Motion Perception 1. PSY305 Lecture 12 JV Stone

Transcription

1 Motion Perception 1 PSY305 Lecture 12 JV Stone 1

2 Structure Human visual system as a band-pass filter. Neuronal motion detection, the Reichardt detector. The aperture problem. 2

3 The visual system is a temporal band-pass filter bullet Motion not visible (too fast) Visible motion Motion not visible (too slow) 3

4 Temporal contrast sensitivity function Refresh rate of computer screen t Time Temporal flicker frequency = 1/t Hertz (Hz). Eg If the black-white-black cycle repeats 10 times each second then the disc has a temporal frequency of 10Hz. Experimenter adjusts contrast between black and white discs to determine contrast sensitivity at each teporal frequency. For humans, minimal contrast for motion detection required at about 10-20Hz. Note log scale. 4

5 Contrast Dark luminance = 0.95 cd/m 2 Grey luminance = 1 cd/m 2 Bright luminance = 1.05 cd/m 2 Contrast: ( ) / 1 = 0.1/1 = 0.1 contrast = (L bright -L dark )/ L av L av = (L bright +L dark )/2 If the smallest contrast that can be seen at 30Hz is 0.1 then the sensitivity at 30Hz is 1/(smallest contrast) = 1/0.1 = 10. 5

6 Contrast Dark luminance = cd/m 2 Grey luminance = 1 cd/m 2 Bright luminance = cd/m 2 contrast ( )/1 = 0.01/1 = 0.01 contrast = (L bright -L dark )/ L av L av = (L bright +L dark )/2 If the smallest contrast that can be seen at 10Hz is 0.01 then the sensitivity at 10Hz is 1/(smallest contrast) = 1/0.01 =

7 Spatiotemporal contrast sensitivity function Contrast sensitivity Temporal Sensitivity Contrast sensitivity Spatial Sensitivity 1000 Contrast sensitivity Note log scales. 1.0 Spatiotemporal Sensitivity 7

8 Apparent Motion In the physical world, perception of motion usually involves physically continuous motion of objects. However, we also perceive motion when presented with (changing) static images in rapid presentation. This is known as apparent motion and is the basis for the perception of moving images on TV, and movies. 8

9 What is motion to the retina? Motion is characterised by subtle but highly structured changes in retinal luminance over space and over time. Difference between grey-levels in frames 1 and 2 9

10 Neural motion detectors Motion detection invloves the comparison of one retinal receptor s response with that of another receptor s response. This can be modelled as two receptors feeding into a comparator neuron (ie part of a Reichardt detector). Each neural motion detector has a preferred direction and speed. 10

11 Reichardt detector AND Gate C s output C B s output A Retina B 1 sec A s output 1 sec Cells A and B have retinal RFs at nearby retinal locations. A moving dot (red) stimulates cell A then cell B. Another cell C only fires if its inputs are both active at the same time (so has zero output). But 11

12 Reichardt detector 1 sec delay AND Gate CR s output CR B B s output A Retina 1 sec A s output 1 sec If we introduce a delay in A s output then the inputs to C are active at the same time, so C fires, signalling motion across the retina. 12

13 Rightward motion detection Preferred direction Null direction A B A B Δt Δt CR CR V R >0 V R =0 13

14 Leftward motion detection Preferred direction Null direction B A B A Δt Δt CL CL V L >0 V L =0 14

15 Reichardt detector: Combined Left and Right motion detection The outputs of both the L and R modules are subtracted: V D = V R - V L If V D >0 then this implies rightward motion. If V D <0 then this implies leftward motion. A pair of combined L/R modules defines a single Reichardt detector. 15

16 The aperture problem What you see (right-downwards local motion) when (global) motion is horizontal 16

17 Local and global motion 17

18 Local and global motion If we overlay two gratings which move in different directions then we do not see two moving gratings, but a plaid which moves in a single direction. Local motion + = Global motion Local motion 18

19 Local and global motion vectors Aperture Seen local motion vector Unseen vector component of global motion v 1 v 1 θ Global motion vector v 0 Unseen motion is always perpendicular (at right angles) to seen (local) motion. 19

20 Local and global motion vectors Aperture Seen local motion vector Unseen vector component of global motion v 1 v 1 θ Global motion vector v 0 The global motion is the sum of the seen and unseen motion vectors. 20

21 Arrows, vectors and sums of vectors The direction and speed can be described by a vector (which is basically an arrow). The direction of the vector is the motion direction. The length of the vector is the motion speed. In order to know how to combine the seen and unseen local motions, all we need to know is how to add two vectors 21

22 Adding vectors Draw one vector v 1 then draw the other one v 1 as if it originates at the end of v 1. The line which connects the start of v1 with the end of v1 is the sum of v 1 and v 1 (and this sum is the global motion vector v 0 ). v 1 v 1 sum 22

23 Decomposing global motion When we view an object through an aperture, we can only see part v 1 of its global motion v 0. If we see one part v 1 then it follows that there is another part v 1 that we can t see. The global motion can only consist of two parts: global motion = part seen + part unseen. v 0 = v 1 + v 1 v 1 v 1 θ Global motion vector v 0 23

24 Decomposing global motion Even though we cannot see the unseen motion v 1, we do know that it lies along a line at right angles to the motion v 1 that we can see (because motion along this line is the only motion we cannot see!). So we know the orientation of the unseen motion v 1, but we do not know its speed (vector length ). v 1 v 1 θ Global motion vector v 0 24

25 Solving the aperture problem So far, we have established that: 1) The global motion is the sum of the seen and unseen motions. 2) The orientation of the unseen motion lies along the line perpendicular to the seen motion. So, as this line constrains the possible global motion, it is called the constraint line. v x v y θ 1 Local motion vector Constraint line Each dashed red line is a possible global motion vector, consistent with the observed local motion vector and its single constraint line. (v x and v y are axis labels denoting motion in x and y directions) 25

26 Solving the aperture problem using the intersection of constraints (IOC) from two local motion vectors Local motion vector Global motion vector Each local motion vector defines a different constraint line. Local motion vector v y Global motion vector The global motion vector touches all constraint lines, and therefore must be at the intersection of constraint lines. v x Constraint lines Local motion vector 26

27 References Essential Reading Frisby JP and Stone JV Seeing Motion Part 1 Chapter 14. Background Reading Frisby, JP and Stone, J.V. Seeing Motion: Part II Chapter 15. Farah, M. J. (2000). The Cognitive Neuroscience of Vision. Oxford, UK: Blackwells Publishers Ltd. Schenk, T., & Zihl, J. (1997). Visual motion perception after brain damage. I. Deficits in global motion perception. Neuropsychologia, 35, Zeki, S. M. (1993). A vision of the brain. Zeki, S. M., Watson, J. D. G., Lueck, C. J., Friston, K., Kennard, C., & Frackowiak, R. S. J. (1991). A direct demonstration of functional specialization in human visual cortex. Journal of Neuroscience, 11,