Lecture 15: Experience-dependent plasticity

Hebb’s postulate

“Neurons that fire together wire together”

Coordinated electrical activity of neurons are strengthened while uncorrelated connections are gradually weakened and eventually eliminated

Construction phase

Post-natal growth of dendrites, axons, and synapses

Elimination phase

Continued elaboration of the synapses that remain

Critical period

The time when experience and the neural activity that reflects that experience have maximal effect on the acquisition or skilled execution of a particular behavior → (Ex. imprinting)

Far more time for sensorimotor skills and complex behaviors to be acquired (Ex. language in humans)

Local Oscillations (“waves”)

Subthreshold activity that are essential for shaping circuit networks

Prepare for optimal experience-driven activity

Retinal Waves

• Each retina independently generates a pattern of waves of electrical activity that moves across large populations of retinal cells in an orderly fashion

• Initiated in local retinal cells (amacrine cells) → AP firing by ganglion cells → relayed to LGN → V1

• Coherent in each eye, asynchronous between eyes → competitive interaction between the two eyes for V1 representation

Ocular dominance columns

• Alternating series of eye-specific domains in cortical layer 4 (in the V1)

• Cells in layer 4 respond strongly or exclusively to stimulation of either the left or right eye

• Neurons in layers above and below layer 4 integrate inputs from both the left & right eyes and respond to visual stimuli seen by both eyes

Visual Deprivation

Very few cortical cells could be driven from the deprived eye

Recordings from the retina and LGN were normal

Deprived eye gets functionally disconnected from the visual cortex → “cortical blindness” (amblyopia)

The same closing the eye experiment in adulthood didn’t get rid of ocular dominance (only reduced activity) → only in critical period

Ocular dominance column pattern

In monkeys, the stripe-like pattern of geniculocortical axon terminals in layer 4 that defines ocular dominance columns is already present at birth → This pattern reflects the functional segregation of inputs from the two eyes

Occurs even in the absence of meaningful visual experience

Alternating stripes of roughly equal width

• Animals deprived from birth of vision in one eye develop abnormal patterns of ocular dominance stripes in V1

• Altered patterns of activity caused by deprivation

• Stripes related to the open eye are substantially wider

• Stripes representing the deprived eye are correspondingly diminished

• Inputs from the active (open) eye take over some – but not all – of the territory that formerly belonged to the inactive (closed) eye

• Competitive interaction for post-synaptic space

Monocular deprivation and LGN

Lateral geniculate nucleus axons (visual cortex) experience a loss of branches and it’s less dense on the axon

Long-term → The other eye maintains the amount of contacts → there are limits to how much the deprived eye can change

Dark Exposure

A potential treatment for monocular deprivation → was applied to rats during adulthood

Increases the density of spines on visual cortical neuron dendrites

Reactivates cortical synaptic plasticity and permits reactivation of visual capacity

Strabismus

These alignment errors both produce double vision → due to muscles in the eye

Inputs to the LGN from the optimally aligned eye are competitively advantaged → More V1 territory

Suppressed eye eventually comes to have very low acuity → may render an individual effectively blind in that eye

Convergent strabismus

esotropia (“crossed eyes”)

Divergent strabismus

exotropia (“wall eyes”)

Manipulating Competition

• Cut one of the extraocular muscles in one eye during critical period (no longer aligned)

• Test the role of correlated activity in driving the competitive postnatal rearrangement of cortical connections

• Unlike monocular deprivation activity levels in each eye remain the same → but the correlations between the two eyes are altered

Ocular Asynchrony

• Input from both eyes remains active but highly asynchronous

• Ocular dominance pattern is sharper than normal in layer 4

• Cells in all layers of V1 are driven exclusively by one eye or the other

• Prevents binocular interactions in other V1 layers

Orientation Tuning

• Prior to eye opening: there is little/no correlation between relatively broad orientation sensitivities in visual cortex neurons driven by both eyes

→ Fairly low maximal response to preferred orientations

→ Orientations are dissimilar between the two eyes

• Start of critical period: magnitude increases in both eyes, but orientation preference remains dissimilar

→ Increased correlation of visually evoked stimuli → matching of orientation tuning of the right and left eye inputs to single cortical binocularly driven neurons

• If one eye is closed during the critical period, the matching of orientation tuning of binocular inputs does not occur → Cannot be restored once the closed eye is opened

Cataracts

Make the lens or cornea not transparent → functionally equivalent to monocular deprivation in animals

Largely avoided if treated before 4 months of age

Bilateral → less dramatic deficits even if treatment is delayed

→ Equal competition during critical period is worse than complete disruption of visual input

Language development

• Hearing babies begin babbling at ~ 7 months

• Deaf babies exposed to sign language at an early age “babble” with their hands

• Regardless of the modality, early experience shapes language behavior

Learning language

• Critical period for language learning → Decline in fluency of nonnative speakers as a function of age

• Children can usually learn to speak a 2nd language without accent and with fluent grammar until about age 7-8

• After this age, performance gradually declines no matter what the extent of practice or exposure

Gray matter volume

In humans, it increases and then decreases in roughly the same way

Increases more slowly in children with ADHD

→ The rate of decline is equivalent, although the net result is lower ___ in adults with ADHD

White matter volume

Increases throughout early childhood and adolescence