Biopsychology Midterm 2 THE VISUAL SYSTEM

-no animal can see in complete darkness, no light, no vision -light thought of in 2 ways, particles of photons or waves of energy -light=wavelengths, different animals respond to dif waves -important properties of light : wavelength (color) and intensity (brightness) -light enters eye through pupil, muscles within the iris control the size of the pupil. ..-adjusment of pupil size compromise between sensitivity (ability to detect prescense of dimly lit objects, humans need more than cats) & acuity (ability to see details of objects, dependent on many things, ie pupil size, smaller pupil=greater acuity/focus...greater depth of focus, dim light=pupil dialte (big), less acuity and depth of focus) -behind pupil is lens, focuses incoming light on the retina -tension in ligaments holding lens in place is adjusted by ciliary muscles -accommodation: adjusting configuration of the lenses to bring images into focus on the retina -convergene of eyes greatest when inspecting things up close -binocular disparity: difference in position of same image on the two retinas..greater for close objects than distant ones, used to construct 3D from 2D retinal images

Retina is composed of 5 layers of different types of nuerons...enters 1.retinal ganglion cells 2.amacrine cells 3.bipolar cells 4. horizontal cells 5. receptors -inside out arangement of retina, light enters, goes through 4 layers, activates receptors back through layers to retinal ganglion cells, axons gather in bundle and exit eyeball -where these bundles leave eye is the blind spot...gap in receptor layer....fixed by completion: uses information provided by receptors around blind spot to fill in gap -fovea: part of retina, high quality vision, seeing fine details -when seeing an image edges and location are is perceived, conducts to cortex, where perception of image is created from this partial information -two types of receptors in retina, cones and rods -duplexity theory: cones and rods mediate different kinds of vision -CONES: (photopic vision) , in good lighting, provided high acuity/detail color perceptions, usually low convergence on retinal ganglion cells ( few sticking out) -RODS: (scotopic vision), high sensitivity, low acuity....in dim lighting, less detial, many pathways (high convergence) to retinal ganglion cells, none in the fovea, more toward periphery - eyes saccade, adds together information from fixations to provide high acuity, colored perception -visual transduction: convergence of light to nueral signals by the visual receptors...rhodopsin study, rods predominate in this pigment (pigments are anything that absorb light)...in dim light out sensitivity to various wavelengths is a direct consequence of rhodopsins ability to absorb them -rhodopsin is g protein, responds to light not neurotransmitter....when rods in darkness, sodium channels aer partially open, thus rods are slightly depolarized, steady flow of glutamate....when bleached by light, sodium channels closed, hyperpolarizes rods, reduces release of glutamate...case in point: in rods signals are often transmitted through neural systems by inhibition

-most popular brain pathway that carries visual information...retina-geniculate-striate pathway -Information from the left visual field of each eye projects to the right lateral geniculate nucleus (LGN) and vice versa -90% of axons of retinal ganglion cells become part of RGS pathway....leave through bling spot and go to lateral geniculate nuclei (LGN) area in thalamus...then goes to primary visual cortex (or striate cortex), and most of these projections terminate in lower part of cortial layer -each LGN has 6 layers...one channel runs through top 4 layers, parvocellular layers (P layers): composed of nuerons with small cell bodies...other channel runs through the bottoms 2 layers, majority is cones, responsive to color, fine pattern details, stationary/slow objects magnocellular layers (M layers): composed of neurons with large cell bodies, responsive to movement, primarily rods

-edge perception very important for vision -perception of an edge is the perception of a contrast between to adjacent areas in the visual field -horseshoe crab....when single ommatidium is illuminated it fires at rate proportional to the internsity of the light, and when receptor fires, it inhibits it neighbors through lateral inhibition...amount of lateral inhibition is greatest when receptor is greatly illuminated -receptive field of a VISUAL NEURONS is the area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuron....efective stimuli are those that either increase or decrease the rate of firing....Hubel and Wiesel tested these areas, tested response of neurons to various stimuli within its receptive field to identify stimuli that most influence its activity ...did this for for all 3 layers of RGS system...ganglion retinal cells, lateral geniculate neurons and striate neurons.....found 4 commonalites in all 3 layers of cells 1. receptive field in fovea smaller than those at periphery 2. call neurons had recetpive fields that were circular 3. all neurons were monocular: receptive filed in one eye but not the other 4. many had both excititory and inhibitry are seperated by a circular boundry NEURONS IN RGS SYSTEM .... 4th discovery most important, leads to on/off firings depending on location of of the spot of light in the receptive field on center cells: respond to lights shone in the central rgion of their receptive fields by on firing, and lights shined in periphery to inhibition/off firing ....off center is opposite pattern, inhibotion/off in the center, on firing in the periphery....respond best on either on or off completely illuminated, and the other completely dark...the most effective way of mazimizing the firing of an on center or off center cell is to completely illuminate either the on area or off area of its receptive field....if both areas of the cell's receptive field are illuminated together, there is little reaction from the cell STRIATE CORTEX NEURONS the receptive field of most primary visual cortex neurons fall into wither simple or complex category simple cells: have receptive fields divided into on/off regions like RGS system, and are monocular...main difference=on/off regions are straight rather than circless...responds maximally when its perferred straight edge stimulus is in a particular position and in a particular orientation...rectangular receptive field complex cells: more numerous than simple cells, unresponsive to diffuse light ...have larger receptive fields than simple cells, cannot divide into static on/off regions, also unlike monocular simple cells, some complex cells are binocular (respond to stimulation of either eye)

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