Chapter 13 - Chapter 13 The Visual System Motion Perception...

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Chapter 13- The Visual System: Motion Perception, Eye Movements and Action A. Fundamental Aspects of Motion Processing What is motion? - Much understanding in this field came from early experiments on the common housefly; as it turns out, the ways of capturing motion info are fundamentally similar across species. - Scientifically, motion is described as a spatiotemporal event ; the greater the space covered over a certain time, the greater the object’s speed. - Speed and direction are the 2 parameters that define velocity of an object. - A spatiotemporal profile (space-time graph) may show a sinusoidal shape, reflecting a harmonic oscillation in space over time, like sound waves (gradual change in position). Temporal vision - Perception of motion requires visual system to integrate both spatial + temporal info. Speed of sight: - Total time for visual signals to reach higher areas of the visual cortex= 80-120 milliseconds. - Factors include the nature of the stimulus and the precise brain area in question. - It generally takes another 80-100 milliseconds to make a motor response to that stimulus. - Essentially, the speed of sight is rather slow due to the complex sets of processes involved. Temporal resolution: - Important distinction- the visual system may be slow in transmitting/interpreting visual signals, but it is not slow at detecting changes in the stimulus. - Rate of change of a visual stimulus = temporal frequency . o Temporal frequency is specified in terms of cycles of change/second (or hertz ). - Way to test speed of following a changing stimulus= experiment of flicker (visual stimulus that appears/disappears over time in a cyclical manner) perception o Simple pattern is presented at different temporal frequencies to determine the frequency at which flicker perception disappears- i.e. can no longer be resolved. o We are capable of detecting temporal frequencies of stimuli up to 60 Hz , after which the perception of flicker disappears, and the stimulus appears uniform in nature. o Frequency at which this undetectability occurs = critical flicker fusion frequency. o However, flicker tests give only a superficial account of temporal resolution. - More sophisticated experiment= obtaining the contrast threshold of a flickering stimulus at different temporal frequencies; the greater the temporal resolving ability at a particular frequency, the lower the contrast needed to observe the flicker. o Shows the human temporal contrast sensitivity function . o This function confirms CFF of 60 Hz and that optimal resolution = at 10-15 Hz. Time to collision: - To avoid collision w/ a looming trajectory, the visual system needs to evaluate the absolute depth of the incoming object, make an estimate of its speed, and obtain the time to collision (TTC). - Problem: the visual system is not that good at estimating absolute distance of objects.
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