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
Representation In 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 Representation in Congress USA Skin surface
Map in Primary Visual Cortex 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 activation 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 Dissection of eye ep_eye.htmlhttp:// ep_eye.html