The Seeing Brain Flashcards

Question

Contralateral representation in V1

Answer 1

1) Each eye represents both sides of the visual field: the left half of each eye receives light from the right half of the visual field, whereas the right half of each eye receives light from the left half of the visual field. 2) At the optic chiasm, the equivalent sides of each eye merge to form the optic tract. This means that the left visual cortex receives information from the right half of the visual field, whereas the opposite is true for the right visual cortex (they receive information front the contralateral side)

Answer 2

: the region of space that elicits a response from a given neuron

Answer 3

Visual processing along the geniculo-striate pathway 1. Thalamus (LGN)

Answer 4

STOP 1 Contains six layers, three for each eye * Cells have a center- surround receptive field * They respond to differences in light across their receptive field (e.g. presence of light in center, absence in surround, or vice versa)

Answer 5

FROM THE Thalamus (LGN) TO THE Primary Visual Cortex (V1) Next stop: Primary Visual Cortex (also known as V1/Striate Cortex) Extracts basic information from the visual scene (e.g. edges, orientations, wavelength of light) * This information is used by later stages of processing to extract information about shape, color, movement, etc. * Single-cell recordings by Hubel and Wiesel lead to a **hierarchical view** of vision in which simple visual features (e.g. points of light) are combined into more complex ones (e.g. adjacent points of light may combine into a line)

Answer 6

Simple cells: combine information from several LGN center-surround cells • respond to bars of particular orientations, and to points of light along those bars

Answer 7

Simple cells: combine information from several LGN center-surround cells • respond to bars of particular orientations, and to points of light along those bars ▫ Complex cells: combine information from several simple cells. • Respond to orientation too, but not to points of light along the bar ▫ Hypercomplex cells: combine information from several complex cells. • Still orientation selective, but prefer lines of certain lengths.

Answer 8

Orderly mapping between spatial location of light on the retina and location of V1 receptive fields • Notice that fovea gets extra cortical representation (“cortical magnification”)

Answer 9

- Damage to parts of V1 results in blindness for the corresponding region of space. - The patient cannot consciously report objects presented in this region of space)

Answer 10

Damage to parts of V1 results in blindness for the corresponding region of space. - The patient cannot consciously report objects presented in this region of space) - However, the patient is still able to make some visual discriminations in the "blind" area (e.g. orientation, movement direction) – this is called blindsight.

Answer 11

The patient is still able to make some visual discriminations in the "blind" area (e.g. orientation, movement direction) – this is called blindsight. - However, the patient is unaware of seeing anything. - If asked, the patient reports that he/she is guessing, but then perform above chance For an example of blindsight, watch this clip: https://www.youtube.com/watch?v=zOHTpobMFu4

Answer 12

Alternative routes are still available in the event of V1 damage. This doesn’t mean that they can completely replace V1 function (only coarse discriminations are possible). Moreover, the information they provide does not reach consciousness (patient is unaware of seeing anything).

Answer 13

OutsideV1,stillinoccipitallobe • Receptivefieldsgetspatially bigger, less organized • Specializedforprocessing particular stimulus attributes

Answer 14

Damage to V4 results in cerebral achromatopsia ▫ World is perceived as drab grey ▫ Retina and V1 responding normally to different wavelengths ▫ Good example of how perception happens in the brain, not at the receptors (sensation may be intact) Not to be confused with color blindness. ▫ Abnormalities with specific cone types ▫ Problem distinguishing between specific wavelengths of light (typically red vs. green).

Answer 15

cerebral achromatopsia ▫ World is perceived as drab grey ▫ Retina and V1 responding normally to different wavelengths ▫ Good example of how perception happens in the brain, not at the receptors (sensation may be intact)

Answer 16

Motion perception Neurons in area V5 respond to particular directions of movement. V5 is also known as MT (“middle temporal”) Patients with bilateral damage to this region see the world in a series of still frames (akinetopsia) ▫ They can still detect movement in other senses (e.g. hearing, touch), so it’s specific to vision ▫ Akinetopsic patients cannot see an object looming (they see the object once it’s already next to them). They also have trouble performing the simplest actions if they require good movement perception (e.g. pouring tea)

Answer 17

Processing is hierarchical: - early on in processing, simple perceptual features are identified (e.g.: edges, orientation, wavelength) - later on in processing, these features are combined into ever more complex features Processing of different features occurs in parallel: - e.g. motion vs. color of objects - e.g. what is it (identity) vs. where is it (location) Sensory transduction converts sensory information to neural signals • The major primary visual pathway goes from the retina to V1 via the LGN * Neurons have receptive fields * V1 is arranged retinotopically; edges & orientation • V4 = color; V5/MT = motion

Answer 18

▫ Where the object is in space (and this can’t be computed from the retinal image alone) ▫ What the object is (knowledge we’ve stored about what things are)

Answer 19

Dorsal (“where/how”) Pathway (Parietal) Location of objects Visual control of skilled action directed at those objects (reaching, grasping) Ventral (“what”) Pathway (Temporal) Identity of objects Attach meaning/ significance to objects

Answer 20

• Cells fire to more complex and specific stimuli • Receptive fields get bigger ▫ Useful for object recognition ▫ But lose details about object location • Often sensitive to color ▫ Helps to separate objects from their background

Answer 21

Not Perceptual Blindness: patients do not have a visual field defect basic discriminations of acuity, brightness, hue are largely intact Not a Generic Recognition (UNLIKE memory loss) Deficit: limited to information presented visually touch, smell, etc. can be used to support successful object recognition •

Answer 22

Apperception (still a perceptual stage) Features are assembled (“integrated”) into objects. • Why do we perceive the world as a series of objects rather than a jumble of lines? ▫ “Parsing” – grouping lines into objects

Answer 23

Association (semantic stage) Semantic attributes (meaning) are attached. Memories associated with the object (e.g. what is it used for?) come into play, facilitating recognition).

Answer 24

After features (edge orientation, color, motion, etc.) Apperception (still a perceptual stage) Features are assembled (“integrated”) into objects. Association (semantic stage) Semantic attributes (meaning) are attached. Memories associated with the object (e.g. what is it used for?) come into play, facilitating recognition).

Answer 25

Most common after carbon monoxide poisoning, rather than a restricted lesion Impairments: Always impaired: Recognition/naming of drawings, photos, and 3-D objects Generally impaired: Ability to copy, match or parse objects Apperceptive agnosias are complex and heterogeneous E.g. Patient HJA (textbook) can copy objects or draw them from memory; may have problems integrating parts into wholes Common feature is evidence for a disorder of perceptual stage of object recognition

Answer 26

Impaired: Recognition/naming of line drawings, photos, and 3-D objects Intact: Ability to copy and parse complex objects (to some extent the object is perceived, but not recognized)

Answer 27

▫ Apperception: parse edges into objects ▫ Association: link objects to memories (knowledge about the object)

Answer 28

-inability to recognize people from their faces, not inability to perceive faces - Can result from brain damage, but also developmental form Individuals with prosopagnosia show disproportionately impaired recognition of faces Associated with brain lesions, but may also have a genetic component Deficit restricted to visual domain May or may not have additional visual impairments

Answer 29

Evidence for holistic processing in healthy participants: The Margaret Thatcher illusion Take home: Recognizing faces may be “special” because it depends on a particular kind of processing (holistic)

Answer 30

• Recognizing objects takes place in a special processing stream (ventral stream) ▫ Apperception: assembling edges into objects ▫ Association: linking objects to memories (recognition) • Recognizing faces may be at least somewhat different from how we recognize objects

Answer 31

Dorsal (“where/how”) Pathway (Parietal) Location of objects Visual control of skilled action directed at those objects (reaching, grasping) Ventral (“what”) Pathway (Temporal) Identity of objects Attach meaning/ significance to objects

Answer 32

PHOTORECEPTOR Specialized for higher levels of light ▫ Color and detail ▫ Three different kinds of cones specialized for red, blue and green light ▫ Highest density of cones in the fovea of the retina

Answer 33

PHOTORECEPTOR Specialized for low levels of light, (important in night vision) ▫ Lower detail, color insensitive

Answer 34

Light = electromagnetic energy (small portion of the electromagnetic spectrum) Shorter wavelength = higher energy

Answer 35

Region of space that illicit a response from a given neuron. (Come in many different sizes and shapes)

Answer 36

LGN - thalamus 6 layers. (three for each eye) Cells have monocular input. Layers alternate inputs from each of the two eyes. The top four are parvocellular layers, two layers from each eye. Parvo (small) LGN cells receive inputs from (small) midget ganglion cells. The bottom two are magnocellular layers, one layer from each eye. Magno (large) LGN cells receive inputs from (large) parasol ganglion cells.

Answer 37

Simple cells: combine information from several LGN center- surround cells • respond to bars of particular orientations, and to points of light along those bars

Answer 38

Complex cells: combine information from several simple cells. • Respond to orientation too, but not to points of light along the bar

Answer 39

Hypercomplex cells: combine information from several complex cells. • Still orientation selective, but prefer lines of certain lengths.

Answer 40

The visual cortex is subdivided into a number of separate and distinct regions called visual cortical areas. Later in the semester we will discuss how these different cortical areas are identified and what their functions are. For now, we will concentrate only on the primary visual cortex (also called V1, shown in blue). V1 is located in the Calcarine sulcus in the medial occipital lobe of the brain (near the back of the head, just to the left and right of the middle). V1 is "primary" because the LGN sends most of its axons there, so V1 is the "first" visual processing area in the cortex. V1 processes the information coming from the LGN (as described below) and then passes its output to the other visual cortical areas which are (creatively) named V2, V3, V4, etc.