In the back of your mind: Cortical mapping of tactile and proprioceptive paraspinal afferent inputs

Persistent pain alters brain-body representations, highlighting their potential pathological significance. In chronic low back pain (LBP), sparse evidence points towards a shift of the cortical representation of sensory afferents of the back. However, systematic investigations of the cortical representation of tactile and proprioceptive paraspinal afferents along the thoracolumbar axis are lacking. Detailed cortical maps of paraspinal afferent input might be crucial to further explore potential relationships between brain changes and the development and maintenance of chronic LBP. We therefore validated a novel and functional magnetic resonance imaging- (fMRI-)compatible method of mapping cortical representations of tactile and proprioceptive afferents of the back, using pneumatic vibrotactile stimulation ("pneuVID") at varying frequencies and paraspinal locations, in conjunction with high-resolution fMRI. We hypothesised that: (i) high (80 Hz) frequency stimulation would lead to increased postural sway compared to low (20 Hz) stimulation, due to differential evoked mechanoreceptor contributions to postural control (proprioceptive vs tactile); and (ii) that high (80 Hz) versus low (20 Hz) frequency stimulation would be associated with neuronal activity in distinct primary somatosensory (S1) and motor (M1) cortical targets of tactile and proprioceptive afferents (N=15, healthy volunteers). Additionally, we expected neural representations to vary spatially along the thoracolumbar axis. We found significant differences between neural representations of low and high frequency stimulation and between representations of thoracic and lumbar paraspinal locations, in several bilateral sensorimotor cortical regions. Proprioceptive (80 Hz) stimulation preferentially activated sub-regions S1 3a and M1 4p, while tactile (20 Hz) stimulation was more encoded in S1 3b and M1 4a. Moreover, in S1, lower back proprioceptive stimulation activated dorsal-posterior representations, compared to ventral-anterior representations activated by upper back stimulation. As per our hypotheses, we found distinct sensorimotor cortical tactile and proprioceptive representations, with the latter displaying clear topographic differences between the upper and lower back. This thus represents the first behavioural and neurobiological validation of the novel pneuVID method for stimulating muscle spindles and mapping cortical representations of paraspinal afferents. Future investigations of detailed cortical maps will be of major importance in elucidating the role of cortical reorganization in the pathophysiology of chronic LBP.

TMS Motor Mapping Methodology and Reliability: A Structured Review

Motor cortical representation can be probed non-invasively using a transcranial magnetic stimulation (TMS) technique known as motor mapping. The mapping technique can influence features of the maps because of several controllable elements. Here we review the literature on six key motor mapping parameters, as well as their influence on outcome measures and discuss factors impacting their selection. 132 of 1,587 distinct records were examined in detail and synthesized to form the basis of our review. A summary of mapping parameters, their impact on outcome measures and feasibility considerations are reported to support the design and interpretation of TMS mapping studies.

A new protocol for multiple muscle mapping using nTMS

Background: Single-pulse transcranial magnetic stimulation is a safe and non-invasive tool for investigating cortical representation of muscles in the primary motor cortex. While non-navigated TMS has been successfully applied to simultaneously induce motor-evoked potentials (MEPs) in multiple muscles, a more rigorous assessment of the corresponding cortical representation can greatly benefit from navigated transcranial magnetic stimulation (nTMS). Objective: We designed a protocol to map the entire precentral gyrus using neural navigation while recording responses of eight muscles simultaneously. Here, we evaluated the feasibility, validity, and reliability of this protocol. Method: Twenty participants underwent conventional (i.e., muscle-based, grid-constrained) and gyrus-based nTMS mapping. For both protocols, we investigated three different stimulation intensities during two consecutive sessions. Results: The gyrus-based nTMS mapping was received well by all participants and was less time consuming than the grid-constrained standard. On average, MEP amplitudes, latencies, and centre-of-gravity and size of the active areas largely agreed across protocols supporting validity. Intraclass coefficients between sessions unscored the reliability of our protocol. Conclusion: We designed an nTMS protocol for the simultaneous mapping multiple muscles on the cortex. The protocol takes only about ten minutes per participant when including as many as eight muscles. Our assessments revealed that the cortical representation of multiple muscles can be determined with high validity and reliability.

Neural Correlates of Attentional Modulation of Prepulse Inhibition

Prepulse inhibition (PPI) refers to the suppression of the startle reflex when the intense startling stimulus is shortly (20–500 ms) preceded by a weak non-startling stimulus (prepulse). Although the main neural correlates of PPI lie in the brainstem, previous research has revealed that PPI can be top-down modulated by attention. However, in the previous attend-to-prepulse PPI paradigm, only continuous prepulse but not discrete prepulse (20 ms) could elicit attentional modulation of PPI. Also, the relationship between the attentional enhancement of PPI and the changes in early cortical representations of prepulse signals is unclear. This study develops a novel attend-to-prepulse PPI task, when the discrete prepulse is set at 150 ms at a lead interval of 270 ms, and reveals that the PPI with attended prepulse is larger than the PPI with ignored prepulse. In addition, the early cortical representations (N1/P2 complex) of the prepulse show dissociation between the attended and ignored prepulse. N1 component is enhanced by directed attention, and the attentional increase of the N1 component is positively correlated with the attentional enhancement of PPI, whereas the P2 component is not affected by attentional modulation. Thus, directed attention to the prepulse can enhance both PPI and the early cortical representation of the prepulse signal (N1).

Interaction of surface pattern and contour shape in the tilt after effects evoked by symmetry

Integration of multiple properties of an object is a fundamental function of the visual cortex in object recognition. For instance, surface patterns and contour shapes are thought to be crucial characteristics that jointly contribute to recognition. However, the mechanisms of integration and corresponding cortical representations have not been fully clarified. We investigated the integration of surfaces and shapes by examining the tilt after effects (TAEs) evoked by the symmetry of patterns and contours. As symmetry in both pattern and contour evokes TAEs, we can directly measure the interaction between the two. The measured TAEs exhibited mutual transfer between the symmetry of the pattern (SP) and that of the contour shape (SS), i.e., adaptation by SP (SS) evoked TAEs when tested by SS (SP), suggesting the existence of an integrated representation. Next, we examined the interaction between SP and SS when both were simultaneously presented in adaptation. Congruent adaptors wherein their symmetry axes aligned evoked compressive interaction, whereas incongruent adaptors wherein the axes of SP and SS tilted to the opposite directions evoked subtractive interaction. These results suggest the existence of a cortical representation that integrates the properties of the surface and shape with suppressive interactions, which can provide crucial insights into the formation of object representation as well as the integration of visual information in the cortex.

Aversive Conditioning of Spatial Position Sharpens Neural Population-level Tuning in Visual Cortex and Selectively Reduces Alpha-band Activity

Processing capabilities for many low-level visual features are experientially malleable, aiding sighted organisms in adapting to dynamic environments. Explicit instructions to attend a specific visual field location influence retinotopic visuocortical activity, amplifying responses to stimuli appearing at cued spatial positions. It remains undetermined, however, both how such prioritization affects surrounding non-prioritized locations, and if a given retinotopic spatial position can attain enhanced cortical representation through experience rather than instruction. This work examined visuocortical response changes as human observers learned, through differential classical conditioning, to associate specific on-screen locations with aversive outcomes. Using dense-array EEG and pupillometry, we tested the pre-registered hypotheses of either sharpening or generalization around an aversively associated location following a single conditioning session. Specifically, competing hypotheses tested if mean response changes would take the form of a gaussian (generalization) or difference-of-gaussian (sharpening) distribution over spatial positions, peaking at the viewing location paired with a noxious noise. Occipital 15 Hz steady-state visual evoked potential (ssVEP) responses were selectively heightened when viewing aversively paired locations and displayed a non-linear, difference-of-gaussian profile across neighboring locations, consistent with suppressive surround modulation of non-prioritized positions. Measures of alpha band (8 – 12.8 Hz) activity and pupil diameter also exhibited selectively heightened responses to noise-paired locations but did not evince any difference across the non-paired locations. These results indicate that visuocortical spatial representations are sharpened in response to location-specific aversive conditioning, while top-down influences indexed by alpha power reduction exhibit all-or-none modulation.Significance StatementIt is increasingly recognized that early visual cortex is not a static processor of physical features, but is instead constantly shaped by perceptual experience. It remains unclear, however, to what extent the cortical representation of many fundamental features, including visual field location, is malleable by experience. Using EEG and an aversive classical conditioning paradigm, we observed sharpening of visuocortical responses to stimuli appearing at aversively associated locations along with location-selective facilitation of response systems indexed by pupil diameter and EEG alpha power. These findings highlight the experience-dependent flexibility of retinotopic spatial representations in visual cortex, opening avenues towards novel treatment targets in disorders of attention and spatial cognition.