Exp Brain Res (2004) 155: 485–499 DOI 10.1007/s00221-003-1755-3 RESEARCH ARTICLES S. Shuichi Haupt . Friederike Spengler . Robert Husemann . Hubert R. Dinse Receptive field scatter, topography and map variability in different layers of the hindpaw representation of rat somatosensory cortex Received: 4 April 2003 / Accepted: 16 October 2003 / Published online: 27 January 2004 # Springer-Verlag 2004 Abstract We recorded neurons extracellularly in layers II/ III, IV, and V of the hindpaw representation of primary somatosensory cortex in anesthetized rats and studied laminar features of receptive fields (RFs) and representa- tional maps. On average, RFs were smallest in layer IV and largest in layer V; however, for individual penetrations we found substantial deviations from this rule. Within the hindpaw representation, a distinct rostrocaudal gradient of RF size was present in all layers. While layer V RFs were generally largest independent of this gradient, layer IV RFs recorded caudally representing the proximal portions of the paw were larger than layer II/III RFs recorded rostrally representing the digits. The individual scatter of the locations of RFs across laminar groups was in the range of several millimeters, corresponding to about 25% of the average RF diameter. The cutaneous representations of the hindpaw in extragranular layers were confined to the areal extent defined by responsive sites in layer IV. Comparison between RFs determined quantitatively and by handplotting showed a reliable correspondence. Repeated measurements of RFs revealed spontaneous fluctuations of RF size of no more than 5% of the initial condition over an observation period of several hours. The topography and variability of cortical maps of the hindpaw representation were studied with a quantitative interpola- tion method taking into account the geometric centers of RFs and the corresponding cortical recording sites. On average, the overall topography in terms of preservation of neighborhood relations was present in all layers, although some individual maps showed severe distortions of topography. Factors contributing to map variability were overall position of the representation on the cortical surface, internal topography and spatial extent. Interindi- vidual variability of map layout was always highest in the digit representations. Local topographic orderliness was lowest in layer V, but comparable in layers II/III and IV. Within layer IV, the lowest orderliness was observed in the digit representations. Our data emphasize a substantial variability of RF size, overlap and position across layers and within layers. At the level of representational maps, we found a similar degree of variability that often co- varied across layers, with little evidence for significant layer specificity. Laminar differences are likely to arise from the specific input-output pattern, layer-specific cell types and the connectivity between different layers. Our findings emphasizing similarities in the variability across layers support the notion of tightly coupled columnar interactions between different layers. Keywords Cortical layers . Individual variability . Topography . Quantitative reconstruction of maps . Orderliness Introduction A striking structural principle of neocortical organization is its homogeneity in the tangential plane, and inhomo- geneity perpendicular to it. The appearance of laminated patterns is due to the distribution of specific cell types (Peters and Jones 1984). Another anisotropy in the vertical comes from the fact that different layers receive their input from different principal sources, and in turn distribute information to different targets (Jones and Powell 1970; S. S. Haupt . F. Spengler . R. Husemann . H. R. Dinse (*) Institute for Neuroinformatics, Department of Theoretical Biology, Ruhr University Bochum, ND 04, 44780 Bochum, Germany e-mail: hubert.dinse@neuroinformatik.ruhr-uni-bochum.de Tel.: +49-234-3225565 Fax: +49-234-3214209 S. S. Haupt Department of Mechano-Informatics, Faculty of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113 Tokyo, Japan Present address: S. S. Haupt Institute for Ecology, Faculty VII, Technical University Berlin FR 1-1, Franklinstr. 28/29, 10587 Berlin, Germany