Biomedical subjects

C L Baker

Publications and source records attributed to C L Baker.

At least 19 recordsLinked to original sources

PubMed1994

Envelope-responsive neurons in areas 17 and 18 of cat.

1. Single cortical neurons are known to respond to visual stimuli containing Fourier components only in a narrow range of spatial frequencies. This investigation demonstrates that some neurons in cat area 17 and 18 can also respond to certain stimuli that have no Fourier components inside the cell's luminance spatial frequency passband. 2. To study such "non-Fourier" responses, we used envelope stimuli that consisted of a high-spatial-frequency sinusoidal luminance grating (carrier) whose contrast was modulated by a low-spatial frequency sine wave (envelope). There was no Fourier component at the apparent periodicity of the envelope spatial frequency. However, some cells responded to such a "phantom" component of the envelope modulation when it fell inside the cell's luminance spatial frequency passband while all the real Fourier components in the stimuli were outside. 3. We conducted extensive control experiments to eliminate the possibility of producing artifactual responses to the envelope stimuli due to any small residual nonlinearity of the z-linearized CRT screen. The control experiments included 1) testing of screen linearity to ensure that the effect from the residual screen nonlinearity was no larger than the sensitivity level of visual responses and 2) comparing the responses to envelope stimuli with the responses to the equivalent contrast of the artifact produced by the screen nonlinearity. All these control experiments indicated that any effect of screen nonlinearity did not contribute significantly to the neural envelope responses. 4. We performed a statistical analysis to obtain an index of relative strength of envelope responses for each cell and to objectively classify cells as "envelope-responsive" or "non-envelope-responsive."(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Spatial frequency selective mechanisms underlying the motion aftereffect.

The motion aftereffect (MAE) was used to study the spatial frequency selectivity of suprathreshold motion perception. Observers were adapted to drifting sine-wave gratings confined to a retinal eccentricity of approx. 4 deg. The magnitude of the subsequent MAE was measured while viewing a stationary sine-wave grating test surface of one of a number of spatial frequencies. The largest MAE was found when the spatial frequency of the test stimulus was the same as that of the adapting stimulus. This phenomenon held for spatial frequencies between 0.5 and 4 c/deg, and was robust with changes in contrast of either adapting or test gratings. However, at an adapting spatial frequency of 0.25 c/deg, the peak MAE was observed at 0.5 c/deg. Control experiments indicated that this peak shift was not the result of the reduced number of cycles in the stimulus, nor the temporal frequency. There was no measurable MAE at spatial frequencies lower than 0.25 c/deg. These results suggest the existence of a "lowest adaptable channel" for the motion aftereffect.

Adaptation, Ocular↗

PubMed1992

Acute hemarthrosis of the knee.

Rapid swelling of the knee following a blow or twisting injury is considered a significant injury. The history of trauma coupled with a thorough examination should provide an accurate diagnosis in most patients. Although it should not be performed routinely, aspiration of the fluid can be done to aid in making a diagnosis and to alleviate pain. Splinting and re-evaluation are recommended as the initial treatment of an acute hemarthrosis. Ancillary testing that includes x-ray films and MRIs is beneficial. Although arthroscopic evaluation of the knee is not needed in every patient with an acute hemarthrosis, a high percentage of these patients eventually undergo arthroscopy to complete the diagnosis or as a means of early surgical intervention. The decision to surgically repair an injured structure depends on the patient's age, activity level, amount of instability, and associated lesions. Routine arthroscopy is indicated as a means to determine the correct treatment and not merely for diagnosis. With knowledge of the common causes of hemarthrosis and understanding of the knee examination, a trained examiner can make an accurate diagnosis 80% to 90% of the time and prescribe the appropriate treatment.

Acute Disease↗

PubMed1991

Optimal spatial displacement for direction selectivity in cat visual cortex neurons.

Responses of single neurons in cat visual cortex were measured in response to sinewave grating stimuli. Firstly, a neuron's spatial frequency tuning was determined, and subsequent stimuli were set at the optimal spatial frequency for that neuron. Then a "jumping grating" stimulus was used: a sinewave grating subjected to a series of abrupt spatial displacements, while remaining stationary for a fixed exposure time between displacements. The amount of direction selectivity elicited by this stimulus was measured as a function of the amount of spatial displacement. Visual cortex neurons generally showed an optimal spatial displacement, corresponding to somewhat less than one quarter of a spatial period of the neuron's optimal spatial frequency (close to, but systematically less than, "quadrature phase"). In a majority of neurons tested, this optimal displacement was not affected by increasing the exposure time between displacements, indicating that the measurements were not a simple consequence of temporal frequency tuning. These results closely parallel recent human psychophysical data obtained from measurements of motion aftereffect or direction discrimination elicited by jumping grating stimuli.

Animals↗

PubMed1991

Motion detection is dependent on spatial frequency not size.

The maximum displacement for the detection of apparent motion is measured using stimuli made up of micro-patterns randomly distributed across the stimulus field. The micro-patterns were Gabor functions or half-wave rectified Gabor functions. Dmax is shown to be dependent of the spatial frequency content of the stimulus, and to be largely independent of the size of the stimulus elements or the number of them in the stimulus field (within limits). Evidence is shown for Dmax to be considered a measure of motion detection that reveals properties of low level underlying motion mechanisms.

Contrast Sensitivity↗

PubMed1991

Popliteal cyst: a surgical approach.

Twenty-nine adult patients (30 knees) who failed conservative treatment had surgical excision of a non-rheumatoid cyst. To determine the effectiveness of this surgical procedure, we reviewed the cases of 24 patients (25 knees) who were available for subjective and objective follow up. Twenty of the 25 knees (80%) were rated excellent or good. Three knees had fair results. Two knees continued to have problems and ultimately underwent total knee replacement for degenerative arthritis.

Adolescent↗

PubMed1991

The prone position for elbow arthroscopy.

Using the prone position for diagnostic and therapeutic arthroscopy improves intraoperative stability of the arm and allows the surgeon to approach the joint in a more intuitive manner. With this position, indications for elbow arthroscopy are increased and risks to neurovascular structures are reduced. Access to the posterior chamber is facilitated when the patient is in a prone position. This article describes both this arthroscopic technique and portal anatomy.

Arthroscopy↗

PubMed1991

Residual motion perception in a "motion-blind" patient, assessed with limited-lifetime random dot stimuli.

A neurological patient (L.M.) suffering a specific loss of visual motion perception (Zihl et al., 1983) due to extrastriate cortical damage was studied using random dot "limited-lifetime" stimuli with a direction discrimination task. With a stimulus like that of Newsome and Pare (1988), the patient exhibited a severe deficit for motion perception, only being able to perform well for very high values of coherence. Different versions of the stimulus were employed to separate out the effects of limited lifetime versus the effects of additive noise as coherence was lowered. When all "signal" dots had a fixed, specified value of lifetime, and varying percentages of "noise" dots were added, the patient showed a profound deficit. In contrast, a stimulus consisting of no noise dots at all, and signal dots having fixed values of lifetime, revealed relatively good performance for surprisingly brief dot lifetimes. Thus, it is the presence of noisy, incoherent dot motion, rather than brief lifetimes, that causes such poor performance on the stimulus of Newsome and Pare (1988). Most surprising was the finding that the presence of even very small percentages of stationary noise dots was sufficient to disrupt totally direction discrimination of moving signal dots. The findings reported here suggest that one major role of extrastriate cortical processing might be the interpretation of stimuli that suffer from an impaired signal-to-noise ratio; the most commonly encountered form of "noise" would presumably be contamination by irrelevant directional spatio-temporal frequency components.

Humans↗

PubMed1990

Anatomy of the supraclavicular fossa portal in shoulder arthroscopy.

Because of the need for a third portal for operative procedures during glenohumeral joint arthroscopy, we studied the anatomy of the supraclavicular fossa portal with the humerus in various degrees of abduction and the trochar placed at various angles. Our purpose was to establish a "safe zone" of introduction that would avoid damage to the tendinou portion of the rotator cuff. Eight shoulder specimens were studied. Sharp- and blunt-tipped 4-mm trocars were used to enter the joint. The trapezius was penetrated at all angles of humeral abduction and trocar angulation. The trocar penetrated the tendinous portion of the rotator cuff in all specimens at 90 degrees of abduction, seven of eight specimens at 70 degrees, six of eight specimens at 60 degrees, and three of eight specimens at 45 degrees of abduction. No penetration of the musculotendinous portion occurred when the arm was in 30 degrees of abduction or at the side. When it is necessary to use the supraclavicular portal, traction should be released and the humerus should be brought down to at least 45 degrees. The trocar should be introduced laterally at 30 degrees and angled slightly posteriorly to avoid the tendinous portion of the rotator cuff.

Adult↗

PubMed1990

Spatial- and temporal-frequency selectivity as a basis for velocity preference in cat striate cortex neurons.

Measurements were made of the optimal velocity for drifting bar-shaped stimuli to excite striate cortex neurons of the cat. These data were compared to the optimal spatial and temporal frequencies of the same neurons, as determined with drifting sine-wave grating stimuli. A systematic relationship was revealed, whereby those neurons preferring higher velocities of bar motion also preferred lower spatial and higher temporal frequencies of gratings. The optimal bar velocity for a given neuron could be quantitatively predicted from the ratio of that neuron's optimal temporal frequency to its optimal spatial frequency.

Action Potentials↗

PubMed1990

Recombinant human DNase I reduces the viscosity of cystic fibrosis sputum.

Respiratory distress and progressive lung destruction in cystic fibrosis can be attributed to bacterial persistence and the accumulation of viscous purulent secretions in the airways. More than 30 yr ago it was suggested that the large amounts of DNA in purulent secretions contribute to its viscosity and that bovine pancreatic DNase I could reduce the viscosity. To evaluate the potential clinical utility of recombinant human DNase I (rhDNase) in the treatment of cystic fibrosis, we have cloned, sequenced, and expressed rhDNase. Catalytic amounts of rhDNase greatly reduce the viscosity of purulent cystic fibrosis sputum, transforming it within minutes from a nonflowing viscous gel to a flowing liquid. The reduction in viscosity is associated with a decrease in size of DNA in the sputum. Inhalation of a rhDNase aerosol may be a simple direct approach that will help individuals with cystic fibrosis and other patients with pneumonia or bronchitis to clear their airways of purulent secretions.

Amino Acid Sequence↗

PubMed1989

Optimal displacement in apparent motion.

Measurements are made of the optimal displacement for an abruptly displaced sinewave grating to elicit a motion aftereffect. At a fixed nominal eccentricity of 4 deg, spatial frequencies ranging from 0.2 to 1.2 c/deg are effective in producing an aftereffect. At any given spatial frequency, the optimal displacement is slightly less than one quarter spatial cycle. The range of effective spatial frequencies does not correspond to the range of optimal spatial frequencies reported for neurons in primate Area V1, but does correspond to that for Area V2.

Afterimage↗

PubMed1989

Arthroscopic treatment of chronic synovitis of the ankle.

Arthroscopic synovectomy is a valuable tool in treating synovitis of the knee joint. We have identified a group of patients who benefit from ankle synovectomy. To determine the indications and long-term results of arthroscopic synovectomy of the ankle, we evaluated the history, preoperative examination, roentgenograms, and operative data of patients who underwent this procedure and had a minimum of 2 years of follow-up. Sixteen patients who underwent arthroscopic ankle synovectomy were identified. Sprains and inversion-type injuries were common in this young patient group. All had failed conservative therapy. Operative findings revealed hypertrophic synovium in all cases. Good or excellent results were obtained in 75% of patients, using a subjective and functional evaluation scale. Complications were one deep infection, one superficial infection, and one temporary sensory paresthesia. Chronic synovitis, or anterior soft-tissue impingement, was responsive to arthroscopic treatment and results did not deteriorate over time.

Adult↗

PubMed1989

The incidence of Hill-Sachs lesions in initial anterior shoulder dislocations.

We undertook a prospective study using arthroscopy to determine the intraarticular derangement caused by initial anterior shoulder dislocations. Of our 32 patients, 15 (47%) had a grade I, grade II, or grade III Hill-Sachs lesion. The patients were 29 men and 3 women whose ages ranged from 15 to 28 years (mean 22 years). All of the patients studied were United States military personnel or members of their families. None of the patients had had prior injuries of the affected shoulder, and all underwent arthroscopy within 72 h of injury. Few reports describe a Hill-Sachs lesion in a patient following a single anterior dislocation. Early reports of this lesion were based on indirect (radiographic) evidence. Our arthroscopic findings indicate that the incidence of this lesion after one anterior dislocation is higher than previously thought.

Adolescent↗

PubMed1989

Salary, education, and managerial-level differences of physical therapists in Maryland.

The purpose of this study was to determine whether differences existed between a random sample of male and female physical therapists on selected factors. These factors included salary, managerial level, number of courses taken in management or administration, years of work experience in physical therapy, and years of seniority in an organization. A self-reported questionnaire was sent to 400 licensed physical therapists in Maryland who were also American Physical Therapy Association members. Data were analyzed on the 244 respondents using chi-square tests, t tests, and critical ratios. Results showed that female physical therapists, when employed full time in Maryland as department managers, on average, earned significantly less than their male counterparts. A significant difference existed between male and female department managers for full-time work experience in physical therapy. Neither the number of management courses taken nor seniority in an organization were significantly different for male and female physical therapists, regardless of managerial level. Conclusions were that differences existed on several factors between male and female physical therapy department managers and that schools preparing physical therapists for entry into the profession should assess the need for management preparation.

Administrative Personnel↗

PubMed1989

The "motion-blind" patient: low-level spatial and temporal filters.

The "motion-blind" patient previously described by Zihl et al. (1983) was investigated using standard psychophysical procedures with stimuli whose spatial and temporal properties could be separately manipulated. Detection experiments for sinewave grating stimuli of varying spatial and temporal frequency showed sensitivity in this patient to be only slightly impaired. Temporal integration for stimuli of varying spatiotemporal frequency exhibited the expected space-time covariation seen in normal vision. An examination of the suprathreshold discriminative capacity of this patient was undertaken for spatial frequency, contrast, and temporal frequency. Although all of these discriminative functions were impaired, those concerning temporal frequency or velocity were dramatically reduced. No similar loss was seen for spatial frequency discrimination for moving or temporally varying stimuli. No measurable temporal frequency discrimination was present above 6 Hz and no velocity discrimination above 6 degrees/sec. Experiments involving the direction discrimination of suprathreshold drifting gratings of arrays of random dots revealed an inability to perceive direction of movement above a velocity of about 6 degrees/sec. Contrast thresholds contingent on direction of motion of a drifting grating also showed a much greater deficit than simple detection. Apparent motion using 2-flash random dot kinematograms revealed that the residual motion vision of this patient corresponded to the "short-range" motion process of normal vision. This process originally defined by Braddick (1974) operates over restricted space and time intervals. Apparent motion could only be supported by a narrow range of intermediate spatial displacements. These results suggest that this patient does exhibit some residual motion perception, probably corresponding to a severely impaired "short-range" mechanism. The patient's relatively intact ability to perform simple types of discrimination but severe impairment of performance at making judgments relevant to the nature of motion of the same stimuli suggests that while the components necessary for the analysis of motion are intact their more global associations have been disrupted. This implicates an extrastriate locus of the brain damage. Alternative explanations for the nature of the deficit are discussed.

Biomechanical Phenomena↗

PubMed1988

Space-time separability of direction selectivity in cat striate cortex neurons.

The relationship between spatial and temporal factors underlying direction selective velocity tuning in cat striate cortex neurons was studied quantitatively using flashed and smoothly moving light bar stimuli. Responses to double-flash stimuli were obtained for all combinations of a series of spatial and temporal intervals between the two flashes. Nonlinear interactions underlying the unit's direction selectivity were estimated by subtracting the responses to the component single flashes. The extent of systematic interaction between spatial and temporal variables was assessed qualitatively, with contour plots, and quantitatively, by plotting the optimum displacement as a function of temporal separation. Both methods indicated interactions (space-time nonseparability) which were generally very small in magnitude, and uncorrelated either in magnitude or sign with the direction selective velocity tuning measured in the same neurons. These results demonstrate characteristic optimum spatial and temporal intervals for responses to the two-flash stimuli. The optimum spatial offset was independent of the temporal offset chosen, indicating space-time separability.

Animals↗

PubMed1988

Current source density analysis of linear and non-linear components of the primate electroretinogram.

1. We have used the method of current source density analysis to locate the generators of harmonic electroretinogram (ERG) responses to contrast-modulated pattern and uniform-field stimuli in the primate retina. 2. Sinusoidal steady-state analysis was used, with a stimulus temporal frequency of 8 Hz. Fundamental and second-harmonic response components were measured for the uniform-field response. The second harmonic of the average of contrast-reversal pattern responses obtained at a series of spatial phases was also determined in the same experiments. In addition, retinal tissue resistance was measured. All of these measurements were obtained at a series of equally spaced depths in the retina. 3. Retinal resistivity was not observed to vary systematically with depth. In addition, any plausible undetected inhomogeneities of resistivity with depth were found to slightly affect the relative magnitudes of estimated current sources and sinks, but to have little effect on their localization. 4. In a given penetration, the phase lag of each harmonic component was relatively constant with depth in most cases; however the magnitude of this phase lag sometimes varied in different penetrations. To compare data from different penetrations, the constant phase lag for each harmonic was estimated, and the response data phase-shifted so as to bring all data into a standard (cosine) phase. 5. The resulting current source density analyses were found to be quite consistent in overall form for different penetrations and in different animals. These data were averaged to obtain a final estimate of the depth profiles for generators of different ERG components. 6. The uniform-field fundamental response was found to have a predominant source-sink pair in the distal half of the retina (receptor layer to outer plexiform layer). The pattern (second-harmonic) response generators had a quite different depth profile, consisting mainly of a source-sink pair in the proximal 20% of the retina (encompassing the nerve fibre layer to the middle of the inner plexiform layer). The uniform-field second-harmonic response showed a current source density (CSD) depth profile with multiple sources/sinks, as if it contained contributions from the other two.

Action Potentials↗