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NROC34: Lec 6: Visual Motion Detection (in flies)

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Andrew Mason

Visual Motion Detection in fliesSlide 22 broad categories of things that flies tend to do for motion visionocourse stabilization when they fly around have to control where theyre goingoobject fixation interested in an object need to keep track of itcompound eyeohave very highly structuredcomplex anatomyoif youre interested in certain parameters of visual info such as light intensity or color or polarizationcan basically get this with a single detector complex eyesSPATIAL visiongetting info about how these parameters of visual stimulation vary in spaceSlide 3spatial infoodifferences in intensity or color at diff locationsoimage formationSlide 4 Motionmotion changes over time in these patterns of stimulation and this could be due to objects in visual space moving themselves OR could be due to selfmotiontemporal factoroimage changes but there are other sources of change in the visual imageHow are motion signals computedSlide 5 Compound eyesFacetsommatidiasmall corneal lenses that are aligned with a group of photoreceptors rhabdomThese corneal lenses are pointed in certain directions in space and will capture light from a certain range of angles around that directionIt makes up a retinotopic mosaic of visual spaceSlide 6 Retinotopic MosaicDiagramblowup of several facets of a compound eyeThe range of angles around the direction in which a particular direction in which a particular ommatidia is facing that will be able to enter the ommatidial lens and stimulate the associated receptorsacceptance angle for that ommatidiaThe angle between the axes of neighboring ommatidiainterommatidial angleEach ommatidia is looking at a diff region of space and that direction plus the acceptance angle around that direction represents the receptive field of an ommatidiaSize of the receptive fielddetermined by size of acceptance angle and the separation of neighboring receptive fields is determined by the size of the interommatidial angleIf you want full and efficient coverage of visual space youd want those 2 angles to be the same which is generally the case in compound eyesoSize of the acceptance equals the separation between so that you dont have gaps and no overlapoHave continuous sampling of visual spaceoReceptor field is where the ommatidia is lookingthe space that stimulates the photoreceptoroInterommatidial angleangle between the centre of neighboring receptive fields and so if the size of the receptive field is bigger than the separation between receptive fields then you have neighboring receptors w overlapping receptive fieldsget 2 receptors that are sampling the same space if the interommatidial angle is much bigger than the acceptance angle then you have neighboring receptors that are looking at 2 places w gap in betweenSlide 7 Visiondetection of lightEach ommatidium signals the luminance brightness in its receptive fieldmeans being stimulated by photonsPhotons have particlelike characteristics such that theres some randomness in the arrival of photons to represent particular level of illuminationIntensity of lightrate of photonic stimulationaverageof photons per unit timeoBright lightmore photons raining down more quickly than dim lightThese photons are pounding into the membrane of the photopigments of photoreceptor cellsdepolarize and activate photoreceptors to generate stimulusTakes certain amount of stimulation to activate a photoreceptorDetectability of light is limited by the available light NRG how many photons are hitting the detector oThere must be some minimum amount hitting photoreceptor over certain period of time to activate itoBecause of the fact that its a stochastic rainstorm of photonsmeans theres some random fluctuation in any given sample of photons that photoreceptor sees over a given time windowSo this photon noise is going to limit the resolution of sensitivity of visual sys in certain waysSlide 8 Contrast SensitivityHave a set of multiple receptors each looking at a diff region of space and making their measurements of brightness of light and you want to know if theres a difference in stimulation in 2 locationsThis is what you need if you want to perceive visual patternsto be able to discriminate differences in intensity between points in spacethat means spatial contrast
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