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The Journal of Neuroscience, June 14, 2006, 26(24):6473-6484; doi:10.1523/JNEUROSCI.5057-05.2006

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Behavioral/Systems/Cognitive
Receptive Field Properties of the Macaque Second Somatosensory Cortex: Representation of Orientation on Different Finger Pads

Paul J. Fitzgerald, John W. Lane, Pramodsingh H. Thakur, and Steven S. Hsiao

Zanvyl Krieger Mind/Brain Institute, Departments of Neuroscience and Biomedical Engineering, Johns Hopkins University, Baltimore, Maryland 21218

Correspondence should be addressed to Dr. Steven S. Hsiao, Zanvyl Krieger Mind/Brain Institute, Johns Hopkins University, 338 Krieger Hall, 3400 North Charles Street, Baltimore, MD 21218. Email: steven.hsiao{at}jhu.edu

Orientation tuning has been studied extensively in the visual system, but little is known about it in the somatosensory system. Here we investigate tuning in the second somatosensory (SII) region using a motorized stimulator that presented a small oriented bar to the 12 finger pads of digits 2–5 (D2–D5) of the macaque monkey. A subset (23%; n = 218) of the 928 SII region neurons [the same 928 neurons studied by Fitzgerald et al. (2004, 2006)] exhibited tuning, and most of these were tuned on one or two finger pads. All eight 22.5° separated orientations were represented as the preferred orientation of multiple neurons, although not necessarily in equal numbers. A measure of bandwidth indicated that tuning in the SII region is sharp and is similar to the tuning observed in visual cortical areas. In addition, two-dimensional Gaussians that were fit to the tuning curves had very high r² values, indicating that most tuning curves are both unimodal and symmetrical with respect to their preferred orientation. Most tuned neurons had additional untuned pads, although the responsiveness of these pads tended to be less than the responsiveness of tuned pads. Neurons with multiple tuned pads tended to have similar preferred orientations on their tuned pads, which can be interpreted as evidence for integration of information across fingers or as a form of positional invariance. Finally, comparison of the tuning properties showed that there are small but significant differences between the posterior, central, and anterior fields of the SII region.

Key words: somatosensory cortex; tactile; SII; orientation; receptive field; object recognition

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Received Nov. 28, 2005; revised April 26, 2006; accepted April 26, 2006.

Correspondence should be addressed to Dr. Steven S. Hsiao, Zanvyl Krieger Mind/Brain Institute, Johns Hopkins University, 338 Krieger Hall, 3400 North Charles Street, Baltimore, MD 21218. Email: steven.hsiao{at}jhu.edu

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