3.4.1 Vision III Flashcards
What are the two types of ganglion and bipolar cells?
What are the two paths in which information from the LGN is projected to the primary visual cortex? What type of information is carried in each projection?
What is incoming information to the LGN organized into?
Occular dominance columns
Where does the optic tract project to?
LGN - lateral geniculate nucleus (within the thalamus)
What are the different neurons/locations involved in the visual pathway?
In dark conditions, what is the state of photoreceptors and how much NT is being released? In conditions with light?
Dark: Depolarized, High NT release
Light: Hyperpolarized, Less NT release
What is true about the photoreceptors that supply the center and surround areas of a bipolar cell?
Different groups of photoreceptors supply those two distinct areas
What are the two types of bipolar cells? What postsynaptic receptor do they possess to respond to NT release by photoceptors?
Bipolar cells and ganglion cells both have “center-surround” receptive fields; their receptive fields are larger than those of photoreceptors as well as being more complex. For these more complex receptive fields, different areas within the receptive field react differently to stimuli. In the case of the visual system the stimulus is light, with the two extremes being bright light or darkness.
A center-surround receptive field organization means that the stimulus that excites the cell when it lands in the center of the receptive field is the OPPOSITE of the stimulus that excites the cell when it lands in the surrounding area in the receptive field. So a bipolar or ganglion cell that is excited by light in the center of it’s receptive field will also be excited by darkness in the surround. This type of cell is called an “on-center” (because turning the light “on” in the center excites it). By definition, an on-center cell is also excited by darkness in the surrounding part of its receptive field. These cells will give a ‘half-hearted” response when exposed to either just light in the center or dark in the surround, but they will fire the most when BOTH areas of the receptive field are stimulated with the optimal stimulus.
The other type of center-surround cell is called “off-center”, meaning it likes dark in the middle with light in the surround. Rather than being called “dark-center” cells, they are called “off” center because they respond when the light stimulus is shut off.
Receptive fields of bipolar cells are “built” by having photoreceptors with adjacent receptive fields synapse on them. Photoreceptors in the center of the bipolar receptive field either excite the bipolar in the light (on-center) or in the dark (off-center); this occurs by virtue of different postsynaptic receptors that respond to glutamate, as shown in the diagram.
What influences form and function of the visual system?
Early sensory experiences
Describe/draw the firing rate of these ganglion cells
This figure shows the responses of on-center and off-center retinal ganglion cells to (1) a central spot which illuminates part of the receptive field center, (2) a peripheral spot presented only to the surround, (3) central illumination of the entire receptive field center, (4) illumination of the entire receptive field surround and (5) diffuse illumination of the whole visual field. The responses are indicated by action potentials shown at the time course on the bottom right of the figure. (1) The on-center ganglion cell gives a modest response to light onset of a small spot (the light is on during the time period indicated by the bar marked light). (2) The spontaneous activity of the on center ganglion cell is inhibited by onset of a small spot in the receptive field surround. (3) The on center ganglion cell gives a brisk response to onset of a spot which fills the receptive field center. (4) The spontaneous activity of the on center ganglion cell is inhibited by illumination of the entire receptive field surround. In addition, the cell gives a brisk response when the surround stimulus is turned off. (5) Diffuse illumination of the entire visual field results in a weak on response.
What allows the LGN to have a more complex receptive field than bipolar/ganglion cells?
Convergence – combination of the RFs of the ganglion cells
What will be the visual field defects with lesions at the sites 1-6?
A map of which side of the visual field is contained in the LGN?
contralateral side
Which regions of the visual field end up on the lower bank of the calcarine sulcus? The upper bank?
Lower bank: Superior region of visual field
Upper bank: Inferior region of visual field
