The brain and seeing Flashcards
Consequences of lateral inhibition Chevreul Illusion
Chevreul Illusion: brighter areas projected to the retina inhibit the sensitivity of neighbouring retinal areas
Consequences of lateral inhibition Match Bands
Match Bands: an optical phenomenon from edge enhancement due to lateral inhibition of the retina
Lateral inhibition and brightness illusions (Hermann Grid)
Lateral inhibition causes increased inhibition of neurons in the retina at intersections, making the region seem darker
Lateral Inhibition and Brightness illusions (simultaneous brightness contrast)
Surrounding a darker area with a lighter background intensifies the perception of darkness due to lateral inhibition.
Lateral Inhibition and Brightness illusions (White’s illusion)
Lateral inhibition enhances the perception of black at the intersection points, creating the illusion of white circles
Problems with current explanations
Current explanations rely heavily on lateral inhibition, but there are additional factors, such as cortical processing, that contribute to brightness illusions. The exact neural mechanisms are not fully understood.
The visual pathway from eye to the brain
- light enters the eye, stimulating photoreceptors on the retina
- signals are transmitted through the optic nerve to the optic chiasm
- At the chiasm, some fibers cross to the opposite hemisphere
- The signals continue through the optic tract to the LGN of the thalamus
- From the LGN, information is relayed to the primary visual cortex (V1) in the occipital lobe
Information travels from nasal and temporal retina
Nasal retina: axons from the nasal retina cross at the optic chiasm to the opposite hemisphere
Temporal retina: Axons from the temporal retina remain on the same side of the hemisphere
Anatomy of LGN
The LGN is a thalamic nucleus involved in visual processing
It has 6 layers, each receiving input from one eye
Layers 2, 3, 5 process information from the contralateral eye, while 1, 4, and 6 process information from the ipsilateral eye
Type of cells in primary visual cortex (V1)
- Simple cells: respond to specific orientations of light
- Complex cells: respond to specific orientations of light, but less sensitive to the location
- Hypercomplex cells (end-stopped cells): respond to specific orientations of light and have inhibitory regions at the ends of their receptive fields
Selective rearing and V1 structure
- Selective rearing: exposing an animal to only one type of stimulus during development
Result: demonstrates the plasticity of V1, as cells become highly specialized for the presented stimulus
Cellular organization of V1
V1 has a retinotopic map, with neighbouring cells responding to neighbouring areas in the visual field.
Columns in V1 are organized by orientation preference
Area IT and Visual Processing
Area IT (inferotemporal cortex)
- located in the ventral stream
- responsible for complex visual processing, including object recognition and face perception
Visual Acuity Measurement with Edge Detection
- Visual acuity: sharpness of vision
Edge detection methods
- Landolt c tests: identifying the gap in a ring
- Snellen Chart: reading letters of varying sizes
Dimensions of Sine Wave Grating
- Spatial frequency: number of cycles per unit of space
- Amplitude: intensity or brightness of the grating
- Phase: position of the grating within the visual field
- Orientation: direction of the stripes
Dimensions determining perceptual contrast
Spatial frequency and amplitude: These dimensions significantly influence the perceived contrast of a sine wave grating
Contrast sensitivity function
a. definition: Describes an individual’s ability to distinguish between different spatial frequencies
b. Graph shape: Typically a bell-shaped curve, indicating optimal sensitivity to certain frequencies
Adaptation experiments and visual system
a. Adaptation experiments: Prolonged exposure to a stimulus reduces sensitivity
b. Results suggest: The visual system adapts to the prevailing stimulus, adjusting sensitivity to optimize perception in different environments. Adaptation can occur at various levels, from the retina to higher visual areas
Sine wave grating
a repeated number of fizzy dark and light bars, or cycles
Edge detection
an image processing technique for finding the boundaries of objects within images
Cortical cells
- simple cells:
- Receptive fields are linear
- These cells are sensitive to orientation — this is called orientation tuning
- Orientation tuning may be a result of the position of LGN cells that make up one simple cell receptive field
- Kittens reared in selective environments develop more cells that correspond to their environment (horizontally reared cat vs vertically reared cat) - complex cells:
a. have elongated receptive fields
b. these cells are position insensitive
c. complex cells are motion sensitive
d. they are elongated contour moving in their preferred direction - end-stopped cells (hypercomplex)
- similar to complex cells but they like bars of a certain length best
Map and Columns
Just like the LGN, cells in V1 are retinotopically organized
All areas of the retina are not equally represented in V1 — known as cortical magnification
Cortical cells are arranged in columns that run the depth of the cortex
All cells within a column respond to approximately the same place on the retina
These are location columns
Columns and Hypercolumns
All cells within a column have the same orientation sensitivity
These are orientation columns
Moving parallel to the surface of the cortex, the preferred orientation of each column changes
Columns are arranged by ocular dominance
A hypercolumn contains columns that represent all 180° for both the left and right eyes
