Interaction between afferent input from fingers in human somatosensory cortex

Abstract

We recorded somatosensory evoked magnetic fields from eight healthy subjects with a 122-channel whole-scalp SQUID magnetometer. The stimulus sequence consisted of 'standard' stimuli (85%) delivered to palmar side of the left thumb with an interstimulus interval of 0.6 s and of 'deviants' (15%), randomly interspersed among the standards, to little finger, and vice versa. Both stimuli activated four source areas: the contralateral primary somatosensory cortex (SI), the contra-and ipsilateral secondary somatosensory cortices (SII), and the contralateral posterior parietal cortex (PPC). The short-latency (20-40 ms) responses originated in the SI cortex, whereas long-latency responses arose from all 4 areas. At SII and PPC, the deviant stimuli elicited larger responses when presented alone, without intervening standards, than among standards. This implies interaction between afferent impulses from the two fingers and/or partly intermingled cortical representations. Our findings show, in agreement with animal data, different excitatory/inhibitory balance in the various somatosensory areas.

Context effects in haptic perception of roughness

“…This spatial contextual effect supports findings from neurophysiological studies, which show that integration of information received from different fingers occurs along the processing pathway (Biermann et al 1998;Forss et al 1995;Gandevia et al 1983;Iwamura et al 1983). In addition, the present results show that this integration effect is also visible when natural stimuli are used and explored actively.…”

“…Researchers found multi-finger or wide-field receptive fields, which cover more than one finger, in area 1 neurons of the primary somatosensory cortex as well as in the medial part of the cortical finger region (Biermann et al 1998;Forss et al 1995;Iwamura et al 1983). In general, these studies found an inhibition effect of the cerebral signal when multiple fingers were stimulated by mechanical stimulations of high-level intensities.…”

“…The spatial contextual effect in haptic roughness perception could be explained by a comparable mechanism in which signals from the index finger and from the inducer middle finger both fall within the centre of the same receptive field, producing the assimilation effect. These receptive fields could be the multi-finger receptive fields that were found at the level of the somatosensory cortex where integration of information received from different fingers occurs (Biermann et al 1998;Fitzgerald et al 2006;Forss et al 1995;Iwamura et al 1983;Sinclair and Burton 1993).…”

Transient and steady-state responses to mechanical stimulation of different fingers reveal interactions based on lateral inhibition

“…This differs from the effects that would be predicted if lateral inhibition would be responsible for the interaction effect. As mentioned before, lateral inhibition has a much longer timeframe of 150 ms (Forss et al 1995). For the optimal SSSEP stimulation frequencies the time between subsequent tactile stimuli is within this period.…”

“…Lateral inhibition is mediated by a different mechanism involving inhibitory processes at a much longer time frame. The observation that the N20m response to paired stimulation is reduced for inter-stimulus intervals (ISI) shorter than 120-150 ms has been attributed to infield inhibition (Gardner 1984) or lateral inhibition (Forss et al 1995).…”