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Chapter 6 Spatial Vision. The visual system recognizes objects from patterns of light and dark. We will focus on the mechanisms the visual system uses.

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Presentation on theme: "Chapter 6 Spatial Vision. The visual system recognizes objects from patterns of light and dark. We will focus on the mechanisms the visual system uses."— Presentation transcript:

1 Chapter 6 Spatial Vision

2 The visual system recognizes objects from patterns of light and dark. We will focus on the mechanisms the visual system uses to process complex patterns of light and dark and, where we can, talk about the neural basis

3 A fundamental requirement for detecting objects is to determine the location of the edges (boundaries) of the objects

4 Absolute luminance is less important in spatial vision than relative luminance levels …because object contrast is relatively constant The reflectance from real objects does not vary (at least not very much), so contrast = (L T – L B )/L B remains fairly constant under different lighting conditions

5 The visual system responds to luminance differences (e.g., contrast) more than to luminance (the visual system is a very poor light meter), so the brightness of an object is not always predicted by its luminance

6 Brightness Constancy the brightness of objects is relatively invariant even though the absolute luminance varies widely. Brightness is determined largely by relative local contrast,

7 demo

8 Simultaneous contrast : The brightness of an object is not always predicted by its luminance The brightness of an object is affected by the local contrast with surrounding objects. (brightness is increased when viewed against a background with which the object has positive contrast and reduced when the stimulus has negative contrast relative to the background)

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10 Assimilation the brightness of a stimulus covaries with the brightness of a surrounding stimulus

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12 unaltered darkened lightened Gradient blurred On the right: Filtered to equalize luminance while accenting the contrast boundaries. The visual system acts similarly to base brightness on local luminance changes On the left:

13 Absolute luminance is less important in spatial vision than relative luminance levels …because object contrast is relatively constant The reflectance from real objects does not vary (at least not very much), so contrast remains fairly constant under different lighting conditions

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19 Examples: if you can’t see the boundaries, you can’t see the object “good camouflage”

20 The visual system is so wired to see edges, it detect them when there is very little information to specify the edge.

21 The visual system “creates” boundaries that are not there!

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23 A fundamental requirement for detecting objects is to determine the location of the edges (boundaries) of the objects The visual system recognizes objects from spatial patterns of light and dark

24 Discovered by Ernst Mach (1865) Mach also measured the speed of sound; “Mach One” is named in his honor Mach Bands are a brightness enhancement created by the visual system and demonstrate the accentuation of luminance changes at surface boundaries

25 Mach bands - regions of increased or decreased brightness caused by the response of the visual system to luminance boundaries

26 Off On Off On Off On Off On Receptive field positions relative to light and dark panels Neural responses The responses of retinal neurons underlie both the dependence of brightness on contrast and the brightness enhancement of Mach Bands

27 Receptive field demo OS/Center_surround.html

28 Off On Off On Off On Off On Receptive field positions relative to light and dark panels Neural responses The responses of retinal neurons underlie both the dependence of brightness on contrast and the brightness enhancement of Mach Bands

29 Center-surround organization a more detailed look at how the surround interacts with the center

30 Light on in the receptive field center produces depolarization in the bipolar cell and increased firing rate (spikes/second) in the on-center ganglion cell

31 Light on in the receptive field surround produces hyperpolarization in the bipolar cell and decreased firing rate (spikes/second) in the on- center ganglion cell

32 Thus, when both the whole center and the whole surround are stimulated with light, surround cancels almost all the depolarization caused by light in the center. The result is a very small depolarization and slight increase in ganglion cell firing (the center is slightly stronger than the surround) The depolarization caused by light on in center is opposed by the hyperpolarization produced by light on in the surround

33 Off On Off On Off On Off On Receptive field positions relative to light and dark panels Neural responses The responses of retinal neurons underlie both the dependence of brightness on contrast and the brightness enhancement of Mach Bands

34 The sensitivity (of the visual system) to spatial luminance changes is measured with gratings of varying spatial frequency and contrast

35 Sensitivity of spatial luminance changes is measured with gratings of varying frequency and contrast

36 The spatial CSF is the inverse of the contrast threshold to detect various spatial frequencies

37 Spatial Frequency (cycles/deg) Contrast Sensitivity Snellen 6/6 Equivalent Low spatial frequency rolloff Cutoff high spatial frequency Peak contrast sensitivity The spatial CSF is “band pass.” We are more sensitive to mid-range spatial frequencies (3 – 10 cycles/deg)

38 As shown in Chapter 5, the cutoff high spatial frequency (the highest spatial frequency that can be resolved at a contrast of 1.0) is very close (nearly identical) to the spatial resolution acuity (MAR)

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40 Do neurons with different receptive-field sizes produce the human spatial CSF?

41 Contrast adaptation demonstrates that there are multiple spatial frequency channels in the human visual system Blakemore and Campbell (1969); contrast adaptation with 7.1 cycles/deg

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45 So far in this chapter: 1) Detecting the boundaries of objects is essential for seeing objects 2) The center-surround receptive fields of retinal ganglion cells cause the visual system to emphasize luminance changes (contrast) that typically occur at the edges of objects (this produces Mach bands) 3) The sensitivity of the visual system to spatial changes is measured by the spatial contrast sensitivity function (CSF) 4) the individual neurons have a band pass spatial CSF with more narrow tuning than the whole CSF due to receptive field size 5) the spatial CSF is produced by neurons with more narrow tuning than the whole CSF (e.g., “channels”)

46 The spatial CSF varies with luminance As mean luminance decreases, peak contrast sensitivity decreases, the spatial frequency of the peak decreases, cutoff (acuity) decreases and low frequency rolloff becomes less

47 The spatial CSF shifts toward lower spatial frequencies with increasing retinal eccentricity

48 Parvocellular pathway neurons Magnocellular pathway neurons

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50 Changes after lesions of M or P LGN layers

51 The band-pass shape of the spatial CSF is the same in many species (but peak at different spatial frequencies)

52 Good fit; strong response; Therefore, lowest threshold Light in center & light in Surround; center slightly Stronger. Will respond but contrast threshold is high (low sensitivity) Light & dark in center cancel each other; light and dark in surround Cancel each other; infinite threshold (cut-off) Sensitivity profile of the Receptive-field center and surround

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56 The surround adds to the center (when light increases in the center and decreases in the surround)

57 Applying the spatial CSF to spatial vision: All spatial luminance patterns are comprised of sine-wave gratings of particular spatial frequencies and contrasts and we look at these with our spatial CSF

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59 Fourier Analysis: can determine the component sine waves & contrasts of complex waveforms Fourier Synthesis: can construct square wave grating using the component sine waves & contrasts

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62 Spatial frequencies and relative contrasts of the images along the horizontal line

63 But, we look at this image with our visual system which does not see spatial frequencies above about 60 c/deg, and is more sensitive to mid-range spatial frequencies than to low ones. We “filter” the visual scene through our visual system

64 The high frequencies are not detected by our visual system and the low spatial frequencies are attenuated by our visual system, emphasizing the mid-range spatial frequencies

65 O.K. The visual scene contains many spatial frequencies. Do we have to have high spatial frequencies (near the acuity limit) to “see objects”, or can we “see objects” with just low & intermediate spatial frequencies?

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67 We CAN “see objects” with just low & intermediate spatial frequencies!

68 Good contrast sensitivity is more important than resolution for some spatial tasks

69 It is useful to have high contrast sensitivity when contrast is low.

70 It is useful to have high contrast sensitivity when contrast is low.

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72 Mobility: Pelli-Robson chart performance is a good predictor of time for patients with age-related macular degeneration to complete an obstacle course & number or errors (collisions). Low vision patients: reading speed is affected by spatial CSF losses Possible new treatments: selective contrast enhancement at certain spatial frequencies to boost face recognition


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