W2 Flashcards
(20 cards)
How does resolution relate to vision?
Our brains do a lot of work to help us see!
The image that lands in our retina is blurry, unevenly focused, poor quality and inverted.
Our brain corrects retinal images (flips, enhances and fills gaps) to create a high resolution perceived image.
Why does the structure of the retina limit its resolution and energy efficiency?
Resolution: If the whole retina is high-res, blind spot would take up most of the retina. Even if every photoreceptor was connected to the brain, the blindspot would be HUGE.
Energy: If all the retina cells were active all the time, the amount of energy required would be huge.
What’s the solution for the retina?
Compression! Where we only transmit important information, focusing on changes over time and across space.
How does encoding change over space work?
Spatial encoding across different locations involves varying neural activity to represent environmental features and their relationships. Mechanisms like lateral inhibition enhance this by sharpening borders and emphasising changes in sensory input across space.
Imagine seeing red. Initially, the red receptors are very active.
Your brain tries to highlight this red area.
This activates both red and green receptors (red is a mix of red and green). Nearby red cells also activate.
Lateral inhibition occurs: active red cells inhibit nearby red cells, especially smaller red areas. This emphasizes borders and contrast.
If there’s no change but contrast, it will inhibit green, whereas on the border it’s the red that follows.
The purpose of red is to show you what’s changed/context.
How does encoding changes over time work?
We have red (R), green (G), and blue (B) cells.
Temporal inhibition turns off cells that have been active for a long time. This is called adaptation.
Adaptation is slow to build up and slow to fade away.
This is how the brain compresses signals that remain constant over time.
Color after-effect?
Staring at red fatigues red receptors. Looking at white then makes it appear bluish-green (missing red).
Edge processing and gap filling?
Brain spreads edge excitation. Red-green edge? Red cells on the red side, and green cells on the green side, get inhibited. Brain fills the gap, sometimes incorrectly perceiving yellow instead of white if details are lacking.
How does the eye gather visual information from the world?
Through receptive fields (in the sensory periphery).
What are receptive fields?
Each photoreceptor in the eye has a small receptive field, a specific area of the visual scene that it’s responsible for capturing.
These individual receptive fields collectively provide the brain with a complete picture of what we see.
What are On-Center/Off-Surround Cells?
Retinal ganglion cells with two receptive field zones:
On-Center: Light excites, cell activated, increasing action potentials.
Off-Surround: Light inhibits, cell inhibited, reducing action potentials.
How are they studied?
Light is shone on different areas of a rodent’s retina.
Electrical activity (action potentials) is recorded to map receptive fields of each cells.
Different cells respond differently to light depending where it hits..
Why do On-Center/Off-Surround Cells matter ?
Help detect contrast, shapes, and motion.
The brain reconstructs images from activation patterns.
No activation = no signal, but the brain processes input continuously.
What is the simultaneous contrast illusion?
The simultaneous contrast illusion occurs because of center-surround antagonism in visual processing.
When a grey bar is placed on different backgrounds, on-center ganglion cells in the retina respond differently:
against a dark background, less inhibition makes the bar appear lighter
while against a light background, more inhibition makes it appear darker.
This contrast exaggeration creates the illusion that identical shades look different.
How does the retina process images?
The retina emphasizes edges by responding strongly to changes in light intensity across space. Uniform areas produce weak signals, resulting in a retinal output dominated by edges.
What’s the impact of this edge-focused processing, as seen in the Craik-O’Brien-Cornsweet illusion?
Because the retina primarily signals edges, an edge-only image and a normal image (after retinal processing) produce similar output. The brain interprets these similar outputs as similar images, leading to perceptual illusions where brightness differences are minimized.
What is Troxler fading?
Troxler fading is the disappearance of a fixed peripheral stimulus due to sustained attention on a central point
How do signaling changes over time affect visual perception?
Over time: Prolonged exposure to a stimulus leads to reduced neural activity (inhibition) to conserve energy. This makes the stimulus appear to fade (“Troxler fading”). A negative afterimage appears when the stimulus is removed due to delayed inhibition.
How do signalling changes over space affect visual perception?
Over space: A stimulus’s lack of sharp edges makes it harder to pinpoint its exact location. This, combined with inhibition, can cause the brain to “fill in” the gaps, potentially leading to inaccurate perception.
What is the lateral geniculate nucleus?
The LGN is a relay station in the thalamus that processes visual info from the retina before sending it to the visual cortex.
Outline the lateral geniculate nucleus gateway to the cortex?
The lateral geniculate nucleus (LGN) acts as a relay station between the retina and the visual cortex.
Parvocellular layers (layers 3-6) receive input from parvocellular (P) ganglion cells, processing color and fine detail.
Magnocellular layers (layers 1-2) receive input from magnocellular (M) ganglion cells, processing motion and depth.
Koniocellular layers receive input from K ganglion cells, processing blue-yellow color.
Each LGN layer represents one half of the visual field, and while each LGN receives input from both eyes, individual cells within the LGN are monocular (receive input from only one eye).
