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Suresh AK, Saal HP, Bensmaia SJ (2016) Edge orientation signals in tactile afferents of macaques. Journal of Neurophysiology, 116(6):2647-2655    
Mechanoreceptive afferents have recently been implicated in the extraction of a stimulus feature, namely orientation. However, afferents produce nearly deterministic responses to repeated stimulation so virtually any stimulus feature can be faithfully decoded from their responses as long as other stimulus features are held constant. Here, we examine the degree to which afferents convey information about edge orientation when other stimulus features also vary and discuss what constitutes a neural code at the sensory periphery
The orientation of edges indented into the skin has been shown to be encoded in the responses of neurons in primary somatosensory cortex in a manner that draws remarkable analogies to their counterparts in primary visual cortex. According to the classical view, orientation tuning arises from the integration of untuned input from thalamic neurons with aligned but spatially displaced receptive fields (RFs). In a recent microneurography study with human subjects, the precise temporal structure of the responses of individual mechanoreceptive afferents to scanned edges was found to carry information about their orientation. This putative mechanism could in principle contribute to or complement the classical rate-based code for orientation. In the present study, we further examine orientation information carried by mechanoreceptive afferents of Rhesus monkeys. To this end, we record the activity evoked in cutaneous mechanoreceptive afferents when edges are indented into or scanned across the skin. First, we confirm that information about the edge orientation can be extracted from the temporal patterning in afferent responses of monkeys, as is the case in humans. Second, we find that while the coarse temporal profile of the response can be predicted linearly from the layout of the RF, the fine temporal profile cannot. Finally, we show that orientation signals in tactile afferents are often highly dependent on stimulus features other than orientation, which complicates putative decoding strategies. We discuss the challenges associated with establishing a neural code at the somatosensory periphery, where afferents are exquisitely sensitive and nearly deterministic