CHAPTER 10

1. CHAPTER 10 Vision and visual perception Form Vision

2. Review: eye to Brain pathway • Rods or cones bipolar cells  ganglion cells • Ganglion cell  optic nerve; • Optic nerves  optic chiasm • Optic chiasm  optic tracts (L and R) • Optic tracts  lateral geniculate nuclei of thalamus • Optic radiations leave LGN  occipital cortex • Several areas of occipital cortex • Areas 17, 18 and 19

4. Two kinds of pathways • Ambient pathway: • Spatial vision pathway • Where things are, not what they are • Magnocelluar pathway • Focal vision • Object vision pathway • What things are, but not where they are • Parvocellular pathway • Cortical blindness • Damage to brain, not eyes or even optic tract • Typically lose form vision first, then all of it • Very rare to just lose ambient pathway

6. Form Vision • Incoming information organized into a retinotopic map • exists in the visual cortex • adjacent retinal receptors activate adjacent cells in the visual cortex. • Form vision: • detection of an object’s boundaries and features (such as texture) • Differs from spatial location (that is spatial vision!). • Primarily using cones • Processed using lateral inhibition • On/off cell activity • each neuron’s activity inhibits the activity of its neighbors • in turn, its activity is inhibited by them. • the inhibition is delivered by horizontal cells to nearby synapses of receptors with bipolar cells.

8. Form Vision • In on center cells • light in the center increases firing • light in the off surround reduces firing below resting levels. • Only responds if center of receptive field is stimulated • Other ganglion cells have an off center • Thus an on surround. • Work the opposite

11. Form Vision • Simple cells • respond to a line or an edge that is at a specific orientation • May also respond to specific place on the retina. • Complex cells: • continue to respond when a line or edge moves to a different location. • According to spatial frequency theory • visual cortical cells function like a Fourier frequency analysis of the luminosity variations in a scene. • Analyze the shape of the sine wave patterns • visual cortical cells can detect not just edges but gradations of change.

13. Do we process input individually, globally or hierarchically? • Modular processing • segregation of the various components of processing into separate locations. • Process each unit or module separately; then build into meaningful unit • Hierarchical processing: • lower levels of the nervous system analyze their information • pass the results on to the next higher level for further analysis. • Layered analysis • Distributed vs. localized processing: • Some neuroscientists reject modular notion, • visual functions distributed not separate locations • E.g., occurs across a relatively wide area of the brain.

14. Two routes and two types of vision • Remember: • Visual information follows two routes from the retina through the brain • Again, are the focal vs. spatial pathways • Parvocellular system: form pathway • Focal vision • Object vision pathway • What things are, but not where they are • Magnocellular system.: spatial pathway • Ambient pathway • Spatial vision pathway • Where things are, not what they are

15. Parvocellular pathway • Parvocellular ganglion cells: • located mostly in the fovea. • Thus mostly involves cones • They have circular receptive fields • Receptive fields: • small and color opponent • These smaller receptive fields best for discrimination of fine detail and color. • Thus this is the form pathway • Form pathway = figures, specific shapes and critical information • Tells us the WHAT

16. Magnocellular pathway • Magnocellular ganglion cells • larger circular receptive fields • Mainly rods • brightness opponent • respond only briefly to stimulation. • specialized for brightness contrast and for movement • This is the spatial pathway. • Parvocellular and magnocellular pathways travel to • the lateral geniculate nucleus (LGN of thalamus) • then to the primary visual cortex, also known as V1. • One tract lies on top of other: magnocellular is on top (dorsal)!!

17. Perception of Objects, Color, and Movement • Two pathways = highly interconnected, • parvocellular system dominates the ventral stream that flows from the visual cortex into the temporal lobes, • magnocellular system dominates the dorsal stream from the visual cortex to the parietal lobes. • Ventral stream = what of visual processing • parvocellular • Dorsal stream = where of visual processing • magnocellular

18. The Perception of Objects, Color, and Movement • Beyond cortical V1 • the ventral stream passes through V2 and into V4, • mostly concerned with color perception. • Then projects to the inferior temporal cortex • lower boundary of the temporal lobe. • This area shows remarkable specialization for object recognition • Damage to this area = loss of much of form vision, even if pathway is intact.

20. The Perception of Objects, Color, and Movement • The dorsal stream proceeds to • V2 through to V5, • also known as Medial temporal or MT • Why? It is on the middle temporal gyrus in the monkey. • Neurons there have strong directional sensitivity • Contributes to the perception of movement. • The dorsal stream then travels to posterior parietal cortex • located just behind the somatosensory cortex. • Primary role: locate objects in space • Significant behavioral implications of this function VERY important • Loss of this area = loss of spatial vision even if pathway intact

21. Visual disorders • Object agnosia: • impaired ability to recognize objects • Often due to damage of inferior temporal cortex • Also if parvocellular pathway damaged • Prosopagnosia : • Special type of object agnosia • inability to visually recognize familiar faces. • Can no longer recognize individual people or sometimes recognize person vs object • Both object agnosia and prosopagnosia are caused by damage to the inferior temporal cortex (part of the ventral stream).

22. Visual disorders • Fusiform face area • Specialized “area”, although somewhat disperse • face-responsive neurons are often intermingled with object-responsive neurons, • But, part of the fusiform gyrus on the underside of the temporal lobe is critical for face recognition • Hence, the name! • Damage to this produces the prosopagnosia

23. Other Vision disorders • Blindsight: Patients blinded by damage to V1 • can locate and track the movement of objects; discriminate colors, • But say they are guessing, unaware of abilities. • Occassionally also due to damage to magnocellular pathway • Color agnosia: loss of the ability to perceive colors • Relies on Color constancy: critical behavior • The ability to recognize the “natural color “ of object despite of illuminating wavelength • If could not, objects would seem to change colors as the sun shifted position through the day or as we went indoors into artificial light • Movement agnosia: inability to perceive movement. • Often involves damage to medial temporal and posterior parietal • Unable to integrate incoming information

24. Sensory neglect • Function of posterior parietal cortex: • combines input from visual, auditory, and somatosensory areas • helps the individual locate objects in space • Helps orient the body in the environment. • Damage here impairs several abilities: • reaching for objects • often produces sensory neglect • patient ignores visual, touch, and auditory stimulation on the side opposite the injury. • Neglect not due to any defect in visual processing • Due to deficit in attention.

25. Sensory neglect

26. The Perception of Objects, Color, and Movement • Many researchers have wondered: where is all the information about a visual object brought back together?. • Suggestions: • ultimate understanding of an object occurs in superior temporal gyrus • Area receives input from both dorsal and ventral neural streams • Alternatively: in part of the parietal cortex where damage causes neglect. • Other investigators suspect frontal areas where both streams converge. • Bottom line: meaning and understanding require careful integration of information by several brain areas working actively together • Loss of any one disrupts this process • Delicate balance