visual and auditory Flashcards
what is the structure of the retina?
The retina is the sensory organ of vision
Three main layers: Photoreceptor layer Rods and cones Intermediate layer Bipolar, horizontal, and amacrine cells Ganglion cell layer
Retinal ganglion cells: midget & parasol
what are the functions of the different cells and layers of the retina?
Cones and rods take in light and respond to different wavelengths of light
Intermediate layer takes infor from mainly cones
Horizontalcells take info from both cones and rods
Biopolar cells
Ganglion layer- information being sent.
what is the fovea?
he fovea centralis is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina.
what happens when a photon of light hits a cone or rod?
Rods and cones respond to light intensity In darkness, rods & cones constantly release neurotransmitter (glutamate)
Light is absorbed by a pigment in rods and cones Rhodopsin in rods, cone opsins in cones, iodopsin
Causes change in shape of photopigment that triggers a G-protein cascade that reduces glutamate release
So, paradoxically, photoreceptors are inhibited (deactivated) by light!
S cones respond to lower wavelgenths of light < rodes < “red cones(L)” < green cones(M)
what does the intermediate layer do?
Contains bipolar, horizontal, and amacrine cells Bipolar cells transfer information from rods & cones to retinal ganglion cells Site of lateral inhibition that creates opponent receptive fields Transforms light (brightness) information into contrast information
what do bipolar cells do?
ON and OFF bipolar cells differ in how they respond to input from photoreceptors: ON bipolar cells are inhibited by input OFF bipolar cells are excited by input Opposite to intuition!
what are retinal ganglion cells?
Parasol Large dendritic trees Combine inputs from many bipolar cells Midget Small dendritic trees Combine inputs from few bipolar cells Dendritic trees larger in periphery for both
What is the Physiology of the retina?
Photoreceptors translate light into neural signals for light intensity (signal transduction)
Signals for light intensity are then converted into signals for contrast (differences in light intensity) by bipolar and ganglion cells
what about visual receptive fields?
To understand how the retina works, we need to know about visual receptive fields (RFs) RFs: the region of sensory space that evokes a response in a neuron The part of the visual field where a stimulus causes a neuron to respond RFs have a position and a size RFs can have both excitatory and inhibitory subregions
what are on and off recpetive fields?
Respond in opposite directions to contrast changes ON RFs respond to an increase in light intensity OFF RFs respond to a decrease in light intensity
what is lateral inhibition?
Neighbouring bipolar cells inhibit each other through horizontal cells (lateral inhibition)
summerise retina lphysiology
Photoreceptors respond to light intensity Lateral inhibition transforms this response to code for contrast (light differences) Retinal ganglion cell RFs code for Differences in light and dark over time (flicker) Differences in light and dark over space (contrast) Differences in colour Retinal ganglion cell RFs are circular No information about direction or orientation
what is the LGN?
Lateral geniculate nucleus
what is the anatomy of the LGN?
The LGN consists of six layers The layers differ in terms of: The kind of cells they contain What type of visual input they receive Which eye they receive input from
what are the two main visual pathways of the LGN?
Magnocellular (M) pathway Inner two layers (1 & 2)
Receive input from parasol ganglion cells Parvocellular (P) pathway
Outer four layers (3,4,5,6) Receive input from midget ganglion cells
Receptive fields in LGN are similar to those of retinal ganglion cells (circular centre-surround)
what is the function of the LGN?
Relay station between eye and brain
Response properties similar to retinal ganglion cells
But receives massive feedback from cortex – 10x as many connections as from the eye!
Eye
First site of attentional gating/enhancement Sleep-related gating of sensory input to cortex (reticular formation)
what is V1
primary visual cortex
Also known as striate cortex (from Stria of Gennari – line of Gennari) First site of visual processing in cortex
the posterior occipital lobe
Topographic (retinotopic) organisation Contains a “map” of the visual field Detailed maps of orientation, colour, spatial scale, motion direction, 3D depth Projects to most higher visual areas in cortex For each part of the visual scene, V1 computes: orientation, spatial frequency, motion, colour, depth