Contemporary Cleaning and Shaping of the Root Canal System.

Prevention or treatment of apical periodontitis is aimed at disinfecting the root canal system so that the periradicular tissues are not vulnerable to attack from microbiota within the tooth. This is achieved by a process of cleaning and shaping the root canal space. The fundamental principles underlying this process have not changed in decades. In contrast, the armamentarium available to the clinician continues to evolve rapidly. The use of specially designed nickel titanium (NiTi) files to create a glidepath allows the clinician to manage tight curved canals more predictably in situations which would otherwise have proved too difficult using conventional techniques. Other files, designed to shape the canal, have been developed using metallurgic principles which permit NiTi files to be more flexible and resistant to cyclic fatigue. These newer systems also require fewer instruments to prepare a canal and some, which have adopted a reciprocating (rotational) motion, may only require one file. Progress is also being made in enhancing efficacy of irrigant activity using negative apical pressure systems, sonic and ultrasonic agitation techniques. These contemporary techniques used to clean and shape the root canal system should result in improved confidence and predictability when managing endodontic disease.

Visual object agnosia is associated with a breakdown of object-selective responses in the lateral occipital cortex.

Patients with visual object agnosia fail to recognize the identity of visually presented objects despite preserved semantic knowledge. Object agnosia may result from damage to visual cortex lying close to or overlapping with the lateral occipital complex (LOC), a brain region that exhibits selectivity to the shape of visually presented objects. Despite this anatomical overlap the relationship between shape processing in the LOC and shape representations in object agnosia is unknown. We studied a patient with object agnosia following isolated damage to the left occipito-temporal cortex overlapping with the LOC. The patient showed intact processing of object structure, yet often made identification errors that were mainly based on the global visual similarity between objects. Using functional Magnetic Resonance Imaging (fMRI) we found that the damaged as well as the contralateral, structurally intact right LOC failed to show any object-selective fMRI activity, though the latter retained selectivity for faces. Thus, unilateral damage to the left LOC led to a bilateral breakdown of neural responses to a specific stimulus class (objects and artefacts) while preserving the response to a different stimulus class (faces). These findings indicate that representations of structure necessary for the identification of objects crucially rely on bilateral, distributed coding of shape features.

Regenerative endodontics: regeneration or repair?

Recent advances in biotechnology and translational research have made it possible to provide treatment modalities that protect the vital pulp, allow manipulation of reactionary and reparative dentinogenesis, and, more recently, permit revascularization of an infected root canal space. These approaches are referred to as regenerative procedures. The method currently used to determine the origin of the tissue secreted during the repair/regeneration process is largely based on the identification of cellular markers (usually proteins) left by cells that were responsible for this tissue production. The presence of these proteins in conjunction with other indicators of cellular behavior (especially biomineralization) and analysis of the structure of the newly generated tissue allow conclusions to be made of how it was formed. Thus far, it has not been possible to truly establish the biological mechanism controlling tertiary dentinogenesis. This article considers current therapeutic techniques to treat the dentin-pulp complex and contextualize them in terms of reparative and regenerative processes. Although it may be considered a semantic argument rather than a biological one, the definitions of regeneration and repair are explored to clarify our position in this era of regenerative endodontics.

When the brain remembers, but the patient doesn't: converging fMRI and EEG evidence for covert recognition in a case of prosopagnosia.

The role of the occipito-temporal cortex in visual awareness remains an open question and with respect to faces in particular, it is unclear to what extent the fusiform face area (FFA) may be involved in conscious identification. An answer may be gleaned from prosopagnosia, a disorder in which familiar faces are no longer recognized. This impairment has sometimes been reported to be associated with implicit processing of facial identity, although the neural substrates responsible for unconscious processing remain unknown. In this study, we addressed these issues by investigating the functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) responses to familiar and unfamiliar faces in a well-known prosopagnosic patient (P.S.). Our fMRI results show that faces known prior to the onset of prosopagnosia produce an increase in activation in the lateral fusiform gyrus encompassing the FFA, as well as the right middle frontal gyrus, when compared to unknown faces. This effect is not observed with photographs of celebrities dating after the onset of prosopagnosia. Furthermore, electrophysiological responses show that previously familiar faces differ from unfamiliar ones at around 550 msec. Since covert processing of familiarity is associated with activation in FFA, this structure does not appear to be sufficient to produce awareness of identity. Furthermore, the results support the view that FFA participates in face individuation.

Neural response to the behaviorally relevant absence of anticipated outcomes and the presentation of potentially harmful stimuli: A human fMRI study.

Adaptive behavior requires the ability to react to potentially harmful stimuli, characterized by high negative inherent emotional salience (iES) (e.g., spiders, snakes), and to the unexpected non-occurrence of anticipated events. When presented simultaneously, threatening stimuli and unexpected absence of anticipated outcomes induce distinct electrocortical responses in different time periods. In this study, we used fMRI to test whether processing of the absence of anticipated outcomes (prediction errors) was anatomically dissociated from the processing of iES or whether iES simply modulated activity of areas processing the non-occurrence of anticipated outcomes. Participants saw two alternating pairs of faces and indicated for each pair which one would have a declared target stimulus on its nose. Depending on the condition, the target stimulus was either a spider (high iES stimulus) or a disk (low iES stimulus). The target stimulus switched to the other face after several consecutive correct responses, with the switch being indicated by the appearance of the alternative stimulus (disk when the spider was the declared target; spider when the disk was the declared target). We found that the spider induced stronger activation in visual areas than the disk. By contrast, the absence of anticipated outcomes specifically activated the orbitofrontal cortex (OFC), irrespective of the iES of the outcome stimulus. The findings support a generic role of the OFC in outcome monitoring.

Seeing the phantom: a functional magnetic resonance imaging study of a supernumerary phantom limb.

OBJECTIVE: Supernumerary phantom limb (SPL) is a rare neurological manifestation where patients with a severe stroke-induced sensorimotor deficit experience the illusory presence of an extra limb that duplicates a real one. The illusion is most often experienced as a somesthetic phantom, but rarer SPLs may be intentionally triggered or seen. Here, we report the case of a left visual, tactile, and intentional SPL caused by right subcortical damage in a nondeluded woman. METHODS: Using functional magnetic resonance imaging, we investigated the multimodal nature of this phantom, which the patient claimed to be able see, use, and move intentionally. The patient participated in a series of sensorimotor and motor imagery tasks involving the right, the left plegic, and the SPL's hand. RESULTS: Right premotor and motor regions were engaged when she imagined that she was scratching her left cheek with her left plegic hand, whereas when she performed the same task with the SPL, additional left middle occipital areas were recruited. Moreover, comparison of responses induced by left cheek (subjectively feasible) versus right cheek scratching (reportedly unfeasible movement) with the SPL demonstrated significant activation in right somesthetic areas. INTERPRETATION: These findings demonstrate that intentional movements of a seen and felt SPL activate premotor and motor areas together with visual and sensory cortex, confirming its multimodal dimension and the reliability of the patient's verbal reports. This observation, interpreted for cortical deafferentation/disconnection caused by subcortical brain damage, constitutes a new but theoretically predictable entity among disorders of bodily awareness.

Spatial attention and memory versus motor preparation: premotor cortex involvement as revealed by fMRI.

Recent studies in both monkeys and humans indicate that the dorsal premotor cortex participates in spatial attention and working memory, in addition to its well known role in movement planning and execution. One important question is whether these functions overlap or are segregated within this frontal area. Single-cell recordings in monkeys suggest a relative specialization of the rostral portion of dorsal premotor cortex for attention and/or memory and of the caudal region for motor preparation. To test whether this possibility also holds true in humans, we used functional magnetic resonance imaging (fMRI) to compare, in the same set of subjects, brain activation related to strong spatial attention and memory demands to that elicited by long motor preparatory periods. The behavioral protocol was based on a task that had proved effective for dissociating neuronal properties related to these two functions in the monkey brain. The principle of the monkey task was that a first cue guided the focus of spatial attention and memory, whereas a second one instructed an arm movement. Based on this principle, two tasks were developed. One maximized spatial attention and memory demands by presenting long series of stimuli (4, 8, or 12) before the motor instructional cue, whereas the other extended the motor preparation phase by imposing long and variable delays (1-5.5 s) between the onset of the instructional cue and movement execution. The two tasks and their respective control conditions were arranged in two blocked-design sequences. The results indicate that the brain networks underlying the two functional domains overlap in the caudate nucleus and presupplementary motor area, and possibly in lateral prefrontal cortex as well, but involve different dorsal premotor fields. Motor preparation primarily recruited a dorsal premotor area located caudally, within the precentral gyrus (together with the supplementary motor area), whereas spatial attention and memory preferentially activated a more rostral site, in and anterior to the precentral sulcus (in addition to the posterior parietal cortex). These findings strengthen the idea that the primate dorsal premotor cortex contributes to both motor and nonmotor processes. Moreover, they corroborate emerging evidence from monkey physiology suggesting a relative functional segregation within this cortex, with attention to short-term storage of visuospatial information engaging a more rostral region than motor preparation.