2. The Visual system Flashcards
(24 cards)
What structures in the eye focus light onto the retina?
Cornea: primary refractive surface
Lens: fine-tunes focus via accommodation
Aqueous and vitreous humors: transparent media
Tags: Eye Anatomy, Optics
How is light converted into a neural signal in photoreceptors?
Photon absorption by opsin → conformational change in retinal
Activation of transducin → phosphodiesterase reduces cGMP
Closure of cGMP-gated channels → hyperpolarization of photoreceptor
Tags: Phototransduction, Retina
How do rods and cones differ in distribution and function?
Rods: high density in peripheral retina, high light sensitivity, achromatic
Cones: concentrated in fovea, lower sensitivity, mediate colour vision (S, M, L types)
Tags: Photoreceptors, Retina
What are the peak sensitivities of the three cone types?
S-cones: ~420 nm (blue)
M-cones: ~530 nm (green)
L-cones: ~560 nm (red)
Tags: Photoreceptors, Colour Visio
What are the main cellular layers of the retina from outer to inner?
Photoreceptor layer
Outer nuclear (photoreceptor cell bodies)
Inner nuclear (bipolar, horizontal, amacrine cells)
Ganglion cell layer
Tags: Retina, Histology
What defines an on-centre/off-surround ganglion cell receptive field?
Centre region: light increases firing (“on”)
Surround region: light decreases firing via lateral inhibition
Tags: Ganglion Cells, Receptive Field
How do red/green and blue/yellow opponent receptive fields arise?
Centre: input from one cone type (e.g. L-cone for red)
Surround: input from opponent cone type (e.g. M-cone for green)
Tags: Ganglion Cells, Colour Opponency
What are the differences between midget and parasol retinal ganglion cells?
Midget: small RFs, colour-opponent, project to parvocellular LGN
Parasol: large RFs, luminance-sensitive, project to magnocellular LGN
Tags: Ganglion Cells, Pathways
What do koniocellular (bistratified) ganglion cells encode?
Input primarily from S-cones (blue)
Project to koniocellular layers of LGN
Tags: Ganglion Cells, Pathways
What are the three cell pathways in the LGN and their retinal inputs?
Magnocellular: parasol cells (motion, luminance)
Parvocellular: midget cells (form, colour)
Koniocellular: bistratified cells (blue-yellow signals)
What characterizes magnocellular LGN neurons?
Large RFs, monochromatic, high temporal resolution, low spatial resolution
Tags: LGN, Magnocellular
What defines parvocellular LGN neurons?
Small RFs, colour-opponent, high spatial resolution, low temporal resolution
What are key features of koniocellular LGN neurons?
Located between magno/parvo layers, driven by S-cones, ON-centre only
Tags: LGN, Koniocellular
How are LGN centre-surround receptive fields organized?
Spatial opponency: centre excitatory vs. surround inhibitory
Similar to their retinal ganglion inputs
What is cortical magnification in the LGN?
Disproportionate representation of foveal inputs over peripheral inputs
Tags: LGN, Retinotopy
Which LGN layers project to V1 layer 4C subdivisions?
Magnocellular → 4Cα
Parvocellular → 4Cβ (and 4A)
Tags: V1, Laminar Organization
What are ocular dominance columns?
Alternating stripes in V1 where neurons preferentially respond to one eye’s input.
Tags: V1, Columns
What is the role of orientation columns in V1?
Clusters of neurons tuned to the same edge orientation, enabling detection of bars and edges.
Tags: V1, Columns
What defines a simple cell’s receptive field in V1?
Adjacent excitatory and inhibitory subregions
Tuned to specific orientation and position
How do complex cells differ from simple cells?
Orientation-selective but without fixed subregions
Respond to stimuli anywhere within their RF; some are direction-selective
Tags: V1, Complex Cells
What are end-stopped cells?
V1 neurons that respond optimally to stimuli of a preferred length or to corners, with reduced firing for longer stimuli.
Tags: V1, Hypercomplex
How are eccentricity and polar angle mapped in visual cortex?
Expanding/contracting ring stimulus → eccentricity maps
Rotating wedge stimulus → polar angle maps
How are borders between V1, V2, etc., identified?
By detecting reversals in retinotopic progression in combined eccentricity and polar maps.
Tags: V1, Visual Areas
How does receptive field complexity change from retina to V1?
Retina/LGN: circular centre–surround
V1 simple: oriented bars with subregions
V1 complex/hypercomplex: orientation with position invariance and end-stopping
Tags: Hierarchy, Receptive Fields
