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Chapter 13

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PSYC 212
Remy Allard

Chapter 13The Visual System Motion Perception Eye Movements and ActionVisual system uses motion information for diverse purposesoTo recreate true motion of objects in real world so we know what is moving and how its movingoHelp avoid collisions as objects move towards uswe move towards themoHelp define its shapeset it apart from the backgroundoTo deduce object identityoUsed to direct eye movements either in a reflexive manner nystagmus or purposeful mannerA Fundamental Aspects of Motion ProcessingAll animals have ability to processuse motion information1 What is MotionMotion described as spatiotemporal a physical term that encompasses the twin qualities of space and time motion is an exampleAn object moves across a certain amount of space in a given amout of timeThe greater the space covered the greater the speedAnother qualitydirectionSpeed and direction define velocity the rate and direction of an objects movement2 Temporal visionPerception of motion requires visual system to processintegrate both spatialtemporal informationThe speed of sightoNeural pathways mediating visual experience are well knownoFrom the moment light photons reach the photoreceptors series of cumulative delays are introduced beginning with conversion of light energy into biological signals then transmission of those signals through hierarchy of visual pathways to temporal cortex and beyond where neural processes that mediate object recognition take placeoTotal time for visual signals to reach higher areas of visual cortex80120 millisecondsDependent on factors like nature of stimulus and precise brain areaoGenerally takes 80100 milliseconds to make motor response to stimulusoSpeed of sight is slow in part due to complex set of processes that must unfold at neurological level before able to perceive the stimulusTemporal resolutionoVisual systemslow to transmit and interpret visual signals AND slow at detecting changes in stimulusoImportant because visual system is somewhat slow at registering incoming informationoSlow at detecting changes that occur in stimulus sequencehave impact on other aspects of vision that rely on temporal processing including motion perceptionoRate of change of visual stimulustemporal frequency the rate at which a stimulus changes in some manner per unit timeSpecified in terms of cycles of change per second hertzTo see take flicker a visual stimulus that appears and disappears in a cyclical manner over timeVisual stimulus typically used is simple patternpresented in alternation fashion at different temporal frequencies with goal of determining frequency at which flicker perception disappearsAble to detect stimulus alternations up to 60 HzCritical clicker fusion CFFa condition at which the temporal change in a stimulus is too rapid for clicker detection The flickering stimulus becomes fused with the background and appears to be uniformoFlicker perception experiment provides superficial account of temporal vision because does not give information on how sensitive our visual system is to different temporal frequenciesoTemporal resolutionthe ability to distinguish physical stimuli that are applied at different moments in time A system has high temporal resolution if it can detect objects that are alternated rapidlyOne way to proceed is to obtain contrast threshold of flickering stimulus usually sinewave grating at different temporal frequenciesThe greater our temporal resolving ability at a particular frequency the lower the contrast needed to just observe the flickeroTemporal contrast sensitivity functionthe sensitivity profile of the visual system to different temporal frequencies Often referred to as TSFDerived from threshold measuresReinforces notion that CFF frequency occurs around 60 Hz under photopic conditionsoOur visual system optimally detects temporal frequencies 1015 Hz rangethat our temporal sensitivity declines on either side of the rangeoScotopic vision more sluggish compared to photopic vision at all frequenciesdisplays much lower CFF frequenciesoNight visionnot well suited for detecting temporal changes in visual stimulusTime to collisionoVisual system needs to evaluate the absolute depth of the incoming object make an estimate of its speed of approachFrom these two proceed to obtain the time to collision TTCthe time required for an object to collide with an observerVisual system isnt very good at estimating absolute distance of objectsoTTC processing system relies on sluggishinaccurate ability would not be able to give precise values that allow us to react rapidlyoSuggested to derive TTC values from way the retinal image changes as an object approaches us
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