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C L Baker

Publications and source records attributed to C L Baker.

At least 73 records · Page 4Linked to original sources
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↗
PubMed1988

Spatial and temporal determinants of directionally selective velocity preference in cat striate cortex neurons.

1. Direction-selective properties of neurons in cat striate cortex (area 17) were studied with flashed and continuously moving bar stimuli. Receptive fields were characterized by measurement of static and dynamic parameters, which were correlated with the velocity preference exhibited by the same cells. 2. Each neuron was found to be direction selective to a limited range of velocities. This behavior was characterized by measuring the optimal velocity (Vopt) to elicit responses in the preferred and null directions that were maximally distinct. 3. A bar stimulus flashed sequentially at two nearby locations in the receptive field also produced direction-selective behavior, which was characterized by an optimal displacement (Dopt) to drive maximally distinct responses in the preferred versus null directions. 4. The static spatial receptive field properties were quantified by measurement of the receptive field size (2 sigma) and the spatial subunit wavelength (lambda). The latter quantity was measured as twice the separation between adjacent ON and OFF regions in simple cells and as twice the optimal separation for lateral inhibition between two simultaneously flashed bars in complex cells. 5. Direction-selective velocity preference for continuously moving stimuli, Vopt, was found to be highly correlated with lambda and with the Dopt for 2-flash motion; Vopt was also correlated to a lesser degree with 2 sigma. These results suggest a fundamental linkage between spatial frequency preference, velocity preference, and spatial tuning to 2-flash motion. 6. The range of measured direction-selective velocity preference values (Vopt) spanned about a 100-fold range, whereas the corresponding values of Dopt or lambda spanned substantially smaller ranges. This discrepancy suggested that the dynamic range of velocity preference among cortical neurons might be determined jointly by the measured spatial properties and by a temporal property that covaries with the measured spatial properties. 7. Temporal properties of striate cortical neurons were assessed from responses to flashed stimuli having a prolonged duration ("step responses"). Neurons typically responded in the following manner: after some latency (L), a transient rise in spike frequency occurred, which then adapted to some sustained level. The adaptation dynamics (extent of sustained vs. transient behavior) were quantified by the first-order time constant (AT) of the adaptation decay, and by the ratio of initial transient rise to final sustained level [adaptation ratio (AR)].(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Miyakawa patellectomy.

A Miyakawa patellectomy was done in seventeen patients (twenty knees) for either osteoarthritis or chondromalacia of the patella, or both, secondary to malfunction of the extensor mechanism. Patients who had had a patellar fracture were not included in the series. The Miyakawa patellectomy realigns the extensor mechanism, with the proper tension, and centers the functional pull of the quadriceps tendon and patellar ligament. A superficial strip of the quadriceps tendon is pulled distally to fill the void that was left by removal of the patella and to maintain proper length. The musculotendinous portions of the vastus lateralis and vastus medialis are advanced over this defect in the midline and are sutured to the quadriceps tendon. The average length of follow-up was 13.8 years (range, 3.6 to 31.7 years). Nineteen of the twenty knees had a good or excellent subjective result and eighteen had a good or excellent objective result. No subsequent surgical treatment was needed for problems that were caused by malfunction of the extensor mechanism of the knee.

Adult↗
PubMed1988

Triplet pregnancy with delivery on three separate days.

Obstetric complications of triplet pregnancies may make the delivery of extremely premature infants in these multiple gestations inevitable. To our knowledge, this case represents the first report of triplets delivered on three separate days, 16 days apart, using the technique of delayed interval delivery. One of the infants survived.

Adult↗
PubMed1986

Arthroscopic treatment of transchondral talar dome fractures.

Flake fractures of the talar dome involving the cartilaginous surface and subchondral bone have been called transchondral talar dome fractures, a term specific for both anatomical and traumatic etiology of this lesion, and more descriptive than the previously used "osteochondritis dissecans" of the talus. The diagnosis is made by standard ankle radiographs and the lesion should be suspected in all "sprained ankles" that have continued disability following standard treatment. Surgical treatment has been recommended for Stage 3 and Stage 4 (detached/displaced fragments). Surgical treatment for removal of symptomatic lesions has, in the past, involved an arthrotomy for exposure with removal of the fracture fragment and curetting and drilling of the donor bed. Posteromedial fractures are often hard to reach through a standard approach and exposure may require an osteotomy of the medial malleolus. The use of the arthroscope in both confirmation of diagnosis and surgical treatment has proved to be beneficial in the treatment of these symptomatic fractures. The technique allows direct visualization of the pathology with adequate debridement with minimal iatrogenic trauma and early mobilization and return to preinjury status. The series includes 10 ankles in nine patients with eight posteromedial lesions, six Stage 3 and two Stage 4, and two anterolateral lesions, both Stage 4. All patients were treated arthroscopically for removal of the loose fragment and curettage of the osteochondral bed. Subjective and objective follow-up at a minimum of 6 months revealed good results in nine of the ten patients.

Adolescent↗
PubMed1986

Spatial receptive-field properties of direction-selective neurons in cat striate cortex.

Responses of direction-selective neurons in cat striate cortex (area 17) were studied with flashed-bar stimuli. Spatial parameters of interactions within the receptive field giving rise to direction selectivity and of receptive-field subunits were quantitatively determined for the same cells and correlated. A bar stimulus flashed sequentially at two nearby locations in the receptive field produced direction-selective behavior comparable with that elicited by continuously moving stimuli. Each cell exhibited a characteristic optimal spatial displacement, Dopt, for which responses in the presumed preferred and null directions were maximally distinct. In all cases, Dopt was much smaller than the receptive-field size. The spatial structure of receptive fields in simple cells was studied using single narrow-bar stimuli flashed at different locations in the receptive field. The resulting line-weighting function exhibited alternating regions of ON and OFF responses having a characteristic spatial period or wavelength, lambda. Spatial subunit structure in complex cells was determined by flashing two bars simultaneously in the receptive field. The response as a function of bar separation was again a wavelike function having a spatial wavelength, lambda. Values of the optimal displacement for direction selectivity, Dopt, showed a clear relationship with the spatial wavelength, lambda, for a given unit. Dopt was also correlated to a somewhat lesser degree with receptive-field size. Generally, the ratio of Dopt to lambda was approximately 1/10 to 1/4, in agreement with theoretical predictions by Marr and Poggio. Taken together with the findings of Movshon et al., these results indicate a systematic relationship between Dopt and the spatial frequency of a sinusoidal grating, which is optimal for that cell. Such a relationship is consistent with the results of human psychophysical experiments on apparent motion.

Animals↗
PubMed1986

Differences between electroretinograms of cat and primate.

We compared the electroretinogram (ERG) evoked by pattern and uniform field stimulation using steady-state analysis in cat, monkey, and human. Evidence is provided that the pattern-evoked ERG is different in cat and primate. In primate it exhibits a resonance at 8 Hz, a spatial band-pass characteristic, contrast linearity, and no scotopic component. None of these properties are seen in the response to 8-Hz modulation in cat. The ERG evoked by a sinusoidally modulated uniform field of light is composed of a fundamental and a second harmonic component. Although the properties of the fundamental response are similar in cat and primate, the second harmonic response exhibits important differences in its temporal response and luminance dependence. The correspondence between the properties of the pattern-evoked ERG and those of the second harmonic component of the uniform field stimulus in primates suggests a common generator that is different from that of the fundamental response to uniform field stimulation. These differences in the properties of the pattern ERG in cat and primate may suggest either a different generator in cat or one with substantially different properties. This should be taken into account in animal models for the generators of the human pattern ERG response.

Animals↗
PubMed1985

A motion aftereffect from an isoluminant stimulus.

We investigate whether a motion aftereffect (MAE) can be induced by an isoluminant stimulus which contains colour contrast but no luminance contrast. We created a red/green chromatic stimulus, composed of red and green monochromatic grating added in antiphase, and corrected for the chromatic aberrations of the eye. We varied the ratio of the red to green luminances in the stimulus and found that there is no luminance ratio at which the MAE disappears. The results suggest that isoluminant stimuli can induce a MAE which is as great and sometimes greater than that induced by luminance contrast.

Color↗
PubMed1985

Eccentricity-dependent scaling of the limits for short-range apparent motion perception.

The ability to report the direction of apparent motion when an array of random dots is displaced fails when the displacement exceeds a limiting value (dmax). We find that dmax increases rapidly with retinal eccentricity, in a manner different from spatial measures such as acuity which are believed to depend on the "magnification factor" of projection to area 17. The minimum displacement giving detectable motion (dmin) shows a shallower increase with eccentricity which is more compatible with the variation of cortical magnification. The dependence of apparent motion on the timing variables (exposure duration, inter-stimulus interval) changes negligibly with eccentricity. Consequently the dynamic range and the upper limit of detectable velocities increases greatly with eccentricity. The increase of dmax with eccentricity means that the perception of apparent motion will show an approximate invariance with display scale, even though dmax has a locally fixed value depending on receptive field structure.

Fixation, Ocular↗
PubMed1985

Temporal properties of the short-range process in apparent motion.

A study is reported of the perception of random-dot two-frame apparent motion in which the durations of each exposure and the interstimulus interval between them were varied. The results are largely consistent with the rule that, for optimal motion detection, a portion of each exposure must fall within the same time interval of about 40 ms. In addition, motion perception is separably dependent on the displacement from one exposure to the next and on the time interval between those exposures, rather than on the 'velocity' implied by their ratio.

Computers↗
PubMed1985

The pattern evoked electroretinogram: its variability in normals and its relationship to amblyopia.

Electroretinograms evoked by pattern stimuli (contrast reversing gratings) were measured under steady state conditions in the normal and amblyopic eyes of 14 amblyopic individuals having Snellen acuities in the range 20/100 to 20/600. These ERGs were measured as a function of spatial frequency, and compared with the psychophysical threshold losses to the same stimuli. In all cases the authors compared the normal and fellow amblyopic eye's response while taking into account the variability of right-left eye comparisons of normal individuals for these psychophysical and electrophysiological tests. When factors such as optical focus, fixation alignment, and fixation stability have been individually optimized, no pattern ERG deficit was observed in a spatial frequency range where there were obvious psychophysical deficits to the same stimuli. Our results do not substantiate previous claims of a pattern ERG anomaly in many severely amblyopic eyes.

Adolescent↗
PubMed1984

Assessment of retinal function in severely amblyopic individuals.

A quantitative psychophysical and electroretinographic assessment of severely amblyopic human eyes revealed no electrophysiological retinal analogue of the psychophysical deficit, when objectively refracted. Electroretinograms to pattern and uniform field flicker were normal in all but one amblyope. In this amblyope, although the electroretinographic deficit was uncorrelated with the psychophysical deficit, the reduced amplitude of the pattern-evoked response was also seen in the second harmonic component to uniform field stimulation. The deficit for each stimulus was only present at high signal levels. The similarity of the pattern evoked response and the second harmonic response to uniform field stimulation in normal and amblyopic eyes suggests a similar underlying process.

Adult↗
PubMed1984

Transchondral talar dome fractures: not just a sprained ankle.

Transchondral talar dome fractures are a rare but not unknown cause of continued disability following a "sprained ankle." The diagnosis is made with standard ankle roentgenograms and an awareness of the lesion. Surgical treatment is recommended in acute and chronic cases, based on the amount of displacement of the fracture and on the persistence of symptoms after nonoperative treatment. Nine patients were surgically treated for symptomatic transchondral talar dome fractures at Brooke Army Medical Center between July 1980 and July 1982. Seven of the nine injuries were initially misdiagnosed as a sprain. Average follow-up for the group was 15.5 months. Seven patients were treated with excision of the fracture fragment, curettement of the donor site, and multiple K-wire drilling of the fracture bed. One patient was treated with open reduction and internal fixation of the fracture fragment. Another was treated with multiple drilling of the cartilage-covered fracture fragment in situ. Postoperatively, all patients were kept non-weight-bearing for six weeks and did active range of motion exercises. After six weeks the patients participated in a supervised functional rehabilitation program. Results were scored by points for functional activity and range of motion. Eight of nine patients had good results and one had a fair result. There were no poor results.

Ankle Joint↗
PubMed1984

Human pattern-evoked electroretinogram.

We have recorded electroretinographic (ERG) responses to grating patterns whose spatial, temporal, and contrast parameters were varied. The resultant evoked potential is dependent on spatial frequency and it exhibits a spatial band-pass characteristic. The peak frequency is dependent on retinal eccentricity. These findings are independent of temporal frequency, contrast, or mean luminance in the photopic range. These results suggest that the pattern ERG originates from a postreceptoral site.

Cerebral Cortex↗
PubMed1984

Linear and nonlinear components of human electroretinogram.

We compared electroretinographic (ERG) responses to uniform-field and a variety of pattern stimuli using both transient and steady-state analyses. Evidence is provided that for all of these stimuli, the peak at high temporal frequencies in the steady-state response corresponds to the fast wave of the transient response and that the peak at low temporal frequencies corresponds to the slow wave of the step response. A variety of contrast-modulated grating stimuli were used to demonstrate that the fast, high-frequency response can be regarded as the sum of two components, an "odd-symmetric" component, which behaves linearly and is independent of spatial frequency, and an "even-symmetric" component, which behaves nonlinearly and has a band-pass spatial-frequency dependence. The prevailing distinction that is made between pattern and uniform-field ERGs is a consequence of the fact that the uniform-field ERG is dominated by the odd-symmetric (linear) component, whereas the so-called pattern (contrast reversal) ERG reveals the even-symmetric (nonlinear) component in isolation. Since a uniform field can also drive the nonlinear component, the present dichotomy ("luminance" versus "pattern") can be better understood in terms of the linear and nonlinear components of the response rather than in terms of the stimuli that produce them.

Electroretinography↗
PubMed1983

Acute posterolateral rotatory instability of the knee.

We reviewed the clinical records and operative notes of seventeen consecutive patients who were treated by surgical repair for acute posterolateral rotatory instability in order to determine the diagnostic features of the instability and the effectiveness of our surgical repair in thirteen patients who returned for objective and subjective evaluation after a mean follow-up of 53.3 months. Sixteen of the seventeen knees had a positive external-rotation recurvatum test; fifteen, a positive adduction-stress test at 30 degrees of knee flexion; and twelve, a positive posterolateral-drawer test. Associated rotatory instabilities were found in ten of the seventeen knees, with anterolateral rotatory instability being the most frequent. Two patients had associated peroneal-nerve palsy. One or more components of the arcuate ligament complex were injured in all seventeen knees. None of the thirteen patients who were followed required subsequent reconstruction for any chronic instability. Of these thirteen, the results in 85 per cent were rated good subjectively and in 77 per cent, good objectively. Eighty-five per cent of these patients had returned to athletic activity at their preinjury level; the remaining 15 per cent did not participate in sports activities. A positive posterolateral-drawer test or external rotation recurvatum test, or both, was diagnostic of posterolateral rotatory instability. The adduction stress test at 30 degrees of knee flexion was usually positive, but was not diagnostic. Accurate diagnosis and treatment of posterolateral rotatory instability in the acute stage can result in subjectively and objectively acceptable knee function.

Adolescent↗