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Biomedical subjects

J M Crook

Publications and source records attributed to J M Crook.

45 records · Page 3Linked to original sources
PubMed1988

Visual resolution of macaque retinal ganglion cells.

1. The visual resolving ability of different types of macaque retinal ganglion cells was estimated at different retinal eccentricities, by measuring the amplitude of modulated responses to black-white gratings of spatial frequencies near the resolution limit for each cell. 2. The resolving ability of tonic, spectrally opponent ganglion cells was usually similar to that of phasic, non-opponent ganglion cells at similar eccentricities, except that at eccentricities greater than 10 deg some tonic ganglion cells with remarkably high resolution (up to ca. 15 cycles/deg) were found. Our cell sample was limited within the central 2 deg of the visual field, however. 3. Only a small proportion of phasic ganglion cells showed an increase of mean firing level to gratings near the resolution limit. The maintained firing of tonic ganglion cells was higher than that of phasic ganglion cells. 4. With red-black or green-black gratings, the resolution of phasic ganglion cells was unaffected. For red or green on-centre ganglion cells, a marked deterioration of resolving ability occurred when the grating was of a colour to which a cell responded poorly (green-black gratings for red on-centre cells, and red-black gratings for green on-centre cells). A slight improvement in resolving ability occurred when the grating was of an excitatory colour. 5. For a sub-sample of cells, we compared resolution limit with centre size as determined from area-threshold curves. For both phasic and tonic ganglion cells, resolution limit (the period length just resolved) was about half the centre diameter, as is the case for cat ganglion cells. This implies that the centre sizes of phasic and tonic monkey ganglion cells are similar at most eccentricities. 6. We attempt to relate these results to primate retinal anatomy and visual resolution, determined behaviourally.

Action Potentials↗
PubMed1987

Dosimetric methods in the optimization of radiotherapy for carcinoma of the uterine cervix.

Pelvic failures and late radiation sequelae were analyzed using the dosimetric parameters of ICRU Report 38 for 338 patients with Stage I-III carcinoma of the uterine cervix treated by radiation alone and followed for a minimum of 2 years. The pelvic recurrence rates were: Stage IB 5.1% (N = 118, 1% pelvis alone), Stage IIA 15.1% (N = 53, 9.4% pelvis alone), Stage IIB 15.8% (N = 76, 9.2% pelvis alone) and Stage IIIB 28.9% (N = 76, 17.1% pelvis alone). For Stages I and II pelvic failure was unrelated to cumulated lateral parametrial dose (CDPW) or reference volumes, but for Stage IIIB was higher for CDPW above 65 Gy. Overall complication rates were: grade 3-10.1% and grade 2-18.1% but were much lower for 176 patients treated with stem and ovoids (S + O: grade 3-5.7%, grade 2-15.7%) than for 43 receiving vaginal cylinders (grade 3-37.2%, grade 2-28%). Grade 3 rectal complications associated with cylinders were related to a maximal vaginal application over 1.50 cGy X m2 of total reference air kerma (or 2080 mgh) and cumulated rectal reference doses (CDRref) above 75 Gy. For the S + O group, grade 2 and 3 rectal complications increased with increasing reference volumes (hwt and HWT) and showed dose thresholds for CDRref and CDRmean (grade 3: 75 Gy). Prospective use of zones of risk defined graphically on a dose-volume plot (CDRref vs HWT) has reduced our severe complication rate without reducing local control. This technique requires individualization of patient therapy, rapid access to computerized dosimetry and the establishment of center- and applicator-specific risks of complications.

Adult↗
PubMed1987

Reference volume, milligram-hours and external irradiation for the Fletcher applicator.

The ICRU Report 38 recommends the use of reference point doses and reference volumes for the reporting of intracavitary gynecologic therapy. The reference volume enclosed by the 60 Gy isodose for intracavitary therapy alone (which can be represented by hwt) and for combined external and intracavitary therapy (HWT) has no conceptual counterpart in the standard intracavitary systems. We report the relationship of the reference volumes (hwt and HWT) to milligram-hours (mgh) radium equivalent and to dose of external irradiation (XRT) for the Fletcher system. HWT and hwt are directly proportional to mgh, the proportionality constant depending on XRT but not appreciably on moderate changes in source geometry. HWT increases slowly with increasing XRT to about 30 Gy, then increases dramatically with even small increases in XRT. The reasons for this behavior and the possible clinical significance are discussed.

Brachytherapy↗
PubMed1987

Dose-volume analysis and the prevention of radiation sequelae in cervical cancer.

We report a 9 year (1975-1983) experience of treatment of carcinoma of the uterine cervix by radiation alone. Computerized dosimetry conforming to the ICRU 38 recommendations was performed for all 348 patients analyzed. Late sequelae were graded as mild (grade 1), moderate (grade 2) and severe (grade 3). The overall rates were grade 3: 9.8%, grade 2: 18%, and grade 1: 19.5%. Of the moderate to severe sequelae, 48% were rectal, 15% rectosigmoid and 21% urinary. The complication rate was highly dependent on the type of intracavitary applicator: lowest for patients treated by two insertions of a standard Fletcher-Suit applicator (grade 3: 5.4%, grade 2: 14.4%) and highest for those receiving uterine stem plus vaginal line sources (grade 3: 29.5%, grade 2: 26%). For 183 patients treated with stem and standard ovoids, moderate and severe rectal and bladder sequelae were analyzed according to critical organ reference doses and reference treatment volumes as defined by ICRU 38, with the addition of a mean rectal dose. Zones of low, moderate and high risk could be defined on dose-volume plots using these parameters. Modification of treatment plans at the Cancer Institute G. F. Leclerc (CGFL) of Dijon according to these concepts produced a reduction in moderate and severe sequelae (grade 3: 14.4-3.4%) without a concurrent increase in pelvic failures. Although the zones of risk proposed can be used directly only with standard Fletcher-Suit applicators and comparable computer dosimetry, the concept can be applied to other systems.

Adult↗
PubMed1987

Thresholds to chromatic spots of cells in the macaque geniculate nucleus as compared to detection sensitivity in man.

1. The relation between wavelength and psychophysical threshold for chromatic spots on a white background provides evidence for the existence of chromatic channels in the primate visual system. To find the physiological substrate of this task, we compared increment thresholds of different cell types in the macaque lateral geniculate nucleus with human psychophysical thresholds to the same stimuli, using two spot sizes, 4 and 0.4 deg. 2. At different wavelengths, different opponent cell classes in the parvocellular layers of the nucleus were most sensitive, so that at long wavelengths (greater than 600 nm) red on-centre cells were most sensitive, while at short wavelengths (less than 500 nm) S-cone, blue on-centre cells were most sensitive, from 500 to about 550 nm green on-centre cells being most sensitive. A rare cell type with inhibition from S-cones was most sensitive at about 570 nm, although its maximum contrast increment sensitivity was poor compared with that of other cell types. Variation in strength of cone opponency caused a considerable range in threshold in each of the opponent cell classes of the parvocellular layers. 3. On- and off-centre cells from the magnocellular layers were more sensitive than opponent cells to white and yellow spots (as is the case with achromatic gratings). 4. With different wavelengths and spot sizes, the most sensitive cells found approached (to within 0.1-0.3 log units) human psychophysical sensitivity, suggesting that the most sensitive cells available may underlie detection. 5. Measurements of psychophysical chromatic discrimination thresholds, both with nearly monochromatic spots and with spots of differing saturation (purity), support this hypothesis. When magnocellular cell sensitivity corresponded to psychophysical threshold, a suprathreshold stimulus, capable of activating opponent cells, was required for chromatic discrimination.

Animals↗
PubMed1986

Neuronal responses to borders with and without luminance gradients in cat visual cortex and dorsal lateral geniculate nucleus.

We investigated responses of neurones in cortical areas 17 and 18 and in the dorsal lateral geniculate nucleus (dLGN) of the cat to a phase shift in a moving line pattern forming a border without a luminance gradient ("subjective contour"). In both areas 17 and 18, S cells and B cells respond only slightly or not at all along the phase shift while C cells respond strongly. The response of C cells is strongest for line patterns with medium line separation and decreases with smaller and larger separation. In the dLGN the relative magnitude of neuronal responses along a phase shift is similar to that of C cells. However, C cells respond uniformly along the entire phase shift, whereas geniculate cells merely respond to individual line ends along the phase shift. In addition we compared responses along a phase shift and those to a luminance gradient formed by a dotted line whose dots were separated by the same distance as the line ends along the phase shift. S cells and B cells respond preferentially to dotted lines whereas C cells and geniculate cells respond equally well along both phase shifts and dotted lines. Possible explanations for these results in terms of receptive field structure and differences in inhibitory input to the cells are discussed. Differential neurone responses may account for the perceptual distinctness of the contours with and without luminance gradients.

Animals↗
PubMed1980

Visceral anatomy of the Amphisbaenia.

Study of the visceral anatomy of 41 specimens of amphisbaenians representing 13 genera shows that they share a very distinct structure which differs from that found in either snakes or typical lizards. The left lung is large while the right is rudimentary or absent (unique); the kidneys are freely suspended in the coelom by a mesentery (unique); the spleen is usually embedded in the anterior end of the pancreas (as in snakes); the gall bladder lies in a notch in the liver, and the kidneys lie opposite each other (as in lizards). The distinctness of this pattern supports the recognition of the Amphisbaenia as a separate suborder of the Squamata.

Animals↗
PubMedDate not supplied

GABA-induced inactivation of functionally characterized sites in cat striate cortex: effects on orientation tuning and direction selectivity.

Microiontophoresis of gamma-aminobutyric acid (GABA) was used to reversibly inactivate small sites of defined orientation/direction specificity in layers II-IV of cat area 17 while single cells were recorded in the same area at a horizontal distance of approximately 350-700 microns. We compared the effect of inactivating iso-orientation sites (where orientation preference was within 22.5 deg) and cross-orientation sites (where it differed by 45-90 deg) on orientation tuning and directionality. The influence of iso-orientation inactivation was tested in 33 cells, seven of which were subjected to alternate inactivation of two iso-orientation sites with opposite direction preference. Of the resulting 40 inactivations, only two (5%) caused significant changes in orientation tuning, whereas 26 (65%) elicited effects on directionality: namely, an increase or a decrease in response to a cell's preferred direction when its direction preference was the same as that at an inactivation site, and an increase in response to a cell's nonpreferred direction when its direction preference was opposite that at an inactivation site. It is argued that the decreases in response to the preferred direction reflected a reduction in the strength of intracortical iso-orientation excitatory connections, while the increases in response were due to the loss of iso-orientation inhibition. Of 35 cells subjected to cross-orientation inactivation, only six (17%) showed an effect on directionality, whereas 21 (60%) showed significant broadening of orientation tuning, with an increase in mean tuning width at half-height of 126%. The effects on orientation tuning were due to increases in response to nonoptimal orientations. Changes in directionality also resulted from increased responses (to preferred or nonpreferred directions) and were always accompanied by broadening of tuning. Thus, the effects of cross-orientation inactivation were presumably due to the loss of a cross-orientation inhibitory input that contributes mainly to orientation tuning by suppressing responses to nonoptimal orientations. Differential effects of iso-orientation and cross-orientation inactivation could be elicited in the same cell or in different cells from the same inactivation site. The results suggest the involvement of three different intracortical processes in the generation of orientation tuning and direction selectivity in area 17: (1) suppression of responses to nonoptimal orientations and directions as a result of cross-orientation inhibition and iso-orientation inhibition between cells with opposite direction preferences; (2) amplification of responses to optimal stimuli via iso-orientation excitatory connections; and (3) regulation of cortical amplification via iso-orientation inhibition.

Animals↗