• Major sensory and motor systems are topographically mapped (see retino-, tono-, and somatotopic maps in figs 4.2/3-5). Locations in a three dimensional sensory world are represented in the central nervous system in such a way that neighboring locations in the sensory world  are represented in neighboring neurons in the nervous system. These topographical maps constitute an inner model of the body.
• Topography might actually "minimize" the amount of "wires" necessary to connect neurons and areas in the brain dealing with the same object at the same time.
• The maps are "use dependent," and reorganize
• in the long range (see figs 4.2/3 and 4.2/6, cortical magnification of important parts)
• in the shorter temporal range as well (individual practice can change them, see fig 4.2/7)

	fig 4.2/2 retinotopic re-presentation (ref 6)	fig 4.2/3 retinotopic map (monkey) (ref 6, 426, 29-7) Notice that the central portion of the retina (dark disc in the upper part of the figure) is overrepresented in the cortex. That is, more neurons deal with the central retina than with the peripheral in the cortex. This distortion is known as cortical magnification. Recall figs I.2/5-6: receptor density is highest, convergence is lowest, RF sizes are the smallest, and acuity is the best in the central retina (also called fovea). This is the region (the fovea) that is overrepresented in the cortex.  (Roedick, 1998, The first steps in seeing, p356)	fig 4.2/4 tonotopic map (cat) (ref 3, 357, 8.32)
fig 4.2/5 somatotopic map (human) (ref 5, 182, 1)	fig 4.2/6 somatotopic maps: use dependence (ref 6) Another example of cortical magnification and use depence of maps. The highest acuity will be  in those body regions which are magnified in the primary somatosensory cortices.	fig 4.2/7 use dependent receptive fields somatosensory cortex (ref 11, 227, 9.4) Reorganization induced by training a monkey in a behavioral task engaging a small skin locus (Recanzone, Merzenich et al.). The cortical representation of the trained skin portion (black circle) showed a dramatic increase; the number of RFs increased; RF overlap increased. Acuity also improved.	fig 4.2/8 referred (phantom) pain:  somatotopy (ref 5, 186, 3) Representation of the amputated hand can be found on the face (Ramachandran). That is, stimulation of the face can elicit sensations felt in the phantom hand. This observation indicates that phantom pain might be related to the proximity of maps of different body parts within the somatosensory cortex. Following input removal (amputation), neighboring cortical regions invade the silent areas, and transfer their input there.

KEYTERMS
    lecture: topographic organization, retino-, tono-, somatotopic maps,  use dependency, cortical reorganization, cortical magnification.
    book, Chapter 4: cortical magnification, retinotopic organization, scotomas, visual field, blindsight.

STUDY QUESTIONS:

• What do the somatotopic maps of different creatures tell us about their "life styles"?
• In what sense are retino-, tono-, and somatotopic maps  similar?
• How are acuity and cortical magnification related?
• Why is it good to have topography?

Multiple Choice Questions:

• Topographic cortical organization means that neighboring locations in the sensory world are represented in neighboring neurons in the nervous system. a. True. b. False.
• In the case of touch, cortical magnification refers to a. larger number of somatosensory cortical cells compared to mechanoreceptors; b. larger cortical representation of larger body areas such as the back; c. larger cortical representation of body parts with better tactile sensitivity, such as the fingers; d. the increase in size of tactile cortical receptive fields.
• Patches of blindness within a person's visual field are termed: a) visual voids, b) visual windows, c) scotomas, d) retinal blanks.