Perception

• Sensory Maps (cortical maps)

• Vision • Visual Attention

• Other sensory systems– Hearing (Audition)– Touch (Somatosensory)– Taste & Smell

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

RepresentationIn cortex

Based on sq footage (skin), New Jersey is overrepresented Wyoming is underrepresented

Based on # of people (nerve endings, sensory acuity) no difference bt NJ & WY

Representationin Congress

USA

Skin surface

Map in Primary Visual Cortex

1

2

3

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

1 2 3

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?

Cortical Maps•

• Learning•Violin players (pantev)

Commonalities among Perceptual Systems• Sensory receptors do transduction

– (ex: mechanical --> electrical)

• Sensory receptors elicit graded potentials – response amplitude is equivalent to stimulus intensity

Link to other lectures

• http://www.pc.rhul.ac.uk/courses/Lectures/PS1061/L6/PS1061_6.htm

• Dissection of eye

• http://www.geocities.com/centennial3d/sheep_eye.html