Removal Of Cortex Research Articles

Surgery of Temporal Lobe Epilepsy: Modalities, Advantages, Disadvantages and Outcomes

From the standard anterior temporal to selective resections, several modalities have evolved in the surgical treatment of temporal lobe epilepsy. In this review, the various surgical approaches, their advantages and disadvantages are discussed from the standpoint of results on seizure tendency, cognitive function, quality of life and technical aspects. Numerous studies, including our own with intracranial recording and reoperations, have provided evidence that the vast majority of temporal lobe seizures arise within the mesial structures. As a result, more and more selective procedures are being carried out directed at the amygdala and hippocampal formation. However, there is still no clear evidence that one selective approach is superior to the more standard temporal resection, which entails removal of the anterior temporal cortex, amygdala and hippocampus, or to any other so-called selective approach in terms of seizure control, cognitive function and quality of life. Technically, in our hands, the transcortical transventricular approach combined with neuronavigation has proven to be safe and less invasive compared with anterior temporal resection. Our experience suggests that the type of resection should be chosen case by case, on an individual basis, determined mainly by symptomatology, imaging, intracranial recording, neuropsychological findings and, at times, by the surgeon‘s experience.

429 Gestion chirurgicale des corps étrangers intraoculaires selon leur localisation

Ocular trauma due to intraocular foreign bodies remains a major cause of visual loss. Three-port pars plana vitrectomy is a standard procedure to remove intraocular foreign bodies. This operation may be complicated with retinal tear/detachment and proliferative vitreoretinopathy, which are poor prognostic factors. The purpose of our study was to assess whether different surgical approach based on the intraocular foreign body location influences the rate of postoperative complications and the final outcome. 107 patients, who underwent pars plana vitrectomy and removal of posterior segment intraocular foreign body were included in the study. All patients were divided in three groups intraoperatively according to the position of the intraocular foreign body in the posterior segment: In 23 patients (21%) the intraocular foreign body was entrapped in the vitreous cavity with no retina trauma (group A), in 48 patients (45%) it was floating into the vitreous after causing retinal trauma (group B) and in 36 patients (34%) it was embedded in the retina (group C). Almost 45% of the cases achieved visual acuity better than 0.3 postoperatively. During vitrectomy removal of the vitreous cortex was achieved in 10 cases in group A (43.5%) and in 65 cases in groups B and C (77%). None of the cases in group A was complicated with proliferative vitreoretinopathy when the vitreous cortex was not removed, while 20% of cases were complicated with proliferative vitreoretinopathy, when the cortex was removed during vitrectomy. In groups B and C the rate of proliferative vitreoretinopathy was lower, when the vitreous cortex was removed, compared to the rate of proliferative vitreoretinopathy when the vitreous cortex was left (16% versus 36%). Visual outcome remained poor when the retina had been directly traumatized by the intraocular foreign body, as in groups B and C. In group A the risk to develop proliferative vitreoretinopathy after vitrectomy was greater, when vitreous cortex was removed, while in groups B and C the risk was greater when vitreous cortex was not removed. Therefore, an attempt to remove vitreous cortex is suggested in cases where the intraocular foreign body has traumatized the retina, as in groups B and C. This study indicates that intraoperative assessment of intraocular foreign body localization in the posterior segment contributes to an optimal surgical management and has also a prognostic value for such cases.

Role of subdural electrocorticography in prediction of long-term seizure outcome in epilepsy surgery

Since prediction of long-term seizure outcome using preoperative diagnostic modalities remains suboptimal in epilepsy surgery, we evaluated whether interictal spike frequency measures obtained from extraoperative subdural electrocorticography (ECoG) recording could predict long-term seizure outcome. This study included 61 young patients (age 0.4-23.0 years), who underwent extraoperative ECoG recording prior to cortical resection for alleviation of uncontrolled focal seizures. Patient age, frequency of preoperative seizures, neuroimaging findings, ictal and interictal ECoG measures were preoperatively obtained. The seizure outcome was prospectively measured [follow-up period: 2.5-6.4 years (mean 4.6 years)]. Univariate and multivariate logistic regression analyses determined how well preoperative demographic and diagnostic measures predicted long-term seizure outcome. Following the initial cortical resection, Engel Class I, II, III and IV outcomes were noted in 35, 6, 12 and 7 patients, respectively. One child died due to disseminated intravascular coagulation associated with pseudomonas sepsis 2 days after surgery. Univariate regression analyses revealed that incomplete removal of seizure onset zone, higher interictal spike-frequency in the preserved cortex and incomplete removal of cortical abnormalities on neuroimaging were associated with a greater risk of failing to obtain Class I outcome. Multivariate logistic regression analysis revealed that incomplete removal of seizure onset zone was the only independent predictor of failure to obtain Class I outcome. The goodness of regression model fit and the predictive ability of regression model were greatest in the full regression model incorporating both ictal and interictal measures [R(2) 0.44; Area under the receiver operating characteristic (ROC) curve: 0.81], slightly smaller in the reduced model incorporating ictal but not interictal measures (R(2) 0.40; Area under the ROC curve: 0.79) and slightly smaller again in the reduced model incorporating interictal but not ictal measures (R(2) 0.27; Area under the ROC curve: 0.77). Seizure onset zone and interictal spike frequency measures on subdural ECoG recording may both be useful in predicting the long-term seizure outcome of epilepsy surgery. Yet, the additive clinical impact of interictal spike frequency measures to predict long-term surgical outcome may be modest in the presence of ictal ECoG and neuroimaging data.

Comparison of surgical procedures for vitreous surgery in diabetic macular edema

Results of vitreous surgery alone in patients with diabetic macular edema (Vit group) were compared with results of surgery combined with peeling of the internal limiting membrane (ILM group), and surgery combined with removal of the residual vitreous cortex after its visualization using triamcinolone acetonide (TA) and a postoperative intravitreal TA injection (TA group). Visual acuities after the operation, degree of improvement in visual acuity (VA), and intra- and postoperative complications were evaluated. The study included 141 consecutive patients (168 eyes) who could be followed up for at least 12 months after the operation. The Vit group consisted of 66 eyes, the ILM group of 62 eyes, and the TA group of 40 eyes. Patient background factors, and visual acuities 6 and 12 months after operation were evaluated first, and at the final examination, the degree of improvement in VA, the incidences of intra- and postoperative complications, and the incidence of intraocular pressure > or =25 mmHg were studied. Preoperative VA was compared with the 6- and 12-month postoperative VA and the VA at the final examination. The preoperative to 6 months postoperative improvements and those between the 12-month postoperative and final examination did not differ among the three groups. However, the degree of improvement in VA between 6 and 12 months after the operation was significantly smaller in the TA group than in the other two groups. The incidences of intra- and postoperative complications did not differ among the three groups, but the incidence of intraocular pressure >/=25 mmHg was slightly higher in the TA group. Surgical results did not differ among the three operative techniques examined in this study.

Role of residual vitreous cortex removal in prevention of postoperative vitreous hemorrhage in diabetic vitrectomy

To evaluate whether residual vitreous cortex removal has a role in the prevention of postoperative vitreous hemorrhage in diabetic vitrectomy. Retrospective, comparative, nonrandomized study on a consecutive series of patients who underwent vitrectomy for diabetic eye disease. Recurrence of postoperative vitreous hemorrhage was compared in patients following surgery with and without residual vitreous cortex removal. During the study period, 59 patients had vitrectomy for diabetic disease. In 31 patients (group 1), the residual vitreous cortex was not removed and in 28 patients (group 2) the residual vitreous cortex was removed during surgery. Both groups were matched for number of patients, age, sex, duration of follow-up, and diagnosis. Recurrent postoperative vitreous hemorrhage (RVH) was noted in ten patients in group 1 (10/31, 32.25%) and in two patients in group 2 (2/28, 7.14%). The difference in postoperative vitreous hemorrhage between the two groups was statistically significant (P = 0.017). Logistic regression analysis showed that residual vitreous cortex removal and presence of intravitreal gas tamponade were associated with reduced risk of postoperative vitreous hemorrhage. However, only residual vitreous cortex removal was statistically significant factor in reducing risk of postoperative vitreous hemorrhage [odds ratio = 0.174; confidence interval (CI) = 0.0331-0.919; P = 0.040]. Residual vitreous cortex removal may be beneficial in reducing recurrence of postoperative vitreous hemorrhage following diabetic vitrectomy. Further randomized studies should be performed to evaluate the potential of this surgical technique in preventing postoperative recurrent vitreous hemorrhage following diabetic vitrectomy.

Processing noxious information at the subnucleus reticularis dorsalis (SRD) of anesthetized cats: Wind-up mechanisms

With the exception of one monkey’s study, where wind-up was not reported, electrophysiological data from SRD neurons were obtained in rodents where they show wind-up. This work was designed to examine the response properties of SRD neurons in anesthetized cats to study how general the data from rats may be. Since cat’s SRD cells showed wind-up, its underlying mechanisms were approached, an issue not previously addressed at supraspinal level. Electrical stimulation, extracellular (combined with microiontophoresis) and intracellular techniques revealed that Aδ information reaches the SRD via the ventrolateral cord, whereas C information preferentially follows a dorsal route. Wind-up was usually generated by spinal and peripheral stimulation, but it was also evoked either by stimulating the nucleus reticularis gigantocellularis (NRGc), even after spinal cord section and bilateral full thickness removal of the cerebral cortex, or by applying microiontophoretic pulses of l-glutamate at 0.3–1 Hz. Wind-up relied on afferent repetitive activity gradually depolarizing the SRD neurons leading 3–4.5 Hz subthreshold membrane rhythmic activity to threshold. Riluzole retarded wind-up generation and decreased the number of spikes per stimulus during wind-up. GABA or glycine abolished spontaneous and sensory-evoked activity and bicuculline, but not strychnine, increased spontaneous and stimulus-evoked activity. These results demonstrate that wind-up at the SRD is not merely the reflection of spinal wind-up, but (i) can be locally generated, (ii) is partially dependent upon persistent sodium currents, and (iii) is under the modulation of a tonic GABAa-dependent inhibition decreasing SRD excitability. Injury and/or inflammation producing tonic C-fiber activation will surpass tonic inhibition generating wind-up.

In the flicker of an eye

In the flicker of an eye

Critical illness-related corticosteroid insufficiency and community-acquired pneumonia: back to the future!

In the past decade, emphasis in the study of the pathogenesis of infectious diseases has shifted from determining the function of the cellular players in the inflammatory response to the mediators that orchestrate activation and regulation of this response 1. According to current thinking, pulmonary sepsis begins when pathogens invade the sterile lower respiratory tract, leading to activation of the innate immune response to produce local and systemic inflammation 2. Whereas restricted inflammation is beneficial, excessive or persistent inflammation incites tissue destruction and disease 3; it is the lack of regulation (dysregulated systemic inflammation) of this vital response that is central to the pathogenesis of sepsis-associated morbidity and mortality 2, 4. Among patients with community-acquired pneumonia (CAP), nonsurvivors, unlike survivors, exhibit persistent elevation in plasma inflammatory cytokine levels over time 5–7. An intact hypothalamic-pituitary-adrenal (HPA) axis with effective intracellular glucocorticoid anti-inflammatory activity is indispensable for host survival after exposure to an infectious agent 8. That the integrity of the axis is fundamental to surviving critical illness has been well demonstrated by the increased risk of death following: removal of the adrenal cortex in lipopolysaccharide-challenged animals 9; bilateral adrenal haemorrhage in patients with severe sepsis 10; and inadvertent suppression of …

Comparative quantification of ingress of trypan blue into the anterior chamber after microcoaxial, standard coaxial, and bimanual phacoemulsification: Randomized clinical trial

To compare ocular surface fluid ingress into the anterior chamber at the end of microcoaxial, standard coaxial, and bimanual phacoemulsification using trypan blue as the quantifying tracer. Iladevi Cataract and IOL Research Centre, Ahmedabad, India. This prospective randomized observational study comprised 180 consecutive patients who had microcoaxial, standard coaxial, or bimanual phacoemulsification. Trypan blue was applied over the conjunctival surface, and the amount of ingress was assessed after cortex removal (time point 1) and at the end of the surgery after intraocular lens insertion and stromal hydration (time point 2). Logs of dilution were used for statistical analysis. At time point 1, a statistically significant difference was observed in the ingress of trypan blue in the aqueous aspirate. The bimanual group had the highest ingress and the microcoaxial group, the lowest. The amount of ingress in the standard coaxial group fell between the other 2 groups (P< .001). At time point 2, there was no statistically significant difference between the microcoaxial group and the standard coaxial group (P = 1.00); however, in the bimanual group, trypan blue ingress was statistically significantly higher than in the other 2 groups (P< .001). At both time points, trypan blue ingress was statistically significantly higher in the bimanual group than in the standard coaxial and microcoaxial groups. At time point 2, there was no statistically significant difference between the standard coaxial and microcoaxial groups.

Primary Phacoemulsification and Aspiration Combined with 25-Gauge Single-Port Vitrectomy for Management of Acute Angle Closure

To describe a technique using phacoemulsification and aspiration (PEA) combined with 25-gauge single-port vitrectomy as a primary treatment for acute angle closure (AAC). Seventeen consecutive cases of AAC were treated with 1) transconjunctival limited single-port vitrectomy with a 25-gauge vitrector and 2) transcorneal PEA and cortex removal followed by implantation of foldable intraocular lenses (IOL). Intraocular pressure (IOP) control was achieved in all 17 eyes examined. Mean preoperative IOP was 51.8+/-13.1 mmHg, and mean IOP on postoperative day 1 was 18.3+/-8.5 mmHg. Additional anti-glaucoma surgery was necessary in one eye. IOL could not be implanted because of zonular dialysis in one eye. Postoperative complications were seen in three cases (one retinal hemorrhage and two papilledema). The PEA procedure is efficient as a primary treatment of AAC. Single-port vitrectomy with a 25-gauge vitrector facilitated PEA and IOL implantation.

Preservation of a subcutaneous pocket for vagus nerve stimulation pulse generator during magnetoencephalography

Patients with epilepsy and an implanted vagus nerve stimulation (VNS) device who are referred for consideration of definitive epilepsy surgery (removal of the epileptogenic cortex) may require magnetoencephalography (MEG), a study requiring explantation of the pulse generator, as part of their evaluation. Nonetheless, these patients may not wish to abandon palliative VNS therapy should definitive surgery prove unsuccessful or impossible. To avoid obliteration of the pocket by scar tissue after the pulse generator is explanted, the authors have preserved the dead space in several patients with insertion of a similarly sized silicone block. This block is easily replaced with the pulse generator if continued VNS therapy is appropriate, and is left in place in patients who appear to no longer require VNS therapy. Upon completion of MEG, if pulse generator replacement proves desirable, atraumatic retrieval of the electrode connector pin and body is easy. Silicone block implantation during what may prove to be temporary device explantation facilitates reuse of the original pulse generator implantation site and atraumatic distal electrode wire retrieval.

Die Kombination von Pharmakologie und Vitrektomie kann die Grenzen der mechanischen vitreoretinalen Chirurgie überwinden

While the limits of mechanical vitrectomy are being approached, scientists and vitreoretinal surgeons are looking for a combination of pharmacology and vitrectomy, i. e., pharmacology-assisted vitrectomy. This review shows the limits of conventional vitreoretinal surgery by presenting clinical and ultrastructural data related to vitreoretinal surgical techniques and outcomes. The limits of conventional vitreoretinal surgery include: 1. Incomplete vitreous cortex removal. 2. Surgery of advanced stages of the disease. 3. The lack of neuroprotection and antiproliferative agents. 4. Ill-defined cleavage planes when removing tissue from the retina. Pharmacology-assisted vitrectomy helps to overcome the limits of conventional vitreoretinal surgery by addressing: Item 1. Pharmacological vitreolysis. Item 2. Earlier, less traumatic intervention. Item 3. Neuroprotection and antiproliferative agents. Item 4. Selective staining of distinct structures of the vitreoretinal interface, such as cortical vitreous, cellular proliferation, and internal limiting membrane.

Unimpaired sentence comprehension after anterior temporal cortex resection

Functional imaging studies have demonstrated involvement of the anterior temporal cortex in sentence comprehension. It is unclear, however, whether the anterior temporal cortex is essential for this function. We studied two aspects of sentence comprehension, namely syntactic and prosodic comprehension in temporal lobe epilepsy patients who were candidates for resection of the anterior temporal lobe. MethodsTemporal lobe epilepsy patients (n=32) with normal (left) language dominance were tested on syntactic and prosodic comprehension before and after removal of the anterior temporal cortex. The prosodic comprehension test was also compared with performance of healthy control subjects (n=47) before surgery. ResultsOverall, temporal lobe epilepsy patients did not differ from healthy controls in syntactic and prosodic comprehension before surgery. They did perform less well on an affective prosody task. Post-operative testing revealed that syntactic and prosodic comprehension did not change after removal of the anterior temporal cortex. DiscussionThe unchanged performance on syntactic and prosodic comprehension after removal of the anterior temporal cortex suggests that this area is not indispensable for sentence comprehension functions in temporal epilepsy patients. Potential implications for the postulated role of the anterior temporal lobe in the healthy brain are discussed.

Neurophysiological properties of cells filling the neonatal medial prefrontal cortex lesion cavity

Removal of the medial prefrontal cortex (mPFC) of the rat during the initial 7–12 days of life results in spontaneous filling of lesion cavity that is accompanied by recovery of cognitive and motor functions. To date, it remains uncertain whether tissue filling the lesion cavity is actually supporting the functional improvement. In the present study, we examined whether spontaneous neuronal activity could be recorded in adulthood from the tissue that fills the lesion cavity. We recorded EEG and multiunit activity in adulthood from the mPFC and the motor cortex of rats that had received neonatal mPFC lesions on post-natal day 10 (P10) or their non-lesioned littermate controls. We found similarities in both the firing pattern and firing rate of cells from the filled-in region compared to that of controls, although the power associated with peak frequencies in the delta, alpha, and beta range in the EEG recorded from the filled-in region was lower compared to controls. Overall, our results suggest that the cells found in the lesion cavity have similar neurophysiological properties to those found in normal tissue and thus should be capable of at least partially supporting the observed recovery of function.

INCIDENCE AND CLINICAL EVOLUTION OF POSTOPERATIVE DEFICITS AFTER VOLUMETRIC STEREOTACTIC RESECTION OF GLIAL NEOPLASMS INVOLVING THE SUPPLEMENTARY MOTOR AREA

We report the incidence and clinical evolution of postoperative deficits and supplementary motor area (SMA) syndrome after volumetric stereotactic resection of glial neoplasms involving the posterior one-third of the superior frontal convolution. We investigated variables that may be associated with the occurrence of SMA syndrome. The postoperative clinical status of 27 consecutive patients who underwent resection of SMA gliomas was retrospectively reviewed. Neurological examination results were recorded 1 day, 1 week, 1 month, and 6 months postoperatively. The extent of tumor resection, the percentage of SMA resection, violation of the cingulate gyrus, and operative complications were tabulated. The overall incidence of SMA-related deficits was 26% (7 of 27 patients), with 3 patients having complete SMA syndrome and 4 patients having partial SMA syndrome. Two additional patients (7.5%) had other postoperative deficits, including one with mild facial weakness and one with transient aphasia. The resection of low-grade gliomas was associated with a higher incidence of SMA syndrome, an outcome that likely reflects more complete removal of functional SMA cortex in this subset of patients. Intraoperative monitoring localized the precentral sulcus within the preoperatively defined tumor volume in 6 (22%) of 27 patients, thereby precluding gross total resection. All 27 patients had excellent outcomes at the 6-month follow-up examination. When the resection of SMA gliomas is limited to the radiographic tumor boundaries, the incidence and severity of SMA syndrome may be minimized. With the use of these resection parameters, patients with high-grade SMA gliomas are unlikely to experience SMA syndrome. These findings are helpful in the preoperative counseling of patients who are to undergo cytoreductive resection of SMA gliomas.

Intraocular lens calculation by intraoperative autorefraction in myopic eyes

The purpose was to provide a formula to calculate intraocular lens (IOL) power by intraoperative autorefraction in myopic refractive lens exchange (RLE). This was a prospective, noncomparative consecutive case series, including 82 eyes of 82 patients with age superior to 45 years and mean preoperative spherical equivalent (SE) -12.84 diopters (D) [standard deviation (SD) 6.24, range -3 to -27]. Autorefraction was performed by a hand-held instrument during phacoemulsification after cortex removal and the IOL power calculated from the resulting aphakic SE by two existing restricted formulas (personal for high myopia and Ianchulev for low myopia). In each eye, subjective refraction 2 months postoperatively and implanted IOL were used to retrospectively calculate the IOL power achieving plano (predicted final adjusted IOL power). The relation between aphakic SE at intraoperative autorefraction (x, independent variable) and predicted final adjusted IOL power (y, dependent variable) was studied. A relation between SE at intraoperative autorefraction and predicted final adjusted IOL power for the whole series range was expressed by the parabola y=0.07x(2)+1.27x+1.22. Previous restricted formulas achieved a good predictability in their specific range with a mean postoperative SE of -0.58 D (SD 0.73, range -3 to 0.75) and a mean defocus equivalent of 0.69 D (SD 0.62, range 0 to 3). The parabolic relation accounts for previous different regression lines in different myopic ranges. Intraoperative autorefraction is a reliable alternative to standard biometry in myopic eyes. A table for clinical use is provided.

Restoration of Acoustic Orienting Into a Cortically Deaf Hemifield by Reversible Deactivation of the Contralesional Superior Colliculus: The Acoustic “Sprague Effect”

Removal of all contiguous visual cortical areas of one hemisphere results in a contralateral hemianopia. Subsequent deactivation of the contralesional superior colliculus (SC) nullifies the effects of the visual cortex ablation and restores visual orienting responses into the cortically blind hemifield. This deficit nullification has become known as the "Sprague Effect." Similarly, in the auditory system, unilateral ablation of auditory cortex results in severe sound localization deficits, as assessed by acoustic orienting, to stimuli in the contralateral hemifield. The purpose of this study was to examine whether auditory orienting responses can be restored into the impaired hemifield during deactivation of the contralesional SC. Three mature cats were trained to orient toward and approach an acoustic stimulus (broadband, white noise burst) that was presented centrally, or at one of 12 peripheral loci, spaced at 15 degrees intervals. After training, a cryoloop was chronically implanted over the dorsal surface of the right SC. During cooling of the cooling loop to temperatures sufficient to deactivate the superficial and intermediate layers (SZ, SGS, SO, SGI), auditory orienting responses were eliminated into the left (contracooled) hemifield while leaving acoustic orienting into the right (ipsicooled) hemifield unimpaired. This deficit was temperature-dependently graded from periphery to center. After the effectiveness of the SC cooling loop was verified, auditory cortex of the middle and posterior ectosylvian and anterior and posterior sylvian gyri was removed from the left hemisphere. As expected, the auditory cortex ablation resulted in a profound deficit in orienting to acoustic stimuli presented at any position in the right (contralesional) hemifield, while leaving acoustic orienting into the left (ipsilesional) hemifield unimpaired. The ablations of auditory cortex did not have any impact on a visual detection and orienting task. The additional deactivation of the contralesional SC to temperatures sufficient to cool the superficial and intermediate layers nullified the deficit caused by the auditory cortex ablation and acoustic orienting responses were restored into the right hemifield. This restoration was temperature-dependently graded from center to periphery. The deactivations were localized and confirmed with reduced uptake of radiolabeled 2-deoxyglucose. Therefore deactivation of the right superior colliculus after the ablation of the left auditory cortex yields a fundamentally different result from that identified during deactivation of the right superior colliculus before the removal of left auditory cortex in the same animal. Thus the "Sprague Effect" is not unique to a particular sensory system and deactivation of the contralesional SC can restore either visual or acoustic orienting responses into an impaired hemifield after cortical damage.

SUCCESSFUL TREATMENT OF PERSISTENT MACULAR HOLES USING “HEAVY SILICONE OIL” AS INTRAOCULAR TAMPONADE

Silicone oil tamponade has been suggested in the treatment of persistent macular hole, but visual outcome is often poor. We describe two patients who underwent reoperation for persistent macular hole using "heavy silicone oil" (HSO) tamponade. Two patients who underwent vitrectomy, removal of the posterior vitreous cortex, peeling of the internal limiting membrane, and long-acting gas tamponade had persistent macular hole 3 months after surgery. The patients underwent reoperation using an HSO (Oxane Hd, Bausch & Lomb) as internal tamponade. This tamponade did not require postoperative posturing and was removed after 3 months. Optical coherence tomography (OCT) was performed, and visual outcome was determined. OCT images showed that the HSO bubble conformed well with the retinal surface in the foveal region. Closure of the macular hole was achieved in both patients. Visual acuity increased from 20/100 to 20/40 in Patient 1 and from 20/600 to 20/100 in Patient 2. HSO can be a useful tool in the treatment of persistent macular hole. OCT images showed that the tamponade was effective in the upright position in the foveal region. OCT allowed determination of the time of tamponade removal according to the anatomical stage of hole closure.

The role of the dorsal vertebral cortex in the stability of transpedicular screws

The aim of this biomechanical study was to investigate the role of the dorsal vertebral cortex in transpedicular screw fixation. Moss transpedicular screws were introduced into both pedicles of each vertebra in 25 human cadaver vertebrae. The dorsal vertebral cortex and subcortical bone corresponding to the entrance site of the screw were removed on one side and preserved on the other. Biomechanical testing showed that the mean peak pull-out strength for the inserted screws, following removal of the dorsal cortex, was 956.16 N. If the dorsal cortex was preserved, the mean peak pullout strength was 1295.64 N. The mean increase was 339.48 N (26.13%; p = 0.033). The bone mineral density correlated positively with peak pull-out strength. Preservation of the dorsal vertebral cortex at the site of insertion of the screw offers a significant increase in peak pull-out strength. This may result from engagement by the final screw threads in the denser bone of the dorsal cortex and the underlying subcortical area. Every effort should be made to preserve the dorsal vertebral cortex during insertion of transpedicular screws.

Impairment and Facilitation of Transverse Patterning after Lesions of the Perirhinal Cortex and Hippocampus, Respectively

We have recently suggested that certain effects of perirhinal cortex removals in monkeys can be attributed to the lesion compromising complex configural representations of visual stimuli. On this view, monkeys with perirhinal cortex lesions will be impaired on acquisition of discrimination problems that possess high "feature ambiguity," that is, those in which many of the same features belong to both rewarded and unrewarded stimuli. A subclass of feature-ambiguous problems includes "configural" discrimination problems in which all features are ambiguous. In the present study, we tested control monkeys and monkeys with bilateral lesions of perirhinal cortex on a configural discrimination problem, the transverse-patterning task (i.e., A+ vs. B-, B+ vs. C-, C+ vs. A-), using complex 2-dimensional visual stimuli. In addition, we investigated the effects of lesions to another structure that has been implicated in configural learning, the hippocampus. Monkeys with perirhinal cortex lesions were impaired, whereas monkeys with selective hippocampal lesions were facilitated, on acquisition of the transverse-patterning task. These data do not provide support for mass action theories of medial temporal lobe function, which cannot account for the opposing effects of the 2 lesions. These results are, however, compatible with a view that perirhinal cortex, and not the hippocampus, contains complex configural representations of visual stimuli critical to the solution of the transverse-patterning task.