The neural representations of the whiskers (Fig. 1.3A) and the neuronal computations associ-ated with the processing of tactile stimuli provided by them have been one of the central elds of study of sensory neuroscience, ever since Woolsey and Van der Loos (1970) described the topographical representation of the whisker pad in primary somatosensory cortex (S1). This representation is known as the barrel cortex (BC), and is comprised of a eld of topographi-cally organized cortical columns, where each represents one of the macrovibrissae in the ve whisker rows and six to seven whisker columns (Fig. 1.3B; also see Brecht et al., 1997). These cortical columns, which are characterized by prominent cytochrome oxidase staining in layer 4 (L4), are separated by septal compartments with distinct functions. Thus, the whisker/barrel cortex system is 'quantized' into single whiskers and their corresponding cortical columns, and involves active sensory strategies. This makes it a particularly valuable system for the study of sensory processing and sensorimotor integration on both behavioral and neural level.
The latter includes the trigeminal nuclei, where the whisker aerents are rst processed, the thalamic relay stages, and, most importantly, the BC as the primary somatosensory area of the neocortex. In addition, studies on sensorimotor integration also encompass primary mo-tor cortex (M1), as well as other cortical and subcortical areas involved in the generation of whisking patterns and their control.
The ow of sensory information to the BC, the computations performed within it, and the projections to and interactions with other areas have been subject to detailed study, and have been reviewed in depth elsewhere (Lübke and Feldmeyer, 2007; Diamond et al., 2008; Bosman et al., 2011; Feldmeyer et al., 2013). In a short and somewhat simplied view, summarized in Fig. 1.4, the information reaches the BC through three channels: the lemniscal, the extralemniscal, and the paralemniscal pathways. The lemniscal is the most canonical one, which leads through the principal trigeminal nucleus and the ventral posterior medial nucleus of the thalamus to BC, where it targets mostly L4 neurons in barrels, but also L5B and L6 (Fig. 1.3C, Fig. 1.4). This pathway provides fast inputs with small receptive elds (RFs). The extralemniscal and paralemniscal pathways both originate in the spinal trigeminal nucleus, although in dierent substructures, and both provide slower inputs with larger RFs. They are also subject to stronger inuence by top-down modulation related to whisker movements.
The paralemniscal pathway gets relayed in the posterior medial thalamic nucleus, and targets mostly L5A, as well as L2/3 and L1 (Fig. 1.3C, Fig. 1.4). The extralemniscal pathway
Figure 1.3: Overview over the whisker system of rats. A, Information from whisker touch is relayed through the brainstem (1) and the thalamus (2) to the barrel cortex (3). B, Whiskers are arranged in an orderly array, and there is a topographical mapping of whiskers onto barrels in the barrel cortex. C, The main two processing pathways are the lemniscal pathway (red) which routes information to the barrel cortex through the ventral posterior medial nucleus of the thalamus (VPM), and the paralemniscal pathway (green), leading through the posterior medial nucleus of the thalamus (POm). D, Within cortex, S1 is strongly and reciprocally connected to primary motor cortex (M1) and secondary somatosensory cortex (S2), as well as the contralateral barrel cortex. Reproduced with permission from Petersen (2007).
pathways exist, which are involved in multi-whisker processing, and preferentially target the septa. This diversity of input streams and compartments is an important limitation to the interpretation of extracellular recordings, which almost unavoidably pool barrel and septal units, and are insensitive to cortical microcircuitry, which might be spatially intermingled and still computationally separated.
Within BC, the information owing through the lemniscal pathway is relayed by L4 neu-rons to neuneu-rons in L2/3, which in turn excite L5 neuneu-rons through their apical dendrites (Fig.
1.4). Partly already in L2/3, but most strongly in L5A, the lemniscal inputs then interact with paralemniscal inputs reaching the cortex at longer latencies, and carrying information regarding a broader set of whiskers. In addition to the vertical ow of information between layers, there are strong horizontal, intralaminar connections, which are mostly local in L4, but are connecting dierent barrels in L2/3 and L5B. These tactile information ows are con-stantly shaped by a multitude of top-down inputs from M1, secondary somatosensory cortex (S2), and secondary thalamic nuclei, amongst others. There are also inputs from the con-tralateral BC, which convey information related to the ipsilateral whisker eld. This view of
VPM POm
striatum brainstem
M1 S2
thalamus L6
L5B L5A
L4 L2/3
Figure 1.4: Schematic overview over the most important projections to and from, as well as connections within BC. Lemniscal inputs are marked in red, paralemniscal in green, projec-tions to other areas in blue and connecprojec-tions within BC in violet. Thick-tufted and slender-tufted L5 neurons are more prominent in L5B and L5A, respectively, as shown here. However, cell type and layer do not map completely in this case. Layer thicknesses and cell sizes do not reect actual anatomical size relationships. L1 is not shown. VPM, ventral posterior medial nucleus of the thalamus; POm, posterior medial nucleus of the thalamus.
connections to and within BC is again simplied and does not take into account interactions between barrels and septa, which receive extralemniscal inputs, as well as other local and long-range inputs.
The predominant cortico-cortical long-range projections from BC come from L2/3, tar-geting S2 and M1 in particular (Fig. 1.3D). Some authors (Bureau et al., 2006) dierentiate within L2/3 a lemniscal, touch-representing circuit projecting to S2 and a paralemniscal, whisking-representing circuit projecting to M1. As in other primary sensory cortices, L5 is the main source of projections to subcortical structures, which include the striatum and brain-stem nuclei, in particular the trigeminal nucleus. While it is known that L6 is involved in feedback loops with the thalamus, its connectivity patterns are as yet less clear.
It should be mentioned that BC has become the area of choice for many questions which are only indirectly related to sensorimotor processing. This includes plasticity (Wilbrecht et al., 2010), development (Matsui et al., 2013), glial functioning (Houades et al., 2008), and neuromodulatory mechanisms (Constantinople and Bruno, 2011). There have also been numerous studies of BC development and function in disease models, including Rett syndrome (Moroto et al., 2013), fragile X-syndrome (Harlow et al., 2010), Alzheimer's Disease (Beker et al., 2012) and, of particular relevance to the study of social representations in BC, autism (Smith et al., 2011). These diverse approaches and the substantial body of information on cortical connectivity and function, molecular mechanisms, and development, might provide a basis to use social touch and BC activity as a multi-faceted and at the same time dissectable model for structure-function relationships in models of autism and other disorders.