block 8- developmental plasticity Flashcards
(19 cards)
Does early experience alter nervous system development?
Yes. At birth, the nervous system is still developing — many neural connections (synapses) aren’t fully formed.
However, newborn animals still interact with their environment, and these early experiences influence how neural circuits develop.
The brain is especially plastic during early life.
Sensory input helps shape which connections are strengthened or pruned.
This experience-dependent development is crucial for normal brain function later in life.
why do we study influence of environment on visual system development
- easy sense to control so ideal for study
congenital cataracts
Cataracts: clouding of one the lens prevents light going through the eye = prevents visual
-if removed in later life (10-20 years of age) permenetly disrupted vision
-if removed in infancy= vision not impaired
-raising monkeys in darkness (for first 3-5 months) had same effect
The mammalian visual system- MAYBE CHEAT SHEET
The brain has two hemispheres, and they are connected at the optic chiasm — this is where some visual information crosses over from one side of the brain to the other.
Mammals are forward-looking, meaning we have two eyes at the front of the head.
→ This setup allows for binocular vision, which the brain uses to perceive depth.
👁️ How Light Becomes a Visual Image:
Light enters the eyes and hits the retina at the back of each eye.
The retina contains photoreceptors (rods and cones) that detect light.
These photoreceptors send signals to retinal ganglion cells.
The axons of the ganglion cells form the optic nerve, which carries visual information toward the brain.
🧭 Key Brain Structures Involved:
Optic Chiasm:
Where visual info from the left and right fields of view cross over to the opposite hemisphere.
Lateral Geniculate Nucleus (LGN) in the thalamus:
Acts as a relay station and filters out noise, helping the brain focus on important visual input.
Visual Cortex (in the occipital lobe):
This is where the brain builds a visual image of the world.
It processes shape, colour, movement, and depth to form conscious visual perception.
Hubel and weisel
-studied cats/monkeys
-When a kitten is born it appears blind
-after ca.10 days first evidence of visual responses
-gradually, vision improves, animals develop the ability to discriminate objects and patterns
what happens if one yeye is kept shut at birth in Hubels study?
-results in permanent blindness inthe closed eye when later opened
- only occurs if vision is covered during ‘critical period’(first 12 weeks of the life)
-does not occur if you do the same experiment in adults
Why does suturing one eye shut during development cause permenant blindness
- Because the visual cortex depends on input from both eyes during a critical developmental period to form proper connections.
If one eye is deprived of light (e.g., sutured shut) during this time:
Neurons in the visual cortex stop responding to input from the deprived eye.
The brain “rewires” to favor the open eye.
Even if the eye is reopened later, the visual cortex doesn’t recover, leading to functional blindness in that eye.
what is happening to the development of the visual cortex ?
-Layer 4 is a specific layer that receives the main input from the lateral geniculate nucleus (LGN) — the brain’s visual relay center.
👁️ What’s Special About Layer 4?
LGN neurons send their axons to layer 4, and they do it in a very organized way:
Inputs from the left eye and right eye are kept separate in “eye-specific columns.”
These are also called ocular dominance columns — distinct stripes or bands in layer 4, each dominated by one eye’s input.
- makes it easier to study experimentally
-
transneuronal labeling-CHEAT SHEET
This is a technique where a labelled substance is injected into one eye and is transported across synapses, so scientists can track the pathway that visual information takes.
🔬 Step-by-Step Explanation:
Inject Radioactive Proline into One Eye
Proline is an amino acid that neurons take up.
It’s labelled with a radioactive isotope (like ¹²⁵I, tritium, or carbon-14) so it can be detected later.
Transport Within the Visual Pathway
The retinal ganglion cells (RGCs) in the injected eye take up the proline.
The proline is transported along the axon of the RGCs to the LGN in the thalamus.
From the LGN, the signal crosses the synapse (transynaptic labelling) and continues along LGN axons to the visual cortex (specifically layer 4 of V1).
Prepare the Brain for Analysis
The animal is euthanized, and the brain is removed.
The cortex is cut into thin slices (serial sections).
Autoradiography
Brain slices are placed on photographic film.
The radioactivity exposes the film like light does in a camera.
After developing the film, you see dark bands where the radioactive proline is — these correspond to the ODCs (regions of cortex receiving input from the injected eye).
📷 What You See:
A striped pattern in layer 4 of the visual cortex.
Each stripe (or column) shows the area receiving input from the labelled eye.
The unlabelled eye’s inputs appear as the gaps between the stripes.
so how does occular dominace colums re;ate to the critical period?
At first, when LGN neurons first connect to layer 4 (C4) of the visual cortex, the inputs from left and right eyes are mixed — no clear ODCs yet.
These eye-specific inputs gradually separate during a special time called the “critical period”.
🕒 What Is the Critical Period?
It’s a time early in development when the brain is especially sensitive to experience.
Visual input (seeing through both eyes) is crucial during this time for normal formation of ODCs.
If one eye is deprived (e.g. patched or closed) during this period, the columns for that eye shrink, and the columns for the other eye expand — this is how the brain adapts based on use.
what experiment is used to determine if sensory information change wiring of c4 during the critical period?
Researchers suture one eye shut in a kitten or monkey during the critical period of development. Later, they:
Reopen the eye after the critical period ends.
Use transneuronal labeling (e.g., radioactive tracers) to trace inputs from each eye to layer 4 of the primary visual cortex.
Compare the size of the territory occupied by each eye’s input to that in control animals (with both eyes open during development).
📌 Key Finding:
The open eye dominates much more territory in layer 4, showing that sensory input during the critical period reshapes cortical wiring.
what are the findings of the experiment used to determine if sensory information change wiring of c4 during the critical period?
In normal animals (both eyes open):
ODCs from both eyes take up equal space in layer 4 of the visual cortex.
In animals with one eye closed during the critical period:
The ODCs for the open eye expanded, taking up more space.
The ODCs for the closed eye shrank, becoming narrower.
This shift was permanent, even after reopening the eye.
📌 Key Conclusions (Hubel & Wiesel):
Yes, sensory experience during the critical period changes the wiring of the visual cortex.
The brain’s response to the closed eye is permanently reduced because its input no longer reaches as much of the visual cortex.
This shows that early visual experience is essential for normal brain development.
The critical period is a window of time when the brain is highly plastic (changeable), but this plasticity decreases in adulthood.
How do environmental cues affect how nerve fibres occupy territory in the visual cortex?
Monkey experiments showed that ocular dominance columns (ODCs) only form if both eyes are active during the critical period.
If one eye is removed or inactive, ODCs do not form properly.
If both eyes are sutured shut (Swindale, 1981), ODCs also fail to form.
➤ Conclusion: Balanced input from both eyes is necessary for normal ODC development.
Electrical activity also matters:
Rakic (1981) and Stryker & Harris (1986) used TTX (a sodium channel blocker) to silence action potentials.
Blocking activity in both eyes → no ODCs.
Blocking one eye only → ODCs from the active eye expand, taking over territory from the silent eye (Chapman et al., 1986).
🔑 Key Takeaway:
Formation of ocular dominance columns depends not just on visual input, but also on balanced neural activity from both eyes during a critical developmental window.
how do environmenyal cues instruct nerve fibres to occupy territory in C4
During the critical period of development, the formation of ocular dominance columns (ODCs) in layer 4 of the visual cortex is guided by neural activity from the eyes.
👁️ Activity-Dependent Competition:
The two eyes compete for cortical territory based on how frequently they fire.
More active input from one eye leads to greater territory in C4.
Balanced activity → ODCs form evenly for both eyes.
🔬 What experiments show:
If one eye is less active (e.g. via suture or TTX), the more active eye takes over more space in C4.
If both eyes are inactive, ODCs fail to form — showing that activity, not just presence of input, is essential.
✅ Key Takeaway:
Environmental cues (like visual input) guide cortical wiring by influencing the relative neural activity of each eye. This instructs how nerve fibres occupy space in layer 4 of the visual cortex during early development.
“neurones that fire together wire together”
During the critical period, inputs from both eyes are active and fire in a coordinated way.
The visual cortex receives balanced, synchronous activity from both sets of inputs.
Because neurons from both eyes “fire together”, their connections to the cortex strengthen and are maintained.
Result: ODCs for both eyes form equally in layer 4 of the visual cortex.
👁️❌ One Eye Closed (Monocular Deprivation)
If one eye is sutured shut during the critical period:
That eye’s neurons don’t fire in sync with the active, open eye.
The inputs from the closed eye are out of step and ineffective.
Meanwhile, the open eye’s neurons fire regularly and strongly.
According to the rule:
The open eye’s inputs “wire” more strongly into the visual cortex.
The closed eye’s connections weaken or shrink because they don’t fire together with the postsynaptic cells.
mechanisms for activity-dependent synaptic competition
Neurons that are active (i.e. fire action potentials regularly) stimulate the postsynaptic cell.
This postsynaptic activation triggers the release of neurotrophic factors — chemical signals that support synaptic growth and survival.
Examples include BDNF (Brain-Derived Neurotrophic Factor) and other neurotrophins.
These trophic factors are released locally — meaning only the active synapses receive the benefit.
The active presynaptic terminals (those that “fired together” with the postsynaptic neuron) are stabilized and strengthened.
The inactive terminals (that didn’t coincide with postsynaptic firing) don’t get the trophic support — so they weaken and may be pruned away.
🧪 Why This Explains ODC Formation (in critical period):
When both eyes are open, neurons from both eyes fire synchronously, and both sets of synapses receive trophic support, maintaining balance.
When one eye is closed, its neurons are less active, so:
They don’t trigger neurotrophin release.
Their synapses don’t get strengthened.
They lose cortical territory to the more active, trophic-supported inputs from the open eye.
what is activity dependent competetition?
Activity-dependent competition is a general principle where neurons compete for synaptic territory based on their level of activity, especially during the critical period of development.
🧠 Key Points:
During the critical window, more active synapses are strengthened and maintained, while less active ones are weakened or eliminated.
This process helps organize neural circuits, such as forming ocular dominance columns in the visual cortex.
After the critical period, synaptic connections become more stable and less plastic — changes are harder to induce.
🔍 Beyond Layer 4:
While clearly seen in layer 4 ODCs, this competitive process likely occurs in other brain areas, though it’s harder to visualize due to less organized input patterns.
why dont amphibiand have occukar dominace
-because inputs from the left and right eyes project to the opposite side of the brain: no competition
-when inected withba transplanted eye they start to compete and occular domiance columns develop
does the rewired auditory cortex adapt its function to process visual information?
Yes, it can — if the brain is rewired during early development.
🧪 Key Experiment Steps (Newton & Sur, 2004):
Rewire visual inputs to go to the auditory cortex instead of the normal visual pathway (done in young ferrets).
Let the animal grow up with this new wiring.
Train the ferret to respond differently to:
Visual stimuli (like a flash of light)
Auditory stimuli (like a tone)
Rewards (juice) are given for correct choices.
Then, damage the normal visual pathway (retinogeniculate).
Now the only place receiving visual input is the auditory cortex.
🎯 Key Findings:
Ferrets can still see and respond correctly to visual stimuli — meaning the auditory cortex is now processing visual information.
The auditory cortex developed features typical of the visual cortex, such as:
Ocular dominance columns (normally only found in visual cortex)
A retinotopic map (an organized layout of visual space)
Responses to orientation, direction, and velocity of movement — all key visual features.
Lesion the auditory cortex (now handling vision), and the ferret loses visual abilities — confirming that the auditory cortex had truly adapted to handle visual perception.
🧠 What Does This Prove?
The brain is highly plastic, especially during development.
Function is not hard-wired to location — it depends on inputs and experience.
The auditory cortex can become a visual cortex — if given visual input early enough.
