Intra-Areal Visual Topography in Primate Brains Mapped with Probabilistic Tractography of Diffusion-Weighted Imaging.

Noninvasive diffusion-weighted magnetic resonance imaging (dMRI) can be used to map the neural connectivity between distinct areas in the intact brain, but the standard resolution achieved fundamentally limits the sensitivity of such maps. We investigated the sensitivity and specificity of high-resolution postmortem dMRI and probabilistic tractography in rhesus macaque brains to produce retinotopic maps of the lateral geniculate nucleus (LGN) and extrastriate cortical visual area V5/MT based on their topographic connections with the previously established functional retinotopic map of primary visual cortex (V1). We also replicated the differential connectivity of magnocellular and parvocellular LGN compartments with V1 across visual field positions. Predicted topographic maps based on dMRI data largely matched the established retinotopy of both LGN and V5/MT. Furthermore, tractography based on in vivo dMRI data from the same macaque brains acquired at standard field strength (3T) yielded comparable topographic maps in many cases. We conclude that tractography based on dMRI is sensitive enough to reveal the intrinsic organization of ordered connections between topographically organized neural structures and their resultant functional organization.

Time course influences transfer of visual perceptual learning across spatial location.

Visual perceptual learning describes the improvement of visual perception with repeated practice. Previous research has established that the learning effects of perceptual training may be transferable to untrained stimulus attributes such as spatial location under certain circumstances. However, the mechanisms involved in transfer have not yet been fully elucidated. Here, we investigated the effect of altering training time course on the transferability of learning effects. Participants were trained on a motion direction discrimination task or a sinusoidal grating orientation discrimination task in a single visual hemifield. The 4000 training trials were either condensed into one day, or spread evenly across five training days. When participants were trained over a five-day period, there was transfer of learning to both the untrained visual hemifield and the untrained task. In contrast, when the same amount of training was condensed into a single day, participants did not show any transfer of learning. Thus, learning time course may influence the transferability of perceptual learning effects.

Individual Differences in the Alignment of Structural and Functional Markers of the V5/MT Complex in Primates.

Extrastriate visual area V5/MT in primates is defined both structurally by myeloarchitecture and functionally by distinct responses to visual motion. Myelination is directly identifiable from postmortem histology but also indirectly by image contrast with structural magnetic resonance imaging (sMRI). First, we compared the identification of V5/MT using both sMRI and histology in Rhesus macaques. A section-by-section comparison of histological slices with in vivo and postmortem sMRI for the same block of cortical tissue showed precise correspondence in localizing heavy myelination for V5/MT and neighboring MST. Thus, sMRI in macaques accurately locates histologically defined myelin within areas known to be motion selective. Second, we investigated the functionally homologous human motion complex (hMT+) using high-resolution in vivo imaging. Humans showed considerable intersubject variability in hMT+ location, when defined with myelin-weighted sMRI signals to reveal structure. When comparing sMRI markers to functional MRI in response to moving stimuli, a region of high myelin signal was generally located within the hMT+ complex. However, there were considerable differences in the alignment of structural and functional markers between individuals. Our results suggest that variation in area identification for hMT+ based on structural and functional markers reflects individual differences in human regional brain architecture.

Post-thyroidectomy tracheomalacia: minimal risk despite significant tracheal compression.

BACKGROUND: Tracheomalacia is a feared complication of goitre surgery, but considered rare in the Western World. This study aimed to estimate the risk of tracheomalacia in a contemporary series of patients with goitres causing significant tracheal compression. METHODS: A retrospective review was conducted of thyroidectomies performed in a UK tertiary referral centre over a 30 month period. Anaesthetic, operative, radiological, and pathological data were obtained from medical notes and hospital software systems. RESULTS: Of 334 patients who underwent thyroid surgery, preoperative CT scan was performed in 101 (30%). Tracheal compression was reported in 62 patients (19%) with minimum tracheal diameter ranging from 2 to 15 mm (mean 7.6 mm) due to multinodular goitre (n=50), malignancy (n=10), or thyroiditis (n=2). Critical compression <5 mm was observed in 18 patients (6%) and 35 patients had compression to 6-10 mm. Awake fibreoptic intubation was performed in eight patients (six of those with tracheas <5 mm) and asleep fibreoptic intubation was performed in one. Standard intubation was performed otherwise. All patients were recovered on a general surgical ward. None required tracheostomy or tracheal stenting. The incidence of tracheomalacia was 0 (95% confidence interval 0.0-4.8%). Mean length of stay was 2.4 days in those with tracheas <5 mm and 2.0 days in those >5 mm. CONCLUSIONS: We found no evidence of tracheomalacia in high-risk patients with significant tracheal compression. This supports prior work on retrosternal goitres suggesting that the risk of tracheomalacia is minimal in modern thyroid surgery. For risk management, however, we would still advocate that such patients be managed in units with multispeciality support.

Mapping the visual brain: how and why.

Over the past 15 years, techniques for identifying visual areas using magnetic resonance imaging (MRI) in human subjects have been applied widely to multiple populations. This review will cover the basic techniques of using functional MRI and very high-resolution structural MRI to determine boundaries between different areas of the visual cortex. Recent applications of these methods to ophthalmological patient populations are discussed, and the future potential applications of very high field strength MRI are considered.

Double-blind, placebo-controlled analgesic study of ibuprofen or rofecoxib in combination with paracetamol for tonsillectomy in children.

BACKGROUND: The analgesics used for paediatric tonsillectomy may be associated with side-effects such as sedation, respiratory depression and vomiting (opioids) or increased bleeding [non-steroidal anti-inflammatory drugs (NSAIDs)]. In our institution, we employ a combination of paracetamol, NSAID and opioid, although there is no published evidence of analgesic benefit from adding NSAIDs to paracetamol in children. METHODS: This randomized, double-blinded clinical study examined the analgesic effectiveness of combining paracetamol (20 mg kg(-1)) with rofecoxib (0.625 mg kg(-1)), ibuprofen (5 mg kg(-1)) or placebo as premedication for (adeno)tonsillectomy (n=98) in children aged 3-15 yr. Intravenous fentanyl 1-2 microg kg(-1) was given intraoperatively. Regular oral paracetamol (15 mg kg(-1), 4 hourly) was given after operation and could be supplemented on request from the child with oral ibuprofen 5 mg kg(-1) or oral codeine 1 mg kg(-1). The primary outcome variable was need for early supplementary analgesia (within 2 h after surgery). RESULTS: The addition of ibuprofen to paracetamol reduced the need for early analgesia from 72% to 38% of children (difference 34%; 95% confidence interval 4-64%). The addition of rofecoxib to paracetamol did not significantly alter the need for early analgesia (68 vs 72%). Pain scores were higher in those children who required early analgesia. There were no differences between the groups in operative blood loss or complications, total 24-h analgesic consumption, pain scores at 4 and 8 h, vomiting or antiemetic use. CONCLUSION: This study provides evidence to support the combination of ibuprofen (but not rofecoxib) with paracetamol for perioperative analgesia in children.

Responses of macaque V1 neurons to binocular orientation differences.

Interocular differences in orientation occur during binocular viewing of a surface slanted in depth. These orientation disparities could be exploited by the visual system to provide information about surface slant, but gradients of positional disparity provide an equally effective means to the same end. We examined the encoding of orientation disparities in V1 neurons that were recorded from two awake fixating monkeys. Monocular orientation selectivity was measured separately in each eye. Although the preferred monocular orientation in the left and right eyes was highly correlated (r = 0.98), 19 of 61 cells showed a significant interocular difference in preferred orientation (IDPO). By itself, an IDPO does not imply a specific binocular selectivity for orientation differences. We therefore examined the response to 25 binocular combinations of orientations by pairing each of five orientations in one eye with five in the other. Forty-four of 64 neurons showed responses that reflected the monocular orientation tuning selectivity; the preferred orientation disparity changed when the monocular orientation was changed in either eye. The remaining third (20 of 64) responded to a consistent orientation disparity in a way that was not simply predictable from monocular orientation selectivity. However, nearly all of these neurons were selective for positional disparity, and several characteristics of the responses suggest that the apparent selectivity for orientation disparities was just a consequence of the positional disparity sensitivity. Neither the data presented here nor previous data from the cat (Blakemore et al., 1972; Nelson et al., 1977) support the idea that a population of neurons early in the visual system has a separate encoding scheme for orientation disparities.

Visual discrimination learning in the water maze: a novel test for visual acuity.

Learning about space, the environment and specific objects comprising three-dimensional arrangements requires processing of visual information. As learning and memory experiments in mammals rely heavily on normal processing of visual cues, drug-induced disruption of acquisition learning or memory formation necessitates the important control for visual acuity. A popular task used frequently for rats is the Morris water maze. However, previously used visual tasks in the water maze only control for gross visual disturbances. Here we describe a new training procedure enabling visual acuity to be tested in the water maze. Animals were trained to discriminate between two cue cards containing a pattern of vertical black and white stripes. Cards were presented in two adjacent quadrants separated by a barrier with the escape platform located in front of the smaller stripes (1 cm wide). Once 80% correct responses were attained, the wider cue card (normally 5 cm wide stripes) was randomly changed to gratings of 1,2,3,4,5, and 10 cm width. Animals learned the discrimination with acuity of 1.5 c/deg. A detailed analysis of the swim patterns further suggests that, independent of the grating used, animals make a choice immediately after release and swim along the walls towards the cue. In a further acuity test taken a few weeks later when animals were given saline infusions, performance was better than in the first test suggesting an effect of learning. This novel test may prove useful in determining subtle drug-induced deficits in visual acuity that may contribute to disruption of spatial performance in the water maze.

Modeling V1 neuronal responses to orientation disparity.

The contribution of interocular orientation differences to depth perception, at either the neuronal or the psychophysical level, is unclear. To understand the responses of binocular neurons to orientation disparity, we extended the energy model of Ohzawa et al. (1990) to incorporate binocular differences in receptive-field orientation. The responses of the model to grating stimuli with interocular orientation differences were examined, along with the responses to random dot stereograms (RDS) depicting slanted surfaces. The responses to combinations of stimulus orientations in the two eyes were left-right separable, which means there was no consistent response to the binocular orientation difference. All existing neuronal data concerning orientation disparity can be well described by this type of model (even a version with no disparity selectivity). The disparity sensitive model is nonetheless sensitive to changes in RDS slant, although it requires narrow orientation bandwidth to produce substantial modulation. The disparity-insensitive model shows no selectivity to slant in this stimulus. Several modifications to the model were attempted to improve its selectivity for orientation disparity and/or slant. A model built by summing several disparity-sensitive models showed left-right inseparable responses, responding maximally to a consistent orientation difference. Despite this property, the selectivity for slant in RDS stimuli was no better than the simple disparity-selective model. The range of models evaluated here demonstrate that interocular orientation differences are neither necessary nor sufficient for signaling slant. In contrast, within the framework of the energy model, positional disparity sensitivity appears to be both necessary and sufficient.

Analgesic efficacy of ketorolac 0.5% ophthalmic solution (Accular) in paediatric strabismus surgery.

This prospective double-blind study was designed to assess the analgesic efficacy of ketorolac 0.5% ophthalmic solution compared with placebo in 30 healthy children undergoing extraocular muscle recession for correction of strabismus. After paracetamol 20 mg.kg-1 preoperatively, a standard anaesthetic was given. There were no significant differences in Children's Hospital of Eastern Ontario Pain Scale (CHEOPS) and faces pain scale (FPS) scores, requirement for supplementary analgesia or in postoperative vomiting between the two groups over the following 24 h. This study did not demonstrate improved postoperative analgesia when topical ketorolac eye drops were given in addition to paracetamol. This observed lack of efficacy may reflect difficulties in the use of CHEOPS and FPS in this age group with this pain model.

Reversible neural inactivation reveals hippocampal participation in several memory processes.

Studies of patients and animals with brain lesions have implicated the hippocampal formation in spatial, declarative/relational and episodic types of memory. These and other types of memory consist of a series of interdependent but potentially dissociable memory processes-encoding, storage, consolidation and retrieval. To identify whether hippocampal activity contributes to these processes independently, we used a novel method of inactivating synaptic transmission using a water-soluble antagonist of AMPA/kainate glutamate receptors. Once calibrated using electrophysiological and two-deoxyglucose techniques in vivo, drug or vehicle was infused chronically or acutely into the dorsal hippocampus of rats at appropriate times during or after training in a water maze. Our findings indicate that hippocampal neural activity is necessary for both encoding and retrieval of spatial memory and for either trace consolidation or long-term storage.

Decreased mivacurium requirements and delayed neuromuscular recovery during sevoflurane anesthesia in children and adults.

UNLABELLED: The purpose of this study was to compare the mivacurium infusion requirements and neuromuscular recovery in adults and children during propofol/opioid and sevoflurane anesthesia. Seventy-five adult and 75 pediatric patients were randomized to receive propofol/opioid 0.5 or 1.0 minimum alveolar anesthetic concentration (MAC) (age-related) sevoflurane anesthesia. Plasma cholinesterase (PChE) activity was measured. Neuromuscular blockade was monitored by train-of-four (TOF) stimulation every 10 s and adductor pollicis electromyography. A bolus of 2 x the 95% effective dose of mivacurium (0.25 mg/kg) was followed by an infusion titrated to maintain 90%-95% blockade. Mivacurium doses were recorded every 5 min. At the end of surgery, the infusion was stopped, and recovery from mivacurium was monitored until TOF > or =0.7. PChE concentrations were within the normal range (adults 4-12 KU/L, children 6-16 KU/L) and correlated with mivacurium dose. Mivacurium infusion rates were higher in children than in adults: at 30 min, the rates in children were 13.1 +/- 6.4, 8.1 +/- 4.7, and 5.2 +/- 2.9 microg x kg(-1) x min(-1) at 0, 0.5, and 1.0 MAC sevoflurane, respectively; the corresponding rates in adults were 5.9 +/- 3.1, 4.3 +/- 1.7, and 2.9 +/- 0.7 microg x kg(-1) x min(-1) (P < 0.01). Sevoflurane decreased mivacurium requirements, maximal decreases at 45 min in children and 10 min in adults, and delayed neuromuscular function recovery. Children recovered twice as quickly as adults, achieving TOF > or =0.7 at 9.8 +/- 2.5, 11.4 +/- 2.8, and 19.6 +/- 6.3 min compared with 19.9 +/- 5.4, 26.4 +/- 8.3, and 32.9 +/- 9.8 min in adults (P < 0.0001). In conclusion, mivacurium requirements were correlated with PChE, were greater in children than in adults, and were reduced by sevoflurane. Neuromuscular recovery occurred more rapidly in children and was delayed by sevoflurane. IMPLICATIONS: The mivacurium infusion requirement to maintain constant 90%-95% neuromuscular block during anesthesia is correlated with plasma cholinesterase activity. It is increased in children and reduced by the inhaled anesthetic sevoflurane. Despite the larger dose administered to children, recovery from block occurred more rapidly in children than in adults and was delayed by sevoflurane.

Temporal aspects of slant and inclination perception.

Linear transformations (shear or scale transformations) of either horizontal or vertical disparity give rise to the percept of slant or inclination. It has been proposed that the percept of slant induced by vertical size disparity, known as Ogle's induced-size effect, and the analogous induced-shear effect, compensate for scale and shear distortions arising from aniseikonia, eccentric viewing, and cyclodisparity. We hypothesised that these linear transformations of vertical disparity are processed more slowly than equivalent transformations of horizontal disparity (horizontal shear and size disparity). We studied the temporal properties of the stereoscopic slant and inclination percepts that arose when subjects viewed stereograms with various combinations of horizontal and vertical size or shear disparities. We found no evidence to support our hypothesis. There were no clear differences in the build-up of percepts of slant or inclination induced by step changes in horizontal size or shear disparity and those induced by step changes in vertical size or shear disparity. Perceived slant and inclination decreased in a similar manner with increasing temporal frequency for modulations of transformations of both horizontal and vertical disparity. Considerable individual differences were found and several subjects experienced slant reversal, particularly with oscillating stimuli. An interesting finding was that perceived slant induced by modulations of dilation disparity was in the direction of the vertical component. This suggests the vertical size disparity mechanism has a higher temporal bandwidth than the horizontal size disparity mechanism. However, conflicting perspective information may play a dominant role in determining the temporal properties of perceived slant and inclination.

An analogue approach to audit in anaesthesia.

The predicted outcome of anaesthesia and surgery was compared with the immediate outcome in 508 patients by means of two 100 mm linear analogue scales. The results were used to obtain a statistically based rule by which the anaesthetist may consistently select three groups of patients for audit: group 1, patients in whom immediate outcome of anaesthesia and surgery is worse than predicted; group 2, patients whose outcome is better than predicted; and group 3, the remaining patients. The rule, which is simply adapted to departmental audit, does not necessarily need a computer but is suited to the computer as it is numerically based.

Plasma observations near neptune: initial results from voyager 2.

The plasma science experiment on Voyager 2 made observations of the plasma environment in Neptune's magnetosphere and in the surrounding solar wind. Because of the large tilt of the magnetic dipole and fortuitous timing, Voyager entered Neptune's magnetosphere through the cusp region, the first cusp observations at an outer planet. Thus the transition from the magnetosheath to the magnetosphere observed by Voyager 2 was not sharp but rather appeared as a gradual decrease in plasma density and temperature. The maximum plasma density observed in the magnetosphere is inferred to be 1.4 per cubic centimeter (the exact value depends on the composition), the smallest observed by Voyager in any magnetosphere. The plasma has at least two components; light ions (mass, 1 to 5) and heavy ions (mass, 10 to 40), but more precise species identification is not yet available. Most of the plasma is concentrated in a plasma sheet or plasma torus and near closest approach to the planet. A likely source of the heavy ions is Triton's atmosphere or ionosphere, whereas the light ions probably escape from Neptune. The large tilt of Neptune's magnetic dipole produces a dynamic magnetosphere that changes configuration every 16 hours as the planet rotates.

Plasma observations near uranus: initial results from voyager 2.

Extensive measurements of low-energy positive ions and electrons in the vicinity of Uranus have revealed a fully developed magnetosphere. The magnetospheric plasma has a warm component with a temperature of 4 to 50 electron volts and a peak density of roughly 2 protons per cubic centimeter, and a hot component, with a temperature of a few kiloelectron volts and a peak density of roughly 0.1 proton per cubic centimeter. The warm component is observed both inside and outside of L = 5, whereas the hot component is excluded from the region inside of that L shell. Possible sources of the plasma in the magnetosphere are the extended hydrogen corona, the solar wind, and the ionosphere. The Uranian moons do not appear to be a significant plasma source. The boundary of the hot plasma component at L = 5 may be associated either with Miranda or with the inner limit of a deeply penetrating, solar wind-driven magnetospheric convection system. The Voyager 2 spacecraft repeatedly encountered the plasma sheet in the magnetotail at locations that are consistent with a geometric model for the plasma sheet similar to that at Earth.