11 Visual Processing in the Brain
Visual Information Processing in the Brain
Overview
Explains how the brain processes visual information through various pathways and structures.
Retina Processing
Rods and Cones
-Inhibitory contacts=neuron connections that reduce activity in other neurons
The retina consists of rods and cones that make synapses with horizontal and bipolar cells.
Horizontal cells create inhibitory contacts with bipolar cells, enhancing contrast.
Optic Nerves
- ganglion cells=a type of cell in the retina that collects and send visual info to the brain
Optic nerves from both eyes intersect at a part of the brain called optic chiasm, where axons cross to the opposite brain side.
Most ganglion cell axons send signals to the lateral geniculate nucleus (LGN) in the thalamus for processing and organizing before sending it to the brain’s visual cortex (the part that actually understands what you're seeing)
Contrast Enhancement
Lateral Inhibition
-Lateral inhibition=a process that makes you focus on one point of your vision while bluring the other for maximum focus?
The retina uses lateral inhibition to make the edges of objects appear sharper and more defined.
How its done:
Excite nearby bipolar cells=retina receptors send signals to the bipolar cells
Simultaneously inhibit neighboring cells=send a "quieting" signal to the surrounding cells, which reduces their activity.
*thus making contrast between light and dark areas stronger, which helps you see borders and details more clearly
Receptive Fields in Visual Cells
Definition
Each cell in the visual system has their receptive field(area of the retina that the cell responds to)
Bipolar & ganglion cells not just responding to one receptor They combine signals from multiple receptors, thus receives input from several receptors in its receptive field, creating a stronger or more complex response.
Field Structure
ganglion cell has "field" of vision that looks like a circle:
center=clearer vision; excited (the ganglion cell fires)
surrounding area=blurry vision; inhibited (the ganglion cell’s activity is reduced)
* improving your ability to see edges clearly
signals processed:
retina (from ganglion cells)→optic nerve→lateral geniculate nucleus (LGN) in the brain→ other parts of the brain for further processing.
Primary Visual Cortex (Area V1)
part of the brain that is the first section in the visual process, with the function of processing basic visual information(lines, edges, colors, and motion) before sending it to more specialized areas for higher-level processing.
Function & Damage
Information from LGN send to the primary visual cortex in the occipital lobe (V1).
Damage to V1 results in the absence of conscious vision, visual imagery, or visual dreams, unlike vision loss due to eye injuries.
Types of Receptive Fields
Simple Cells
cells that are apart of the V1 but the function is specifically to detecting fixed lines and edges in one specific area
Complex Cells
cells that are apart of the V1, similar to simple cells. But more flexible, able to respond to fixed line and edges regardless of location.
Differentiates from simple cells, emphasizing movement patterns over precise locations.
End-stopped Cells
These cells are similar to complex cells, in that they can detect these features across a large area of the visual field (not just in one specific location). however they stop responding if the lines are too long or too far away.
good at detecting things like edges of objects or boundaries in the visual scene.
Columnar Organization
Organization in Visual Cortex
In visual cortex (particularly in Area V1), neurons (cells) with similar functions are grouped together in columns
each column is dedicated to detecting a certain aspect of the visual world, such as lines at specific angles.
Development of the Visual Cortex
Sensitive Periods
During early development, experience significantly influences synapse responsiveness.
Closing one eye in kittens leads to unresponsiveness in the visual cortex; re-opening doesn't restore function.
Prolonged closure in both eyes leads to sluggish responses, shifting focus towards auditory and tactile stimuli.
Dorsal and Ventral Streams
Overview
Visual information travels along two distinct pathways:
ventral (what)=identifying and recognizing an object
dorsal (where/how)=visually guided movement
Disruption in one stream can affect the other.
Specific Cases
DF Patient: Cannot recognize or name objects but can interact with them effectively.
JS Patient: Cannot determine object location but can recognize objects.
Function of Each Stream
Ventral Stream: Processes identification and recognition of objects
damage interferes with this function.
Dorsal Stream: Involved in visually guided movements
Damage to this part can cause disturbances in visual coordination and motor organs
Shape and Detail Analysis
Primary and Secondary Visual Cortex
V1(simple cells)→V2(complex cells dominate) →specializing further based on information type(texture & depth)
Inferotemporal cortex:recognize objects by processing the visual information from V1 and V2.
Damage to this area can result in visual agnosia, where you can see objects but not recognize them for what they are.
Recognition of Faces
Areas such as the amygdala and fusiform gyrus are crucial for face recognition; damage leads to prosopagnosia, where individuals can't recognize faces holistically.
Color Perception
V4 and Color constancy
V4 area is vital for color perception; color appearance is affected by surrounding contexts.
Color constancy is the ability to recognize consistent object colors despite lighting changes.
Motion Perception
Areas Involved
Middle temporal cortex (MT) and medial superior temporal cortex (MST) specialize in motion. MT responds to motion speed/direction, while MST processes complex motion patterns.
Motion Blindness
Motion blindness (Akinetopsia) arises from damage to MT and MST, impairing movement perception.