Figure 1. Anatomical basis for the ‘one barrel–one vibrissa’ hypothesis. Each peripheral axon in the trigeminal nerve innervates one whisker follicle. Centrally the axons are aggregated in whisker-related bundles that terminate in several nuclei in the brainstem. Illustrated from top to bottom these are: principal sensory nucleus and spinal trigeminal complex subnuclei oralis, interpolaris and caudalis. Barrelettes are present in all but the subnucleus oralis. Whisker-related aggregations of afferent fibres and cell bodies are also observed in the contralateral thalamus and cerebral cortex. Inset: Cauterization of the middle row of whiskers on the day of birth (postnatal day (PND) 0) leads to an abnormal cortical barrel field.

Figure 2. Information processing in the whisker–barrel cortex. Each thalamic barreloid conveys multi-whisker information (indicated by colour mixture) to its corresponding cortical barrel where neuronal responses become focused on the barrel's principal whisker (denoted by solid colours). By means of horizontal connections linking neighbouring barrel-associated columns, single-whisker information leaving the barrel is combined in supragranular and infragranular layers, creating spatially and temporally complex receptive fields. Supragranular (layer II/III) intercolumnar connections into the central (green) barrel–column are denoted by the horizontal blue and yellow arrows; connections involving the infragranular layers are not shown. Barrels are only sparsely interconnected, indicated by the absence of connecting arrows in layer IV, although neurons in other layers do provide some inputs to them.

Figure 3. Effects of whisker trimming on cortical receptive fields. In normally reared adult animals, several days of whisker cutting (in this case, of the whisker associated with the yellow barrel in Figure 2) leads to a loss of responsiveness in the corresponding barrel and increased responsiveness to the remaining whiskers in the barrels corresponding to neighbouring, intact (nontrimmed) whiskers. Note that neurons in the latter now respond less selectively to green versus blue whiskers (compare with Figure 2), possibly as a result of strengthened intercolumnar connections in the supragranular layers. With neonatal whisker trimming followed by regrowth in adulthood of the yellow whisker, neurons in the yellow barrel also become highly responsive to the yellow whisker and to previously trimmed (e.g. green) and nontrimmed whiskers (not shown). These effects, which remain even after months of regrowth, may reflect alterations in barrel circuitry whereby neuronal receptive fields more closely mimic those of the thalamic barreloid neurons.