Intracortical Pathways Mediate Nonlinear Fast Oscillation (>200 Hz) Interactions Within Rat Barrel Cortex

Spike-wave discharges in Sprague-Dawley rats reflect precise intra- and interhemispheric synchronization of somatosensory cortex

Spike-wave discharges (SWDs) are among the most prominent electrical signals recordable from the rat cerebrum. Increased by inbreeding, SWDs have served as an animal model of human genetic absence seizures. Yet, SWDs are ubiquitous in inbred and outbred rats, suggesting they reflect normal brain function. We hypothesized that SWDs represent oscillatory neural ensemble activity underlying sensory encoding. To test this hypothesis, we simultaneously mapped SWDs from wide areas (8 × 8 mm) of both hemispheres in anesthetized rats, using 256-electrode epicortical arrays that covered primary and secondary somatosensory, auditory and visual cortex bilaterally. We also recorded the laminar pattern of SWDs with linear microelectrode arrays. We compared the spatial and temporal organization of SWDs to somatosensory-evoked potentials (SEPs), as well as auditory- and visual-evoked potentials (AEPs and VEPs) to examine similarities and/or differences between sensory-evoked and spontaneous oscillations in the same animals. We discovered that SWDs are confined to the facial representation of primary and secondary somatosensory cortex (SI and SII, respectively), areas that are preferentially engaged during environmental exploration in the rat. Furthermore, these oscillations exhibit highly synchronized bilateral traveling waves in SI and SII, simultaneously forming closely matched spread patterns in both hemispheres. We propose that SWDs could reflect a previously unappreciated capacity for rat somatosensory cortex to perform precise spatial and temporal analysis of rapidly changing sensory input at the level of large neural ensembles synchronized both within and between the cerebral hemispheres.NEW & NOTEWORTHY We simultaneously mapped electrocortical SWDs from both cerebral hemispheres of Sprague-Dawley rats and discovered that they reflect systematic activation of the facial representation of somatosensory cortex. SWDs form mirror spatiotemporal patterns in both hemispheres that are precisely aligned in both space and time. Our data suggest that SWDs may reflect a substrate by which large neural ensembles perform precise spatiotemporal processing of rapidly changing somatosensory input.

High frequency oscillations in the intact brain

High frequency oscillations (HFOs) constitute a novel trend in neurophysiology that is fascinating neuroscientists in general, and epileptologists in particular. But what are HFOs? What is the frequency range of HFOs? Are there different types of HFOs, physiological and pathological? How are HFOs generated? Can HFOs represent temporal codes for cognitive processes? These questions are pressing and this symposium volume attempts to give constructive answers. As a prelude to this exciting discussion, we summarize the physiological high frequency patterns in the intact brain, concentrating mainly on hippocampal patterns, where the mechanisms of high frequency oscillations are perhaps best understood.

High-frequency oscillations and other electrophysiological biomarkers of epilepsy: underlying mechanisms

In the normal mammalian brain, neuronal synchrony occurs on a spatial scale of submillimeters to centimeters and temporal scale of submilliseconds to seconds that is reflected in the occurrence of high-frequency oscillations, physiological sharp waves and slow wave sleep oscillations referred to as Up-Down states. In the epileptic brain, the well-studied pathologic counterparts to these physiological events are pathological high-frequency oscillations and interictal spikes that could be electrophysiological biomarkers of epilepsy. Establishing these abnormal events as biomarkers of epilepsy will largely depend on a better understanding of the mechanisms underlying their generation, which will not only help distinguish pathological from physiological events, but will also determine what roles these pathological events play in epileptogenesis and epileptogenicity. This article focuses on the properties and neuronal mechanisms supporting the generation of high-frequency oscillations and interictal spikes, and introduces a new phenomenon called Up-spikes.

Gap junction blockade eliminates supralinear summation of fast (> 200 Hz) oscillatory components during sensory integration in the rat barrel cortex

The vibrissa-barrel system of rodents has become one of the dominant models for studying sensory information processing. Fast oscillations (>200 Hz) have been shown to play an important role in cortical integration of inputs from several whiskers. The mechanism subserving such integration remains, however, unknown. To address this issue, we examined the influence of the gap junction blocker (carbenoxolone, CBX) topically applied on the cortical surface on the high frequency component evoked by multiple-whisker stimulation. The magnitude of the fast oscillatory response to simultaneous stimulation of three whiskers was shown to be higher compared to its linear prediction (defined as the sum of corresponding single whisker responses). Application of CBX eliminated this supra-linear enhancement of fast oscillations. These results indicate that gap junctions are involved in the synchronization of cortical high frequency oscillations and integration of multiple whisker responses.

Large-scale organization of rat sensorimotor cortex based on a motif of large activation spreads

Parcellation according to function (e.g., visual, somatosensory, auditory, motor) is considered a fundamental property of sensorimotor cortical organization, traditionally defined from cytoarchitectonics and mapping studies relying on peak evoked neuronal activity. In the adult rat, stimulation of single whiskers evokes peak activity at topographically appropriate locations within somatosensory cortex and provides an example of cortical functional specificity. Here, we show that single whisker stimulation also evokes symmetrical areas of suprathreshold and subthreshold neuronal activation that spread extensively away from peak activity, effectively ignoring cortical borders by spilling deeply into multiple cortical territories of different modalities (auditory, visual and motor), where they were blocked by localized neuronal activity blocker injections and thus ruled out as possibly caused by "volume conductance." These symmetrical activity spreads were supported by underlying border-crossing, long-range horizontal connections as confirmed with transection experiments and injections of anterograde neuronal tracer experiments. We found such large evoked activation spreads and their underlying connections regardless of whisker identity, cortical layer, or axis of recorded responses, thereby revealing a large scale nonspecific organization of sensorimotor cortex based on a motif of large symmetrical activation spreads. Because the large activation spreads and their underlying horizontal connections ignore anatomical borders between cortical modalities, sensorimotor cortex could therefore be viewed as a continuous entity rather than a collection of discrete, delineated unimodal regions, an organization that could coexist with established specificity of cortical organization and that could serve as a substrate for associative learning, direct multimodal integration and recovery of function after injury.

Representation of moving wavefronts of whisker deflection in rat somatosensory cortex

Rats rhythmically sweep their whiskers over object features, generating sequential deflections of whisker arcs. Such moving wavefronts of whisker deflection are likely to be fundamental elements of natural somatosensory input. To determine how moving wavefronts are represented in somatosensory cortex (S1), we measured single- and multiunit neural responses in S1 of anesthetized rats to moving wavefronts applied through a piezoelectric whisker deflector array. Wavefronts consisted of sequential deflections of individual whisker arcs, which moved progressively across the whisker array. Starting position (starting arc), direction, and velocity of wavefronts were varied. Neurons responded strongly only when wavefront starting position included their principal whisker (PW). When wavefronts started at neighboring positions and swept through the PW, responses to the PW arc were suppressed by <or=95%, and responses over the entire wavefront duration were suppressed by <or=60% compared with wavefronts that initiated with the PW. Suppression occurred with interarc deflection delays of >or=5 ms, was maximal at 20 ms, and recovered within 100-200 ms. Suppression of PW arc responses during wavefronts was largely independent of wavefront direction. However, layer 2/3 neurons showed direction selectivity for responses to the entire wavefront (the entire sequence of SW and PW arc deflection). Wavefront direction selectivity was correlated with receptive field somatotopy and reflected differential responses to the specific SWs that were deflected first in a wavefront. These results indicate that suppressive interwhisker interactions shape responses to wavefronts, resulting in increased salience of wavefront starting position, and, in some neurons, preference for wavefront direction.

Two-Dimensional Coincidence Detection in the Vibrissa/Barrel Field

Coincidence detection in visual and auditory cortex may also be critical for feature analysis in somatosensory cortex. We examined its role in the rat posteromedial barrel subfield (PMBSF) using high-resolution arrays of epipial electrodes. Five vibrissae, forming an arc, row, or diagonal, were simultaneously or asynchronously stimulated to simulate contact with a straight edge of different angles at natural whisking velocities. Results indicated supralinear responses for both slow-wave and fast oscillations (FOs, about 350 Hz) at intervibrissa delays <2 ms. FO represented the earliest and most precisely tuned response to coincident vibrissa displacement. There was little difference in the spatiotemporal pattern of slow-wave or FO responses in the row, arc, or diagonal. This equivalence of function suggests that the PMBSF may be capable of working as a two-dimensional integrative array, processing spatial features based on coincidence detection despite the direction that the vibrissae pass across an object.

Active sensation: insights from the rodent vibrissa sensorimotor system

Rats sweep their vibrissae through space to locate objects in their immediate environment. In essence, their view of the proximal world is generated through pliable hairs that tap and palpate objects. The texture and shape of those objects must be discerned for the rat to assess the value of the object. Furthermore, the location of those objects must be specified with reference to the position of the rat's head for the rat to plan its movements. Recent in vivo and in vitro electrophysiological measurements provide insight into the algorithms and mechanisms that underlie these behavioral-based computations.

Temporal patterns of field potentials in vibrissa/barrel cortex reveal stimulus orientation and shape

During environmental exploration, rats rhythmically whisk their vibrissae along the rostrocaudal axis. Each forward extension of the vibrissa array establishes rapid spatiotemporal contact with an object under investigation. This contact presumably produces equally rapid spatiotemporal patterns of population responses in the vibrissa representation of somatosensory cortex [the posterior medial barrel subfield (PMBSF)] reflecting features of a stimulus. We used extracellular mapping to identify object features based on spatiotemporal patterns of evoked potentials. Spatiotemporal modeling of evoked potential patterns accurately reconstructed linear versus curved stimuli and detected orientation changes as small as 5 degrees. Whiskers forming arcs in the PMBSF, essential for this reconstruction, may represent a fundamental processing module. We propose that the PMBSF may function as a spatial frequency analyzer, with intrarow processing integrating a complementary set of spatial frequencies from the arcs in a single whisk.