1. Neuroscience: Exploring the Brain, 3e
Chapter 10: The Central Visual System
کارگاه تخصصی توانبخشی دیداری عصبی
ارائه توسط: دکتر مهدی علیزاده
بهار 1396

2. Introduction Neurons in the visual system
Neural processing results in perception
Parallel pathway serving conscious visual perception originate in the retina
Progress to lateral geniculate nucleus, primary visual cortex & higher order visual areas in temporal and parietal lobes
Overlapping neuronal receptive fields
Sensitive to different facets of the visual input

3. The Retinofugal Projection
The Optic Nerve, Optic Chiasm, and Optic Tract

4. The Retinofugal Projection
Right and Left Visual Hemifields
Left hemifield projects to right side of brain
Ganglion cell axons from nasal retina cross, temporal retinal axons stay ipsilateral

5. The Retinofugal Projection
Visual deficits from lesions in the retinofugal projection

6. The Retinofugal Projection
Nonthalamic Targets of the Optic Tract:
Hypothalamus: Biological rhythms, including sleep and wakefulness
Pretectum: Size of the pupil; certain types of eye movement
Superior colliculus: Orients the eyes in response to new stimuli

7. The Lateral Geniculate Nucleus (LGN)

8. The Lateral Geniculate Nucleus (LGN)
Inputs Segregated by Eye and Ganglion Cell Type

9. The Lateral Geniculate Nucleus (LGN)
Receptive Fields
Receptive fields of LGN neurons: Identical to the ganglion cells that feed them
Magnocellular LGN neurons: Large, monocular receptive fields with transient response
Parvocellular LGN cells: Small,monocular receptive fields with sustained response

10. The Lateral Geniculate Nucleus (LGN)
Nonretinal Inputs to the LGN
Primary visual cortex provides 80% of the synaptic input to the LGN
Brain stem neurons provide modulatory influence on neuronal activity

11. Anatomy of the Striate Cortex

12. Anatomy of the Striate Cortex
Retinotopy
Map of the visual field onto a target structure (retina, LGN, superior colliculus, striate cortex)
Central visual field overrepresented
Discrete point of light: Activates many cells in the target structure due to overlapping receptive fields
Perception: Based on the brain’s interpretation of distributed patterns of activity

13. Anatomy of the Striate Cortex
Retinotopy

14. Anatomy of the Striate Cortex
Lamination of the Striate Cortex
Layers I - VI
Spiny stellate cells: Spine- covered dendrites; layer IVC
Pyramidal cells: Spines; thick apical dendrite;
layers III, IV, V, VI
Inhibitory neurons: Lack spines; All cortical layers; Form local connections

15. Anatomy of the Striate Cortex
Inputs to the Striate Cortex
Magnocellular LGN neurons: Project to layer IVC
Parvocellular LGN neurons: Project to layer IVC
Koniocellular LGN axons: Bypasses layer IV to make synapses in layers II and III

16. Anatomy of the Striate Cortex
Ocular Dominance Columns
Studied with transneuronal autoradiography from retina, to LGN, to striate cortex.

17. Anatomy of the Striate Cortex
Ocular Dominance Columns
Present in layer IV of macaque monkeys - alternating inputs from two eyes

18. Anatomy of the Striate Cortex
Inputs to the Striate Cortex
First binocular neurons found in striate cortex - most layer III neurons are binocular (but not layer IV)

19. Anatomy of the Striate Cortex
Outputs of the Striate Cortex:
Layers II, III, and IVB: Projects to other cortical areas
Layer V: Projects to the superior colliculus and pons
Layer VI: Projects back to the LGN

20. Physiology of the Striate Cortex
Monocular Receptive Fields
Layer IVC: Similar to LGN cells
Layer IVC: Insensitive to the wavelength
Layer IVC: Center-surround color opponency
Binocular Receptive Fields
Layers superficial to IVC: First binocular receptive fields in the visual pathway
Two receptive fields - one for each eye

21. Physiology of the Striate Cortex
Cortical Receptive Fields
Orientation Selectivity

22. Physiology of the Striate Cortex
Cortical Receptive Fields
Direction Selectivity
Neuron fires action potentials in response to moving bar of light

23. Physiology of the Striate Cortex
Cortical Receptive Fields
Simple cells: Binocular; Orientation-selective; Elongated on-off region with antagonistic flanks responds to optimally oriented bar of light
Possibly composed of three LGN cell axons with center-surround receptive fields

24. Physiology of the Striate Cortex
Cortical Receptive Fields
Complex cells: Binocular; Orientation-selective; ON and OFF responses to the bar of light but unlike simple cells, no distinct on-off regions

25. Physiology of the Striate Cortex
Cortical Receptive Fields
Blob Receptive Fields:
Circular
Monocular
No orientation or direction selectivity
Majority of color-sensitive neurons outside layer IVC
Specialized for analysis of object color

26. Physiology of the Striate Cortex
Parallel Pathways: Magnocellular; Koniocellular; Parvocellular

27. Physiology of the Striate Cortex
Cortical Module
Each module capable of analyzing every aspect of a portion of the visual field

28. Beyond Striate Cortex Dorsal stream
Analysis of visual motion and the visual control of action
Ventral stream
Perception of the visual world and the recognition of objects

29. Beyond Striate Cortex
The Dorsal Stream (V1, V2, V3, MT, MST, Other dorsal areas)
Area MT (temporal lobe)
Most cells: Direction-selective; Respond more to the motion of objects than their shape
Beyond area MT - Three roles of cells in area MST (parietal lobe)
Navigation
Directing eye movements
Motion perception

30. Beyond Striate Cortex
The Ventral Stream (V1, V2, V3, V4, IT, Other ventral areas)
Area V4
Achromatopsia: Clinical syndrome in humans- caused by damage to area V4; Partial or complete loss of color vision
Area IT
Major output of V4
Receptive fields respond to a wide variety of colors and abstract shapes

31. From Single Neurons to Perception
Visual perception
Identifying & assigning meaning to objects
Hierarchy of complex receptive fields
Retinal ganglion cells: Center-surround structure, Sensitive to contrast, and wavelength of light
Striate cortex: Orientation selectivity, direction selectivity, and binocularity
Extrastriate cortical areas: Selective responsive to complex shapes; e.g., Faces

32. From Single Neurons to Perception
From Photoreceptors to Grandmother Cells
Grandmother cells: Face-selective neurons in area IT?
Probably not: Perception is not based on the activity of individual, higher order cells
Parallel Processing and Perception
Groups of cortical areas contribute to the perception of color,motion, and identifying object meaning

33. Concluding Remarks Vision
Perception combines individually identified properties of visual objects
Achieved by simultaneous, parallel processing of several visual pathways
Parallel processing
Like the sound produced by an orchestra of visual areas rather than the end product of an assembly line

34. Anatomy of the Striate Cortex
Cytochrome Oxidase Blobs
Cytochrome oxidase: mitochondrial enzyme used for cell metabolism
Blobs: Cytochrome oxidase staining in striate cortex
Each blob centered on an ocular dominance column in layer IV
Receive koniocellular inputs from LGN

35. Differences in the Sensitivity of M and P Cells to Stimulus Features

36. Luminance contrast is a measure of the difference between the brightest and darkest parts of the stimulus—M cells respond when contrast is as low as 2%, whereas P cells rarely respond to contrasts less than 10% Spatial frequency is the number of repetitions of a pattern over a given distance. For example, alternating light and dark bars each occurring 10 times over a visual angle of one degree have a spatial frequency of 10 cycles per degree Temporal frequency is how rapidly the pattern changes over time; turning the bars of a grating on and off 10 times per second would produce a temporal frequency of 10 Hz. The M cells tend to have lower spatial resolution and higher temporal resolution than P cells. The P cells respond to changes in color (red/green and blue/yellow) regardless of the relative brightness of the colors, whereas M cells respond weakly to changes of color when the brightness of the color is matched

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