visual system - receptive fields

receptive field

region of sensory space in which a stimulus can evoke a response from a sensory neuron, subregion of the multidimensional parameter space that describes a sensory stimulus to which a neuron responds

features of receptive fields in visual system

receptive field size (complexity often increases along visual pathway), receptive field of central neuron is the sum of receptive fields of all cells that have convergent input to that neuron

receptive field circuitry for center response in light

when glutamate release from photoreceptor is low (light is on), metabotropic receptors on the on-center bipolar cell causes cell to depolarize

receptive field circuitry for center response in light

when glutamate release from photoreceptor is low (light is on), ionotropic receptors on the off-center bipolar cell causes cell to hyperpolarize

receptive field circuitry for center responses in dark

when glutamate release from photoreceptors is high (light is off), metabotropic receptors on the on-center bipolar cell causes cell to hyperpolarize

receptive field circuitry for center responses in dark

when glutamate release from photoreceptors is high (light is off), ionotropic receptors on the off-cener bipolar cell causes cell to depolarize

receptive field circuitry responsible for generating receptive-field surround responses

lateral inhibition to generate receptive field surround responses

lateral inhibition pathway

horizontal cells receive glutamate from photoreceptors and depolarize in response, horizontal cells release GABA into neighboring photoreceptors to modulate neurontransmitter release on bipolar cells, GABA release from horizontal cells hyperpolarizes photoreceptors

lateral inhibition of on-center summary

light in surround depolarizes photoreceptors in the center, dark in the surround hyperpolarizes photoreceptors in the center

lateral inhibition of off-center cells summary

off center cell can be excited by dark in the center, or light in the surround, suppresses inhibition that’s normally coming into the center

optic nerve

bundle of axons from retinal ganglion cells that are stuck together encoding visual scene, flows out of the eye

optic chiasm

crossing of subset of retinal ganglion axons, axons in medial portion of each optic nerve crosses to the opposite side at this point

optic tract

rebundling of axons after they’ve crossed the chiasm

lateral geniculate nucleus

visual subdivision of the thalamus, where they make a synapse and then thalamic cells project to the cortex

primary visual cortex (V1)

anatomic target of thalamic input

lateral portion of optic nerve

carries action potentials from retinal ganglion cells in temporal retina and conveys info to contralateral visual field

medial portion of optic nerve

carries action potentials from retinal ganglion cells in nasal retinal, conveys info from ipsilateral visual field

where is the left part of the visual field encoded

right half of nasal half of left eye and right half of temporal half of right eye

resulting optic tract

contains only representation of contralateral visual field

right optic nerve

contains all info for right eye

lesion of right optic nerve

results in complete vision loss in right eye

midline/medial axons

show info for far left side of left eye and far right side of right eye

optic tract info

contains info from both eyes about contralateral field

right side of optic tract lesion

vision loss in left side of both eyes

types of cells in retina

retinal P cells, retinal M cells

retinal P cells

responsible for color, fine textures, patterns, longer latency, fine detail, project tot he parvocellular layers fo the LGN

retinal M cells

responsible for motion detection, shorter latency, courser detail, project to the magnocellular layers of LGN

LGN 6 major cellular layers

magnocellular (lower 2 layers) and parvocellular (upper 4 layers)

magnocellular layers

larger receptive fields, better sensitivity, motion processing (coarse details)

parvocellular layers

smaller receptive fields, better acuity, color processing (fine details)

primary visual cortex (V1)

from LGN visual info travels to occipital lobe, representation of visual field and V1 is divided into 12 section, each half of the visual field if mapped on opposite side of cortex

sensory surface in visual system

retina

cortical magnification of V1

disproportionate representation devoted to the fovea

ocular dominance columns

neurons in a column respond mainly to stimulus of one eye

orientation tuning

cell fires best when bar is positioned with a specific orientation

major pathways of visual analysis

dorsal and ventral

dorsal pathway

“where” pathway, location and action

ventral pathway

“what” pathway, object recognition, inferotemporal cortex

inferotemporal cortex

in ventral pathway, has part that is responsible for recognizing faces in monkeys and people (likes all features organized in specific way)

prosapagnosia

failure to identify faces