Monitoring of cerebral blood flow autoregulation: physiologic basis, measurement, and clinical implications.

Cerebral blood flow (CBF) autoregulation is the physiologic process whereby blood supply to the brain is kept constant over a range of cerebral perfusion pressures ensuring a constant supply of metabolic substrate. Clinical methods for monitoring CBF autoregulation were first developed for neurocritically ill patients and have been extended to surgical patients. These methods are based on measuring the relationship between cerebral perfusion pressure and surrogates of CBF or cerebral blood volume (CBV) at low frequencies (<0.05 Hz) of autoregulation using time or frequency domain analyses. Initially intracranial pressure monitoring or transcranial Doppler assessment of CBF velocity was utilised relative to changes in cerebral perfusion pressure or mean arterial pressure. A more clinically practical approach utilising filtered signals from near infrared spectroscopy monitors as an estimate of CBF has been validated. In contrast to the traditional teaching that 50 mm Hg is the autoregulation threshold, these investigations have found wide interindividual variability of the lower limit of autoregulation ranging from 40 to 90 mm Hg in adults and 20-55 mm Hg in children. Observational data have linked impaired CBF autoregulation metrics to adverse outcomes in patients with traumatic brain injury, ischaemic stroke, subarachnoid haemorrhage, intracerebral haemorrhage, and in surgical patients. CBF autoregulation monitoring has been described in both cardiac and noncardiac surgery. Data from a single-centre randomised study in adults found that targeting arterial pressure during cardiopulmonary bypass to above the lower limit of autoregulation led to a reduction of postoperative delirium and improved memory 1 month after surgery compared with usual care. Together, the growing body of evidence suggests that monitoring CBF autoregulation provides prognostic information on eventual patient outcomes and offers potential for therapeutic intervention. For surgical patients, personalised blood pressure management based on CBF autoregulation data holds promise as a strategy to improve patient neurocognitive outcomes.

Cerebral Autoregulation during Orthostatic Challenge in Congenital Central Hypoventilation Syndrome.

Rationale: Congenital central hypoventilation syndrome (CCHS) is a rare autonomic disorder with altered regulation of breathing, heart rate (HR), and blood pressure (BP). Aberrant cerebral oxygenation in response to hypercapnia/hypoxia in CCHS raises the concern that altered cerebral autoregulation may contribute to CCHS-related, variably impaired neurodevelopment. Objectives: To evaluate cerebral autoregulation in response to orthostatic challenge in CCHS cases versus controls. Methods: CCHS and age- and sex-matched control subjects were studied with head-up tilt (HUT) testing to induce orthostatic stress. Fifty CCHS and 100 control HUT recordings were included. HR, BP, and cerebral oxygen saturation (regional oxygen saturation) were continuously monitored. The cerebral oximetry index (COx), a real-time measure of cerebral autoregulation based on these measures, was calculated. Measurements and Main Results: HUT resulted in a greater mean BP decrease from baseline in CCHS versus controls (11% vs. 6%; P < 0.05) and a diminished increase in HR in CCHS versus controls (11% vs. 18%; P < 0.01) in the 5 minutes after tilt-up. Despite a similar COx at baseline, orthostatic provocation within 5 minutes of tilt-up caused a 50% greater increase in COx (P < 0.01) and a 29% increase in minutes of impaired autoregulation (P < 0.02) in CCHS versus controls (4.0 vs. 3.1 min). Conclusions: Cerebral autoregulatory mechanisms appear to be intact in CCHS, but the greater hypotension observed in CCHS consequent to orthostatic provocation is associated with greater values of COx/impaired autoregulation when BP is below the lower limits of autoregulation. Effects of repeated orthostatic challenges in everyday living in CCHS necessitate further study to determine their influence on neurodevelopmental disease burden.

High-resolution perioperative cerebral blood flow autoregulation measurement: a practical and feasible approach for widespread clinical monitoring.

A growing body of evidence demonstrates that excursions of BP below or above the limits of cerebral blood flow autoregulation are associated with complications in patients with neurological injury or for those undergoing cardiac surgery. Moreover, recent evidence suggests that maintaining MAP above the lower limit of cerebral autoregulation during cardiopulmonary bypass reduces the frequency of postoperative delirium and is associated with improved memory 1 month after surgery. Continuous measurement of BP in relation to cerebral autoregulation limits using a virtual patient monitoring platform processing near-infrared spectroscopy digital signals offers the hope of bringing this application to the bedside.

Critical Closing Pressure by Diffuse Correlation Spectroscopy in a Neonatal Piglet Model.

The critical closing pressure (CrCP) of the cerebral vasculature is the arterial blood pressure (ABP) at which cerebral blood flow (CBF) ceases. Because the ABP of preterm infants is low and close to the CrCP, there is often no CBF during diastole. Thus, estimation of CrCP may become clinically relevant in preterm neonates. Transcranial Doppler (TCD) ultrasound has been used to estimate CrCP in preterm infants. Diffuse correlation spectroscopy (DCS) is a continuous, noninvasive optical technique that measures microvascular CBF. Our objective was to compare and validate CrCP measured by DCS versus TCD ultrasound. Hemorrhagic shock was induced in 13 neonatal piglets, and CBF was measured continuously by both modalities. CrCP was calculated using a model of cerebrovascular impedance, and CrCP determined by the two modalities showed good correlation by linear regression, median r 2 = 0.8 (interquartile range (IQR) 0.71-0.87), and Bland-Altman analysis showed a median bias of -3.5 (IQR -4.6 to -0.28). This is the first comparison of CrCP determined by DCS versus TCD ultrasound in a neonatal piglet model of hemorrhagic shock. The difference in CrCP between the two modalities may be due to differences in vasomotor tone within the microvasculature of the cerebral arterioles versus the macrovasculature of a major cerebral artery.

Selected 2019 Highlights in Congenital Cardiac Anesthesia.

This article is a review of the highlights of pertinent literature published in 2019, which is of interest to the pediatric cardiac anesthesiologist. After a search of the United States National Library of Medicine PubMed database, several topics emerged in which significant contributions were made in 2019. The authors of this manuscript considered the following topics noteworthy and were included in this review: advances in pediatric heart transplantation, blood management in pediatric cardiac surgery, the impact of nutrition on outcomes in congenital heart surgery, and the use of vasopressin in patients after Fontan palliation.

Observed and calculated cerebral critical closing pressure are highly correlated in preterm infants.

BACKGROUND: Cerebrovascular critical closing pressure (CrCP) is the arterial blood pressure (ABP) at which cerebral blood flow ceases. Preterm ABP is low and close to CrCP. The diastolic closing margin (diastolic ABP minus CrCP) has been associated with intraventricular hemorrhage in preterm infants. CrCP is estimated from middle cerebral artery cerebral blood flow velocity (CBFV) and ABP waveforms. However, these estimations have not been validated due to a lack of gold standard. Direct observation of the CrCP in preterm infants with hypotension is an opportunity to validate synchronously estimated CrCP. METHODS: ABP and CBFV tracings were obtained from 24 extremely low birth weight infants. Recordings where diastolic CBFV was zero were identified. The gold standard CrCP was delineated using piecewise regression of ABP and CBFV values paired by rank ordering and then estimated using a published formula. The measured and estimated values were compared using linear regression and Bland-Altman analysis. RESULTS: Linear regression showed a high degree of correlation between measured and calculated CrCP (r2 = 0.93). CONCLUSIONS: This is the first study to validate a calculated CrCP by comparing it to direct measurements of CrCP from preterm infants when ABP is lower than CrCP.

Elevated arterial blood pressure after superior cavo-pulmonary anastomosis is associated with elevated pulmonary artery pressure and cerebrovascular dysautoregulation.

BACKGROUND: Elevated arterial blood pressure (ABP) is common after superior bidirectional cavopulmonary anastomosis (BCPA). The effects of elevated ABP after BCPA on cerebrovascular hemodynamics are unknown. We sought to determine the relationship between elevated ABP and cerebrovascular autoregulation after BCPA. METHODS: Prospective, observational study on infants with single-ventricle physiology after BCPA surgery. Continuous recordings of mean ABP, mean cavopulmonary artery pressure (PAP), near-infrared spectroscopy measures of cerebral oximetry (regional cerebral oxygen saturation (rSO2)), and relative cerebral blood volume index were obtained from admission to extubation. Autoregulation was measured as hemoglobin volume index (HVx). Physiologic variables, including the HVx, were tested for variance across ABP. RESULTS: Sixteen subjects were included in the study. Elevated ABP post-BCPA was associated with both, elevated PAP (P<0.0001) and positive HVx (dysautoregulation; P<0.0001). No association was observed between ABP and alterations in rSO2. Using piecewise regression, the relationship of PAP to ABP demonstrated a breakpoint at 68 mm Hg (interquartile range (IQR) 62-70 mm Hg). Curve fit of HVx as a function of ABP identified optimal ABP supporting robust autoregulation at a median ABP of 55 mm Hg (IQR 51-64 mm Hg). CONCLUSIONS: Elevated ABP post-BCPA is associated with cerebrovascular dysautoregulation, and elevated PAP. The effects, of prolonged dysautoregulation within this population, require further study.

A Novel Electrocardiogram Algorithm Utilizing ST-Segment Instability for Detection of Cardiopulmonary Arrest in Single Ventricle Physiology: A Retrospective Study.

OBJECTIVE: We evaluated ST-segment monitoring to detect clinical decompensation in infants with single ventricle anatomy. We proposed a signal processing algorithm for ST-segment instability and hypothesized that instability is associated with cardiopulmonary arrests. DESIGN: Retrospective, observational study. SETTING: Tertiary children's hospital 21-bed cardiovascular ICU and 36-bed step-down unit. PATIENTS: Twenty single ventricle infants who received stage 1 palliation surgery between January 2013 and January 2014. Twenty rapid response events resulting in cardiopulmonary arrests (arrest group) were recorded in 13 subjects, and nine subjects had no interstage cardiopulmonary arrest (control group). INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Arrest data were collected over the 4-hour time window prior to cardiopulmonary arrest. Control data were collected from subjects with no interstage arrest using the 4-hour time window prior to cardiovascular ICU discharge. A paired subgroup analysis was performed comparing subject 4-hour windows prior to arrest (prearrest group) with 4-hour windows prior to discharge (postarrest group). Raw values of ST segments were compared between groups. A 3D ST-segment vector was created using three quasi-orthogonal leads (II, aVL, and V5). Magnitude and instability of this continuous vector were compared between groups. There was no significant difference in mean unprocessed ST-segment values in the arrest and control groups. Utilizing signal processing, there was an increase in the ST-vector magnitude (p = 0.02) and instability (p = 0.008) in the arrest group. In the paired subgroup analysis, there was an increase in the ST-vector magnitude (p = 0.05) and instability (p = 0.05) in the prearrest state compared with the postarrest state prior to discharge. CONCLUSIONS: In single ventricle patients, increased ST instability and magnitude were associated with rapid response events that required intervention for cardiopulmonary arrest, whereas conventional ST-segment monitoring did not differentiate an arrest from control state.

Does hypothermia impair cerebrovascular autoregulation in neonates during cardiopulmonary bypass?

BACKGROUND: Autoregulation monitoring has been proposed as a means to identify optimal arterial blood pressure goals during cardiopulmonary bypass, but it has been observed that cerebral blood flow is pressure passive during hypothermic bypass. When neonates cooled during cardiopulmonary bypass are managed with vasodilators and controlled hypotension, it is not clear whether hypothermia or hypotension were the cause of impaired autoregulation. AIM: We sought to measure the effect of both arterial blood pressure and hypothermia on autoregulation in a cohort of infants cooled for bypass, hypothesizing a collinear relationship between hypothermia, hypotension, and dysautoregulation. METHODS: Cardiopulmonary bypass was performed on 72 infants at Texas Children's Hospital during 2015 and 2016 with automated physiologic data capture, including arterial blood pressure, nasopharyngeal temperature, cerebral oximetry, and a cerebral blood volume index derived from near infrared spectroscopy. Cooling to 18°C, 24°C, and 30°C was performed on 33, 12, and 22 subjects, respectively. The hemoglobin volume index was calculated as a moving correlation coefficient between mean arterial blood pressure and the cerebral blood volume index. Positive values of the hemoglobin volume index indicate impaired autoregulation. Relationships between variables were assessed utilizing a generalized estimating equation approach. RESULTS: Hypothermia was associated with hypotension, dysautoregulation, and increased cerebral oximetry. Comparing the baseline temperature of 36°C with 18°C, arterial blood pressure was 44 mm Hg (39-52) vs 25 mm Hg (21-31); the hemoglobin volume index was 0.0 (-0.02 to 0.004) vs 0.5 (0.4-0.7) and cerebral oximetry was 59% (57-61) vs 88% (80-92) (Median, 95% CI of median; P<.0001 for all three associations by linear regression with generalized estimation of equations with data from all temperatures measured). CONCLUSIONS: Arterial blood pressure, temperature, and cerebral autoregulation were collinear in this cohort. The conclusion that hypothermia causes impaired autoregulation is thus confounded. The effect of temperature on autoregulation should be delineated before clinical deployment of autoregulation monitors to prevent erroneous determination of optimal arterial blood pressure. Showing the effect of temperature on autoregulation will require a normotensive hypothermic model.

Ventilatory and Orthostatic Challenges Reveal Biomarkers for Neurocognition in Children and Young Adults With Congenital Central Hypoventilation Syndrome.

BACKGROUND: Children and young adults with congenital central hypoventilation syndrome (CCHS) are at risk of cognitive deficits. They experience autonomic dysfunction and chemoreceptor insensitivity measured during ventilatory and orthostatic challenges, but relationships between these features are undefined. RESEARCH QUESTION: Can a biomarker be identified from physiologic responses to ventilatory and orthostatic challenges that is related to neurocognitive outcomes in CCHS? STUDY DESIGN AND METHODS: This retrospective study included 25 children and young adults with CCHS tested over an inpatient stay. Relationships between physiologic measurements during hypercarbic and hypoxic ventilatory challenges, hypoxic ventilatory challenges, and orthostatic challenges and neurocognitive outcomes (by Wechsler intelligence indexes) were examined. Independent variable inclusion was determined by significant associations in Pearson's analyses. Multivariate linear regressions were used to assess relationships between measured physiologic responses to challenges and neurocognitive scores. RESULTS: Significant relationships were identified between areas of fluid intelligence and measures of oxygen saturation (SpO2) and heart rate (HR) during challenges. Specifically, perceptual reasoning was related to HR (adjusted regression [ß] coefficient, -0.68; 95% CI, 1.24 to -0.12; P = .02) during orthostasis. Working memory was related to change in HR (ß, -1.33; 95% CI, -2.61 to -0.05; P = .042) during the hypoxic ventilatory challenge. Processing speed was related to HR (ß, -1.19; 95% CI, -1.93 to -0.46; P = .003) during orthostasis, to baseline SpO2 (hypercarbic and hypoxic ß, 8.57 [95% CI, 1.63-15.51]; hypoxic ß, 8.37 [95% CI, 3.65-13.11]; P = .002 for both) during the ventilatory challenges, and to intrachallenge SpO2 (ß, 5.89; 95% CI, 0.71-11.07; P = .028) during the hypoxic ventilatory challenge. INTERPRETATION: In children and young adults with CCHS, SpO2 and HR-or change in HR-at rest and as a response to hypoxia and orthostasis are related to cognitive outcomes in domains of known risk, particularly fluid reasoning. These findings can guide additional research on the usefulness of these as biomarkers in understanding the impact of daily physical stressors on neurodevelopment in this high-risk group.

Automated prediction of cardiorespiratory deterioration in patients with single-ventricle parallel circulation: A multicenter validation study.

Objectives: Patients with single-ventricle physiology have a significant risk of cardiorespiratory deterioration between their first- and second-stage palliation surgeries. Detection of deterioration episodes may allow for early intervention and improved outcomes. Methods: A prospective study was executed at Nationwide Children's Hospital, Children's Hospital of Philadelphia, and Children's Hospital Colorado to collect physiologic data of subjects with single ventricle physiology during all hospitalizations between neonatal palliation and II surgeries using the Sickbay software platform (Medical Informatics Corp). Timing of cardiorespiratory deterioration events was captured via chart review. The predictive algorithm previously developed and validated at Texas Children's Hospital was applied to these data without retraining. Standard metrics such as receiver operating curve area, positive and negative likelihood ratio, and alert rates were calculated to establish clinical performance of the predictive algorithm. Results: Our cohort consisted of 58 subjects admitted to the cardiac intensive care unit and stepdown units of participating centers over 14 months. Approximately 28,991 hours of high-resolution physiologic waveform and vital sign data were collected using the Sickbay. A total of 30 cardiorespiratory deterioration events were observed. the risk index metric generated by our algorithm was found to be both sensitive and specific for detecting impending events one to two hours in advance of overt extremis (receiver operating curve = 0.927). Conclusions: Our algorithm can provide a 1- to 2-hour advanced warning for 53.6% of all cardiorespiratory deterioration events in children with single ventricle physiology during their initial postop course as well as interstage hospitalizations after stage I palliation with only 2.5 alarms being generated per patient per day.

The utilization of caudal hydromorphone for fast-tracking in congenital cardiac surgery in a tertiary-care Children's hospital: An audit.

STUDY OBJECTIVE: Our study sought to audit our institutional practice of routine single-shot caudal epidural hydromorphone injection in children undergoing congenital cardiothoracic surgery to assess perioperative pain control and evaluate for any caudal complications. DESIGN: Retrospective observational study of all patients that received a caudal hydromorphone injection as part of the anesthetic for their cardiac surgical operation between January 2017 and July 2019. SETTING: Pediatric Cardiothoracic Operating Room (OR), Cardiac Intensive Care Unit. PATIENTS: One hundred and twenty-seven patients that received caudal hydromorphone as part of their anesthetic for a cardiac surgical operation. INTERVENTIONS: Caudal epidural injection performed immediately following induction of anesthesia utilizing only hydromorphone. MEASUREMENTS: The primary outcome was well-controlled pain, defined as a score of <4/10 on rFLACC or verbal pain scoring. Secondary outcome measures included in-OR extubation, pain service duration (from first assessment to "sign-off"), complications related to the caudal block, intensive care unit (ICU) length of stay (LOS), and Hospital LOS. MAIN RESULTS: One hundred and nine patients were included in the final analysis. Pain was "well-controlled" on average in 96.3% of patients (105/109). Average pain in the 24-h post-block period was 1.67 (SD = 2.37), with median pain score of 0 [0-3]. Peak pain score remained <4/10 for the entire 24-h post-block period in 22% of patients. 77.1% of caudal hydromorphone patients were extubated in the operating room. The median time to heparinization post-block was 108 min, beyond the ASRA recommendation of 60 min for neuraxial procedures. There were two caudal-related complications: one subcutaneous injection, and one instance of a time to heparinization of less than 60 min (56 min). Neither caudal complication led to patient harm. CONCLUSION: Caudal hydromorphone injection can safely contribute to achieving "well-controlled" pain in the pediatric cardiac surgical population when used as a component of a perioperative pain control plan.

Automated Prediction of Cardiorespiratory Deterioration in Patients With Single Ventricle.

BACKGROUND: Patients with single-ventricle physiology have a significant risk of cardiorespiratory deterioration between their first and second stage palliation surgeries. OBJECTIVES: The objective of this study is to develop and validate a real-time computer algorithm that can automatically recognize physiological precursors of cardiorespiratory deterioration in children with single-ventricle physiology during their interstage hospitalization. METHODS: A retrospective study was conducted from prospectively collected physiological data of subjects with single-ventricle physiology. Deterioration events were defined as a cardiac arrest requiring cardiopulmonary resuscitation or an unplanned intubation. Physiological metrics were derived from the electrocardiogram (heart rate, heart rate variability, ST-segment elevation, and ST-segment variability) and the photoplethysmogram (peripheral oxygen saturation and pleth variability index). A logistic regression model was trained to separate the physiological dynamics of the pre-deterioration phase from all other data generated by study subjects. Data were split 50/50 into model training and validation sets to enable independent model validation. RESULTS: Our cohort consisted of 238 subjects admitted to the cardiac intensive care unit and stepdown units of Texas Children's Hospital over a period of 6 years. Approximately 300,000 h of high-resolution physiological waveform and vital sign data were collected using the Sickbay software platform (Medical Informatics Corp., Houston, Texas). A total of 112 cardiorespiratory deterioration events were observed. Seventy-two of the subjects experienced at least 1 deterioration event. The risk index metric generated by our optimized algorithm was found to be both sensitive and specific for detecting impending events 1 to 2 h in advance of overt extremis (receiver-operating characteristic curve area: 0.958; 95% confidence interval: 0.950 to 0.965). CONCLUSIONS: Our algorithm can provide 1 to 2 h of advanced warning for 62% of all cardiorespiratory deterioration events in children with single-ventricle physiology during their interstage period, with only 1 alarm being generated at the bedside per patient per day.

Prediction of imminent, severe deterioration of children with parallel circulations using real-time processing of physiologic data.

OBJECTIVES: Sudden death is common in patients with hypoplastic left heart syndrome and comparable lesions with parallel systemic and pulmonary circulation from a common ventricular chamber. It is hypothesized that unforeseen acute deterioration is preceded by subtle changes in physiologic dynamics before overt clinical extremis. Our objective was to develop a computer algorithm to automatically recognize precursors to deterioration in real-time, providing an early warning to care staff. METHODS: Continuous high-resolution physiologic recordings were obtained from 25 children with parallel systemic and pulmonary circulation who were admitted to the cardiovascular intensive care unit of Texas Children's Hospital between their early neonatal palliation and stage 2 surgical palliation. Instances of cardiorespiratory deterioration (defined as the need for cardiopulmonary resuscitation or endotracheal intubation) were found via a chart review. A classification algorithm was applied to both primary and derived parameters that were significantly associated with deterioration. The algorithm was optimized to discriminate predeterioration physiology from stable physiology. RESULTS: Twenty cardiorespiratory deterioration events were identified in 13 of the 25 infants. The resulting algorithm was both sensitive and specific for detecting impending events, 1 to 2 hours in advance of overt extremis (receiver operating characteristic area = 0.91, 95% confidence interval = 0.88-0.94). CONCLUSIONS: Automated, intelligent analysis of standard physiologic data in real-time can detect signs of clinical deterioration too subtle for the clinician to observe without the aid of a computer. This metric may serve as an early warning indicator of critical deterioration in patients with parallel systemic and pulmonary circulation.

Bilateral control and interhemispheric coordination in the avian song motor system.

Birdsong is a complex learned motor behavior controlled by an interconnected network of vocal control nuclei that are present in both cerebral hemispheres. Unilateral lesions of song nuclei in the left or the right hemisphere result in different effects on song structure, suggesting that normal song output results from the activation of two parallel but functionally different motor pathways. Because each syringeal half is innervated primarily by ipsilateral motor structures and activity in both halves is tightly coordinated during singing, motor commands originating from both hemispheres must be tightly coordinated to produce the appropriate vocal output. This coordination occurs despite the absence of direct interhemispheric connections between song control nuclei. In this article, we discuss how motor commands in nucleus HVC, a key forebrain song control region, are coordinated by precisely timed inputs that act to synchronize premotor activity in both hemispheres. Synchronizing inputs are tightly linked to syllable and note onset, which suggests that bilaterally organized circuits in the midbrain or brainstem act in specifying higher-order song features, such as duration, order, and possibly even structure of individual song syllables. The challenge ahead lies in identifying the networks that generate the synchronizing timing inputs and to determine how these inputs specify the motor commands in HVC. Resolving these issues will help us gain a better understanding of how pattern-generating networks in the midbrain/brainstem interface with forebrain circuits to produce complex learned behaviors.

Recovery of impaired songs following unilateral but not bilateral lesions of nucleus uvaeformis of adult zebra finches.

Zebra finches utilize neural circuits in both cerebral hemispheres to produce their learned songs. Although direct reciprocal connections do not exist between song control nuclei across hemispheres, premotor activity in these nuclei during singing is precisely and continuously coordinated between the hemispheres. We hypothesized that this interhemispheric coordination is mediated by bilateral feedback projections from medullary and midbrain song control nuclei to the thalamic song control nucleus uvaeformis (Uva). Consistent with our hypothesis, bilateral lesions of Uva severely impaired singing. This impairment was long-lasting, as it persisted for at least 35 days after the lesions. Unilateral lesions of Uva on either side also resulted in an immediate singing impairment. However, song recovered substantially after less than 15 days, suggesting a possible compensation by the unlesioned side. Although the acoustic structure of individual syllables recovered fully after unilateral lesioning, subtle changes in the sequencing of syllables were observed after song recovery, suggesting that the lesion led to an alteration in the functioning of the remaining song control network. These results demonstrate that the adult songbird brain can adjust to damage to certain parts of the song control network and recover from their associated singing deficits. The well-identified and localized central neural pathways mediating birdsong production provide an advantageous model system to analyze systematically the sensorimotor contexts and the specific sites and mechanisms for behavioral recovery following partial damage to a behavior-producing neural circuit.