. Thin stripes: Parvocellular information. Colour coded. Interstripes: Parvocellular information. Orientation sensitive, no colour coding Extrastriate area V2 Each set of stripes contains a retinotopic map → 3 interleaved maps in V2. From V2, information is segregated into 2 pathways – ventral and dorsal Dorsal stream = WHERE? pathway Ventral stream = WHAT? pathway Form and colour Location & motion Geniculocortical path Retina Retinotectal path LGN Superior colliculus V1 Pulvinar contrast,ocular dom inance, orientation& spatial frequencyselectivity V2 Parietal cortex binocular integration featurelinking guidanceof action visual attention visual im agery, visual memory Ventral stream Dorsal stream Prefrontal cortex memory V4 V3 colour dynamicform? Frontal eyefields(FEF) IT objects eyemovem ents M T motion M ST opticflow motioncausedbyeyes Cortical receptive fields: a summary Clinical relevance of receptive fields and the visual pathway Visual pathway Knowledge of the visual pathway and patterns of visual field defects can help the clinician tentatively locate the lesion. E.g. if we find a visual field defect that looks like no.3, we might suspect a compressive lesion at the optic chiasm. More on this later in your programme… Perimetry (visual field assessment) Novel perimetry for identifying changes in visual field sensitivity in glaucoma The Hill of Vision (fovea) Optic nerve (blind spot) The Hill of Vision changes in eye disease Eye disease can change our hill of vision. The hill (aka ‘island of vision’) can become flatter (e.g. our eyes are only able to see dimmer lights) The area that it fills in the ‘sea of darkness’ becomes bigger (we can’t see so far out to the edge) There are local dips in the surface (our vision becomes patchy) The Hill of Vision changes in eye disease Gradual changes to the hill of vision can go unnoticed for years, especially if only in one eye. Sometimes these changes are associated with serious underlying health conditions Changes to your visual field can have a big impact on one’s life May not be able to drive May have mobility issues May have consequences for being able to to work in certain occupations Visual field testing aims to detect these changes ‘Threshold’: The lowest level of a stimulus that can be detected by the visual system A ‘Just Noticeable Difference (JND)’ (e.g. lowest brightness of a spot, finest detail detectable, minimum difference in brightness between a stimulus and the background on which it is placed) Static perimetry Stimulus brightness Stimuli (small spots of light) change from non-seeing (too dim) to seeing (bright enough to see) in order to find ‘threshold’ Testing is done with stimuli in different locations to find the height of the hill of vision at that particular location in the visual field This is the technique you will be using in clinics this year Commercially-available static perimeters Examples of commercially-available perimeters Humphrey Field Analyzer 3 Henson Pro Octopus 900 iCare COMPASS Threshold will depend on… Size of stimulus Visibility determined by total amount of light falling within a receptive field. i.e. we need a critical number of quanta falling on a receptive field for the stimulus to be seen. Spatial summation refers to the absorption of quanta over space. i.e. over the area of a receptive field. If summated number of quanta absorbed by receptive field exceeds threshold then stimulus will be seen. Small bright light has same visibility as larger dim light, provided total amount of light falling on receptive field is equivalent. Two or more stimuli within 1 receptive field will also be added together to give single response. B = = A C Responses A, B and C are equal as long as total amount of light falling on receptive fields is the same! A, B, and C will be perceived as being the same Ricco’s Law describes spatial summation Threshold Luminance × Area = CONSTANT If we double the stimulus area, the amount of light required to reach threshold is halved. Ricco’s law applies within a critical area (i.e. over a finite range of small stimulus sizes). This is known as complete spatial summation The limit to complete spatial summation, i.e the critical area, is called ‘Ricco’s Area’ Thought that Ricco’s Area represents receptive field size Critical area is larger towards periphery and in lower light levels. Ricco’s area aka The critical area aka The area of complete spatial summation Ricco’s Law describes spatial summation Beyond critical area, summation is partial (or incomplete): e.g. Piper’s Law Threshold will depend on… Duration of Stimulus Visibility also determined by total amount of light falling on one receptive field within a given time frame. Temporal summation refers to the absorption of quanta over time (rather like the exposure time on a camera) A short bright stimulus can have the same effect as a long dim stimulus, if the total light is equivalent. Several short stimuli can have the same effect as one long stimulus if the total light is equivalent (but will only be seen as one flash). Several bright flashes of light of short duration have the same visibility as one light of longer duration, provided the total amount of light falling on receptive field is equivalent. Two or more stimuli within the receptive field will be added together to give single response. 5ms 5ms 30 ms 15ms 5ms A = B = C Responses A, B and C are equal as long as total amount of light falling on receptive fields is the same! A, B, and C will be perceived as being the same Bloch’s Law describes temporal summation Threshold Luminance × Duration = CONSTANT If we halve luminance, we must double duration to maintain visibility. OR If we double stimulus duration, we only need half the luminance to bring it to threshold (i.e. threshold is halved). Bloch’s law applies within a critical duration, during which complete temporal summation occurs. This is known as the ‘critical duration’ At absolute threshold critical duration is up to 100ms Bloch’s Law describes temporal summation Threshold Luminance × Duration = CONSTANT For stimuli longer than the critical duration, summation is partial or incomplete (Bloch’s law no longer holds) (Note: Just as Bloch’s law is analogous to Ricco’s law [Bloch’s = temporal, Ricco’s = spatial], the critical duration (temporal) is analogous to Ricco’s area (spatial), i.e. the same principle applies. Relevance for Perimetry? Glaucoma Biomarker 1: Glaucoma Biomarker 2: Redmond et al, Invest Ophthalmol Vis Sci 2010:51:6540-8 Mulholland et al, Invest Ophthalmol Vis Sci 2015:56:6473-82 Ricco’s area Changes in temporal summation in glaucoma Ricco’s area Changes in spatial summation in glaucoma Ricco’s area in Age-related Macular Degeneration Hunter et al (TVST, 2023) Holm-Bonferroni corrected p-values Proof-of-concept for Area-Modulation Perimetry Rountree et al, Invest Ophthalmol Vis Sci 2018:8:2172 Importance of spatial & temporal summation for perimetry Both spatial summation and temporal summation are altered in glaucoma Spatial (but not temporal) summation is altered in age-related macular degeneration These phenomena indicate that receptive fields are disrupted somewhere along the visual pathway Current perimetric stimuli have a fixed area and vary in luminance (the Goldmann III stimulus). The design of the current standard visual field test stimulus is unchanged for 40+ years and the choice of stimulus was not a scientific standard Stimuli with a fixed luminance, varying in area, (designed to measure changes in spatial summation) appear to have better performance characteristics in identifying glaucomatous damage than the current clinical standard The finding of altered temporal summation in glaucoma suggests that perimetry could be improved by reducing the duration of the stimulus (currently 200ms) and that the current duration actually masks an important functional biomarker for the condition Developmental Pathway Funding Scheme (DPFS) MR/V038516/1 £1.82 million, 2021–25 www.revamp-vision.com Importance of spatial & temporal summation for perimetry Unanswered questions…. – Are receptive fields altered in glaucoma? – If so, receptive fields of which cells? – Where in the visual pathway is the most likely location of the altered receptive fields? Research is ongoing…….. References and Recommended Reading Recommended reading Schwartz (1999). Visual Perception. A Clinical Orientation. Chapters 2, 12, 13, 14 & 15. Livingstone and Hubel “Segregation of Form, Color, Movement, and Depth: Anatomy, Physiology, and Perception” (PDF on Blackboard) Useful website… http://www.webvision.med.utah.edu/ Other References Hubel & Wiesel (1959). Journal of Physiology, 148, 574-591. Hubel & Wiesel (1962). Journal of Physiology, 160, 106-154. Hubel & Wiesel (1968). Journal of Physiology, 195, 215-243. Hubel & Wiesel (1965). Journal of Neurophysiology, 28, 1041-1059. Hubel & Wiesel (1974). Journal of Comparative Neurology, 177, 361-380. LeVay, Hubel & Wiesel (1975). Journal of Comparative Neurology, 159, 559-575. Bruce, Desimone & Gross (1981). Journal of Neurophysiology, 46, 369-384. Questions? Dr Tony Redmond [email protected] @tony_redmond