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Biomedical subjects

H Farid

Publications and source records attributed to H Farid.

17 recordsLinked to original sources
PubMed2001

Synchrony does not promote grouping in temporally structured displays.

It has been proposed that the human visual system can use temporal synchrony to bind image regions into unified objects, as proposed in some neural models. Here we present experimental results from a new dynamic stimulus suggesting that previous evidence for this hypothesis can be explained with the well-established mechanisms of early visual processing, thus obviating the need to posit new synchrony-sensitive grouping mechanisms (see also ref. 5 for a critique of the binding by neural synchrony hypothesis).

Cortical Synchronization↗
PubMed2001

The perception of texture on folded surfaces.

Do judgments of texture similarity reflect surface texture or image texture? To find out, we had observers view a rectangular surface that was folded into three panels, much like a brochure. Each panel was textured with an oriented noise pattern and the observers' task was to determine which side panel matched the center panel in surface texture. Information about surface geometry was conveyed by binocular disparity and by the boundaries of the rectangular surface. We found that observers were often consistently wrong, selecting the texture that differed in the image and not on the surface. In sharp contrast, when observers judged the texture orientation on each panel individually, their judgments were accurate reflections of the surface texture. So even when observers can recover surface texture, their judgments of texture similarity may still be based on image texture.

Adolescent↗
PubMed2001

Occlusion junctions do not improve stereoacuity.

Occlusion geometry gives rise to interocular shifts in the positions of binocularly viewed contour junctions. Since these shifts do not give rise to normal binocular disparities, they have been called 'pseudodisparities'. Previous work has shown that the unmatched contour segments of a partially occluded contour at occlusion junctions can be used to recover the geometry of the occluding surface through the construction of 'illusory' contours. Here, experiments were performed to determine whether such junction shifts could enhance stereoscopic depth detection when the relative disparity between the contours was below threshold. Our results showed that stereoscopic depth detection does not improve when pseudodisparity is present. We conclude that the visual system is less sensitive to pseudodisparity than to conventional disparity information. We suggest that the primary role of pseudodisparity is to overcome conditions of camouflage.

Cues↗
PubMed2001

Blind removal of lens distortion.

Virtually all imaging devices introduce some amount of geometric lens distortion. A technique is presented for blindly removing these distortions in the absence of any calibration information or explicit knowledge of the imaging device. The basic approach exploits the fact that lens distortion introduces specific higher-order correlations in the frequency domain. These correlations can be detected by using tools from polyspectral analysis. The amount of distortion is then estimated by minimizing these correlations.

Journal Article↗
PubMed2000

The effect of 3D structure on motion segmentation.

A smooth surface imaged on the retina produces a smooth flow field. Thus, the visual system may group regions of smoothly varying flow to segment surfaces. We tested this idea by having observers perform a segmentation task on several stimuli that differed in their 3D interpretations but were all matched in the smoothness of their 2D flow fields. Performance varied across conditions with the best performance occurring when the stimulus stimulated a rigid plane. This result suggests that while observers may use deviations from smoothness to segment a broad class of motion stimuli, they use a more precise strategy to segment stimuli with a familiar 3D interpretation.

Depth Perception↗
PubMed2000

A new approach to the design of uniquely folded thermally stable proteins.

A new computer program (CORE) is described that predicts core hydrophobic sequences of predetermined target protein structures. A novel scoring function is employed, which for the first time incorporates parameters directly correlated to free energies of unfolding (deltaGu), melting temperatures (Tm), and cooperativity. Metropolis-driven simulated annealing and low-temperature Monte Carlo sampling are used to optimize this score, generating sequences predicted to yield uniquely folded, stable proteins with cooperative unfolding transitions. The hydrophobic core residues of four natural proteins were predicted using CORE with the backbone structure and solvent exposed residues as input. In the two smaller proteins tested (Gbeta1, 11 core amino acids; 434 cro, 10 core amino acids), the native sequence was regenerated as well as the sequence of known thermally stable variants that exhibit cooperative denaturation transitions. Previously designed sequences of variants with lower thermal stability and weaker cooperativity were not predicted. In the two larger proteins tested (myoglobin, 32 core amino acids; methionine aminopeptidase, 63 core amino acids), sequences with corresponding side-chain conformations remarkably similar to that of native were predicted.

Aminopeptidases↗
PubMed2000

Sympathetic nervous system plays a role in postnatal eyeball enlargement in the rabbit.

PURPOSE: To examine the role of ocular sympathetic activity in the enlargement of the rabbit eyeball during postnatal growth. METHODS: Fourteen New Zealand albino rabbits aged 5 weeks underwent unilateral surgical transection of the cervical sympathetic trunk caudal to the superior cervical ganglion. Postoperative enlargement of both eyeballs was monitored by measuring the axial length and corneal diameters every 2 weeks for 22 weeks (7-27 weeks of age). Rabbits were housed under a 12-hour light/12-hour dark cycle, and the measurements were made in the middle of the light period. At a final age of 30 to 31 weeks, the refractive state of the whole eye was determined on both sides by measurement through the central cornea with a refractometer. Rabbits were then killed, eyeballs enucleated, and their ocular volumes determined. RESULTS: From 9 weeks of age the axial length and corneal diameters were significantly shorter (P < 0.05) in the decentralized eye (surgical side) compared with the intact eye. This reduction remained statistically significant throughout the study period. However, the final refractive states of the two eyes were found not to be different. The mean ocular volume determined after postmortem enucleation was 4.5% less in the decentralized eye than in the intact eye (P < 0.05). CONCLUSIONS: Sympathetic nervous system activity is involved in the normal enlargement of the rabbit eyeball during postnatal growth. However, removal of the ocular sympathetic tone at the age of 5 weeks does not significantly alter the refractive state of the eye when measured in young adulthood.

Aging↗
PubMed1999

Separating reflections from images by use of independent component analysis.

The image of an object can vary dramatically, depending on lighting, specularities, reflections, and shadows. It is often advantageous to separate these incidental variations from the intrinsic aspects of an image. We describe a method for photographing objects behind glass and digitally removing the reflections from the surface of the glass, leaving the image of the objects behind the glass intact. We describe the details of this method, which employs simple optical techniques and independent component analysis and show its efficacy with several examples.

Image Processing, Computer-Assisted↗
PubMed1998

Range estimation by optical differentiation.

We describe a novel formulation of the range recovery problem based on computation of the differential variation in image intensities with respect to changes in camera position. This method uses a single stationary camera and a pair of calibrated optical masks to measure this differential quantity directly. We also describe a variant based on changes in aperture size. The subsequent computation of the range image involves simple arithmetic operations and is suitable for real-time implementation. We present the theory of this technique and show results from a prototype camera that we have constructed.

Humans↗
PubMed1998

Twenty-four-hour change in axial length in the rabbit eye.

PURPOSE: To study the 24-hour changes in axial length, lens thickness, and anterior chamber depth in rabbits and to examine the role of ocular sympathetic activity on these changes. METHODS: Young adult rabbits were entrained to a daily 12-hour light-12-hour dark cycle. Axial length, lens thickness, and anterior chamber depth were measured using ultrasonic techniques. In the first group of 12 rabbits, measurements were taken in the middle light phase and in the early dark phase. In the second group of 12 rabbits, measurements were taken in constant dark every 2 hours for a period of 24 hours. The latter group of rabbits underwent unilateral transection of the cervical sympathetic trunk. Three to four weeks later, axial length, lens thickness, and anterior chamber depth were measured again in constant dark every 2 hours for 24 hours. RESULTS: Under the light- dark condition, axial length and anterior chamber depth were larger in the early dark phase than in the middle light phase. Lens thickness changed in the opposite direction. Under the constant-dark condition, axial length and anterior chamber depth changed gradually during the 24-hour period. The trough appeared in the late subjective light phase, and the peak appeared in the late subjective dark phase. Lens thickness remained relatively constant. In the eyes with decentralized ocular sympathetic nerves, 24-hour changes in axial length and anterior chamber depth occurred. However, magnitudes of nocturnal enlargement were relatively smaller than those in the intact eyes. Although larger in the decentralized eyes, lens thickness appeared unchanged in either eye for 24 hours. CONCLUSIONS: Consistent 24-hour changes in axial length and anterior chamber depth occur in young adult rabbits. These changes are driven endogenously. Significant portions of the nocturnal enlargements of axial length and anterior chamber depth are unrelated to ocular sympathetic activity.

Animals↗
PubMed1996

Prediction and evaluation of side-chain conformations for protein backbone structures.

A common approach to protein modeling is to propose a backbone structure based on homology or threading and then to attempt to build side chains onto this backbone. A fast algorithm using the simple criteria of atomic overlap and overall rotamer probability is proposed for this purpose. The method was first tested in the context of exhaustive searches of side chain configuration space in protein cores and was then applied to all side chains in 49 proteins of known structure, using simulated annealing to sample space. The latter procedure obtains the correct rotamer for 57% and the correct chi 1 value for 74% of the 6751 residues in the sample. When low-temperature Monte-Carlo simulations are initiated from the results of the simulated-annealing processes, consensus configurations are obtained which exhibit slightly more accurate predictions. The Monte-Carlo procedure also allows converged side chain entropies to be calculated for all residues. These prove to be accurate indicators of prediction reliability. For example, the correct rotamer is obtained for 79% and the correct chi 1 value is obtained for 84% of the half of the sample residues exhibiting the lowest entropies. Side chain entropy and predictability are nearly completely uncorrelated with solvent-accessible area. Some precedents for and implications of this observation are discussed.

Algorithms↗
PubMed1996

Surgical management of massive splenomegaly.

Recent reports document an increased mortality and morbidity associated with splenectomy for massive splenomegaly (<1000 g), with a morbidity that is 2 to 10-fold higher than that seen for splenectomy for normal size spleens. Preoperative angiographic embolization of the splenic artery has been advocated as a means to decrease this morbidity and mortality. In a retrospective review of 100 splenectomies performed at Kaiser Permanente (Los Angeles, CA), 20 were performed for splenomegaly, average weight 1811 g (1050-3700 g), and 80 were normal sized spleens. Mortality for normal sized spleens is 1.25 per cent, and for those performed for splenomegaly is zero. Likewise, the morbidity for splenectomy of normal sized spleens was 21.25 per cent, but for splenomegaly, a 20 per cent morbidity rate was observed. Average blood loss with splenomegaly was 696 mL, slightly higher than the 600 mL blood loss for normal sized spleens. Sixty-seven per cent of patients with splenomegaly required no transfusion, and none required more than two units. Of patients with normal size spleens, 75 per cent required no transfusion, and 96 per cent required two units or less. Splenectomy for splenomegaly is possible without an increase in morbidity or mortality. In this series, preoperative embolization was not performed; however, the morbidity and mortality rates compare favorably with series in which it was performed. Preoperative embolization of the splenic artery may be unnecessary and may expose the patient to additional expense, risk, and discomfort.

Adolescent↗
PubMed1995

Methods to decrease the morbidity of abdominoperineal resection.

Recent reports stress that abdominoperineal resections (APR) are associated with many complications, including hemorrhage, long hospitalization, and delayed closure of an open perineal wound. Thirty-five patients underwent an abdominoperineal resection for cancer at Kaiser Permanente Medical Center, Los Angeles, from January 1989 to December 1993. All patients, except two, had their perineal wound closed after closure of the peritoneum and insertion of closed system suction catheters. Ninety-one per cent of patients achieved successful primary healing. Three patients (8.5%) suffered perineal wound dehiscence. Overall morbidity was 55 per cent, with urinary retention being the most common, occurring in 23 per cent of patients. This was managed successfully by early in-and-out self catheterization. There was no incidence of urinary tract infections. There were no operative deaths. Length of stay averaged 8.6 days, with a median of 7 days. Five patients had previous radiation therapy. Of those, two (40%) had perineal wound dehiscence, compared to only one of 33 (3.3%) patients without previous radiation. APRs can be done with minimal mortality, although with an increased morbidity in irradiated patients. Primary closure and drainage of the perineal wound significantly lowers the complication rate, as does early Foley removal and self in-and-out catheterization.

Adenocarcinoma↗
PubMed1994

Hepatic resections: changing mortality and morbidity.

Hepatic resections have been associated with high mortality and morbidity; therefore, this procedure was not used frequently. With increasing safety, acceptance of the procedure grew. A review of recent series still reveals a strikingly high incidence of complications. However, in a review of the last 100 hepatic resections performed at Kaiser Permanente Medical Center, Los Angeles, the average blood loss was 1351 mL, (range 90-10,000 mL) with a median of 900 mL. Blood transfusion requirements averaged 1.5 units, with 50 per cent of patients requiring no transfusion, and 80 per cent of patients requiring two units or less. Cirrhosis, present in 15 per cent of our patients, was associated with increased blood loss and morbidity. The use of the Cavitron Ultrasonic Aspirator (CUSA) and the liver clamp have had a major impact on reducing intraoperative blood loss and transfusion requirement. Operative time was significantly reduced from previous reports, to an average of 221 minutes (90 to 410 minutes). Hospital stay averaged 7 days, with a median of 6 days. Both the mortality rate of 2 per cent and the morbidity rate of 22 per cent represent significant improvements. Utilizing modern techniques, hepatic resections can be done with acceptable mortality and mortality, and may lead to its increased application.

Adolescent↗
PubMed1994

Gastrointestinal complications associated with cardiopulmonary bypass procedures.

Gastrointestinal complications after cardiopulmonary bypass (CPB) procedures are rare, but when they do occur, they carry a significant incidence of morbidity and mortality. Over a 5-year period spanning 1988-1992, 4923 CPB procedures were performed and 64 patients were identified who suffered a GI complication, giving an incidence of 1.3 per cent. The most frequent complications were GI bleeding (40%) and pancreatitis (34%). Other complications included acute cholecystitis (11%), perforated duodenal ulcer (8%), ischemic bowel (5%), and diverticulitis (2%). Complications occurred most frequently in patients undergoing procedures with longer pump and cross-clamp times, such as valvular and combination (CABG/valve) procedures. Redo procedures and the use of an intra-aortic balloon pump increased the risk of developing a GI complication 2.5 and 12 times, respectively. Patients were treated aggressively both medically and surgically, but suffered a higher mortality (16%) as compared to those not suffering a GI complication (3%). We conclude that GI complications after CPB procedures are infrequent but lethal. Clinical features are often subtle, and a high index of suspicion is needed for early diagnosis and aggressive treatment.

Adolescent↗