Basic Visual Processes

1. Basic Visual Processes Anatomy and physiology of the visual pathway and a few other pertinent facts to get us started

2. Anatomy of the eye

3. Visual angles • Distances in eyes are always given as angles: Thumb width at arm’s length= 1.5 degrees Fist width at arm’s length=8-10 degrees Hand span at arm’s length=20 degrees One degree at retina=0.3 mm Visual angle of one cone=0.0084 degrees = 0.5 minutes of arc

4. Measuring the effects of optics • Line and point spread functions • If we assume linearity, then we can use these to predict what any stimulus presented to the lens will produce on the retina

5. The point spread function -Measuring these involves a number of very tricky problems -essentially, what is involved is measuring the reflected light that leaves the eye as a pinpoint of light is presented to it. -remember that what we’re looking at here is what gets through the optics and not necessarily what is seen.

6. Contrast and spatial frequency

7. Gratings Square wave gratings Sine wave gratings

8. Varying contrast and frequency -variation in spatial frequency is usually given in cycles/degree Contrast=Imax/Imin/2*Iav so can be changed by varying top or bottom of fraction

9. Spatial frequency Jean Fourier Any complex waveform can be represented as the sum of a series of sine waves of different frequencies and amplitudes

10. Measuring optics • Modulation transfer function • A different and handy way to represent spread functions -what is measured is the ability of the eye’s optics to transmit variation in contrast at a variety of different spatial frequencies

11. Measuring optics • Chromatic aberration Because refraction by any medium depends on wavelength, the optical quality of the eye varies with wavelength. Resolving power is lowest at short wavelengths

12. The retina • Receptor subtypes

13. The beginning of vision -presenting a flash of light to a photoreceptor produces an electrical response

14. Disks in outer segments called lamellae contain a photopigment

15. TRANSDUCTION In darkness, there’s a continuous current in the outer segment caused by the circulation of sodium. In light, sodium circulation slows down and receptors hyperpolarize

16. Rhodopsin -- the magic photopigment

17. Through the wizardry of biochemistry, sodium channels close

18. The sensitivity of visual transduction • 7 photons produce a perceptible response (Hecht, Schlaer and Pirenne) • 1 photon closes about 106 ion channels

19. The receptor mosaic • Spatial frequency limits of the receptor mosaic • -using interferometry to get past optics -when light waves collide, the effects can be destructive or constructive. Thomas Young showed that you can set up conditions where these patterns of construction and destruction produce a fine grating. In fact, this was an early, powerful demonstration of the wave nature of light.

20. Laser interferometry -takes advantage of diffraction to produce very fine gratings behind eye’s optics

21. The receptor mosaic • Aliasing -Nyquist frequency is the spatial frequency above which confusion can occur. -demonstration of aliasing

22. The receptor mosaic Can measure the sampling frequency of the cones using aliasing. The prediction is that when the Nyquist frequency is exceeded, the apparent spatial frequency of gratings should begin to decrease because of aliasing. The frequency at which this happens depends on the types of cones involved. Long and Medium wavelength cones – 60 cycles/deg Short wavelength cones – 3 cycles/deg This corresponds to retinal spacing of 10 arc minutes for S-cones and 0.5 arc minutes for M and L cones

23. The S-cone mosaic

24. The S-cone mosaic • Spatial frequency of S-cones makes a nice match with point spread function

25. Using adaptive optics to visual living photoreceptors Using the same technology as in the Hubble telescope, you can compensate for optical imperfections using a deformable mirror.

26. Spectacular images of in vivo receptors

27. Cone mosaic summary • It is possible to measure the optical properties of the eye separately from visual function per se using various forms of refractometry. • It is possible to measure some simple properties of the photoreceptor array using laser interferometry • It is also possible to visualize the cone mosaic directly using various staining techniques or, in vivo, using adaptive optics • All of these measurements suggest ultimately that the receptor array is a very good optimized match for the eye’s optics. In other words, the organization of receptors is as good as, but no better than, the eye’s optics • There are significant differences between L and M cones and S cones. Again, though, these differences match the way that the eye’s optics respond to light with different spectral properties

28. Photoreceptor array The images show how the photoreceptor array varies with retinal eccentricity

29. The duplex retina

30. Retinal circuitry Note important differences in connections of rods and cones Cones can often have fairly direct access to retinal ganglion cells via bipolar cells. Rods most often synapse with rod bipolar cells which, in turn, synapse with amacrine cells Note the midget and parasol retinal ganglion cells.

31. Midget and parasol RG cells

32. Dendritic field sizes of RG cells Parasol dendritic fields are much larger than midget fields. It takes many more midget cells to cover visual field than parasol cells

33. Lateral geniculate nucleus About 90-95% of retinal ganglion cell axons land here. Layers 1 & 2 are magnocellular and layers 3-6 are parvocellular So-called intercalated layers (between 6 layers) may be a special koniocellular pathway, dedicated to S-cone transmission

34. M and P pathway response to contrast *Note that these show response to contrast and not illumination. This is a very important distinction – visual systems respond (non-linearly) over about 5-6 orders of magnitude for illumination but the range of illumination in a typical scene is rarely more than one order of magnitude. By and large, visual systems are linear for contrast.

35. Behavioural effects of lesions to P or M layers of monkey LGN Spatial frequency effects of P Temporal frequency effects on both but M system very important for high frequencies Filled circles = magnocellular lesion Open circles = parvocellular lesion

36. Retinal ganglion cell receptive fields Steven Kuffler Linear summation of centre and surround responses

37. Graphical representations of the receptive field 1-d 2-d

38. Can measure CSF for single cells -low frequency dropoff tells about size of RF -high frequency dropoff tells about size of centre of RF

39. CSF for a typical LGN cell

40. Adding the time dimension Notice that temporal properties of centre and surround stimulation differ a bit One way of describing this is to say that such receptive fields are space-time inseparable. If they were separable, the spatial profile of the rf would differ over time only by a scalar value. There ARE rfs like this, but not in retina or LGN.

41. Phase reversing gratings -this is one common way to measure the temporal properties of rf’s

42. Notice how CSF varies depending on both spatial and temporal frequency

43. The DOG operator -this is a very common and useful way of modelling the spatial and temporal structures of receptive fields

44. Adaptation and contrast normalization -Notice that the CSF does depend on intensity, but only really for low intensities. At higher intensities, contrast sensitivity changes very slowly.

45. The retinogeniculostriate pathway

46. The structure of cortex Cortex is laminar and connections are very precisely organized

47. Orientation selectivity

48. Hubel and Wiesel’s simple hierarchical model of visual cortical processing

49. Columnar organization of VI

50. Ocular dominance