[First case series of femoral lengthening using a Precice® intramedullary nail in our center]. / Primera serie de casos de alargamiento femoral mediante clavo intramedular Precice® en nuestro centro.

INTRODUCTION: femoral lengthening using an intramedullary nail is one of the surgical options in the treatment of severe lower limb dysmetria in routine clinical practice. MATERIAL AND METHODS: a retrospective descriptive study was carried out on a series of five patients with a mean age of 15.4 years, who underwent femoral lengthening surgery using a Precice® intramedullary nail. The etiology in all cases was idiopathic. Preoperative and definitive postoperative theoretical lengthening or dysmetry was measured, as well as lengthening accuracy, distraction rate and index (mm/day and days/cm, respectively) and consolidation index (days/cm). Intraoperative and postoperative complications were identified in all cases. RESULTS: mean follow-up was 21 months (12-42), with no loss to follow-up. The mean duration of the surgical procedure was 126 minutes (105-160). The preoperative theoretical dysmetry was 38 ± 2.7 mm. The final mean lengthening was 41 ± 7.5 mm. The mean accuracy was 108% (91-125) and the distraction rate was 0.9 ± 0.4 mm/day. The distraction rate was 13.9 ± 5.1 days/cm and the consolidation rate was 26.6 ± 9.1 days/cm. Bone consolidation was observed in all patients with a mean of 113 ± 58 days. Regarding complications, a total of four minor muscular complications were found. CONCLUSION: the Precice® intramedullary nail is a good treatment option for cases of severe femoral shortening, providing good clinical and radiological results with a low rate of complications and implant failure.

INTRODUCCIÓN: el alargamiento femoral mediante clavo intramedular es una de las opciones quirúrgicas en el tratamiento de las dismetrías severas de miembros inferiores en la práctica clínica habitual. MATERIAL Y MÉTODOS: se realizó un estudio descriptivo retrospectivo de una serie de cinco pacientes con una media de edad de 15.4 años, intervenidos de alargamiento femoral mediante clavo intramedular Precice®. La etiología en todos los casos fue idiopática. Se midió la dismetría o alargamiento teórico prequirúrgico y el definitivo postquirúrgico, así como la precisión del alargamiento, la tasa y el índice de distracción (mm/día y días/cm respectivamente) y el índice de consolidación (días/cm). Se identificaron las complicaciones intra y postoperatorias en todos los casos. RESULTADOS: la media de seguimiento fue de 21 meses (12-42), sin pérdidas en el seguimiento. La duración media del procedimiento quirúrgico fue de 126 minutos (105-160). La dismetría teórica prequirúrgica fue de 38 ± 2.7 mm. El alargamiento medio final fue de 41 ± 7.5 mm. La precisión media fue de 108% (91-125) y la tasa de distracción de 0.9 ± 0.4 mm/día. El índice de distracción fue de 13.9 ± 5.1 días/cm y el índice de consolidación, de 26.6 ± 9.1 días/cm. La consolidación ósea se observó en la totalidad de los pacientes con una media de 113 ± 58 días. Con respecto a las complicaciones, se encontraron un total de cuatro complicaciones menores de índole muscular. CONCLUSIÓN: el clavo intramedular Precice® es una buena opción de tratamiento para casos de acortamiento femoral severo aportando buenos resultados clínicos y radiológicos con una baja tasa de complicaciones y fallo del implante.

[Treatment of intrinsic brain tumors located in motor eloquent areas. Results of a protocol based in navigation, tractography and neurophysiological monitoring of cortical and subcortical structures]. / Tratamiento de los tumores cerebrales intrínsecos de áreas motoras elocuentes. Resultados de un protocolo basado en la navegación, tractografía y monitorización neurofisiológica de estructuras corticales y subcorticales.

OBJECTIVES: The role of the microsurgical management of intrinsic brain tumors is to maximize the volumetric resection of the tumoral tissue minimizing the postoperative morbidity. The purpose of our paper has been to study the benefits of an original protocol developed for the microsurgical treatment of tumors located in eloquent motor areas where the navigation and electrical stimulation of motor subcortical pathways have been implemented. MATERIALS AND METHODS: A total of 17 patients operated on for resection of cortical or subcortical tumors in motor areas were included in the series. Preoperative planning for multimodal navigation was done integrating anatomic studies, motor functional MRI (f-MRI) and subcortical pathways volumes generated by diffusion tensor imaging (DTI). Intraoperative neuromonitorization included motor mapping by direct cortical and subcortical electrical stimulation (CS and sCS) and localization of the central sulcus using cortical multipolar electrodes and the N20 wave inversion technique. The location of all cortical and subcortical stimulated points with positive motor response was stored in the navigator and correlated with the cortical or subcortical motor functional structures defined preoperatively. RESULTS: The mean tumoral volumetric resection was 89.1±14.2% of the preoperative volume, with a total resection (≥100%) in twelve patients. Preoperatively a total of 58.8% of the patients had some motor deficit, increasing 24 hours after surgery to 76.5% and decreasing to 41.1% a month later. There was a great correlation between anatomic and functional data, both cortically and subcortically. However, in six cases it was not possible to identify the central sulcus and in many cases fMRI gave contradictory information. A total of 52 cortical points submitted to CS had positive motor response, with a positive correlation of 83.7%. Also, a total of 55 subcortical points had positive motor response, being in these cases 7.3±3.1 mm the mean distance from the stimulated point to the subcortical tract. CONCLUSIONS: The integration of preoperative and intraoperative anatomic and functional studies allows a safe functional resection of the brain tumors located in eloquent areas, compared to the tumoral resection based on anatomic imaging studies. Multimodal navigation allows the integration and correlation among preoperative and intraoperative anatomic and functional data. Cortical motor functional areas are anatomically and functionally located preoperatively thanks to MRI and fMRI and subcortical motor pathways with TDI and tractography. Intraoperative confirmation is done with CS and N20 inversion wave for cortical structures and with sCS for subcortical pathways. With this protocol we achieved a mean of 90% of volumetric resection in cortical and subcortical tumors located in eloquent motor areas with an increase of neurological deficits in the immediate postoperative period that significantly decreased one month later. Ongoing studies will define the safe limits for functional resection taking into account the intraoperative brain shift. Finally, it must be demonstrated if this protocol has any benefit for patients concerning disease free or overall survival.

Primera serie de casos de alargamiento femoral mediante clavo intramedular Precice® en nuestro centro / First case series of femoral lengthening using a Precice® intramedullary nail in our center

Resumen: Introducción: el alargamiento femoral mediante clavo intramedular es una de las opciones quirúrgicas en el tratamiento de las dismetrías severas de miembros inferiores en la práctica clínica habitual. Material y métodos: se realizó un estudio descriptivo retrospectivo de una serie de cinco pacientes con una media de edad de 15.4 años, intervenidos de alargamiento femoral mediante clavo intramedular Precice®. La etiología en todos los casos fue idiopática. Se midió la dismetría o alargamiento teórico prequirúrgico y el definitivo postquirúrgico, así como la precisión del alargamiento, la tasa y el índice de distracción (mm/día y días/cm respectivamente) y el índice de consolidación (días/cm). Se identificaron las complicaciones intra y postoperatorias en todos los casos. Resultados: la media de seguimiento fue de 21 meses (12-42), sin pérdidas en el seguimiento. La duración media del procedimiento quirúrgico fue de 126 minutos (105-160). La dismetría teórica prequirúrgica fue de 38 ± 2.7 mm. El alargamiento medio final fue de 41 ± 7.5 mm. La precisión media fue de 108% (91-125) y la tasa de distracción de 0.9 ± 0.4 mm/día. El índice de distracción fue de 13.9 ± 5.1 días/cm y el índice de consolidación, de 26.6 ± 9.1 días/cm. La consolidación ósea se observó en la totalidad de los pacientes con una media de 113 ± 58 días. Con respecto a las complicaciones, se encontraron un total de cuatro complicaciones menores de índole muscular. Conclusión: el clavo intramedular Precice® es una buena opción de tratamiento para casos de acortamiento femoral severo aportando buenos resultados clínicos y radiológicos con una baja tasa de complicaciones y fallo del implante.

Abstract: Introduction: femoral lengthening using an intramedullary nail is one of the surgical options in the treatment of severe lower limb dysmetria in routine clinical practice. Material and methods: a retrospective descriptive study was carried out on a series of five patients with a mean age of 15.4 years, who underwent femoral lengthening surgery using a Precice® intramedullary nail. The etiology in all cases was idiopathic. Preoperative and definitive postoperative theoretical lengthening or dysmetry was measured, as well as lengthening accuracy, distraction rate and index (mm/day and days/cm, respectively) and consolidation index (days/cm). Intraoperative and postoperative complications were identified in all cases. Results: mean follow-up was 21 months (12-42), with no loss to follow-up. The mean duration of the surgical procedure was 126 minutes (105-160). The preoperative theoretical dysmetry was 38 ± 2.7 mm. The final mean lengthening was 41 ± 7.5 mm. The mean accuracy was 108% (91-125) and the distraction rate was 0.9 ± 0.4 mm/day. The distraction rate was 13.9 ± 5.1 days/cm and the consolidation rate was 26.6 ± 9.1 days/cm. Bone consolidation was observed in all patients with a mean of 113 ± 58 days. Regarding complications, a total of four minor muscular complications were found. Conclusion: the Precice® intramedullary nail is a good treatment option for cases of severe femoral shortening, providing good clinical and radiological results with a low rate of complications and implant failure.

[Clinical guide to the management of patients with Beckwith-Wiedemann syndrome]. / Guía clínica para el seguimiento de pacientes con síndrome de Beckwith-Wiedemann.

Beckwith-Wiedemann syndrome (BWS) is characterized by congenital overgrowth, macroglossia and omphalocele or umbilical hernia. Children with BWS may also have all or some of the following features: asymmetry (hemihypertrophy) of the limbs, torso or face, hypoglycemia, organomegaly, ear pits or creases, and embryonal tumors. The frequency of BWS is approximately 1:14,000 births. We present a guide for the management of children with BWS aimed at helping pediatricians and general practitioners or specialists in the clinical follow-up of these patients. This guide has been structured according to different age groups and is based on published evidence.

[Holt-Oram syndrome: characterization of a novel mutation]. / Síndrome de Holt-Oram: caracterización de una nueva mutación.

INTRODUCTION: Cardiomyelic syndromes encompass congenital heart disease and skeletal malformations of the upper limbs and are related to mutations in transcription factors with T-Box domains. Holt-Oram syndrome is caused by a dominant mutation in the TBX5 gene that alters the three-dimensional structure of the protein and its DNA binding function. Several point mutations and deletions in TBX5 have been reported in patients with the Holt-Oram syndrome phenotype. PATIENTS AND METHODS: The proband was a boy with a large atrial septal defect ostium secundum type and a ventricular septal defect, diagnosed by clinical findings (heart murmur) and echocardiography. He also presented slightly hypoplastic thumbs with distal bilateral placement and an implantation index of 0.19 (compared with an average of 0.50 for his gestational age at birth). The boy was referred to the department of medical genetics to rule out 22q11.2 microdeletion syndrome. RESULTS: Karyotype and fluorescence in situ hybridization at locus D22S75 were both normal. Because of his clinical findings, molecular study for Holt-Oram syndrome was indicated, leading to the finding of a mutation at intron 7 of TBX5, probably producing a splicing alteration of the gene and resulting in a protein truncated at its C-terminal end. The proband's parents presented the wild type sequence of the gene, thus indicating that the mutation was produced de novo, although a possible germinal mosaicism in the parents could not be ruled out. CONCLUSIONS: Holt-Oram syndrome is the most frequent cause of cardiomyelic syndrome. All children with heart malformations and abnormalities of the upper limbs such as absent, hypoplastic, distally placed or triphalangic thumbs should undergo molecular studies for this syndrome.

Changes in circadian rhythms and sleep quality with aging: mechanisms and interventions.

Literature is reviewed indicating that aging is characterized by changes in circadian rhythms and sleep quality. The most marked change is an attenuation of amplitude. An advance of phase, a shortening of period, and a desynchronization of rhythms are also evident. The mechanisms underlying these changes are unknown. However, age-related changes in the retina, suprachiasmatic nucleus, and pineal gland seem relevant along with behavioral changes such as a reduction in physical activity and exposure to photic stimulation. Changes in circadian rhythms are frequently associated with a reduction in nighttime sleep quality, a decrease in daytime alertness, and an attenuation in cognitive performance; reversing such changes could enhance the quality of life for a large and rapidly increasing percentage of the population. Reversal appears possible by increasing melatonin levels with either appropriately timed exposure to photic stimulation and/or appropriately timed administration of exogenous melatonin. These interventions may increase aspects of genetic expression that have changed with aging. A hypothesis concerning the potential benefits of enhanced circadian amplitude is also offered.

Sleep-promoting and hypothermic effects of daytime melatonin administration in humans.

Sleep-promoting and hypothermic effects of orally administered melatonin during the daytime were assessed using a placebo-controlled, double-blind, cross-over design. Following a 7-hour nighttime sleep opportunity, healthy young male subjects (n = 8) were given either a placebo or one of three doses of melatonin (1 mg, 10 mg, and 40 mg) at 1000 hours. Sleep was polygraphically assessed in a 4-hour sleep opportunity from 1200 to 1600 hours. All doses of melatonin significantly shortened the latency to sleep onset. Melatonin also significantly increased total sleep time and decreased wake after sleep onset (WASO). Sleep following melatonin administration contained significantly more stage 2 and less stage 3-4, while stage 1 and rapid eye movement (REM) sleep were unaffected. In addition to the sleep-promoting effects, melatonin completely suppressed the normal diurnal rise of core body temperature. These data suggest that melatonin may be an effective method of promoting sleep for individuals attempting to sleep during their subjective day, such as shiftworkers and individuals rapidly traveling across multiple time zones.

Cognitive activity in sleep and responsiveness to external stimuli.

The relationship between responsiveness to auditory stimuli presented during sleep and cognitive activity during sleep was assessed. Sixteen college-aged women were instructed while awake to turn off a tone by taking a deep breath. The tone was then presented during Stage 2 and REM sleep. Subjects were awakened after select trials to assess the relationship between responding and reports of ongoing cognitive activity. Consistent with the view that cognitive activity reduces responsiveness, significantly fewer responses were found on report (cognitive activity) trials relative to no-report (no cognitive activity) trials in analyses involving all trials and Stage 2 trials alone. Trained judges then rated the subjects' reports of cognitive activity as indicating incorporation or not indicating incorporation of the tone and/or the breathing response. Incorporation was associated with a reduced likelihood of responding relative to no incorporation in analyses involving all trials. No difference in responding was found between no-incorporation trials and no-report trials, suggesting that reduced responsiveness is associated with cognitive activity only when incorporation occurs. These findings support hypotheses that the reduced responsiveness to external stimulation during sleep is at least in part due to ongoing cognitive activity.

Time-of-night patterns in PLMS activity.

Time-of-night patterns for periodic leg movement in sleep (PLMS) activity were assessed in 72 patients diagnosed as having PLMS (46 with only PLMS, 10 with narcolepsy and PLMS and 16 with sleep apnea and PLMS). Two distinct patterns of PLMS activity were identified. For one pattern, PLMS activity was high initially upon falling asleep and then decreased across the remainder of the night with the majority of the activity occurring during the first half of the night. For the second pattern, PLMS activity was relatively evenly distributed across the night with slightly greater activity in the middle of the night. These patterns were highly reliable across two nights of recording for group 1 and less so for group 2 even though there was significant variability between nights in the number of PLMS. Patients in group 2 were younger (except for those with sleep apnea) and were sleepier during the day. Implications of these findings for pharmacologic management are discussed.

Topographical and temporal patterns of brain activity during the transition from wakefulness to sleep.

Changes in electroencephalographic (EEG) spectral power, coherence and frequency were examined for the last minute of wakefulness and the first minute of sleep via topographical mapping. Data were also analyzed across sequential 1-minute samples of wake, stage 1 and stage 2 sleep. Not all brain regions exhibited the same EEG changes during the transition and not all brain regions were found to change at the same time. Brain sites closest to the midline (e.g. F4, C4, P4, O2) showed significant changes in EEG power (increases in theta and decreases in alpha power) during the transition to sleep, whereas brain sites most lateral to the midline (e.g. Fp2, F8, T4) showed little change. Decreases in alpha coherence were observed from wakefulness to sleep for brain site comparisons furthest away from each other (e.g. T3 vs. T4, T5 vs. T6, F7 vs. F8, F3 vs. O1, F4 vs. O2). Spectral analysis of EEG activity revealed that the time of significant change in EEG power varies among brain regions. Decreases in alpha power continued to occur later into the transition period for the posterior regions of the brain (O2, P4).

Sleep fragmentation and daytime sleepiness.

It has been noted that clinical populations complaining of excessive daytime sleepiness (EDS) frequently have disrupted or fragmented nocturnal sleep. The relation between sleep fragmentation and daytime sleepiness has not been systematically studied. This study was designed to use correlational techniques evaluating the relation between these variables in patients complaining of EDS, patients complaining of insomnia, and asymptomatic controls. The four groups studied included patients complaining of EDS with sleep apnea (n = 15) or with periodic leg movements (n = 15), patients complaining of insomnia (n = 15), and healthy volunteers with no sleep complaint (n = 10). One night of polysomnography followed by a Multiple Sleep Latency Test was obtained for each subject. Each recording was evaluated using standard criteria and also by a four-level arousal scoring system. Across all subjects, the total number of arousals correlated significantly with sleepiness index (r = 0.48, p less than 0.001). Closer analysis of the data shows that, depending upon the sleep complaint, different types of arousals are predictive of degree of daytime sleepiness. It is concluded that the number and type of nocturnal arousals play an important role in subsequent daytime sleepiness.

Effects of menstrual cycle phase and oral contraceptives on alertness, cognitive performance, and circadian rhythms during sleep deprivation.

The influence of menstrual cycle phase and oral contraceptive use on neurobehavioral function and circadian rhythms were studied in healthy young women (n = 25) using a modified constant routine procedure during 24 h of sleep deprivation. Alertness and performance worsened across sleep deprivation and also varied with circadian phase. Entrained circadian rhythms of melatonin and body temperature were evident in women regardless of menstrual phase or oral contraceptive use. No significant difference in melatonin levels, duration, or phase was observed between women in the luteal and follicular phases, whereas oral contraceptives appeared to increase melatonin levels. Temperature levels were higher in the luteal phase and in oral contraceptive users compared to women in the follicular phase. Alertness on the maintenance of wakefulness test and some tests of cognitive performance were poorest for women in the follicular phase especially near the circadian trough of body temperature. These observations suggest that hormonal changes associated with the menstrual cycle and the use of oral contraceptives contribute to changes in nighttime waking neurobehavioral function and temperature level whereas these factors do not appear to affect circadian phase.

Acute effects of bright light and caffeine on nighttime melatonin and temperature levels in women taking and not taking oral contraceptives.

Caffeine and bright light effects on nighttime melatonin and temperature levels in women were tested during the luteal phase of the menstrual cycle (n=30) or the pseudo luteal phase for oral contraceptive users (n=32). Participants were randomly assigned to receive either bright (5000 lux) or dim room light (<88 lux) between 20:00 and 08:00 h under a modified constant routine protocol. Half the subjects in each lighting condition were administered either caffeine (100 mg) or placebo in a double-blind manner at 20:00, 23:00, 02:00 and 05:00 h. Results showed that the combination of bright light and caffeine enhanced nighttime temperature levels to a greater extent than did either caffeine or bright light alone. Both of the latter groups had higher temperature levels relative to the dim light placebo condition and the two groups did not differ. Temperature levels in the bright light caffeine condition were maintained at near peak circadian levels the entire night in the luteal and pseudo luteal phase. Melatonin levels were reduced throughout the duration of bright light exposure for all women. Caffeine reduced the onset of melatonin levels for women in the luteal phase, but it had little effect on melatonin levels for oral contraceptive users. The results for women in the luteal phase of the menstrual cycle are consistent with our previous findings in men. The results also suggest that oral contraceptives may alter the effects of caffeine on nighttime melatonin levels.

Caffeine and light effects on nighttime melatonin and temperature levels in sleep-deprived humans.

The effects of caffeine ingestion and exposure to bright light, both separately and in combination, on salivary melatonin and tympanic temperature were assessed in humans. Four treatments during a 45.5 h sleep deprivation period were compared: Dim Light-Placebo, Dim Light-Caffeine, Bright Light-Placebo and Bright-Light Caffeine. The Dim Light-Caffeine condition (200 mg twice each night) relative to the Dim Light-Placebo condition suppressed nighttime melatonin levels and attenuated the normal decrease in temperature. Combining caffeine ingestion with bright light exposure (> or = 2000 lux) suppressed melatonin and attenuated the normal nighttime drop in temperature to a larger degree than either condition alone; i.e. effects were additive. Circadian effects were also observed in that the amplitude and phase of the temperature rhythm were altered during treatment. These findings establish that the human melatonin system is responsive to caffeine. Other evidence suggests that caffeine may influence melatonin and temperature levels through antagonism of the neuromodulator adenosine.

Habituation of P300 to target stimuli.

Two experiments examined the effects on N100, N200, and P300 of 1) repeated target presentation, 2) interblock intervals, 3) knowledge of session end, and 4) reversal of targets and nontargets. In the first experiment, subjects (N = 40) performed an oddball task across six test blocks (35 targets per block). Statistical analyses showed that repeated target presentation resulted in P300 amplitude decreases (habituation) and P300 latency increases within and across test blocks (11.3 microV to 5.4 microV, 330 msec to 348 msec), and also resulted in N200 amplitude increases within blocks and N200 latency increases within and across blocks. Interblock intervals resulted in partial recovery of these habituation effects. Knowledge of session end increased P300 amplitude, while reversal of targets and nontargets had no effect. The latencies of N200 and P300 were positively correlated (r .55), while the amplitudes were not. The second experiment demonstrated P300 habituation at multiple scalp sites (Fz, Cz, Pz). Two factors that may relate to habituation of the P300 are decreased arousal level and degree of involvement in the task.

The P300 component in sleep.

The present investigation utilized the P300 component of the auditory evoked potential as an index of information processing (discrimination) in sleep. Auditory evoked potentials were recorded to target and nontarget stimuli during sleep stages 3/4, 2 and REM under two probability conditions. Corresponding "nontone" waveforms were generated in each sleep stage, representing averaged EEG activity with no tones presented. Target P300 amplitude was higher than both corresponding "nontone" targets and tone nontargets. Probability did not affect the target-nontarget relationship. Latency of P300 increased and amplitude decreased from wakefulness through sleep; however, neither amplitude nor latency differed among sleep stages. Amplitude and latency of N200 increased during sleep. While N200 amplitude was highest in Stage 3-4, N200 latency did not differ among sleep stages. These findings suggest that the P300 recorded in sleep indexes similar cognitive processes as the P300 recorded in wakefulness. That P300 as well as N200 latency increased in sleep suggests that processes indexed by these components may slow during sleep.

Motor responsiveness to stimuli presented during sleep: the influence of time-of-testing on sleep stage analyses.

The relationship between time-of-night of testing (circadian factors) and motor responsiveness to stimuli presented during different stages of sleep was examined. Nine males slept for two nonconsecutive nights in the laboratory. On Night 1, tympanic temperature was assessed at 30 min intervals. On Night 2, responsiveness was assessed with an incremental series of tones presented in sleep stages 2, 3/4, and REM throughout the night. Subjects were instructed to make a microswitch closure to the tones. Results showed that for all stages, responsiveness decreased across thirds of the night. Because the distribution of each sleep stage differed across the night, the effects on responsiveness due to time-of-testing and to sleep stage were confounded. When time-of-testing was held constant, responsiveness was greater in stages 2 and REM than in stage 3/4. When time-of-testing was not held constant, effects nearly opposite of the latter could be demonstrated.

Immediate effects of different light intensities on body temperature and alertness.

The effects of different light intensities on temperature and alertness were investigated. It was hypothesized that temperature and alertness would be affected by certain light intensities but only during the melatonin release period (after 2100 h). Fifteen subjects were tested under three levels of light known to suppress melatonin (500, 1000, and 5000 lx) and a level known not to affect melatonin (50 lx). Subjects were tested on four occasions from 1700 until 2300 h. Tympanic temperature and measures of alertness (EEG power and frequency and self-reports) were obtained before and after melatonin onset. There were no differences in any measure prior to the melatonin onset, increases in temperature and alertness occurred only after melatonin onset. Temperatures and self-reported alertness scores obtained under light intensities of 500, 1000, and 5000 lx were elevated relative to those obtained under 50 lx but were not significantly different from each other. The results suggest that melatonin may be involved in mediating the effects of light on temperature and alertness and that 500 lx may be near the threshold for significant melatonin suppression, temperature enhancement, and increases in alertness.

Time of day, repeated testing, and interblock interval effects on P300 amplitude.

Time of day, repeated testing, and interblock interval effects on P300 amplitude were investigated. Subjects (N = 50) were tested using a standard oddball paradigm in either morning or afternoon sessions consisting of six test blocks per session. Amplitude of P300 was significantly higher in the morning than in the afternoon for all test blocks. In addition, amplitude of P300 habituated across test blocks from a mean of 9.59 microV on Block 1 to a mean of 4.98 microV on Block 6. Inserting a one-hour interval between Blocks 2 and 3 attenuated the rate of habituation. The results indicate that time of day, repeated testing, and interblock intervals affect P300 amplitude. Amplitude changes due to time of day may reflect circadian variations in cognitive resources indexed by the P300 component, while decrements due to repeated testing may reflect changes in allocation of resources across test sessions.