Perception

• Sensory Maps (cortical maps)

• Plasticity– Phantom limb– Blind & braille

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Cortical Sensory Maps: A correspondence between an external dimension and an internal representation.

Internal: Somatosensory Cortex External: Skin surface

- there is a topographic organization - Some skin areas are overrepresented in cortex

“Whoa! That was a good one! Try it, Hobbs – just poke his brain right where my finger is”

Cortex (internal)Skin (external)

2-point discrimination

Map in Primary Visual Cortex

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Visual Field

1. Central vision (fovea)

3. Peripheral vision

2. Parafoveal vision

Based on visual field, - fovea is overrepresented in visual cortex- peripheral vision (in blue) is underrepresented

The fovea has larger visual acuity: It is easier to discriminate two nearby point in the fovea than in the periphery

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Cortical Maps: Remapping due to lesion (Phantom limbs)

Braille reading activates ‘visual’ areas in blind subjects

Cortical Maps: Remapping due to long-term sensory deprivation (blindness)

But is this activation causally effective, or epiphenomenal?

2. ‘Lesioning’ occipital cortex with TMS disrupts tactile discrimination, but only in the blind

1. A congenitally blind woman suffered a bilateral occipital strokes & became alexic (couldn’t read Braille anymore)

Visual cortex is necessary for Braille reading in the blind

Cortical Maps: Remapping due to SHORT-TERM sensory deprivation (blindfolded for 5 days)

Scientific American Frontiers: Changing Your Mind, 2000 Alvaro Pascual-Leone

impact on performance

(reading Braille)

impact on cortical activation

(touch finger with a brush)

Day 1:Somato-sensory activation

Day 5:Occipital activatio

n too

• Subject blinfolded for 5 days

• Discrimination task: stimulate finger with pairs of Braille letters

• TMS in visual cortex for 10 mins: decreases accuracy

• Take off blindfold and allow vision

• TMS in visual cortex after a few hours of vision: does not affect performance!

Is occipital activation necessary for performance?

• hearing: how high a frequency can you hear?

• http://www.world-voices.com/software/nchtone.html