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Lecture 3

Lecture 3.docx

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Matthias Niemeier

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Lecture 3  the central visual system Short answer question  how do bipolar cells in the fovea (and elsewhere) differ as to how they are activated / inhibited by light o on center vs off center bipolar cells  on center cells  on center cells fire action potentials when light is on it o dark in center o surrounded by light  hyperpolarize  off center cells fire action potentials when dark is on it o light in center o surrounded by darkness  hyperpolarize Introduction  neurons in the visual systems o neural processing resulting in perception  parallel pathway serving conscious visual perception originate in the retina o progress to  lateral geniculate nucleus (LGN)  primary visual cortex  visual area 1 (V1)  higher order visual areas  dorsal stream o parietal lobe  ventral stream o temporal lobe  neuronal receptive fields o sensitive to different facets of the visual input The norwegian silly crab  the major events in the life of a crab o foe o food o love Ganglion cells + / - Motor control Upper visual field Motion + Run Pink + Love Green N/A N/A Lower visual field Motion - Run Pink + Love Green + Eat  no green necessary in upper visual field (no food sources in sky)  no motion detection necessary in lower visual field (no predators on the ground)  pink detection in upper and lower visual field (essential for reproduction / survival) Overview  visual pathways  anatomy of the striate cortex  physiology of the striate cortex  extrastriate cortex and beyond  from neurons to face perception  visual deficits Visual pathways  from eye to the optic tract  visual hemifields  what does the optic tract target?  functional organization of the lateral geniculate nucleus (LGN) The retinofugal projection  right and left visual hemifields  chiasm  partial decussation o decussation = a nerve or tract of nerve fibers that crosses form one side of the central nervous system to the other  targets of the optic tract o right brain  nasal portion of left eye  temporal portion of the right eye o left brain  nasal portion of right eye  temporal portion of left eye What does the optic tract target? The retinofugal projection  nonthalamic targets of the optic tract o hypothalamus  biological rhythms  sleep  wakefulness o pretectum  size of the pupil  certain types of eye movements o superior colliculus  orients the eyes in response to new stimuli Functional organization of the lateral geniculate nucleus  6 layers (not really)  separation between eyes  cellular systems o magno o parvo o konio Lateral geniculate nucleus  first two layers have larger cell bodies (soma) o characteristic of magno-cells  last four layers have smaller cell bodies (soma) o characteristic of parvo-cells  right LGN receives input from left visual field o left LGN receives input from right visual field  the segregation of input by eye and ganglion cell type Retinal output LGN Eye Ganglion cell type Layer LGN cell type Contralateral P-type 6 Parvocellular Ipsilateral Parvo 5 Contralateral 4 Ipsilateral 3 Ipsilateral M-type 2 Magnocellular Contralateral Magno 1 Same as overlying nonM-nonP Ventral to each Konicellular principal layer principal layer Lateral geniculate nucleus  magno-cellular system o large receptive fields o lower spatial frequencies o transient o motion o good luminance contrast o poor color contrast  parvo-cellular system o small receptive fields o higher spatial frequencies  responsible for fine detail detection o sustained o not so good luminance contrast o good color contrast  konio-cellular system o light-dark o color opponency o other unknowns Lateral geniculate nucleus (LGN)  receptive fields o receptive fields of LGN neurons:  similar to the ganglion cells that feed them  shapes of the receptive fields are not as important for LGN neurons as they are for V1 neurons  nonretinal inputs to the LGN o retinal ganglion cell axons  not the main source of synaptic input to the LGN o primary visual cortex  80% of the synaptic input o neurons in the brain stem  modulatory influence on neuronal activity Anatomy of the striate cortex  where is V1 located?  retinotopy  layers in V1  inputs and outputs  blobs Where is V1 located?  medial portion tucked into calcarine fissure  lateral (foveal) portion relatively small Retinotopy  map of the visual field onto a target structure o retina o LGN o superior colliculus o striate cortex  overrepresentation of central visual field  discrete point of light o activates many cells in the target structure  overlapping RFs (RF = ?)  RF = receptive fields  perception o based on the brains interpretation of distributed patterns of activity  there is nobody who actually reads the 'map' Retinotopy  diagram o parallel growth of neurons  retina  LGN  striate cortex  layer IVC Lamination of the striate cortex  layers I- VI  spiny stellate cells o spine covered dendrites  layer IVC  pyramidal cells o spines  thick apical dendrite  layers outside IVC  basal dendrites  local connections  inhibitory neurons o lack spines o all cortical layers o form only local connections Inputs to the striate cortex  magnocellular LGN neurons o project to layer IVCa  IVB  parvocellular LGN neurons o project to layer IVCb  III  koniocellular LGN axons o bypasses layer IV to make synapses in layers II and III  direct projections to layers II and III Ocular dominance columns  Review! Ocular dominance columns vs orientation columns  optical imaging o stripes of ocular dominance  zebra stripes Inputs to the striate cortex  layer IVC innervates superficial (surface?) layers o radial and horizontal connections to II and III  monocular vs binocular dominance  III and IVB o connections to all layers in V1 o layer III involved in stereovision? outputs of the striate cortex  layers II / III / IVB o projects to other cortical areas  layer V o projects to the superior colliculus and pons  layer VI o projects back to the LGN Cytochrome oxidase blobs  cytochrome oxidase is a mitochondrial enzyme used for cell metabolism  blobs o cytochrome oxidase staining in cross sections of the striate cortex appear as blobs  not present in layer IV o Review! Physiology of the striate cortex (Overview)  receptive fields o binocularity o orientation selectivity o direction selectivity o color  pathways vs modules Physiology of the striate cortex  receptive fields o layer IVC  monocular  center surround o layer IVCa  insensitive to wavelength o layer IVCb  center surround color opponency  binocularity o layers superficial to IVC  first binocular receptive fields in the visual pathway  receptive fields o Review! o orientation selectivity  bar of light o direction selectivity  neuron fires action potentials in response to moving bar of light o simple cells  binocular  orientation selective  elongated on / off region  with antagonistic flanks which responds to optimally oriented bar of light  possibly composed of three LGN cell axons with center surround receptive fields  if a bar of light is shining onto the off-center portion of receptive field then action potentials will not fire
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