PubMed Health⌕ Search

Biomedical subjects

C L Baker

Publications and source records attributed to C L Baker.

At least 19 recordsLinked to original sources
PubMed2001

Processing of second-order stimuli in the visual cortex.

Naturally occurring visual stimuli are rich in examples of objects delineated from their backgrounds simply by differences in luminance, so-called first-order stimuli, as well as those defined by differences of contrast or texture, referred to as second-order stimuli. Here we provide a brief overview of visual cortical processing of second-order stimuli, as well as some comparative background on first-order processing, concentrating on single-unit neurophysiology, but also discussing relationships to human psychophysics and to neuroimaging. The selectivity of visual cortical neurons to orientation, spatial frequency, and direction of movement of first-order, luminance-defined stimuli is conventionally understood in terms of simple linear filter models, albeit with some minor nonlinearities such as thresholding and gain control. However, these kinds of models fail entirely to account for responses of neurons to second-order stimuli such as contrast envelopes, illusory contours, or texture borders. Second-order stimuli constructed from sinusoidal components have been used to analyze the neurophysiological mechanisms of such responses; these experiments demonstrate that the same neuron can exhibit three distinct kinds of tuning to spatial frequency, and also to orientation. These results can be understood in terms of a type of nonlinear 'filter-->rectify-->filter' model, which has been widely used in human psychophysics. Finally, several general issues will be discussed, including potential artifacts in experiments with second-order stimuli, and strategies for avoiding or controlling for them; caveats about definitions of first- vs. second-order mechanisms and stimuli; the concept of form-cue invariance; and the functional significance of second-order processing.

Animals↗
PubMed2001

Lateral epicondylitis.

Satisfactory treatment of lateral epicondylitis results from correct diagnosis followed by a well-controlled operative or nonoperative treatment program. Many options for nonoperative and operative treatment exist for lateral epicondylitis. More study is needed on outcomes of both nonoperative treatment and operative treatment so that each patient can attain maximal improvement. Balanced assessments of specific patient populations, along with definitions of the optimal treatment for each group, are required. This will allow physicians to integrate the available information and improve patient care.

Adrenal Cortex Hormones↗
PubMed2001

Centrifugal bias for second-order but not first-order motion.

Limited-lifetime Gabor stimuli were used to assess both first- and second-order motion in peripheral vision. Both first- and second-order motion mechanisms were present at a 20-deg eccentricity. Second-order motion, unlike first-order, exhibits a bias for centrifugal motion, suggesting a role for the second-order mechanism in optic flow processing.

Anisotropy↗
PubMed2000

Absence of a chromatic linear motion mechanism in human vision.

We have investigated motion mechanisms in central and perifoveal vision using two-frame random Gabor kinematograms with isoluminant red-green or luminance stimuli. In keeping with previous results, we find that performance dominated by a linear motion mechanism is obtained using high densities of micropatterns and small temporal intervals between frames, while nonlinear performance is found with low densities and longer temporal intervals [Boulton, J. C., & Baker, C. L. (1994) Proceedings of SPIE, computational vision based on neurobiology, 2054, 124-133]. We compare direction discrimination and detection thresholds in the presence of variable luminance and chromatic noise. Our results show that the linear motion response obtained from chromatic stimuli is selectively masked by luminance noise; the effect is selective for motion since luminance noise masks direction discrimination thresholds but not stimulus detection. Furthermore, we find that chromatic noise has the reverse effect to luminance noise: detection thresholds for the linear chromatic stimulus are masked by chromatic noise but direction discrimination is relatively unaffected. We thus reveal a linear 'chromatic' mechanism that is susceptible to luminance noise but relatively unaffected by color noise. The nonlinear chromatic mechanism behaves differently since both detection and direction discrimination are unaffected by luminance noise but masked by chromatic noise. The double dissociation between the effects of chromatic and luminance noise on linear and nonlinear motion mechanisms is not based on stimulus speed or differences in the temporal presentations of the stimuli. We conclude that: (1) 'chromatic' linear motion is solely based on a luminance signal, probably arising from cone-based temporal phase shifts; (2) the nonlinear chromatic motion mechanism is purely chromatic; and (3) we find the same results for both perifoveal and foveal presentations.

Color Perception↗
PubMed2000

A new anatomical landmark for reliable identification of human area V5/MT: a quantitative analysis of sulcal patterning.

The location of human area V5 (or MT) has been correlated with the intersection of the ascending limb of the inferior temporal sulcus (ALITS) and the lateral occipital sulcus (LO). This study was undertaken to attempt a replication and quantification of these observations using functional magnetic resonance imaging. V5 was significantly activated in 19 hemispheres with alternating, low contrast, random checkerboard patterns. We confirmed the stereotaxic location of V5 and were able to describe a fairly consistent sulcal pattern in the parieto-temporo-occipital cortex. V5 was usually (95%) buried within a sulcus, most commonly within the inferior temporal sulcus (ITS) (11%), the ascending limb of the ITS (ALITS) (53%) and the posterior continuation of the ITS (26%). The average distance from V5 of two identified anatomical landmarks of V5, the junctions of the LO and the ALITS, and the ITS and ALITS, were both 1 cm. However, the LO-ALITS junction often had to be determined by interpolation (47%), and was not always present even with interpolation (21%). In contrast, the ITS-ALITS junction was always present and V5 was usually (90%) located in a sulcus intersecting with this junction, making it a more reliable landmark for localizing V5 with respect to gross morphological features on individual cortical surfaces.

Adult↗
PubMed2000

Anterior cruciate ligament reconstruction: socioeconomic issues and cost effectiveness.

Anterior cruciate ligament ruptures continue to occur in increasing numbers. These injuries and their treatment create significant social and economic problems for patients, their physicians, and for society. By critically evaluating the trade-off between the costs and benefits of various treatment options, one can define a cost-effective approach to the management of this problem. Limited available data suggest that anterior cruciate ligament reconstruction is a highly cost-effective method of treatment for active young adults. The cost per additional quality-adjusted life year provided by surgery compares favorably with figures associated with other highly utilized health care interventions. Such data may prove invaluable within the context of health care reform.

Adolescent↗
PubMed2000

Prevalence of negative-pressure pulmonary edema at an orthopaedic hospital.

Negative-pressure pulmonary edema (NPPE) occurs when a large, negative intrathoracic pressure is generated against an obstructed upper airway, causing fluid to shift into the lung interstitium. Young, healthy, athletic male patients appear to be at increased risk for this disorder, but the prevalence in orthopaedic surgery patients has been unknown. We retrospectively reviewed the charts of 14 patients (11 male, 3 female) with NPPE at our institution over a 15-year period. The patients had 11 different surgical procedures; 16,653 similar procedures were done during this time. The overall prevalence of NPPE (< 0.1%) was not significantly different between male and female patients. Patients with NPPE were significantly younger than those without NPPE. If NPPE is recognized promptly and treated appropriately with intravenous diuretic and oxygen therapy, most patients respond well. Physicians should be vigilant to the potential for NPPE in young, otherwise healthy patients after general anesthesia.

Female↗
PubMed2000

Clinical Evaluation of the Athlete's Shoulder.

OBJECTIVE: To describe the history and physical examination of the athlete's shoulder. BACKGROUND: The complex, highly mobile shoulder joint is very susceptible to athletic injury. A comprehensive history and physical examination lay the groundwork for accurate decision making about the nature of the injury and the appropriate treatment plan. DESCRIPTION: In taking the history,inquire about the patient's lifestyle (dominant hand, occupation, sports, activity level) and then focus on the specific complaint. Ask about the location, quality, and nature of the pain and activities that provoke the pain. If stiffness is a factor, a review of systems and the patient's past medical history are important. Discuss any previously undertaken interventions and their effects. The physical examination consists of inspection, range of motion, palpation, manual muscle testing, and provocative tests. CLINICAL ADVANTAGES: Once the clinical evaluation has been completed, the nature of the injury will, in most cases, be apparent. If necessary, appropriate diagnostic tests are ordered, and then a treatment plan tailored to the athlete and the injury is instituted.

Journal Article↗
PubMed1999

The quality of medical textbooks: bladder cancer diagnosis as a case study.

PURPOSE: This study was conducted to determine whether there is an inherent bias in medical texts that influences physicians to favor certain signs and symptoms over others when evaluating patients with bladder cancer. MATERIALS AND METHODS: Numerous sources, primarily textbooks from various medical specialties, were reviewed (italics are added for emphasis). Attention was paid to the criteria the authors suggested for use in the diagnosis of bladder cancer. RESULTS: Although most authors agree on a core of presenting signs and symptoms, some concentrate strongly on hematuria with the result of down playing the importance of other findings such as irritative voiding symptoms. The concern is that this approach may lead to delay in the diagnosis of tumors, some of which may be highly aggressive, which present with dysuria and pyuria but no hematuria. CONCLUSIONS: Inconsistencies do exist in the literature regarding which diagnostic criteria to use in selecting those patients to be evaluated for bladder cancer. Recommendations are made for ways to improve standardization.

Humans↗
PubMed1999

Arthroscopic release for lateral epicondylitis: a cadaveric model.

At least 10 different surgical approaches to refractory lateral epicondylitis have been described, including an arthroscopic release of the extensor carpi radialis brevis tendon. The advantages of an arthroscopic approach include an opportunity to examine the joint for associated pathology, no disruption of the extensor mechanism, and a rapid return to premorbid activities with possibly fewer complications. A cadaveric study was performed to determine the safety of this procedure. Ten fresh-frozen cadaveric upper extremities underwent arthroscopic visualization of the extensor tendon and release of the extensor carpi radialis brevis tendon. The specimens were randomized with regard to the use of either a 2.7-mm or a 4.0-mm 30 degree arthroscope through modified medial and lateral portals. Following this, the arthroscope remained in the joint, and the portal, cannula track, and surgical release site were dissected to determine the distance between the cannula and the radial, median, ulnar, lateral antebrachial, and posterior antebrachial nerves, and the brachial artery and the ulnar collateral ligament. No direct lacerations of neurovascular structures were identified; however, the varying course of the lateral and posterior antebrachial nerves place these superficial sensory nerves at risk during portal placement. As in previous reports, the radial nerve was consistently in close proximity to the proximal lateral portal (3 to 10 mm: mean, 5.4 mm). The ulnar collateral ligament was not destabilized. Arthroscopic release of the extensor carpi radialis brevis tendon appears to be a safe, reliable, and reproducible procedure for refractory lateral epicondylitis. Cadaveric dissection confirms these findings.

Arthroscopy↗
PubMed1999

Central neural mechanisms for detecting second-order motion.

Single-unit neurophysiology and human psychophysics have begun to reveal distinct neural mechanisms for processing visual stimuli defined by differences in contrast or texture (second-order motion) rather than by luminance (first-order motion). This processing begins in early visual cortical areas, with subsequent extrastriate specialization, and may provide a basis for form-cue invariant analyses of image structure, such as figure-ground segregation and detection of illusory contours.

Animals↗
PubMed1999

Arthroscopic treatment of transchondral talar dome fractures: a long-term follow-up study.

Arthroscopic treatment of transchondral talar dome fractures allows accurate visualization and debridement of the lesion with less postoperative morbidity and earlier mobilization than arthrotomy. Although many studies document the results of open treatment, no reports of long-term results of arthroscopic treatment of these fractures have been published. We reviewed the results in 12 patients who had arthroscopic excision and curettage of a transchondral talar dome fracture and had an average of 10.1 years of follow up (range, 8.1 to 13.9 years). There were 5 medial and 7 lateral lesions. According to the four-stage classification of Berndt and Harty, 2 were stage II; 8 were stage III; and 2 were stage IV. All patients were evaluated with a subjective questionnaire, ankle radiographs, and a physical examination. The long-term subjective and objective results were good or excellent in 10, fair in 1, and poor in 1 patient. All patients' radiographs showed residual subchondral changes at the site of the original lesion, but minimal to no degenerative changes. Patients who are treated with arthroscopic debridement and curettage for transchondral talar dome fractures achieve a predictably high percentage of successful results with low morbidity.

Adolescent↗
PubMed1999

Comparison of motion and stereopsis: linear and nonlinear performance.

To address the issue of whether the luminance-dependent (linear) and contrast-dependent (nonlinear) processes in stereo and motion have a common computational basis, we compare both carrier-dependent and envelope-dependent performance for these two modalities by using the same stimulus and task: two-flash apparent motion/depth for a wide range of displacements. We do this for different densities, bandwidths, contrasts, spatial frequencies, and exposure durations. The results suggest that there is concordance not only between the luminance-dependent (linear) processes of motion and stereo but also between the envelope-dependent (nonlinear) processes of both modalities. Only one exception was found, but we show this to be amenable to an explanation based on a different contrast dependence for the nonlinear mechanisms of stereo and motion. This suggests that the computational basis of linear and nonlinear processes may be similar for stereopsis and motion.

Depth Perception↗
PubMed1999

Motion perception over long interstimulus intervals.

Recent studies using moving arrays of textured micropatterns have suggested that motion perception can be supported by two mechanisms, one quasilinear and sensitive to the motion of luminance-defined local texture, the other nonlinear and coding motion of contrast-defined envelopes of texture (Baker & Hess, 1998; Boulton & Baker, 1993b). Here we used similar patterns to study motion perception under conditions previously shown to isolate the nonlinear mechanism (low micropattern densities and positive interstimulus intervals [ISIs]. We measured direction discrimination for two-flash apparent motion over a much larger range of ISIs, and susceptibility to masking by incoherently moving "distractor" micropatterns. The results suggest that two nonlinear mechanisms can support motion perception under these conditions. One operates only for relatively short ISIs (less than c. 100 msec), is sensitive to small spatial displacements, and is relatively insensitive to distractor masking. The other operates over much longer ISIs, is insensitive to small spatial displacements, and is highly disrupted by distractor masking. These results are in line with previous studies suggesting that three mechanisms support motion perception.

Attention↗
PubMed1999

Cost effectiveness of anterior cruciate ligament reconstruction in young adults.

The cost effectiveness of ligament reconstruction for acute anterior cruciate ligament tears in young adults was compared with the cost effectiveness of nonoperative management. A decision tree was constructed to predict the expected functional outcomes for operative and nonoperative treatment. Outcome probabilities were derived from the surgical and natural history literature. Cost data were based on averaged figures from the senior author's institution. Utility values were determined from a questionnaire administered to 285 local university students. Cost effectiveness was calculated in terms of dollars spent per additional quality adjusted life year provided by the surgical reconstruction for the initial 7 years after an injury. The operative strategy provided 5.10 quality adjusted life years versus 3.49 years for nonoperative treatment, yielding a marginal effectiveness of 1.61 quality adjusted life years. The estimated total costs of the operative and nonoperative strategies were $11,768 and $2333, respectively, for a marginal cost of $9435. The resulting marginal cost effectiveness ratio was $5857 per quality adjusted life year. These data suggest that, when based on functional outcomes, anterior cruciate ligament reconstruction is a cost effective method of treatment for acute tears in young adults. The cost effectiveness ratio predicted compares favorably with those of other health care interventions that aim to improve quality of life.

Activities of Daily Living↗
PubMed1999

Initial biomechanical properties of anterior cruciate ligament reconstruction autografts.

To provide more information to consider when selecting a reconstruction technique, we did a side-by-side comparison of some of the initial biomechanical properties of currently accepted reconstruction methods. Our research hypotheses were that a quadrupled, woven semitendinosus and gracilis graft is as strong as any of the other commonly used graft materials and that quadrupling and weaving the hamstring graft may increase the stiffness of the overall construct Using lower extremity cadaveric specimens harvested from young donors, we fashioned seven each of seven types of graft: 9-mm, 10-mm, and 11-mm-wide patellar tendon graft (PTG); 10-mm-wide central quadriceps tendon graft; doubled semitendinosus graft; tripled semitendinosus graft; and quadrupled, woven semitendinosus and gracilis graft. Specimens were stripped of remaining soft tissue, and anterior cruciate ligament (ACL) constructs were created for biomechanical testing. The tibia was translated anteriorly on the femur, mimicking a pivot shift maneuver, andfailure strength, failure mechanism, and construct stiffness were recorded. No differences in mean strength were detected. The quadrupled, woven graft was significantly stiffer than the doubled semitendinosus graft and no less stiff than any of the PTG constructs. All grafts showed similar and adequate initial absolute strength to reconstruct the ACL. Quadrupling and weaving the semitendinosus and gracilis graft increases the stiffness of the reconstructed specimen to a level statistically similar to that of specimens reconstructed with a PTG.

Adolescent↗
PubMed1998

A nonlinear chromatic motion mechanism.

Previous research has demonstrated two categorically distinct mechanisms mediating apparent motion of kinematograms composed of eccentricity-confined, randomly placed Gabor micropatterns: a quasi-linear mechanism operating for high micropattern densities and short time separations, and a nonlinear mechanism operating at low micropattern densities or longer time separations. Here we compare the performance of these two mechanisms using color (isoluminant) and luminance-defined stimuli. When these stimuli are defined only by their color contrast, the response of the quasi-linear mechanism is severely impaired, while the nonlinear mechanism remains fully operative. This result further strengthens the dichotomy between the two kinds of motion perception, and suggests that when color vision supports motion perception it does so primarily, or perhaps entirely, via a nonlinear mechanism.

Color Perception↗