Hebb's Postulate

• Hebb’s postulate states that when a pre-synaptic neuron and a post-synaptic neuron fire together, the connection between them strengthens.
• Coordinated pre- and post-synaptic activity strengthens connections, maintaining and expanding synapses.
• Uncoordinated activity weakens connections, leading to synapse loss or redirection to another target.

Hebb's Theory Explains Observations

• Behaviors absent at birth develop through experience in early life.
• There is a superior capacity for acquiring skills and abilities in early life.
• Brain growth continues after birth, alongside the acquisition of complex behaviors and development of synaptic connections.
• Hebb predicts a decline in synapse number during adolescence as the brain refines its connections.

Hebb’s Developmental Phases:

• Progressive construction phase involves brain enlargement after birth due to dendrite, axon, and synapse growth (not new neurons).
• Selective elimination phase involves continued brain growth while remaining synapses and target neurons become more specialized and efficient.
• Intrinsic mechanisms create basic neural circuits (axon growth, synapses) that experiences refine.
• Lack of sensory input changes connectivity and behavior, which may be beneficial, like sensory impairments enhancing other senses.
• Sensory deprivation can impair function due to a lack of proper refinement.
• Basic survival behaviors come from intrinsic neural circuits, which form the foundation of behavior.
• Embryonic mechanisms develop more complex behaviors, and animals with advanced behaviors adapt their nervous systems through environmental experience, most strongly during a critical period.

Critical Periods

• Critical periods are specific early-life windows when experience and neural activity have the greatest effect on acquiring a behavior.
• Bird parental imprinting involves baby chicks recognizing parents only if exposed within a short timeframe.
• Songbird and human language communication require prolonged exposure to stimuli over longer critical periods.
• Disruptions in songbird learning impair communication, affecting reproduction and survival.
• Critical periods involve time-sensitive learning and are experience-dependent, requiring sensory input and interaction with the environment.
• They also involve irreversibility, where missed experience results in a behavior never fully developing.
• Each critical period applies to a specific behavior and is highly influenced by environmental factors.
• After the critical period ends, there is reduced plasticity, making learning new skills harder, and missed experiences can cause permanent deficits.
• Critical periods rely on changes in the organization and function of circuits in the cerebral cortex.

Oscillations in Establishing Critical Periods

• During critical periods, action potentials in response to stimuli reveal activity-dependent changes in neural connections.
• In the visual system, sensory experience shapes neural circuits during the critical period.
• Before external stimuli activate neurons, early brain waves (oscillations) prepare neural circuits for later activity.
• Local oscillations shape neural networks for optimal experience-driven plasticity.
• Before birth, the retina shows early electrical activity called retinal waves which activate V1 before actual visual experience.
• Retinal waves help separate input from two eyes.
• Retinal waves within each eye are coordinated but fire asynchronously between the two eyes, creating competition in the LGN.
• Competition follows Hebb’s rule, where stronger signals get reinforced, segregating inputs from each eye.

Critical Periods in Visual System Development

• The visual system is ideal for studying brain plasticity due to its experimental manipulability and structured organization.
• Light hits the eye, travels to the optic nerve, then to the LGN (thalamus), and finally to V1.
• The LGN keeps signals from both eyes separate before passing them to V1.

Ocular Dominance Columns in V1

• In layer 4 of V1, inputs from each eye are organized into alternating strips called ocular dominance columns.
• Left and right eyes are first processed separately before combining in higher layers.
• The brain receives signals from both eyes, but some neurons respond more strongly to one eye than the other.
• The left eye sends signals to both hemispheres, with the strongest connection to the right hemisphere, and vice versa.
• Each hemisphere mainly processes vision from the opposite eye.
• Signals from each eye remain separate in layer 4 but integrate inputs from both eyes in layers above and below layer 4.
• The critical period for vision depends on synchronized activity from visual stimuli received by both eyes.

Visual Deprivation Effect on Ocular Dominance

• Ocular dominance is the strength of a neuron’s response to one eye versus the other (left eye to right hemisphere, right eye to left hemisphere).
• The critical period for eye dominance occurs between 1 week and 1 year after birth, peaking at 4 weeks for cats and birth to 6 months for monkeys.
• In normal adult cats, neurons in V1 respond equally to both eyes except in layer 4, where eye-specific patterns are more distinct.
• Monocular deprivation (closing one eye in kittens) causes neurons in V1 to become unresponsive to the deprived eye, leading to cortical blindness.
• In experiments with cats, shutting one eye from 1 week to 2.5 months after birth resulted in neurons responding only to the non-deprived eye.
• The deprived eye was functionally disconnected from V1, causing permanent blindness.
• Even 1 week of unequal vision significantly reshapes neural connections, but prolonged deprivation up to 12 months had little additional effect.
• Monocular deprivation in adult cats has no effect on ocular dominance.
• Closing the right eye for 2 weeks to 18 months causes the open left eye’s column to grow wider and the deprived right eye’s column to shrink.
• Even 3 days of monocular deprivation during the critical period produced a significant shift in cortical activation.
• Eye dominance is shaped only during the critical period.
• Blocking one eye during the critical period disrupts brain processing vision only in V1 (not the retina and thalamus).
• Early separation is driven by spontaneous retinal activity, not visual experience.
• LGN-to-V1 connections adjust based on visual experience.
• During the critical period, both eyes compete for cortical space based on the strength of visual input.
• Normal vision results in both eyes sharing equal cortical space, forming clear ocular dominance columns.
• Closing one eye causes the open eye to take over connections from the closed eye, weakening the deprived eye’s connections.
• This happens because the active eye takes over more cortical space, not because the deprived eye’s signals disappear.
• If vision imbalance is not corrected during the critical period, it leads to permanent issues in visual processing.
• When both eyes are shut, the results are closer to normal than with monocular deprivation.
• With one eye deprived, the open eye strengthens connections at the expense of the closed eye.

Manipulating Competition

• Strabismus is when an eye muscle is cut, causing the eyes to become misaligned.
• Matching points on both retinas are no longer stimulated at the same time, leading to different activity patterns.
• Cats with strabismus have sharper ocular dominance columns because the differences in activity between the two eyes increase.
• Strabismus blocks binocular convergence, disrupting how the brain integrates signals from both eyes.
• Before the critical period, neurons have low correlation in their responses to visual orientation.
• During the critical period, responses to oriented stimuli from both eyes strengthen significantly, but orientation preferences remain different.
• Over time, the correlation between stimuli from both eyes increases, leading to the alignment of orientation preferences.
• During the critical period, closing one eye causes orientation tuning mismatch between the two eyes, which cannot be corrected later.
• After the critical period, closing one eye has no effect on orientation tuning between the two eyes.
• Competition of signals from both eyes is needed to match the orientation.
• Amblyopia is caused by a lack of normal visual input during early development.
• Strabismus causes misalignment of the eyes (esotropia/inward or exotropia/outward) leading to double vision and difficulty merging images.
• In some cases of strabismus, the brain suppresses input from the weaker eye to avoid double vision, causing the stronger eye to dominate in V1.
• Surgical correction of strabismus during the critical period helps maintain normal vision.
• Cataracts can cause vision loss and can cause permanent vision damage in the affected eye if untreated.
• Surgery before 4 months of age can prevent most damage from cataracts.
• Bilateral cataracts cause less severe deficits as unequal competition during the critical period for normal vision is worse than complete deprivation.