week 2 Flashcards
(32 cards)
What problems does the visual system face regarding resolution and energy?
If the retina were uniformly high resolution, the blind spot would become disproportionately large, and maintaining all photoreceptors in an active state would demand enormous energy and supporting vascular structures. This is why the system uses compression to send only critical information
How does the concept of compression solve problems in visual processing?
Compression reduces the vast amount of visual data by transmitting only essential information—especially changes in the scene—while omitting redundant, unchanging details. This allows for efficient use of neural resources and avoids overwhelming the brain with unnecessary data.
What is a receptive field in the context of the visual system?
A receptive field is the specific region of the retina where the presence of a stimulus will influence the electrical activity of that sensory cell. Each photoreceptor and retinal ganglion cell is responsible for processing a small part of the overall image
How do retinal ganglion cells contribute to forming a visual image?
Retinal ganglion cells gather input from their small receptive fields, which are organized with an on-center/off-surround structure. This arrangement emphasizes contrast by activating when the center is lit and inhibiting when the surrounding area is bright, thereby encoding edges and important features
Explain the on-center/off-surround mechanism in retinal ganglion cells and its role in visual perception
In an on-center/off-surround arrangement, light hitting the center of the receptive field excites the cell, while light in the surrounding region inhibits it. This differential response enhances contrast and sharpens object boundaries, which is crucial for localizing edges and contributing to visual illusions such as simultaneous contrast.
How do simultaneous contrast illusions arise from the receptive field organization?
These illusions occur because the same area can appear to have different brightness levels depending on the light conditions in the surrounding area. The on-center/off-surround receptive field causes a cell to respond differently to a target on a dark versus a light background, altering perceived brightness
Why does retinal output emphasize edges?
Retinal ganglion cells are highly sensitive to changes in light intensity, meaning they focus on transitions or edges in the visual scene. This edge detection simplifies the image by converting it into outlines, which the brain later fills in with additional details
What causes Troxler fading in peripheral vision?
When fixating on a particular point, the neurons that encode luminance from the unchanged surrounding stimulus gradually reduce their response. This neural adaptation leads to the peripheral stimulus fading away, known as Troxler fading
How are negative after-images produced?
After prolonged exposure and subsequent neural adaptation, removing the stimulus results in a lag in the recovery of neuronal activity. This lag causes the perception of the complementary color to appear as a negative after-image
What is the role of the Lateral Geniculate Nucleus (LGN) in visual processing?
The LGN acts as a relay station between the retina and the visual cortex. It processes and reorganizes visual information before sending it to higher-order brain regions, with each layer representing half of the visual field
Describe the differences among K, P, and M cells within the LGN
K Cells: Involved in blue-yellow color processing.
P Cells: Have a slower response, are sensitive to red-green colors, dominate in the fovea, and convey fine detail.
M Cells: Respond rapidly, primarily encode motion and depth, are more active in the periphery, and handle coarse detail.
What are the two main visual pathways emerging from the LGN, and what functions do they serve?
Dorsal Pathway: Processes spatial information such as an object’s location and movement (“where” pathway).
Ventral Pathway: Responsible for object recognition, including detailed form and color processing (“what” pathway).
What are some of the specialized brain areas associated with the ventral stream?
Occipital Facial Area (OFA), Fusiform Facial Area (FFA), Parahippocampal Place Area (PPA), and the Lateral Occipital Cortex (LOC), which are involved in processing faces, objects, and places
What does the statement “Illusions are in the eye, not the mind” imply?
- visual illusions, including those related to color and lightness, result from the early neural processing and inherent design of the eye
- These low-level spatial filtering and feature detection mechanisms interact with top-down information (context and prior knowledge) to shape our perception
How does the raw retinal image differ from our conscious visual experience?
The retinal image is of variable (mostly low) resolution, is inverted and laterally flipped—yet the brain reconstructs and “corrects” it so that we see a high‐resolution, upright, and coherent scene
What is the blind spot, and why is it larger if the retina were uniformly high resolution?
The blind spot is the region with no photoreceptors (due to the optic nerve exit). A uniformly high-resolution retina would require a larger proportion of retinal area to accommodate the necessary blood vessels and connections, thereby expanding the blind spot
What energy constraints influence retinal design, and how does compression help?
If all photoreceptors were active continuously, the energy (and blood supply) demands would be enormous. Compression minimizes transmitted information to only important, changing signals—saving energy and reducing metabolic load
Explain the concept of efficient data transmission in the visual system
Instead of acting like a video camera that records everything, the visual system compresses information by transmitting only significant details (e.g., changes, edges, and movement) while ignoring redundant, constant background data
What are the three key compression mechanisms mentioned in the lecture?
- Spatial Inhibition: Uses lateral inhibition to emphasize changes across space (producing simultaneous contrast illusions).
- Temporal Inhibition: Reduces responses of cells over time to unchanging stimuli, leading to after-effect illusions.
- Filling-In: The brain uses edge information to “fill in” areas that lack a direct signal (as seen in the Craik–O’Brien–Cornsweet illusion).
What consequences result from the brain’s compression strategies?
- The visual system becomes very sensitive to sudden changes and movement but less adept at detecting slow changes
- it has high resolution for black and white but lower for color
- excels at comparing adjacent regions
- struggles with absolute judgments across time
What is a receptive field, and why is it important in vision?
A receptive field is the small region of the visual scene that influences a particular cell’s activity. It ensures that each cell processes only a fraction of the entire image, which—with overlapping fields—allows for the construction of a complete, detailed visual perception
How does center–surround antagonism in retinal ganglion cells lead to simultaneous contrast illusions?
In an on-center/off-surround cell, a light stimulus in the center excites the cell, but if the surrounding area is also lit, it inhibits the response. Thus, the same shade may appear lighter or darker depending on its context, creating simultaneous contrast illusions
Describe the Craik–O’Brien–Cornsweet illusion and its relation to retinal processing
This illusion occurs because retinal ganglion cells primarily respond to edges. When only the borders are detected, the brain “fills in” the interior areas, causing two regions with nearly identical luminance to appear different due to edge information
What causes Troxler fading, and how does it relate to temporal inhibition?
When you fixate on a point, neurons processing the unchanging peripheral stimulus gradually reduce their response (temporal inhibition), making that stimulus fade from awareness—a phenomenon known as Troxler fading
