Figure 1: Possible mechanisms of large-scale brain reorganization and the experimental design.

(a) Location of area 3b in the posterior bank of the central sulcus is shown on an outline diagram of the macaque brain. Central sulcus (CS) is drawn partially opened. D, dorsal; IPS, intraparietal sulcus; LS, lateral sulcus; R, rostral. (b) Normal somatotopy in area 3b showing representation of the hand (blue), chin (orange) and other parts of the face (red) on en face view of the posterior bank of CS reconstructed from histological sections of the brain. Dots mark locations of the recording sites where receptive fields of the neurons were mapped (see ‘Methods’). Scale bar, 1 mm. (c) In normal monkeys inputs from the chin ascend to area 3b via the trigeminal nucleus of the brainstem (Trig) and the ventroposterior medial nucleus (VPM) of the thalamus. Inputs from the hand ascend via cuneate nucleus (Cu) and the ventroposterior lateral nucleus (VPL). (d) In monkeys with chronic lesions of the dorsal columns, neurons in the deafferented hand region of area 3b¹, VPL² and cuneate nucleus (this study) respond to touch on the chin. Possible mechanisms include growth between the normal chin inputs and the deafferented hand regions at the cortical, thalamic or brainstem levels (black arrows)¹³. (e) Inactivation of the normal chin region in area 3b would silence the expanded chin representation if cortical growth is the primary cause for area 3b reorganization. (f) If area 3b reorganization is due to upstream effects of growth at the medullary level, inactivation of the reorganized cuneate nucleus would render silent the expanded chin representation in area 3b.