Acute Stress Effects on Statistical Learning and Episodic Memory

Stress is widely considered to negatively impact hippocampal function, thus impairing episodic memory. However, the hippocampus is not merely the seat of episodic memory. Rather, it also (via distinct circuitry) supports statistical learning. On the basis of rodent work suggesting that stress may impair the hippocampal pathway involved in episodic memory while sparing or enhancing the pathway involved in statistical learning, we developed a behavioral experiment to investigate the effects of acute stress on both episodic memory and statistical learning in humans. Participants were randomly assigned to one of three conditions: stress (socially evaluated cold pressor) immediately before learning, stress ∼15 min before learning, or no stress. In the learning task, participants viewed a series of trial-unique scenes (allowing for episodic encoding of each image) in which certain scene categories reliably followed one another (allowing for statistical learning of associations between paired categories). Memory was assessed 24 hr later to isolate stress effects on encoding/learning rather than retrieval. We found modest support for our hypothesis that acute stress can amplify statistical learning: Only participants stressed ∼15 min in advance exhibited reliable evidence of learning across multiple measures. Furthermore, stress-induced cortisol levels predicted statistical learning retention 24 hr later. In contrast, episodic memory did not differ by stress condition, although we did find preliminary evidence that acute stress promoted memory for statistically predictable information and attenuated competition between statistical and episodic encoding. Together, these findings provide initial insights into how stress may differentially modulate learning processes within the hippocampus.

Statistical learning in epilepsy: Behavioral and anatomical mechanisms in the human brain

OBJECTIVE

Statistical learning, the fundamental cognitive ability of humans to extract regularities across experiences over time, engages the medial temporal lobe (MTL) in the healthy brain. This leads to the hypothesis that statistical learning (SL) may be impaired in patients with epilepsy (PWE) involving the temporal lobe, and that this impairment could contribute to their varied memory deficits. In turn, studies done in collaboration with PWE, that evaluate the necessity of MTL circuitry through disease and causal perturbations, provide an opportunity to advance basic understanding of SL.

METHODS

We implemented behavioral testing, volumetric analysis of the MTL substructures, and direct electrical brain stimulation to examine SL across a cohort of 61 PWE and 28 healthy controls.

RESULTS

We found that behavioral performance in an SL task was negatively associated with seizure frequency irrespective of seizure origin. The volume of hippocampal subfields CA1 and CA2/3 correlated with SL performance, suggesting a more specific role of the hippocampus. Transient direct electrical stimulation of the hippocampus disrupted SL. Furthermore, the relationship between SL and seizure frequency was selective, as behavioral performance in an episodic memory task was not impacted by seizure frequency.

SIGNIFICANCE

Overall, these results suggest that SL may be hippocampally dependent and that the SL task could serve as a clinically useful behavioral assay of seizure frequency that may complement existing approaches such as seizure diaries. Simple and short SL tasks may thus provide patient-centered endpoints for evaluating the efficacy of novel treatments in epilepsy.

Multiple Memory Subsystems: Reconsidering Memory in the Mind and Brain

The multiple-memory-systems framework-that distinct types of memory are supported by distinct brain systems-has guided learning and memory research for decades. However, recent work challenges the one-to-one mapping between brain structures and memory types central to this taxonomy, with key memory-related structures supporting multiple functions across substructures. Here we integrate cross-species findings in the hippocampus, striatum, and amygdala to propose an updated framework of multiple memory subsystems (MMSS). We provide evidence for two organizational principles of the MMSS theory: First, opposing memory representations are colocated in the same brain structures; second, parallel memory representations are supported by distinct structures. We discuss why this burgeoning framework has the potential to provide a useful revision of classic theories of long-term memory, what evidence is needed to further validate the framework, and how this novel perspective on memory organization may guide future research.

More than a moment: What does it mean to call something an 'event'?

Experiences are stored in the mind as discrete mental units, or 'events,' which influence-and are influenced by-attention, learning, and memory. In this way, the notion of an 'event' is foundational to cognitive science. However, despite tremendous progress in understanding the behavioral and neural signatures of events, there is no agreed-upon definition of an event. Here, we discuss different theoretical frameworks of event perception and memory, noting what they can and cannot account for in the literature. We then highlight key aspects of events that we believe should be accounted for in theories of event processing--in particular, we argue that the structure and substance of events should be better reflected in our theories and paradigms. Finally, we discuss empirical gaps in the event cognition literature and what the future of event cognition research may look like.

Hippocampal Mechanisms Support Cortisol-Induced Memory Enhancements

Stress can powerfully influence episodic memory, often enhancing memory encoding for emotionally salient information. These stress-induced memory enhancements stand at odds with demonstrations that stress and the stress-related hormone cortisol can negatively affect the hippocampus, a brain region important for episodic memory encoding. To resolve this apparent conflict and determine whether and how the hippocampus supports memory encoding under cortisol, we combined behavioral assays of associative memory, high-resolution fMRI, and pharmacological manipulation of cortisol in a within-participant, double-blinded procedure (in both sexes). Behaviorally, hydrocortisone promoted the encoding of subjectively arousing, positive associative memories. Neurally, hydrocortisone led to enhanced functional connectivity between hippocampal subregions, which predicted subsequent memory enhancements for emotional associations. Cortisol also modified the relationship between hippocampal representations and associative memory: whereas hippocampal signatures of distinctiveness predicted memory under placebo, relative integration predicted memory under cortisol. Together, these data provide novel evidence that the human hippocampus contains the necessary machinery to support emotional associative memory enhancements under cortisol.SIGNIFICANCE STATEMENT Our daily lives are filled with stressful events, which powerfully shape the way we form episodic memories. For example, stress and stress-related hormones can enhance our memory for emotional events. However, the mechanisms underlying these memory benefits are unclear. In the current study, we combined functional neuroimaging, behavioral tests of memory, and double-blind, placebo-controlled hydrocortisone administration to uncover the effects of the stress-related hormone cortisol on the function of the human hippocampus, a brain region important for episodic memory. We identified novel ways in which cortisol can enhance hippocampal function to promote emotional memories, highlighting the adaptive role of cortisol in shaping memory formation.

Intracranial Entrainment Reveals Statistical Learning across Levels of Abstraction

We encounter the same people, places, and objects in predictable sequences and configurations. Humans efficiently learn these regularities via statistical learning. Importantly, statistical learning creates knowledge not only of specific regularities but also of regularities that apply more generally across related experiences (i.e., across members of a category). Prior evidence for different levels of learning comes from post-exposure behavioral tests, leaving open the question of whether more abstract regularities are detected online during initial exposure. We address this question by measuring neural entrainment in intracranial recordings. Neurosurgical patients viewed a stream of photographs with regularities at 1 of 2 levels: In the exemplar-level structured condition, the same photographs appeared repeatedly in pairs. In the category-level structured condition, the photographs were trial-unique but their categories were paired across repetitions. In a baseline random condition, the same photographs repeated but in a scrambled order. We measured entrainment at the frequency of individual photographs, which was expected in all conditions, but critically also at half that frequency-the rate at which to-be-learned pairs appeared in the 2 structured (but not random) conditions. Entrainment to both exemplar and category pairs emerged within minutes throughout visual cortex and in frontal and temporal regions. Many electrode contacts were sensitive to only one level of structure, but a significant number encoded both levels. These findings suggest that the brain spontaneously uncovers category-level regularities during statistical learning, providing insight into the brain's unsupervised mechanisms for building flexible and robust knowledge that generalizes across input variation and conceptual hierarchies.

Statistical learning in epilepsy: Behavioral, anatomical, and causal mechanisms in the human brain

Statistical learning, the fundamental cognitive ability of humans to extract regularities across experiences over time, engages the medial temporal lobe in the healthy brain. This leads to the hypothesis that statistical learning may be impaired in epilepsy patients, and that this impairment could contribute to their varied memory deficits. In turn, epilepsy patients provide a platform to advance basic understanding of statistical learning by helping to evaluate the necessity of medial temporal lobe circuitry through disease and causal perturbations. We implemented behavioral testing, volumetric analysis of the medial temporal lobe substructures, and direct electrical brain stimulation to examine statistical learning across a cohort of 61 epilepsy patients and 28 healthy controls. Behavioral performance in a statistical learning task was negatively associated with seizure frequency, irrespective of where seizures originated in the brain. The volume of hippocampal subfields CA1 and CA2/3 correlated with statistical learning performance, suggesting a more specific role of the hippocampus. Indeed, transient direct electrical stimulation of the hippocampus disrupted statistical learning. Furthermore, the relationship between statistical learning and seizure frequency was selective: behavioral performance in an episodic memory task was impacted by structural lesions in the medial temporal lobe and by antiseizure medications, but not by seizure frequency. Overall, these results suggest that statistical learning may be hippocampally dependent and that this task could serve as a clinically useful behavioral assay of seizure frequency distinct from existing neuropsychological tests. Simple and short statistical learning tasks may thus provide patient-centered endpoints for evaluating the efficacy of novel treatments in epilepsy.

Hippocampal mechanisms support cortisol-induced memory enhancements

Stress can powerfully influence episodic memory, often enhancing memory encoding for emotionally salient information. These stress-induced memory enhancements stand at odds with demonstrations that stress and the stress-related hormone cortisol can negatively affect the hippocampus, a brain region important for episodic memory encoding. To resolve this apparent conflict and determine whether and how the hippocampus supports memory encoding under cortisol, we combined behavioral assays of associative memory, high-resolution functional magnetic resonance imaging (fMRI), and pharmacological manipulation of cortisol in a within-participant, double-blinded procedure. Hydrocortisone led to enhanced functional connectivity between hippocampal subregions, which predicted subsequent memory enhancements for emotional information. Cortisol also modified the relationship between hippocampal representations and memory: whereas hippocampal signatures of distinctiveness predicted memory under placebo, relative integration predicted memory under cortisol. Together, these data provide novel evidence that the human hippocampus contains the necessary machinery to support emotional memory enhancements under stress.

Mnemonic Content and Hippocampal Patterns Shape Judgments of Time

Our experience of time can feel dilated or compressed, rather than reflecting true "clock time." Although many contextual factors influence the subjective perception of time, it is unclear how memory accessibility plays a role in constructing our experience of and memory for time. Here, we used a combination of behavioral and functional MRI measures in healthy young adults (N = 147) to ask the question of how memory is incorporated into temporal duration judgments. Behaviorally, we found that event boundaries, which have been shown to disrupt ongoing memory integration processes, result in the temporal compression of duration judgments. Additionally, using a multivoxel pattern similarity analysis of functional MRI data, we found that greater temporal pattern change in the left hippocampus within individual trials was associated with longer duration judgments. Together, these data suggest that mnemonic processes play a role in constructing representations of time.

Temporal Dynamics of Competition between Statistical Learning and Episodic Memory in Intracranial Recordings of Human Visual Cortex

The function of long-term memory is not just to reminisce about the past, but also to make predictions that help us behave appropriately and efficiently in the future. This predictive function of memory provides a new perspective on the classic question from memory research of why we remember some things but not others. If prediction is a key outcome of memory, then the extent to which an item generates a prediction signifies that this information already exists in memory and need not be encoded. We tested this principle using human intracranial EEG as a time-resolved method to quantify prediction in visual cortex during a statistical learning task and link the strength of these predictions to subsequent episodic memory behavior. Epilepsy patients of both sexes viewed rapid streams of scenes, some of which contained regularities that allowed the category of the next scene to be predicted. We verified that statistical learning occurred using neural frequency tagging and measured category prediction with multivariate pattern analysis. Although neural prediction was robust overall, this was driven entirely by predictive items that were subsequently forgotten. Such interference provides a mechanism by which prediction can regulate memory formation to prioritize encoding of information that could help learn new predictive relationships.SIGNIFICANCE STATEMENT When faced with a new experience, we are rarely at a loss for what to do. Rather, because many aspects of the world are stable over time, we rely on past experiences to generate expectations that guide behavior. Here we show that these expectations during a new experience come at the expense of memory for that experience. From intracranial recordings of visual cortex, we decoded what humans expected to see next in a series of photographs based on patterns of neural activity. Photographs that generated strong neural expectations were more likely to be forgotten in a later behavioral memory test. Prioritizing the storage of experiences that currently lead to weak expectations could help improve these expectations in future encounters.

Remembering the pattern: A longitudinal case study on statistical learning in spatial navigation and memory consolidation

Distinct brain systems are thought to support statistical learning over different timescales. Regularities encountered during online perceptual experience can be acquired rapidly by the hippocampus. Further processing during offline consolidation can establish these regularities gradually in cortical regions, including the medial prefrontal cortex (mPFC). These mechanisms of statistical learning may be critical during spatial navigation, for which knowledge of the structure of an environment can facilitate future behavior. Rapid acquisition and prolonged retention of regularities have been investigated in isolation, but how they interact in the context of spatial navigation is unknown. We had the rare opportunity to study the brain systems underlying both rapid and gradual timescales of statistical learning using intracranial electroencephalography (iEEG) longitudinally in the same patient over a period of three weeks. As hypothesized, spatial patterns were represented in the hippocampus but not mPFC for up to one week after statistical learning and then represented in the mPFC but not hippocampus two and three weeks after statistical learning. Taken together, these findings suggest that the hippocampus may contribute to the initial extraction of regularities prior to cortical consolidation.

The prevalence and importance of statistical learning in human cognition and behavior

Statistical learning, the ability to extract regularities from the environment over time, has become a topic of burgeoning interest. Its influence on behavior, spanning infancy to adulthood, has been demonstrated across a range of tasks, both those labeled as tests of statistical learning and those from other learning domains that predated statistical learning research or that are not typically considered in the context of that literature. Given this pervasive role in human cognition, statistical learning has the potential to reconcile seemingly distinct learning phenomena and may be an under-appreciated but important contributor to a wide range of human behaviors that are studied as unrelated processes, such as episodic memory and spatial navigation.

Effects of catecholamines on calvarial bone resorption in vitro

Many important diseases in otolaryngology manifest through abnormal bone remodeling or destruction. The mechanisms for such pathological remodeling remain poorly understood. Bone is known to be innervated by norepinephrine-containing sympathetic nerves, and sympathectomy is known to induce bone resorption. The role, however, of norepinephrine as a potential bone-modulatory substance is unknown. Using the calvarial calcium release assay, we conducted the following experiment to evaluate the bone-modulatory activity of norepinephrine, the alpha-agonist octopamine, and the beta-agonist isoproterenol. Each agent was tested at 2 concentrations with and without parathyroid hormone. Norepinephrine was found to have no effect on calcium release. In contrast, octopamine at 10(-8) mol/L exerted a significant stimulatory effect on calcium release, and isoproterenol at 10(-6) mol/L exerted a significant inhibitory effect on parathyroid hormone-induced calcium release. The investigation suggests that a bimodal, concentration-dependent, receptor-specific model for catecholamine-mediated modulation of bone resorption may operate in calvarial bone.

A unique case of the rapid onset of a large cystic hygroma in the adult

Cystic hygroma is an uncommon lymphatic tumor seen rarely in adults, with less than 100 cases reported in the literature. The etiology and pathophysiology of this lesion is still in question. The majority of cystic hygromas occur in the head and neck, particularly in the posterior triangle. Although cystic hygromas tend to enlarge progressively over a span of weeks or months, relatively rapid enlargement over a span of days has been described. We present the unique case of an adult woman who experienced sudden onset of a large cystic hygroma in the neck without history of antecedent swelling, infection, or trauma. Successful surgical removal of the hygroma was performed. A brief review of the literature is presented.

Effect of chemical sympathectomy with 6-hydroxydopamine on osteoclast activity in the gerbilline middle ear bulla

HYPOTHESIS

Chemical sympathectomy with 6-hydroxydopamine (HDA) increases middle ear bulla bone resorption in the Mongolian gerbil.

BACKGROUND

Many diseases of the middle ear have pathologic processes linked to abnormal bone remodeling. Numerous factors controlling bone remodeling have been identified. An understanding of these factors and their role in pathologic remodeling is therefore essential. Sympathectomy, induced both surgically and pharmaceutically, is known to increase middle ear bone resorption, suggesting a role for the central nervous system in bone metabolism. This effect, however, may be confounded by hemodynamic changes induced by hemicranial surgical sympathectomy or by uncertainty in the action of pharmaceutical agents on the sympathetic nervous system. In this experiment, a third modality with unique properties, chemical sympathectomy with HDA, was used to quantify further the effect of sympathectomy on middle ear bone remodeling.

METHODS

Eight gerbils designated experimental received intraperitoneal injections of HDA (75 mg/kg) for 1 week, whereas eight animals designated control received similar injections of saline. One week after injections, the animals were euthanized and bulla bone samples were analyzed histomorphometrically to determine osteoclastic activity. In addition, to assess for any direct effects on bone metabolism, the activity of HDA was determined in vitro using the calvarial calcium release assay.

RESULTS

The in vitro study found HDA to have no direct stimulatory activity on calcium release. The in vivo study showed HDA to increase osteoclastic activity significantly in middle ear bone.

CONCLUSION

HDA-induced sympathectomy increases bone resorption in gerbilline middle ear bone.

First place--resident basic science award 1999. Effects of leukotriene and cyclo-oxygenase inhibition on adaptive bone remodeling in the middle ear

Abnormal bone remodeling is associated with important otolaryngologic diseases. In such diseases, the mechanisms of osteoclastic control underlie the pathologic processes. It is known that strain applied to auditory bullae induces bone resorption-an effect mediated by prostaglandins and blocked by cyclo-oxygenase inhibitors. It is also known that cyclo-oxygenase inhibition shunts arachidonic acid into alternate metabolic pathways, mainly the lipoxygenase pathway with leukotriene production. The role of these metabolites in adaptive bone remodeling is unknown. Using the gerbilline bulla as a model, we infused BW755c (dual lipoxygenase/cyclo-oxygenase inhibitor) and L-663,536 (5-lipoxygenase inhibitor) into animals undergoing middle ear pressurization. After 7 days, the bulla bones were harvested, and osteoclasts were quantified histomorphometrically. The results showed that neither treatment altered pressure-induced resorption. However, BW755c significantly increased resorption in unpressurized bone when compared with control values. Because BW775c blocks both lipoxygenase and cyclo-oxygenase pathways, the results suggest an alternate pathway in middle ear bone resorption.

Sympathectomy, which induces membranous bone remodeling, has no effect on endochondral long bone remodeling in vivo

Sympathectomy has been shown to induce resorption within the membranous middle ear bone of gerbils. It is unknown whether sympathectomy exerts a similar effect on endochondral long bone. In the present study, guanethidine sulfate (GS) and 6-hydroxydopamine (HDA) were administered to gerbils to induce sympathectomy. One week later, samples of middle ear bulla bone and radial long bone were harvested and assessed for osteoclastic activity. Histomorphometric analysis showed both pharmacologic sympathectomy with GS and chemical sympathectomy with HDA significantly increased the osteoclast counts and osteoclast surfaces of bulla bone samples but not radial long bone samples, respectively. In contrast, HDA but not GS increased the osteoclast profile area of both long bone and membranous bone samples when compared with vehicle-treated controls. Sympathectomy, induced both chemically and pharmacologically, thus has been shown to increase resorption in membranous bone but not endochondral long bone in the gerbilline model.

Effect of pharmacological sympathectomy on osteoclastic activity in the gerbilline auditory bulla in vivo

Bone destruction causes hearing loss in various middle ear disorders. The mechanisms of such pathological remodeling are unknown. Unilateral surgical sympathectomy is known to induce resorption within mandibular and auditory bulla bone. Explanation of the cause of this effect, however, may be confounded by hemodynamic changes induced by hemicranial sympathectomy and by uncertainty as to the neuroanatomical origins of sympathetic fibers. In this study, gerbils were infused with guanethidine sulfate (GS) to evaluate the in vivo effects of systemic sympatholysis on auditory bone remodeling. In addition, to discount any direct osteolytic effect, GS was assessed of its bone resorbing activity in vitro by means of the calvarial calcium release assay. The in vitro study revealed GS to have no effect on calcium release. The in vivo study revealed GS to increase both the osteoclast surface and number. Guanethidine-induced sympathectomy has thus been shown to increase remodeling in gerbilline auditory bone, while no direct osteolytic effect could be measured in vitro.

In vivo effects of surgical sympathectomy on intramembranous bone resorption

Bone modeling and remodeling are highly regulated processes in the mammalian skeleton. The exact mechanism by which bone can be modeled at a local site with little or no effect at adjacent anatomic sites is unknown. Disruption of the control of modeling within the temporal bone may lead to various bone disease such as otosclerosis, osteogenesis imperfecta, Paget's disease of bone, fibrous dysplasia, or the erosion of bone associated with chronic otitis media. One possible mechanism for such delicate control may be related to the ubiquitous and rich sympathetic innervation of all periosteal surfaces. Previous studies have indicated that regional sympathectomy leads to qualitative alterations in localized bone modeling and remodeling. In this study, unilateral cervical sympathectomy resulted in significant increases in osteoclast surface and osteoclast number within the ipsilateral bulla of experimental animals. The mechanisms by which sympathectomy leads to increased local bone loss is unknown. Potential mechanisms include disinhibition of resorption, secondary to the elimination of periosteal sympathetics, as well as indirect vascular effects.

A mechanism for sympathectomy-induced bone resorption in the middle ear

BACKGROUND

Recent investigations have demonstrated a link between sympathectomy and osteoclast-mediated bone resorption. The exact nature of this link, however, is unknown. We hypothesize that substance P, a potent vasoconstrictive neuropeptide found in peripheral sensory fibers, including those innervating bone, is the mediator of this phenomenon. To test this theory, the effects of substance P on in vitro calcium release from cultured neonatal mouse calvaria were assessed. In addition, an in vivo study was conducted whereby gerbils were injected with capsaicin to eliminate substance P-containing fibers before sympathectomy with 6-hydroxydopamine. If the effects of 6-hydroxydopamine were eliminated by prior administration of capsaicin, the role of sensory nerves in sympathectomy-induced resorption would be strongly implicated.

IN VITRO STUDY

Substance P at 10(-8) mol/L was incubated with eight newborn Swiss-Webster mouse hemicalvarial explants and compared with explants incubated in control media alone. The neonatal mice were euthanized at day 3, and their hemicalvaria were preincubated in 2 ml of stock media without treatment for 24 hours at 36.5 degrees C as a stabilization period. After the stabilization period, the stock media were replaced with 2 ml of fresh control media or media containing substance P at 10(-8) mol/L. A similar experiment was performed with the addition of indomethacin at 5 x 10(-7). The explants were then incubated for 72 hours with gassing every 12 hours with a mixture of O2, N2, and CO2. At the end of the 72-hour period, the media were analyzed for calcium content by atomic absorption spectrophotometry and compared by one-way analysis of variance with Bonferroni-corrected post hoc tests.

IN VIVO STUDY

Forty-eight Mongolian gerbils were placed into four groups: group 1 received intraperitoneal injections of 6-hydroxydopamine at 75 micrograms/gm body weight on days 1, 2, 6, 7, and 8; group 2 received identical injections of hydroxydopamine, but 12 hours after receiving subdermal injections of capsaicin at 50 micrograms/gm body weight; group 3 received only subdermal injections of capsaicin; and group 4 received only saline injections to serve as controls. Seven days after treatment, the animals were euthanized, and the ventral wall of each animal's right bulla was resected and quantified for osteoclast number and surface with a computer-based histomorphometry system. Analysis was then made by one-way analysis of variance with Bonferroni-corrected post hoc tests.

RESULTS

The results of the in vitro study revealed that substance P at 10(-8) mol/L (11.05 +/- 3.37 micrograms/ml) induced significant calcium release from cultured neonatal mouse calvaria when compared with control bone incubated in base media alone (0.92 +/- 2.85 micrograms/ml, p < 0.01). The process was completely inhibited by 5.0 x 10(-7) indomethacin. The results of the in vivo study showed 6-hydroxydopamine treatment significantly increased both the osteoclast number (NOc/TL = 3.14 +/- 1.33/mm) and the osteoclast surface (OcS/BS = 16.04% +/- 6.95%) of bone when compared with bone from saline-treated controls (NOc/TL = 1.77 +/- 0.79/mm, p < 0.01; OcS/BS = 8.88% +/- 4.15%, p < 0.01). These 6-hydroxydopamine-induced increases were eliminated, however, in animals pretreated with capsaicin before sympathectomy (NOc/TL = 1.86 +/- 0.68/mm, p > 0.05; OcS/BS = 9.92 +/- 3.73, p > 0.05), whereas treatment with capsaicin alone had no effect when compared with bone from saline-treated controls (NOc/TL = 2.02 +/- 0.50/mm, p > 0.05; OcS/BS = 10.28% +/- 2.62%, p > 0.05). Substance P has thus been shown to induce calcium release from membranous bone in vitro, whereas capsaicin, a substance P-specific sensory neurolytic chemical, eliminates the in vivo osteoclast-inductive effects of 6-hydroxydopamine when given 12 hours before treatment. The results indicate that substance P is capable of inducing resorption and that substance P-containing sensory ne