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

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Chapter 13: The Visual System: Motion Perception, Eye Movements and Action A. Fundamental Aspects of Motion Processing 1. What is Motion Motion is described as a spatiotemporal event. The greater the space covered by an object, the greater the speed of the object. The speed and direction is an object define the velocity of an object 2. Temporal Vision The speed of light There’s a certain delay between the appearance of an object, and our perception of the object’s appearance. A delay is introduced between the conversions of light energy (in photons) into biological signals. Another delay is between the transmission of those signals in the visual pathways and the neural processes that mediate object recognition (80-120 milliseconds) It takes another 80-100 milliseconds for our motor signals to respond Temporal Resolution Temporal frequency: the rate of change of a visual stimulus (cycles of change per second) We can test this through flicker perception A simple pattern is presented in an alternating fashion at different temporal frequencies with the goal of determining the frequency at which flicker perception disappears . Meaning that up to about 60 Hz, we can tell something is changing in a pattern (a flicker). Above 60 Hz, the stimulus will appear uniform (blends into the background). The frequency at which temporal changes are no longer detectable is called the critical flicker fusion.
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The problem with this experiment is that it doesn’t give information about how sensitive our visual system is to different temporal frequencies. One way to find the temporal resolution is to obtain the contrast threshold of a flickering stimulus at different temporal frequencies. The greater our ability to see the temporal flicker at a certain frequency, the lower the contrast needed to just observe the flicker. In figure 13.2, we see the temporal contrast sensitivity function o CFF occurs around 60 Hz in photopic vision and the optimal range of temporal frequencies is around 10-15 Hz. o CFF occurs around 15 Hz in scotopic vision (night vision is not well suited for detecting temporal changes) Time to collision In order to obtain the time to collision, we need to estimate the speed of approach of the object and the incoming depth. The problem is that we have trouble estimating the distance of an object. Instead, we look at the way the retinal image changes as an object approaches us. The ratio of an expanding image to its actual size is referred to as tau. o Tau provides an accurate estimate of TTC o It does so without the needing object distance or the rate of movement. o Tau is the ultimate mechanism for allowing us to make quick reactions and avoid collisions. 3. Low-Level Motion Detectors Directional selectivity One way we can detect motion is if our brain captures snapshots of a scene and notices the changes in physical location of the object over time. The problem with this is that it doesn’t represent smooth movement.
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