Neuronal Responses Below Firing Threshold for Subthreshold Cross-Modal Enhancement

Abstract

Multisensory integration (such as somatosensation-vision, gustation-olfaction) could occur even between subthreshold stimuli that in isolation do not reach perceptual awareness. For example, when a somatosensory (subthreshold) stimulus is delivered within a close spatiotemporal congruency, a visual (subthreshold) stimulus evokes a visual percept. Cross-modal enhancement of visual perception is maximal when the somatosensory stimulation precedes the visual one by tens of milliseconds. This rapid modulatory response would not be consistent with a top-down mechanism acting through higher-order multimodal cortical areas, but rather a direct interaction between lower-order unimodal areas. To elucidate the neuronal mechanisms of subthreshold cross-modal enhancement, we simulated a neural network model. In the model, lower unimodal (X, Y) and higher multimodal (M) networks are reciprocally connected by bottom-up and top-down axonal projections. The lower networks are laterally connected with each other. A pair of stimuli was presented to the lower networks, whose respective intensities were too weak to induce salient neuronal activity (population response) when presented alone. Neurons of the Y network were slightly depolarized below firing threshold when a cross-modal stimulus was presented alone to the X network. This allowed the Y network to make a rapid (within tens of milliseconds) population response when presented with a subsequent congruent stimulus. The reaction speed of the Y network was accelerated, provided that the top-down projections were strengthened. We suggest that a subthreshold (nonpopulation) response to a cross-modal stimulus, acting through interaction between lower (primary unisensory) areas, may be essential for a rapid suprathreshold (population) response to a congruent stimulus that follows. Top-down influences on cross-modal enhancement may be faster than expected, accelerating reaction speed to input, in which ongoing-spontaneous subthreshold excitation of lower-order unimodal cells by higher-order multimodal cells may play an active role.

A Model of the Differential Representation of Signal Novelty in the Local Field Potentials and Spiking Activity of the Ventrolateral Prefrontal Cortex

“…This unique pattern of neural responses will be sufficient for vPFC neurons to detect the repetition of the same stimulus, leading us to a hypothesize that LFP habituation may be a neural mechanism underlying novelty detection. This hypothesis is consistent with the functional role of subthreshold oscillations suggested by Hoshino (2011).…”

Dynamic faces speed up the onset of auditory cortical spiking responses during vocal detection

“…It has been reported that the frequency selectivity of auditory response latencies arises from a combination of sharply tuned intracortical excitation at preferred frequencies and broad inhibition at nonpreferred frequencies (the E-I network) (53). In our task, the vocalization is heard 85 ms after the visual input (facial motion) is seen; thus any synaptic input resulting from the auditory component would have a lag of 20-40 ms and would arrive when changes in the auditory cortical network induced by mouth motion already have occurred (54). In this scheme, the resulting auditory cortical response would combine with the E-I network reorganized by the visual input to speed up spiking activity.…”

GABA diffusion across neuronal columns for efficient sensory tuning

“…r G AB A ext (n; t) is the fraction of extrasynaptic GABAa receptors in the open state provoked by ambient GABA in column n. δ P , δ F and δ L denote the amounts of extrasynaptic GABAa receptors on P, F and L cells, respectively. For model parameters and their values, see Table 1 and our previous studies (Hoshino 2007a(Hoshino , b, 2008b(Hoshino , 2011aTotoki et al 2010;Miyamoto et al 2012). The receptor dynamics is defined in Appendix 2.…”