The resilience of the developing reading system: multi-modal evidence of incident and recovery after a pediatric stroke

Decades of neuroscientific findings have elucidated the highly specialized brain areas involved in reading, especially along the ventral occipitotemporal stream where the critical step of recognizing words occurs. We report on a 14-year-old female with temporary dyslexia after a left ventral occipitotemporal ischemic stroke. Our longitudinal multimodal findings show that the resolution of the reading impairment was associated with heightened activity in the left posterior superior and inferior temporal gyri. Our findings highlight the role of the left inferior temporal gyrus in reading and the importance of perilesional and ipsilateral cortical areas for functional recovery after childhood stroke.

Apparent diffusion coefficient scalars correlate with near-infrared spectroscopy markers of cerebrovascular autoregulation in neonates cooled for perinatal hypoxic-ischemic injury

BACKGROUND AND PURPOSE

Neurologic morbidity remains high in neonates with perinatal hypoxic-ischemic injury despite therapeutic hypothermia. DTI provides qualitative and quantitative information about the microstructure of the brain, and a near-infrared spectroscopy index can assess cerebrovascular autoregulation. We hypothesized that lower ADC values would correlate with worse autoregulatory function.

MATERIALS AND METHODS

Thirty-one neonates with hypoxic-ischemic injury were enrolled. ADC scalars were measured in 27 neonates (age range, 4-15 days) in the anterior and posterior centrum semiovale, basal ganglia, thalamus, posterior limb of the internal capsule, pons, and middle cerebellar peduncle on MRI obtained after completion of therapeutic hypothermia. The blood pressure range of each neonate with the most robust autoregulation was identified by using a near-infrared spectroscopy index. Autoregulatory function was measured by blood pressure deviation below the range with optimal autoregulation.

RESULTS

In neonates who had MRI on day of life ≥10, lower ADC scalars in the posterior centrum semiovale (r = -0.87, P = .003, n = 9) and the posterior limb of the internal capsule (r = -0.68, P = .04, n = 9) correlated with blood pressure deviation below the range with optimal autoregulation during hypothermia. Lower ADC scalars in the basal ganglia correlated with worse autoregulation during rewarming (r = -0.71, P = .05, n = 8).

CONCLUSIONS

Blood pressure deviation from the optimal autoregulatory range may be an early biomarker of injury in the posterior centrum semiovale, posterior limb of the internal capsule, and basal ganglia. Optimizing blood pressure to support autoregulation may decrease the risk of brain injury in cooled neonates with hypoxic-ischemic injury.

The rapid formation of a large rotating disk galaxy three billion years after the Big Bang

Observations and theoretical simulations have established a framework for galaxy formation and evolution in the young Universe. Galaxies formed as baryonic gas cooled at the centres of collapsing dark-matter haloes; mergers of haloes and galaxies then led to the hierarchical build-up of galaxy mass. It remains unclear, however, over what timescales galaxies were assembled and when and how bulges and disks--the primary components of present-day galaxies--were formed. It is also puzzling that the most massive galaxies were more abundant and were forming stars more rapidly at early epochs than expected from models. Here we report high-angular-resolution observations of a representative luminous star-forming galaxy when the Universe was only 20% of its current age. A large and massive rotating protodisk is channelling gas towards a growing central stellar bulge hosting an accreting massive black hole. The high surface densities of gas, the high rate of star formation and the moderately young stellar ages suggest rapid assembly, fragmentation and conversion to stars of an initially very gas-rich protodisk, with no obvious evidence for a major merger.

HIV prevention research and global inequality: steps towards improved standards of care

Intensification of poverty and degradation of health infrastructure over recent decades in countries most affected by HIV/AIDS present formidable challenges to clinical research. This paper addresses the overall standard of health care (SOC) that should be provided to research participants in developing countries, rather than the narrow definition of SOC that has characterised the international debate on standards of health care. It argues that contributing to sustainable improvements in health by progressively ratcheting the standard of care upwards for research participants and their communities is an ethical obligation of those in resource-rich countries who sponsor and implement research in poorer ones.

Totally extraperitoneal repair of obturator hernia

BACKGROUND

One distinct advantage of the 1aparoscopic inguinal hernia repair is the opportunity for clear visualization of the direct, indirect, femoral, and obturator spaces. The surgeon should routinely inspect all of them. Obturator hernia accounts for as few as 0.073% of all hernias, but the mortality rate when it is acutely incarcerated can be as high as 70%. There is only one previous report of a totally extraperitoneal repair for obturator hernia. Five such procedures are described.

METHODS

A retrospective review was undertaken to evaluate one surgeon's experience with the totally extraperitoneal repair of obturator hernia over a 4-year period. Four of five cases were completed, and the remaining case was converted to an open procedure.

RESULTS

Three hernias were on the right side, and two on the left. One patient presented with an acutely incarcerated obturator hernia and underwent a small bowel resection for strangulated bowel within the obturator space. The other four hernias were found during totally extraperitoneal repair, and the patients were discharged home several hours later. There was one complication, a midline wound infection in the patient with strangulated bowel. It was treated with dressing changes. There were no other complications, and during a follow-up period of 3 to 48 months, there was no recurrence.

CONCLUSIONS

The laparoscopic totally extraperitoneal approach allows inspection and repair of direct, indirect, femoral, and obturator hernias. This study found this procedure to be feasible, safe, and highly effective for the diagnosis and repair of obturator hernias.

Investigating a possible cause of mesh migration during totally extraperitoneal (TEP) repair

BACKGROUND

In experienced hands, laparoscopic inguinal hernia repair has a low rate of recurrence, but it still can recur, and a number of reasons for this have been identified. In published studies, the majority of such cases seem to result from inadequate dissection leading to missed hernias or suboptimal mesh placement. But even with adequate dissection and proper placement of a sufficiently large mesh, recurrence sometimes happens. A number of investigators have cited mesh migration or dislocation as a possible cause, and this study examined how hip flexion affects the position of newly placed meshes and staples in totally extraperitoneal (TEP) repair of inguinal hernia.

METHODS

After completion of the dissection and reduction of discovered hernias, a 15 x 15-cm polypropylene mesh was placed either unilaterally or bilaterally, as indicated. The preperitoneal space then was desufflated. The operating table, in an extended -20 degrees position during surgery, was placed in a 90 degrees position for approximately 15 s. After reinsufflation, the possibility of mesh migration and folding was investigated. Finally, the mesh was stapled, the table again extended and flexed, and the possibility of mesh migration and staple dislodgement investigated once more.

RESULTS

The mesh did not migrate or become displaced from any potential hernia area, nor did any of the staples become dislodged.

CONCLUSIONS

Concern about mesh migration attributable to patients sitting up immediately after surgery appears to be unfounded, at least according to the findings for the current, small, simulated study group.

Totally extraperitoneal (TEP) hernia repair after an original TEPIs it safe, and is it even possible?

BACKGROUND

There are only scant published reports of totally extraperitoneal (TEP) repair of recurrence after a primary TEP procedure. Furthermore, at least two authors have made the statement that such an operation is virtually impossible.

METHODS

We have been performing TEP repair of recurrence after TEP since we 1996, and here we present a retrospective review of our experience with the procedure. We employ a method not varying greatly from the standard TEP done for primary hernia.

RESULTS

All cases were started laparoscopically, and only one of 20 had to be converted to open. Of these cases, 12 were for same-side recurrence and eight for a contralateral new hernia. With a follow-up of 28-74 months, there have been no fatalities, no complications, and no re-recurrence.

CONCLUSION

We have found that TEP repair of recurrent inguinal hernia after a primary TEP repair is entirely feasible technically as well as entirely safe.

Obstructive symptoms associated with the Lap-Band in the first 24 hours

BACKGROUND

The Lap-Band is a gastric restrictive procedure for the treatment of morbid obesity. We review the etiology of obstructive complications that present in the first postoperative 24 h.

METHODS

Fifty-six Lap-Band procedures were performed by one surgeon between January and September 2002.

RESULTS

Six patients presented with obstruction within 24 h of surgery: gastric slippage in three patients, gastric edema in one patient, and esophageal hypomotility in two patients.

CONCLUSIONS

Placing the band in an esophagogastric position as per Belachew and Weiner reduced our incidence of gastric slippage to none. Endoscopy with placement of a nasogastric feeding tube can relieve obstruction caused by esophageal hypomotility. Gastric edema with no clinical signs of obstruction will resolve with time. Clinicians must be aware of the unique complications that come with the advent of this new procedure.

Laparoscopic extraperitoneal approach to acutely incarcerated inguinal hernia

BACKGROUND

Laparoscopic treatment of acutely incarcerated inguinal hernia is uncommon and still controversial. Those being performed almost all use the transabdominal (TAPP) approach. The authors here present their experience with totally extraperitoneal (TEP) repair of acutely incarcerated hernia.

METHODS

A retrospective review was undertaken to evaluate the authors' experience with this procedure over a 4-year period. There were 16 cases, 5 of which were performed using a conventional anterior repair. These 5 cases were excluded from the review. The surgery for all of the remaining 11 acutely incarcerated hernias was started laparoscopically using the TEP approach. Eight of the cases were completed this way, whereas three were converted to the open procedure. In addition to standard TEP repair techniques, a releasing incision is required for acutely incarcerated direct, indirect, or femoral hernias. With a direct hernia, the opening of the defect is enlarged to allow safe dissection of its contents. A releasing incision is made at the anteromedial aspect of the defect to avoid injury to the epigastric or iliac vessels. With an indirect hernia, several additional steps are required. The epigastric vessels may be divided; an additional trocar may be placed laterally below the linea semicircularis to facilitate dissection of the sac and to assist with suturing of the divided sac; and the deep internal ring is divided anteriorly at the 12 o'clock position toward the external ring, facilitating dissection of the indirect sac. With a femoral hernia, a releasing incision is made by carefully incising the insertion of the iliopubic tract into Cooper's ligament at the medial portion of the femoral ring.

RESULTS

The mean operative time was 50 min (range, 20-120 min), and the length of hospital stay was 5.4 days (range, 1-29 days). During a follow-up period of 9 to 69 months, there was no recurrence, and only two complications. One of these complications was an infected mesh that occurred in a case involving cecal injury. It was treated with continuous irrigation and salvaged. The other complication was a midline wound infection after a small bowel resection for a strangulated obturator hernia.

CONCLUSIONS

Familiarity with the anatomy involved leads to the conclusion that the laparoscopic approach, specifically the TEP procedure, can be used without hesitation even in cases of acutely incarcerated hernia.

Laparoscopic adjustable gastric banding: is there a learning curve?

BACKGROUND

To be certified for laparoscopic placement of adjustable gastric banding, surgeons must have advanced laparoscopic experience. Despite previous exposure to other kinds of laparoscopy, there may a learning curve specific to Lap-Band placement.

METHODS

Sixty consecutive patients were prospectively separated into two groups: the first 30 patients operated on (group 1) and the second 30 patients operated on (group 2).

RESULTS

Both groups were similar statistically in regard to gender, age, and body mass index. Operative time for group 1 was 79 +/- 31.1 min. There were 11 (37%) complications in 10 patients. Operative time for group 2 was 59 +/- 19.9 min. There were two complications (7%). All operations were completed laparoscopically. Operative time was significantly lower in group 2 ( t-test; p = 004). Complications were also significantly lower (chi-square; p = 0.005). The number of reoperations was also reduced and approached statistical significance (chi-square; p = 0.054). Readmissions, although reduced, were not statistically significant. There were no deaths in either group.

CONCLUSIONS

Despite a surgeon's history of advanced laparoscopic experience, there is a definite learning curve associated with the laparoscopically placed adjustable gastric band.

Sometimes a noun is just a noun: comments on Bird, Howard, and Franklin (2000)

Bird, Howard, and Franklin (2000) have proposed a semantic-conceptual explanation of grammatical category-specific deficits that attributes impairments in noun and verb processing to two distinct mechanisms. According to their account, apparent deficits in verb production are not category specific, but rather result from the lower imageability of verbs compared to concrete nouns. Noun deficits are said to result from differences in the distribution of semantic feature types such that damage to sensory features results in disproportionate impairments in naming nouns, especially animate nouns, compared to verbs. However, this hypothesis, which we call the "extended sensory/functional theory" (ESFT), fails on several counts. First, the assumption that representations of living things are more heavily freighted with sensory features than are those of nonliving objects does not have any reliable empirical basis. Second, the ESFT incorrectly predicts associations between deficits in processing sensory features and living things or functional features and nonliving things. Finally, there are numerous cases of patients with grammatical category-specific deficits that do not seem to be consistent with damage at the semantic level. All of this suggests that the ESFT is not a useful model for considering grammatical (or semantic) category-specific deficits.

The use of gated cine phase contrast and MR venography in achondroplasia

OBJECTS

Foramen magnum and jugular foramen stenosis, well-known problems in achondroplasia, may result in brain stem compression and venous outflow obstruction, respectively. We studied a series of children with achondroplasia using gated cine phase contrast (PC) CSF flow studies to evaluate CSF dynamics across the foramen magnum and MR venography (MRV) to depict obstructed venous drainage.

METHODS

Ten patients (9 months to 11 years, mean 2.85 years) were referred for possible brain stem compression. MRI included routine sequences, cine PC with velocity encoding (VENC) = 5 cm/s, and MRV. Six patients, including the asymptomatic patient, had brain stem compression without tonsillar herniation; two had tonsillar herniation; and two had neither brain stem compression nor tonsillar herniation. Abnormal tonsillar movement was seen only with tonsillar herniation. MRV showed steno-occlusive disease of the internal jugular vein (IJV) in nine patients, sigmoid sinus in four, and absent or hypoplastic transverse sinus in seven. Veno-occlusive disease was not progressive. No patient had massive hydrocephalus, although larger ventricles were associated with more profuse venous collateral formation and more severe degrees of IJV stenosis. Three patients have undergone CSF diversion.

CONCLUSIONS

MR imaging may be useful in defining the pathophysiology of brain stem compression and hydrocephalus in achondroplasia.

MR venography in children with complex craniosynostosis

PURPOSE

Chronic venous hypertension due to jugular foramen stenosis has been proposed as an etiology for the hydrocephalus and tonsillar herniation seen in some patients with complex craniosynostosis. We report the use of MR venography (MRV) to evaluate venous outflow obstruction in this clinical setting.

MATERIALS AND METHODS

We studied 17 patients, (ages 4 months to 34 years; mean 7.3 years) with complex craniosynostosis; 8 patients with Crouzon's syndrome, 2 with Apert's, 1 with Pfeiffer's and 6 patients without an eponymous classification. MR imaging included routine imaging sequences and axial 2D TOF MRV. Patterns of venous drainage and the presence of hydrocephalus and tonsillar herniation were noted.

RESULTS

Jugular vein obstruction was seen in 12/17 patients; in 5/8 patients with Crouzon's, 1/2 with Apert's, the single patient with Pfeiffer's and 5/6 patients with nonsyndromic craniosynostosis. The predominant collateral drainage was via the posterior condylar veins. Nine of 12 (75%) of the patients with abnormal MRV had hydrocephalus; 3/8 patients with Crouzon's, 1/2 patients with Apert's, and 5/6 nonsyndromic patients. Two patients had hydrocephalus with normal MRV. Ten patients had tonsillar herniation, which was associated with shunted hydrocephalus in 7/10 patients, and hydrocephalus seen prior to shunt placement in 3/9. Nine of 10 patients with tonsillar herniation had an abnormal MRV, while 1 patient had a normal MRV. Venous pressures measured in 1 patient showed an 8-mm-Hg differential across the skull base.

CONCLUSIONS

The posterior condylar veins appear pivotal in maintaining venous drainage when the jugular bulbs are occluded. Although the association between venous outflow obstruction, hydrocephalus and tonsillar herniation is intriguing, evidence of venous outflow obstruction by MRV may not be indicative of significant intracranial venous hypertension.

Posterior fossa syndrome: identifiable risk factors and irreversible complications

Cerebellar mutism was first described by Rekate et al. in 1985 as a transient condition which occurs after posterior fossa operations in children. Posterior fossa syndrome (PFS) and cerebellar mutism are often used interchangeably in the literature. In our experience, we found cerebellar mutism to be a reversible component of a persistent neurologic syndrome. The cause and identifiable risk factors have not been clearly elucidated in the literature. To further characterize PFS, we reviewed 253 children with posterior fossa tumors who underwent surgical resection. We documented 20 cases of PFS (8%), 12 males and 8 females. Age ranged from 1.5 to 13 years (mean = 6.5). Of the 20, 16 were medulloblastoma, 3 ependymoma and 1 astrocytoma. There was a 21 % incidence (16/76) of PFS in medulloblastoma of the posterior fossa. The incidence for ependymoma was 13% (3/24) and 1% (1/102) for astrocytoma. All 20 cases (100%) had brainstem involvement by the tumor. The most frequent postoperative findings included mutism, ataxia, 6th and 7th nerve palsies and hemiparesis. Mutism had a latency range of 1-7 days (mean = 1.7) and a duration of 6-365 days (mean = 69.2, median = 35). Although mutism resolved in all cases, the remaining neurologic complications which characterized our findings of PFS were rarely reversible. We describe potential risk factors for developing PFS after surgery with hopes of making neurosurgeons more aware of potential problems following the removal of lesions in this area. Early recognition of PFS would further promote patient and family understanding and coping with this syndrome.

NuMA: evaluation of a new biomarker for the detection of low stage colorectal cancer

An enzyme immunoassay for NuMA was evaluated in a retrospective clinical study for its potential utility in the detection of colorectal cancer. The concentrations of NuMA and CEA (Abbott IMx) were measured in sera from 86 patients (presurgical) with colorectal cancer, 72 subjects with benign gastrointestinal diseases, 80 subjects with risk factors for colorectal cancer, and 141 age-matched healthy subjects. Reference values for NuMA and CEA were calculated by two methods: 95% cumulative distribution and ROC analyses versus healthy subjects. By the first method, NuMA and CEA both had approximately 20% sensitivity for colorectal cancer. By the second method (which generated lower reference values), NuMA was more sensitive than CEA for colorectal cancer. This improved sensitivity was most evident in Dukes B subjects. By either analysis method, NuMA was more sensitive than CEA for subjects at risk for developing colorectal cancer, whereas CEA was more specific for benign gastrointestinal diseases.

Open and laparoscopic cholecystectomy in acquired immunodeficiency syndrome: indications and results in fifty-three patients

BACKGROUND

Although acute cholecystitis is one of the most common indications for abdominal surgery in patients with acquired immunodeficiency syndrome (AIDS), previous studies have reported disappointingly high morbidity and mortality among those patients who have undergone cholecystectomy. The aims of this study were to analyze the indications for and the outcome of cholecystectomy performed for acute cholecystitis in patients with AIDS.

METHODS

We retrospectively reviewed the hospital charts of 53 patients with AIDS who underwent open or laparoscopic cholecystectomy from 1992 to 1997. Statistical analysis using the chi-square, Student's t, and Fisher exact tests was conducted to determine whether cause of cholecystitis, type of surgical approach, and CD4+ T-lymphocyte count influenced outcome.

RESULTS

The clinical findings and imaging by ultrasonography were always reliable in establishing diagnosis and guiding treatment of acute cholecystitis. Open cholecystectomy was performed in 24 patients (45%). The procedure was begun laparoscopically in 29 patients (55%) and converted to open in 4 (14%). The pathologic findings showed acalculous cholecystitis in 19 patients (36%) and cholelithiasis in 32 (60%). Morbidity was 34% and mortality was 2%. Type of operative approach, cause of cholecystitis, and CD4+ T-lymphocyte count (greater or less than 50 cells/mm3) did not significantly affect morbidity and mortality. The length of hospital stay was significantly influenced by the CD4+ T-lymphocyte count.

CONCLUSIONS

These findings suggest that in most patients with AIDS, laparoscopic or open cholecystectomy may be performed with significant but acceptable morbidity and low mortality.

Surgical treatment of HIV-related immune thrombocytopenia

Immune related thrombocytopenia has been described extensively in patients infected with the human immunodeficiency virus (HIV). The efficacy and safety of splenectomy performed in 21 patients affected with HIV-related immune thrombocytopenia (platelet count less than 50,000/mm3), between 1992 and 1996, were evaluated. All the patients were symptomatic and had failed medical therapy. Nine of them were affected with acquired immune deficiency syndrome (AIDS), whereas 12 were HIV-positive (non-AIDS). In all the patients, a pre-operative bone marrow biopsy revealed increased megakaryocytes. Follow-up ranged from 5-16 months. The response rate to splenectomy (platelet count greater than 100,000/mm3) in the AIDS group was 83%, as opposed to 100% in the HIV-positive (non-AIDS) group. During the follow-up period, 19 of the 21 patients maintained platelet counts greater than 98,000/mm3; of the two non-responders, one patient expired 3 weeks after surgery, and a second patient had never responded. None of the HIV-positive (non-AIDS) patients developed AIDS during the follow-up period. All the complications observed (24%) were treated without sequelae. Based on these data, splenectomy can be considered safe and effective in treating patients with symptomatic HIV-related thrombocytopenia, when medical therapy has failed. Moreover, splenectomy did not appear to adversely affect the rate of conversion from the HIV-positive to the AIDS status, nor did it accelerate the progression of the disease in patients already diagnosed with AIDS.

Enhancement of the nucleosomal pattern in sequences of lower complexity

Intuitively, the complexity of a given DNA sequence is related to the number of various superimposed biological messages it contains. Here we assess the expectation that in nucleosome DNA sequences of lower linguistic complexity, the nucleosome DNA positioning pattern would be more pronounced than in those of higher linguistic complexity. The nucleosome DNA positioning pattern is one of the weakest (highly degenerate) sequence patterns. It has been extracted recently by specially designed multiple alignment procedures. We applied the most sensitive of these procedures to nearly equal subsets of a nucleosome database separated according to linguistic complexity. The pattern extracted from the subset of the simpler nucleosome sequences not only possesses all major attributes of the known nucleosomal pattern, but is substantially stronger with respect to amplitude in comparison with the total database. This result constitutes the first demonstration that a weak pattern can be significantly enhanced by selective treatment of a lower complexity subset of the sequence ensemble under consideration.

Effects of similarity, difficulty, and nontarget presentation on the time course of visual attention

Long-lasting interference from an initial visual target on a subsequent one has been measured in two paradigms: rapid serial presentation of targets and nontargets at a single location, and simple presentation of two spatially separated targets. We note that comparisons between these paradigms might be invalid, since interference in each paradigm can be attributed to a different source: demands on selective attention, or demands to switch locations. We use a novel target presentation that both minimizes selection demands and eliminates location switching, yet we still find long-lasting interference. We suggest that all three paradigms discussed tap a common attentional limit. We also examine effects of similarity between targets, and effects of discrimination difficulty on the initial target. We find that similarity effects are more pronounced when nontargets are present, and we find no effect of discrimination difficulty on subsequent interference.

The Lassa fever virus L gene: nucleotide sequence, comparison, and precipitation of a predicted 250 kDa protein with monospecific antiserum

The large (L) RNA segment of Lassa fever virus (LAS) encodes a putative RNA-dependent RNA polymerase (RdRp or L protein). Similar to other arenaviruses, the LAS L protein is encoded on the genome-complementary strand and is predicted to be 2218 amino acids in length (253 kDa). It has an unusually large non-coding region adjacent to its translation start site. The LAS L protein contains six motifs of conserved amino acids that have been found among arenavirus L proteins and core RdRp of other segmented negative-stranded (SNS) viruses (Arena-, Bunya- and Orthomyxoviridae). Phylogenetic analyses of the RdRp of 20 SNS viruses reveals that arenavirus L proteins represent a distinct cluster divided into LAS-lymphocytic choriomeningitis and Tacaribe-Pichinde virus lineages. Monospecific serum against a synthetic peptide corresponding to the most conserved central domain precipitates a 250 kDa product from LAS and lymphocytic choriomeningitis virus-infected cells.

Abnormal temporal dynamics of visual attention in spatial neglect patients

When we identify a visual object such as a word or letter, our ability to detect a second object is impaired if it appears within 400ms of the first. This phenomenon has been termed the attentional blink or dwell time and is a measure of our ability to allocate attention over time (temporal attention). Patients with unilateral visual neglect are unaware of people or objects contralateral to their lesion. They are considered to have a disorder of attending to a particular location in space (spatial attention). Here we examined the non-spatial temporal dynamics of attention in patients, using a protocol for assessing the attentional blink. Neglect patients with right parietal, frontal or basal ganglia strokes had an abnormally severe and protracted attentional blink When they identified a letter, their awareness of a subsequent letter was significantly diminished for a length of time that was three times as long as for individuals without neglect. Our results demonstrate for the first time that visual neglect is a disorder of directing attention in time, as well as space.

The Slow Time-Course of Visual Attention

Visual attention is often conceived as a high-speed serial system moving rapidly from one object to another at rates of a few dozen milliseconds per item. We present four experiments demonstrating that this high-speed model is incorrect. Subjects identify two objects, presented at separate times. We measure how long the first object continues to interfere with accuracy on the second, and hence the time-course of the first object's attentional demand. We find interference for a half-second or more-roughly 10 times longer than might be predicted from conventional visual search paradigms. In further experiments, we show that the time-course of interference depends upon the number of attended objects, not the number or complexity of responses. Even objects which require no response, such as nontargets in visual search, can still produce long lasting interference on subsequent identification. We suggest that visual attention is not a high-speed switching mechanism, but instead a sustained state during which representations of relevant objects become available to guide behavior.

Direct measurement of attentional dwell time in human vision

In vision, attentional limitations are reflected in interference or reduced accuracy when two objects must be identified at once in a brief display. In our experiments a brief temporal separation was introduced between the two objects to be identified. We measured how long the object continued to interfere with the second, and hence the time course of the first object's attentional demand. According to conventional serial models, attention is assigned rapidly to one object after another, with a dwell time of only a few dozen milliseconds per item. But we report here that interference lasts for several hundred milliseconds--an order of magnitude more than the prediction of conventional models. We suggest that visual attention is not a high-speed switching mechanism, but a sustained state during which relevant objects become available to influence behaviour. This conclusion is consistent with recent physiological results in the monkey.