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Motion Andy Rider A.Rider@Ucl.Ac.Uk Motion Andy Rider [email protected] Motion aftereffect • If you stare at something that contains motion, and then look at something that’s stationary, you perceive illusory motion in the opposite direction First described by Aristotle Then by Lucretius (384–322 BC) (99–55 BC) 1 Motion aftereffect Addams, R. (1834). An account of a peculiar optical phænomenon seen after having looked at a moving body. London and Edinburgh Philosophical Magazine and Journal of Science, 5, 373–4. Fall of Foyers https://www.youtube.com/watch?v=6MK9qQ_ApHQ Motion aftereffect Addams, R. (1834). An account of a peculiar optical phænomenon seen after having looked at a moving body. London and Edinburgh Philosophical Magazine and Journal of Science, 5, 373–4. Fall of Foyers https://www.youtube.com/watch?v=6MK9qQ_ApHQ 2 Motion aftereffect Storm Illusion https://www.youtube.com/watch?v=OAVXHzAWS60 Motion aftereffect Storm Illusion https://www.youtube.com/watch?v=OAVXHzAWS60 3 Motion aftereffect Wohlgemuth (1911) Direction-selective cells in visual cortex Hubel & Wiesel (1959): Neuron in cat primary visual cortex (V1) Hubel & Wiesel (1968): Neuron in monkey primary visual cortex (V1) 4 Adaptation of direction-selective neurons Vautin & Berkley (1977): Neuron in cat primary visual cortex (V1) Average response Average Blank screen Pattern moving Blank screen Time in preferred direction • Presentation of motion with neuron’s preferred direction causes substantial adaptation Adaptation of direction-selective neurons Vautin & Berkley (1977): Neuron in cat primary visual cortex (V1) Average response Average Blank screen Pattern moving Blank screen Time in anti- preferred direction • Presentation of pattern with neuron’s anti-preferred direction (i.e., opposite to preferred direction) generates a weaker response, and less adaptation • This suggests that adaptation results from responding strongly 5 Sutherland’s (1961) “Ratio model” of MAE Before During After adaptation adaptation adaptation L = Neurons selective for leftward motion R = Neurons selective for Response rightward motion L R L R L R • Perceived motion direction depends on relative activity of neurons tuned to opposite directions of motion • Before adaptation, neurons selective for leftward and rightward motion are equally responsive • Prolonged exposure to rightward motion stimulates rightward-selective neurons more than leftward-selective neurons • After adaptation, rightward-selective neurons are less responsive than leftward-selective neurons • Greater response of leftward-selective neurons interpreted as leftward motion Sutherland’s (1961) “Ratio model” • “the direction in which something is seen to move might depend upon the ratios of firing in cells sensitive to movement in different directions.” • In fact, all modern models of motion perception involve a difference between responses of cells selective for motion in opposite directions Direction Leftward Rightward selectivity (V1) motion detector motion detector Motion opponency L – R • Differencing operation compares responses of leftward and Perception rightward motion detectors 6 Representing negative numbers in the brain • Do the subtraction both ways L R L R L – R R – L • If the subtraction gives a negative number, the output will be zero • The L – R unit responds to positive values of L – R • The R – L unit responds to negative values of L – R Representing negative numbers in the brain • Do the subtraction both ways L = 10 R = 6 L = 10 R = 6 L – R R – L 4 spikes 0 spikes • If the subtraction gives a negative number, the output will be zero • The L – R unit responds to positive values of L – R • The R – L unit responds to negative values of L – R 7 Representing negative numbers in the brain • Do the subtraction both ways L = 6 R = 10 L = 6 R = 10 L – R R – L 0 spikes 4 spikes • If the subtraction gives a negative number, the output will be zero • The L – R unit responds to positive values of L – R • The R – L unit responds to negative values of L – R • In models of the brain, it’s simpler to let the response values go negative Outline of model of motion perception Direction Leftward Rightward selectivity (V1) motion detector motion detector Motion opponency L – R Perception • Not just leftward and rightward motion detectors – opponent pairs tuned to each direction • How are motion detectors constructed? 8 Werner Reichardt (1924–1992) • German Radio engineer • Pioneered application of engineering principles to neuroscience • Most famous for developing a model of motion detection in the fly: the “Reichardt detector” 9 Photoreceptors 0 spikes 0 spikes Delay 0 spikes Multiplication 0 spikes Rightward motion Photoreceptors 10 spikes 0 spikes Delay 0 spikes Multiplication 0 spikes Rightward motion 10 Photoreceptors 10 spikes 0 spikes Delay 0 spikes Multiplication 0 spikes Rightward motion Photoreceptors 10 spikes 0 spikes Delay 0 spikes Multiplication 0 spikes Rightward motion 11 Photoreceptors 0 spikes 10 spikes Delay 10 spikes Multiplication 0 spikes Rightward motion Photoreceptors 0 spikes 10 spikes Delay 10 spikes Multiplication 100 spikes Rightward motion 12 Photoreceptors 0 spikes 0 spikes Delay 0 spikes Multiplication 0 spikes Rightward motion Photoreceptors 0 spikes 10 spikes Delay 0 spikes Multiplication 0 spikes Rightward motion 13 Photoreceptors 0 spikes 10 spikes Delay 0 spikes Multiplication 0 spikes Rightward motion Photoreceptors 0 spikes 10 spikes Delay 0 spikes Multiplication 0 spikes Rightward motion 14 Photoreceptors 10 spikes 0 spikes Delay 0 spikes Multiplication 0 spikes Rightward motion Photoreceptors 10 spikes 0 spikes Delay 0 spikes Multiplication 0 spikes Rightward motion 15 Photoreceptors 0 spikes 0 spikes Delay 10 spikes Multiplication 0 spikes Rightward motion Photoreceptors 0 spikes 0 spikes Delay 0 spikes Multiplication 0 spikes • Short delay to detect fast motion • Long delay to detect slow motion Rightward motion 16 Photoreceptors Delay Delay Multiplication Leftward motion Rightward motion Reichardt detector (Reichardt, 1957, 1961) Photoreceptors Delay Delay Multiplication Opponency L – R (subtraction) Leftward motion Rightward motion 17 18 Aliasing • Happens with movies of periodic patterns when the temporal sampling is too coarse (not enough frames per second) • With Reichardt motion detector, we can also get aliasing in real-world vision (i.e. not movies) when the delay is too long, or the spatial sampling is too coarse (photoreceptors too far apart) • This is a good model of motion perception in insects because there is evidence that aliasing occurs in insect vision • One theory of why zebras have their stripes is that the periodic stripe pattern gives rise to aliasing in the visual systems of biting insects, making them less likely to land on the zebra (How & Zanker, 2014) • But humans and other mammals show little evidence of aliasing • van Santen & Sperling (1984) introduced a fix to the Reichardt detector to prevent aliasing • “Elaborated Reichardt detector” Lion Attack Zebra || Wild Animal Attack https://www.youtube.com/watch?v=H1NTLhwSDpw 19 Aliasing • Happens with movies of periodic patterns when the temporal sampling is too coarse (not enough frames per second) • With Reichardt motion detector, we can also get aliasing in real-world vision (i.e. not movies) when the delay is too long, or the spatial sampling is too coarse (photoreceptors too far apart) • This is a good model of motion perception in insects because there is evidence that aliasing occurs in insect vision • One theory of why zebras have their stripes is that the periodic stripe pattern gives rise to aliasing in the visual systems of biting insects, making them less likely to land on the zebra (How & Zanker, 2014) • But humans and other mammals show little evidence of aliasing • van Santen & Sperling (1984) introduced a fix to the Reichardt detector to prevent aliasing • “Elaborated Reichardt detector” Photoreceptors Delay Delay Multiplication Leftward motion Rightward motion 20 Input receptive fields have same location, but different phase Photoreceptors Delay Delay Multiplication Leftward motion Rightward motion Adelson & Bergen (1985) Edward H. Adelson 21 Space-time x from Adelson & Bergen (1985) t • Motion is tilt in space-time • We can detect it using analogous methods to processing orientation in space • Need receptive fields that are tiled in space- time Space-time Donnie Darko, 2001 22 Space-time x from Adelson & Bergen (1985) t • Motion is tilt in space-time • We can detect it using analogous methods to processing orientation in space • Need receptive fields that are tiled in space- time Spatiotemporal receptive fields • Spatial receptive field gives the neuron’s preferred image (See DeAngelis, Ohzawa & Freeman, 1993) http://ohzawa-lab.bpe.es.osaka-u.ac.jp/ohzawa- lab/teaching/RF/XTinseparable.html • In reality, a neuron has a preferred “movie”, the “spatiotemporal receptive field” 23 Spatiotemporal receptive fields • Spatial receptive field gives the neuron’s preferred image (See DeAngelis, Ohzawa & Freeman, 1993) http://ohzawa-lab.bpe.es.osaka-u.ac.jp/ohzawa- lab/teaching/RF/XTinseparable.html • In reality, a neuron has a preferred “movie”, the “spatiotemporal receptive field” 24 Energy model (Adelson & Bergen, 1985) x x x x • At each point in the image, have simple cells with four t ttt types of receptive field • Put each neuron’s output through a squaring function Energy model (Adelson & Bergen, 1985) x x x x • At each point in the image, have simple cells with four t ttt types of receptive field • Put each neuron’s
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