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Biomedical subjects

H Krastel

Publications and source records attributed to H Krastel.

At least 55 records · Page 3Linked to original sources
PubMed1987

Normal and defective colour vision in large field.

Colour vision is spatially organized. A light stimulus has to strike spectrally different photoreceptors, covering the center and surround of the receptive field of a colour opponent retinal ganglion cell. Otherwise, no colour opponent processing of signals will occur. Vice versa, spatial summation provided by a large field may compensate for weak opponency. This happens not only in congenital, but also in acquired colour vision defects, when opponency is weakened secondary to a reduced receptoral input. Large field colour vision in Daltonians. Large field red-green opponency is a common phenomenon in patients fitting into the criteria of protanopia and deuteranopia. The difference between small and large field colour vision can be demonstrated by the "projection anomaloscope". At a 30 degrees test field, many anopes behave like the respective anomalous observers. The majority of anopes appear to have some "forbidden cones" at their retinal disposal. So, anomaly and anopia share a common photochemical basis, i.e., the anomalous pigment. However, in anopes, the number of those anomalous cones is extremely small. Therefore, anopic observers usually need a very large amount of spatial summation to arrive at a well defined match of the projection anomaloscope. In protanopes the large field match in our experiments was always a protanomalous one, with the exception of one large field protanope. In deuteranopes, however, there was no such constant behaviour in large field matching. We found deuteranomalous matches as well as matches in the vicinity of the normal mid-match point. Contrary to this behaviour of anopes anomalous observers do not significantly alter their matching pattern irrespectively of whether small (1 degree) or large (30 degrees) test fields are used. So-called peripheral colour blindness of normal observer. Results of classical colour perimetry reveal a dichromatism of the intermediate and a monochromatism of the extreme retinal periphery of the normal observer. These results appear to contradict the common everyday experience of colour constancy throughout the visual field. But a threshold correlation of colour constancy at different retinal exentricities can be demonstrated by recording spectral increment thresholds with test field diameters increasing towards the retinal periphery. So, the colour blindness of the retinal periphery is merely an area phenomenon. It can be overcome by large field observation, rendering spatial summation. Congenital achromatopsia. Remnants of colour vision can be demonstrated in many achromats.(ABSTRACT TRUNCATED AT 400 WORDS)

Color Perception↗
PubMed1985

Pupil increment thresholds are influenced by color opponent mechanisms.

Pupillomotor and sensory spectral sensitivity curves of light-adapted eyes were compared. Experimental conditions were designed to favor (despite the threshold criterion) opponent processing of signals: long (500-ms) stimuli of large diameter (60 degrees) were presented centrally on an intense (16 cd/m2) white background. Narrow spacing of monochromatic stimuli permitted the recording of peaks and troughs in the increment spectral sensitivity curve which neither coincided with the characteristics of receptor pigment absorption spectra nor with the V-lambda curve. They are considered as showing influences of opponency mechanisms. Pupillomotor spectral sensitivity paralleled the sensory one, exhibiting the influence of opponency mechanisms on the pupil. Using more physiological conditions, human photopic spectral sensitivity was revealed as a three-peaked function, in sensory as well as in pupillomotor terms.

Adaptation, Ocular↗
PubMed1985

[Value of visual evoked potentials (VEP) in compression of the anterior visual pathway, especially in the area of the chiasm].

We investigated 86 patients by pattern half-field stimulation presenting clinically with temporal field-defects or with a mass causing compression of the visual pathways found by C.T.-scanning. Pattern half-field stimulation proved to be a very sensitive method for detecting a compression of the anterior visual pathways. In 65 cases the results of perimetry and VEP were congruent, 6 times the evoked potential was pathologic the perimetry showing no abnormalities, only 1 false negative VEP was observed. Thus we stress the importance of an investigation by VEP including half-field stimulation in all patients with suspected compression of the anterior visual pathways.

Evoked Potentials, Visual↗
PubMed1983

[Differential diagnostic considerations in aniridia congenita and progressive iris atrophy].

The autosomal dominant mode of inheritance in aniridia congenita and in dysgenesis mesodermalis (Rieger's syndrome) usually facilitates differentiation from progressive iris atrophy, whose occurrence is, with very few exceptions, sporadic. However, due to incomplete penetration, aniridia and Rieger's syndrome cannot be ruled out by family history alone. In addition, aniridia may closely resemble certain stages of iris atrophy. For appropriate genetic counseling, the reliability of differentiation between the dominantly inherited conditions and sporadic dystrophy has to be verified by: a) looking for extraocular manifestations of Rieger's syndrome or b) clear evidence of a progressive course in cases of essential iris atrophy.

Adult↗
PubMed1981

The latent period of the pupil light reflex in lesions of the optic nerve.

The latent period of the pupil light reflex was investigated by means of infrared pupillography in patients suffering from optic neuritis (retrobulbar neuritis, papillitis), papilledema, optic atrophy or anterior ischemic optic neuropathy. The inflammatory diseases of the optic nerve and the optic atrophy cause significant prolongation of the latent period of the pupil light reflex. Conversely, in papilledema no prolonged latent period could be demonstrated. The vascular diseases of the optic nerve affect the pupil latent period also, however, to a lower degree.

Computers↗