Cortical Maps and Plasticity - L1 Dominic McGonigle

How is the cortical map of rodent whiskers arranged?

Preserves the arrangement of whiskers, by placing cells for each whisker in the same order as the whiskers themselves.

Where are senses processed in rodent’s cortex?

Barrel Cortex (aka rodent somatosensory cortex) - called the Barrel cortex as each set of cells for each whisker is surrounded by myelination - appear like barrels when stained.

How can sensory maps also represent visual areas?

The arrangement in the brain represents the arrangement of areas of the visual field - the largest areas represent the most receptor-dense parts of the eye (e.g. the fovea is the part of the eye with the most receptors as it is responsible for the most detailed form of vision) = cortical magnification

What do orientation-selective visual maps show?

Some neurons are more sensitive to stimuli at particular orientations - we record how much the neuron fires when a stimulus is presented at different orientations - neurons with similar orientation preference cluster together in the brain.

Therefore cortical maps of the primary visual cortex are often arranged according to FEATURE SPACE.

What is feature space?

Refers the either the location or orientation of an object in the visual field - one way that cortical maps are arranged (neurons which process similar feature spaces cluster together)

What is a receptive field?

The region of feature space a neuron responds most too (fires most action potentials for)

How are cortical maps similar/different across species?

The more genetically similar a mammal is to humans, the larger their primary visual area = this is the main sensory domain used in humans. However, rodents are more likely to have larger somatosensory areas.

Does a naked mole rat have a primary visual cortex?

YES! Even though they have evolved to have skin covering their eyes, the primary visual cortex is a basic feature of the cortex - shared with distant ancestors.

Who coined the term plasticity?

William James (1890) in his book, ‘Principles of Psychology’

What is the critical period in relation to brain development?

post-natal period in mammals where exposure to sensory stimuli within it can determine brain development and adult behaviour.

What is the critical period for language acquisition?

Up to age 12 are brains are more susceptible to language aquisition

Experiment investigating the critical period for the primary visual cortex development (Blakemore et al)

Placed kittens in an environment from birth until end of critical period with either only vertical lines or only horizontal lines (couldn’t even see its own body due to collar) - found that kittens only brought up with vertical lines, no cells in the V1 area responded only to horizontal (did not develop orientation specificity for horizontal) = and visa versa.

What is the pathway responsible for fine touch?

The Dorsal Column Pathway for fine touch

Afferent vs Efferent Neurons

Afferent carry impulse from periphery TOWARDS central nervous system

Efferent carry impulse AWAY from central to periphery

Describe the order of the Dorsal Column Pathway in the Spinal cord and Brain

1. Primary Afferents = nerve fibres either enter above the midthoracic level or below

2. Secondary neurons = in the medulla (bottom of brain)

3. Tertiary neurons = in the primary somatosensory cortex (S1) = final destination

Study investigating plasticity of the somatosensory cortex in Primates(Merzenich et al, 1978)

First Experiment: Removed middle digit from non-human primates = found activation in the same area of cortex when one side of the second and fourth digits were stimulated = no silent zone - other areas took over.

Second Experiment: Used syndactyly procedure to join together two digits on the hand. = found that some cells on the border between the two digits in the cortex now responded to stimulation on both digits. (before they were distinct areas, after they overlapped) = induced double-digit receptive fields