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Chapter 6
Optic Nerves and the Striate Cortex
| Question | Answer |
|---|---|
| Describe the pathway of the optic nerves. | 1. Ganglion cell axons exit eye through optic disk, forming optic nerve 2. Optic nerves divide - inner halves cross 3. Optic nerves cross at optic chiasm -> optic tracts 4. 90% go to LGN in thalamus (-> striate), 10% go to superior colliculus |
| Why do the inner halves of the optic nerves cross over? | This ensures that information from both eyes regarding the same part of the visual field will be processed in the same places in the brain. |
| What are the optic tracts? | The fiber pathways between the optic chiasm and destinations in the forebrain and brainstem. |
| What is the optic chiasm? | Area of the base of the brain where the optic nerves cross over to form the optic tracts. |
| What is the special case of the special light-sensitive ganglion cells? | Those that release melanopsin leave the optic tract and synapse in the suprachiasmatic nucelus of the hypothalamus, providing the light information used to regulate daily rhythms. |
| What is the superior colliculus? | It is a brain structure in the tectum of the midbrain responsible for processing visual information. It guides movement of the eyes and the head towards newly detected objects in the visual field. Input from the visual cortex moderates its activity. |
| What is the lateral geniculate nucleus? | A nucleus within the thalamus that receives input from the optic tracts. |
| How many layers are there in the LGN? Describe their function. | 6 distinct stacked layers, numbered from ventral to dorsal. They allow us to make connections between neural activity and reality. |
| What layers is the LGN divided into? | The magnocellular layers, the parvocellular layers and the koniocellular layers. |
| What is the magnocellular layer?T | The two most ventral layers (1 and 2) of the LGN. They receive input from the M cells in the retina. |
| What is the parvocellular area? | It is the 4 dorsal layers of the LGN that receive input from the P cells in the retina. |
| What is the koniocellular area? | The area between the magnocellular and the parvocellular area that is made up of very small neurons that receive input from the K cells in the retina. |
| What is the differece between LGN neurons and retinal cells? | The lateral inhibition between center and surround is much stronger than observed among retinal cells. Thus, the contrast between areas of light and dark are boosted. |
| What is the role of the LGN? | LGN activity changes to different levels of awareness and selective attention suggesting that the LGN filters information before sending it.LGN also receives input from the reticular formation and the LGN, modifying flow of information to the cortex. |
| What is the primary visual cortex/striate cortex/area V1? | The location in the occipital lobe for the initial cortical analysis of visual input. |
| Describe the layers of area V1. | It has 6 layers. Layers 2 and 4 are thicker than other layers as they receive most of the input from other parts of the brain. Layer 4 receives input from the LGN. Layers 3, 5 and 6 contain output neurons that communicate with other parts of the brain. |
| What is cortical mapping? | It allows us to use the location of neuroal activity to understand the position of an object in the visual field. |
| What is interesting about the mapping of the striate cortex? What are its implications? | Although the fovea contains 0.01% of the retina's total area, signals from the fovea are processed by 8 to 10% of the striate cortex. This is one reason why focusing an image onto the fovea provides the greatest amount of fine detail. |
| What are simple cortical cells? | A cortical interneuron that responds to stimuli in the shape of a bar or edge with a particular slant or orientation in a particular location on the retina. |
| Describe the receptive field of the simple cortical cell. | They maintain an antagonistic center-surround organization but the shape of the receptive field is more elongated or racetrack-shaped. |
| What is a complex cortical cell? | A cortical interneuron that shows a preferred stimulus size and orientation and sometimes direction of movement but not location within the visual field. |
| What is the role of simple cortical cells? | They probably help in responding to object shape. |
| What is the role of complex cortical cells? | They probably participate in the perception of movement. |
| What are end-stopped cells? | A cortical interneuron that responds best to a stimulus that does not extend beyond the receptive field. This can either be a simple or complex cortical cell. |
| What is the role of end-stopped cells? | They probably participate in our detection of boundaries: viewing corners and similar boundaries is essential to recognition of familiar objects. |
| What is a cortical column?? | A perpendicular organization of neurons in the cortex. |
| Why are cortical columns important? | Neurons in cortical columns communicate with one another but do not form many connections with neighboring columns more than half a millimeter away |
| What is an ocular dominance column? | They are columns of cortex perpendicular to the cortical surface that respond to input from either the right eye or the left eye but not to both. |
| Describe the characteristics of the ocular dominance column. | They are 1mm wide and alternate across the surface of the visual cortex. |
| What are orientation columns? | A column of cortex that responds to lines of a single angle; adjacent coulumns respond to angles shifted about 10 degrees. |
| What are hypercolumns? | A set of orientation columns that respond to a complete rotation of 180 degrees. |
| What are cytochrome oxidase blobs? | They are areas of the striate cortex rich in cytochrome oxidase which responds to colour. |
| What are cortical modules? | A unit of primary visual cortex containing two sets of ocular dominance columns, 16 blobs, and two hypercolumns. |
| How do cortical modules explain perception? | Cortical neurons respond to line orientation, movement and colour. At some point, our visual system puts these separate characteristics back together to form coherent images. |
| What are the physical characteristics of modules? | They make up a 2 mm x 2 mm area of striate cortex. Research suggests that we have about 1000 modules and that they are more accurately viewed as approximations. |
| If you stain Area V2 for cytochrome oxidase, there are thick, thin and interstripe regions. Describe their functions. | The thick stripes form part of the magnocellular pathway and project to the dorsal stream. The thin and interstripe regions continue the parvocellular pathway and project to area V4, which participates in the ventral stream. |
| What are the dorsal and ventral streams? | The dorsal stream proceeds from V1 to the parietal lobe and then to the medial temporal lobe. It is the "where" stream: deals with the analysis of movement, object locations and the coordination of eyes and arms in grasping or reaching. |
| What is the ventral stream? | The ventral stream proceeds from V1 to the inferior temporal lobe. It is the "what" pathway, and deals with object recognition. |
| What is the role of area MT (medial temporal lobe) in the dorsal stream? | It receives input from neurons along the dorsal stream and appears to participate in the processing of motion. It receives input from the magnocellular pathway. Area MT cells respond to movement across large regions of the visual field. |
| What is the role of area MST (medial superior temporal lobe) in the dorsal stream? | Area MST neurons respond to stimulus rotation, expansion and contraction which are large, global movements. Area MST helps use use vision to guide our movements. |
| Some critics have suggested relabelling the dorsal stream as the "how" rather than the "where" stream. Why? | According to them, not only does the dorsal stream tell us an object's location but it also provides information about how to interact with an object. |
| What is the role of area V4 in the ventral stream? | Neurons in V4 respond to both shape and colour. The area projects to the inferior temporal lobe or Area IT. |
| What is the role of area IT in the ventral stream? | Cells in IT respond to many shapes and colors. |
| Which part of the areas in the ventral stream is the fusiform face area a part of? | Area IT. |
| What is the role of the fusiform face area or FFA? | It appears to respond most vigorously to faces and to members of learned categories like species of birds or models of cars. |
| Describe the areas in order of the dorsal stream. | 1. V1 2. Thick stripes in V2 3. Area MT 4. Area MST |
| Describe the areas in order of the ventral stream. | 1. V1 2. Thin and interstripe regions in V2 3. V4 4. Area IT |
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