neurobio 916 ch 10 of Bear's book: the central visual system part 2 (retinofugal projection optic n
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true or false? the striate cortex layers function in almost the same way as the layers of the LGN
false. in the lgn, every layer receives retinal afferents and sends efferents to the visual cortex. in the visual cortex, only a subset of the layers receives input from the LGN or sends output to a different cortical or subcortical area
where do most axons from the LGN terminate in the cortex? are the axons integrated or segrated through this process?
the largest number go to layer IVC.
each stream of info. from the LGN (for ex. from the magnocellular and parvocellular layers serving the right and left eyes) remain anatomically segregated in the layer IVC.
The magnocellular LGN neurons project to layer IVCα and parvocellular LGN neurons project to layer IVCβ.
explain why we should think of the two layers of IVC as stacked on one another "like pancakes"?
because the input from the LGN to the cortex is arragned topographically, we see that layer IVC contains two overlapping retinotopic maps, one is from the magnocellular LGN (α) and the other from the parvocellular LGN (β).
to which part of the cortex do koniocellular LGN axons send info?
to layer II and III
how do the LGN cell layers receive retinal output from different ganglion cells?
The right LGN receives information from the left visual field, and the left LGN receives info. from the right visual field.
At LGN, info from the two eyes is kept separate. In each LGN, the info from the temporal portion of the retina in the eye that is ipsilateral (same side) to the LGN is sent to layers 2,3, and 5 of the LGN. Meanwhile, the info from the nasal portion of the retina in the eye that is contralateral (opposite side) to the LGN is sent to layers 1,4, and 6.
describe striate cortex outputs.
the pyramidal cells in different layers innervate different structures. Layer II, III, and IVB pyramidal cells send their axons to other cortical areas. Layer V pyramidal cells send axons all the way down to the superior colliculus and pons. Layer VI pyramidal cells give rise to the massive axonal projection back to the LGN.
what is mitochondrial oxidase?
an enzyme used for cell metabolism
if a part of the V1 is stained to reveal the presence of cytochrome oxidase, what is observed?
the stain is not uniformly distributed in layers II and III. Rather, the cytochrome oxidase staining in cross sections of striate cortex appears as a colonnade, a series of pillars at regular intervals, running the full thickness of layer II and III and also in layers V and VI. these pillars of cytochrome oxidase rich neurons have come to be called blobs.
how are blobs positioned?
the blobs are in rows, each blob centered on an ocular dominance stripe in layer IV.
between blobs are ___-_____ regions
what is the function of blobs?
they receive direct LGN input from the koniocellular layers, as well as parvocellular and magnocellular input from layer IVC of V1
true or false? the foveal region of the occipital lobe is extremely large.
what are the targets of the optic tract?
most of the optic tract innervates the LGN. Some of the optic tract innervatesthe hypothalamus (including the suprachiasmatic nuclei), and another 10% or so continue past the thalamus and innervate the midbrain (including the pretectum and the superior colliculi).
what is retinotopy?
retinotopy is an organization whereby neighboring cells in the retina feed info to neighboring places in their target structures (for ex. the LGN and striate cortex)
the mapping of the visual field onto a retinotopically organized structure is often distorted. Why?
Because visual space isn't sampled uniformly by the cells with receptive fields in or near the fovea than in the periphery. Thus the representaiton of the visual field is distorted in the striate cortex: the central few degrees of the visual field are overrepresented in the retinotopic map
a discrete point of light can activate a large population of cortical neurons. Explain why this is
this is because, every neuron that contains that point in its receptive field is potentially activated
true or false? most neurons in layers superficial to IVC are binocular.
true or false? the info from both eyes begins to integrate in IVC of the striate cortex/
false. info from both eyes begins to integrate after the info leaves IVC
Hubel and Wiesel found that the best stimulus for neurons in V1 was what?
an elongated bar of light
Explain what it means when a V1 neuron exhibits orientation selectivity
Hubel and Weisel found that when they stimulated V1 neurons with a bar of light, the orientation of the bar was important. The greatest response is given to a bar with a particular orientation; perpendicular bars generally elicit much weaker responses.
Which V1 neurons are orientation selective
most neurons in layers outside of layer IVC are orientation selective
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