Vision

1. Vision

2. The Visible Spectrum Light is electromagnetic energy. One nm = one billionth of a meter

3. Properties of light • hue – determined by wavelength. • saturation – relative purity of light. • brightness – variation in intensity.

4. Wavelength and Frequency As wavelength increase, frequency decreases

5. The Human Eye

6. The Human Eye In order to see things in greatest detail our eyes are moved so that the object being looked at falls on the fovea. Fovea is a central portion of the retina with the greatest visual acuity.

8. rods and cones contain photopigment that provides input to bipolar and horizontal cells. • photoreceptors and bipolar cells do not produce action potentials – instead release neurotransmitters to the ganglion cells. • ganglion cells connect with the optic nerve. Photoreceptors

9. Blind spot Optic disk – where the optic nerve joins the retina – transmits retinal information to the occipital lobes

10. Blind spot Close your LEFT eye and move head closer to or further away from the screen until the central red circle disappears – always fixate the CROSS.

11. Visual Fields

14. temporal nasal VISUAL FIELD temporal nasal RETINA

15. Primary geniculostriate visual pathway

16. retina Primary Geniculastriate Pathway optic nerve optic chiasm optic tract dorsal lateral geniculate nucleus optic radiations striate visual cortex

17. Lateral Geniculate Nucleus The LGNd has six layers each of which gets independent input from either the left or the right eye but not both. There are two major classes of projections, parvocellular (small) and magnocellular (large) projections(known as the P and M pathways).

18. Large ganglion cells Small ganglion cells Centre/Surround Centre/Surround Colour insensitive Colour sensitive Large RFs Small RFs Fast, transient Slow, sustained High contrast sensitivity Low contrast sensitivity Magnocellular Parvocellular

19. Primary Visual Cortex The LGNd projects to primary visual cortex (striate cortex or area V1) in the occipital lobe. The magno and parvo projections are still somewhat segregated in V1.

21. Retinotopic map in striate visual cortex

23. receptive fields of retinal ganglion cells correspond to specific regions in space – hence a retinotopic map of the world in the occipital cortex. • receptive fields in visual cortex also respond selectively to other stimulus properties (e.g., orientation, brightness). Visual receptive fields

24. Centre – surround organization • tuning – different types of cells are “tuned” to respond to different aspects of visual information • e.g., brightness, location, direction of motion, colour etc…

25. centre / surround organization • ON, OFF and OFF/ON cells • ON/OFF cells project primarily to the superior colliculus (midbrain) • the SC is important for directing reflexive saccades Coding information at the retina - brightness

27. Coding information at the retina - colour

28. trichomatic sensitivity AND colouropponency • red – green • blue – yellow • on/off surround organization Coding information at the retina - colour Yellow ON Blue OFF BlueON YellowOFF Red ON Green OFF Green ON Red OFF

29. impossibility of seeing a redish green colour! Coding information at the retina - colour

32. after staring at the green Canadian flag you see a red one because the “green” component of red/green cells has adapted to the stimulus. • some red/green cells are inhibited for a long period. • when looking at neutral light (white light) these cells “rebound” due to the absence of inhibition creating the afterimage. • Big Spanish Castle • can get afterimages for motion – waterfall illusion . Adaptation – negative afterimages

33. 6 layers (bands or striations). • input from magno and parvocellular information processed at layer IV. • disproportionate representation of the fovea (brain would weigh over 30,000 pounds (≈13,600 kg) if the whole visual field had as many neurons dedicated to it as are dedicated to the fovea!!!). Striate cortex

34. cells in striate cortex sensitive to specific orientations. • simple cells – opponent system. • complex cells – no inhibitory surround – direction specific movement detectors (also in MT). • cells organized in columns. Orientation and movement

35. high spatial frequency • many of the cells in striate cortex are actually tuned to different spatial frequencies. • everything you see in the world can be described in terms of spatial frequency. Spatial frequency low spatial frequency

36. Information not lost at low spatial frequencies Gender and can still be extracted from the low frequency image (right) but identity requires the high frequency image (left).

38. Different “modules” sensitive to different visual processes • V4 – colour • MT – motion • FFA – face perception • PPA – place recognition • IT – object recognition Modularity in vision

40. Review Questions 1) A unique feature of the fovea is that it A) contains mostly rods. B) contains mostly cone photoreceptors. C) is devoid of photoreceptors. D) mediates vision in dim light. E) has very poor acuity. 2) The reason for a "blind spot" in the visual field is that A) rods are less sensitive to light than are cones. B) blood vessels collect together and enter the eye at the blind spot. C) the lens cannot focus all of the visual field onto the retina. D) retinal cells die with age and overuse, resulting in blind spots. E) there are no photoreceptors in the retina where the axons exit the eye.

41. 3) Action potentials in the visual system are first observed in the A) bipolar cells. B) horizontal cells. C) ganglion cells. D) photoreceptors. E) axons leaving the internal surface of the retina. 4) Select the correct sequence for processing of information in the primary visual pathway. A) Retina - > dorsal lateral geniculate (DLG) -> striate cortex B) Retina -> striate cortex -> extrastriate cortex -> inferior temporal cortex C) DLG -> retina -> striate cortex -> primary visual cortex D) Retina -> DLG -> inferior temporal cortex -> amygdala E) DLG-> frontal cortex -> amygdala -> extrastriate cortex Recommended web page http://www.tutis.ca/Senses/index.htm