Visual System- Sterzer Flashcards
what are the 2 optical functions of the eye?
- adaptation- gather light reflected from surfaces
2. accommodation- focus it on an a clear image on the retina
what types of photo receptors do we have? where are they located?
rods and cones
located in the back of the retina and oriented away from the retina to propagate info to the brain
characteristics of rods….
- very light sensitive
- more photopigement
- high amplification, single photon detection
- sensitive to scattered light
- not preset in the central fovea
- achromatic (no colour distinction)
characteristics of cones….
- not so light sensitive
- less photopigement
- low amplification
- most sensitive to direct axis rays
- most preset in the central fovea
- trichromatic (3 types: S cones - short WL, M cones - medium WL, L cones- long WL)
Photoreceptors respond w. graded changes in membrane potential which generates greater response from foveal and ganglion cells
T/F?
T
which of the following statements is/are correct?
a. M have lower spatial resolution than P cells but have a faster frequency
b. M and P ganglion cells convey the same type of information
c. P cells are more sensitive to contrast than M cells
d. P cells respond to lower temporal frequency than M cells
e. M cells are more sensitive to contrast than P cells
a, c, d
which of the following statements is/are correct?
a. the organisation of the LGN is by eye (some cells respond to right eye and some to left eye)
b. the organisation of the LGN is by hemifield i.e. left LGN receives info from right hemifield and right LGN from left hemifield
c. the LGN has layers for Ipsy- and contralateral inputs which project to P and M channels
d. the LGN projects to the optic chiasm
e. the LGN is organised in columns
b, c
why can lesions in the temporal lobe cause visual disturbances?
optic graduation doesn’t go straight from LGN to occipital lobe, but curves through the temporal lobe–> lesions can damage subcortical pathways
what are the different visual-cortical pathways V1-V5?
the visual cortex is composed from 5 regions.
- V1- V3 have high phototopic organisation
- V4-V5 sensitive to very specific functional aspects (colour, motion…)
what does retinotopy mean?
- mapping of the visual field from the retina to cortical subregions.
- spatial organisation of neurons in the visual cortex corresponds to the visual field.
- the fovea is reflected to the back of the brain (posterior regions of the visual cortex) whereas the periphery is mapped more anteriorly.
what is polar angle mapping?
polar angle mapping is a method for imaging retinotopic mapping using fMRI.
- subject looks at object–> fMRI imaging shows which position of the wedge of the object activates which part of the visual cortex.
- flattening the fMRI data of the cortex –> detection of exact borders of the activated regions in the visual cortices
which of the following statements is/are correct?
a. P cells project more to the ventral/’what’ stream and M-cells project more dorsally (‘where’) stream
b. The WHAT stream is the ventral stream and cannot act without information from the WHERE stream which is dorsal
c. P cells project to the where/ventral stream and M-cells project to the what/dorsal stream
d. the dorsal (WHERE) stream can act without information from the ventral (WHAT) stream and vice versa
e. there is only one stream in the visual system
a, d
P-cells are more sensitive to shapes and colours and thus project to the ventral stream and M cells are more sensitive to motion and project to the dorsal stream
T/F?
T
what happens to a person with achromatopsia? which visual subregion is damaged?
the person cannot perceive colours due to lesions in V4
what happens to a person with akinetopsia? which visual subregion is damaged?
subject cannot perceive motion due to lesion in V5
which cortical subregion is responsible for perception of objects? what pathological conditions are associated with lesions in these areas?
the fusiform gyrus, fusiform face area (FFA) in the ventral stream and PPA are the subregions where visual processing of objects takes place.
- -> information about objects is not specific to each object, but rather it depends on activation pattern.
- -> example for pathological condition- prosopagnosia- patient can’t recognise faces (lesions in FFA)
the visual cortex is organised in…
a. layers
b. columns
c. columns and layers
d. hemifields
e. none of the above
c
why is the primary visual cortex (V1) called striate cortex?
because of the stria of Gunnari, a heavily myelinated group of cells. (stria = stripe)
how is the primary visual cortex organised?
V1 is organised in layers and columns:
Layers:
there are 6 layers in V1 (similar to the neocortex):
I- molecular layer
II-III- mainly pyramidal cells, receive info from LGN and project it to higher areas
IV- mainly interneurons
V-VI- cells receive info from II and III and send it to lower level areas–> feedback mechanism, info from V and VI is transmitted back to LGN.
Columns:
clusters of cells in V1 are organised across layers from top to bottom in a columnar fashion. This organisation is important for orientation selectivity - graded orientation selectivity of cells in columns.
definition if receptive fields…
a region in space in which the presence of a stimulus will alter the firing rate of a specific neuron (either increase or decrease)
the structure of receptive fields….
a. starts in the retina
b. starts in LGN
c. starts in V1
b. only occurs in the retina
a
what are the types of receptive fields at the level of retinal ganglion cells?
- On centre- respond to objects on the centre
- Off centre- higher response to objects in the periphery
response to stimuli…
a. determined by the borders and not by the light of the pattern.
b. response depends on the light reflected from the pattern to the retina
c. determined both by the light and the borders of a oattern
d. depends on the contrast of the image
a
at the level of V1 there is direction selectivity, as well as orientation selectivity. what cell types are responsible for the orientation selectivity in V1? what determines their response?
- simple cells- receive horizontal input from on- and off- cells in LGN. –> the orientation selectivity of simple cells is determined by the group of concentric cells in LGN which is projecting the information to it.
- Complex cells- simple cell population converge onto 1 complex cell. complex cells respond to vertical bars (don’t respond to horizontal/diagonal like simple cells) –> strength of the response changes according to the position of the stimulus (good for motion processing as position of stimulus changes)
