PubMed Health⌕ Search

Biomedical subjects

W Singer

Publications and source records attributed to W Singer.

At least 127 records · Page 7Linked to original sources
PubMed1983

Direct and indirect visual inputs to superficial layers of cat superior colliculus: a current source-density analysis of electrically evoked potentials.

1. The spatiotemporal pattern of visual inputs to the stratum griseum superficiale (SGS) and stratum opticum (SO) of the cat superior colliculus (SC) has been determined by an analysis of the current sinks occurring during postsynaptic activity following stimulation of each optic nerve (ON) and the optic chiasm (OX). Electrolytic lesions were used to determine the locations of the five major current sinks. 2. Direct SC afferents from the contralateral ON induced three current sinks whose maxima were located a) in the upper part of the SGS, b) in the middle part of the SGS, and c) in the lower part of the SGS and upper part of the SO. These three sinks were generated by three afferent fiber groups conducting in the optic nerve with modal and maximum velocities, respectively, of a) 4 and 5 m/s (slow W-group), b) 7 and 10 m/s (fast W-group), and c) 32 and 43 m/s (Y-group). 3. Indirect SC inputs from the contralateral ON via the ipsilateral visual cortex were identified by comparing the pattern of current sinks generated by OX stimulation before and after cortical ablation. The most prominent and fastest indirect sink (Y-group) was found in ;the lower half of the SGS and uppermost part of the SO. Low-amplitude, long-latency indirect current sinks were also found in the upper and lower thirds of the SGS. 4. The principal conclusions of this report are first, that the SGS is divisible into three physiologic regions according to the spatiotemporal pattern of excitatory synaptic activity generated by the afferent inputs and second, that there is a spatiotemporal matching of the direct collicular afferents from the contralateral retina and the indirect retinal afferents relaying through the ipsilateral visual cortex.

Animals↗
PubMed1983

Mechanisms of experience dependent self-organization of neuronal assemblies in the mammalian visual system.

Neuronal activity and hence sensory signals serve as a shaping factor in the development of the structural and functional organization of the mammalian visual cortex. The electrical responses of cortical neurons control the consolidation and repression of interneuronal connections. The algorithms of this activity dependent selection process closely resemble those proposed by Hebb for adaptative synaptic connections. These local modifications depend in addition on internally generated gating signals. The latter exert a "now print" function by controlling Hebbian modifications as a function of the animal's central state. These permissive gating signals are related to mechanisms which control arousal and visual attention. Evidence as available which suggests that the activation of voltage dependent Ca++ -channels in the dendrites of cortical neurones is the trigger signal for a Hebbian modification. The permissive gating signals appear to control the probability of Ca++ -channel activation. It is concluded that these activity-dependent modifications of neuronal connectivity have an function. They are capable of developing assemblies of cooperating neurones by specifying according to functional criteria which neurones should interact with each other.

Adaptation, Physiological↗
PubMed1982

Central core control of developmental plasticity in the kitten visual cortex: I. Diencephalic lesions.

In five, dark-reared, 4-week-old kittens the posterior two thirds of the corpus callosum were split, and a lesion comprising the intralaminar nuclei was made of the left medial thalamic complex. In addition, the right eye was closed by suture. Post-operatively, the kittens showed abnormal orienting responses, neglecting visual stimuli presented in the hemifield contralateral to the side of the lesion. Sudden changes in light, sound, or somatosensory stimulation elicited orienting responses that all tended toward the side of the lesion. These massive symptoms faded within a few weeks but the kittens continued to neglect visual stimuli in the hemifield contralateral to the lesion when a second stimulus was presented simultaneously in the other hemifield. Electrophysiologic analysis of the visual cortex, performed after the end of the critical period, revealed marked interhemispheric differences. In the visual cortex of the normal hemisphere most neurons were monocular and responded exclusively to stimulation of the open eye, but otherwise had normal receptive field properties. In the visual cortex of the hemisphere containing the thalamic lesion, the majority of the neurons remained binocular. In addition, the selectivity for stimulus orientation and the vigor of responses to optimally aligned stimuli were subnormal on this side. Thus, the same retinal signals, which in the control hemisphere suppressed the pathways from the deprived eye and supported the development of normal receptive fields, failed to do either in the hemisphere containing the thalamic lesion. Apparently, experience-dependent changes in the visual cortex require both retinal stimulation and the functioning of diencephalic structures which modulate cortical excitability and control selective attention.

Animals↗
PubMed1982

Central core control of developmental plasticity in the kitten visual cortex: II. Electrical activation of mesencephalic and diencephalic projections.

Fifteen dark-reared, 4- to 5-week-old kittens were stimulated monocularly with patterned light while they were anesthetized and paralyzed. Six of these kittens were exposed to the light stimuli only, in four kittens the light stimuli were paired with electric stimulation of the mesencephalic reticular formation and in five kittens with electric activation of the medial thalamic nuclei. Throughout the conditioning period, the ocular dominance of neurons in the visual cortex was determined from evoked potentials that were elicited either with electric stimulation of the optic nerves or with phase reversing gratings of variable spatial frequencies. In two kittens, ocular dominance changes were assessed after the end of the conditioning period by analyzing single unit receptive fields. Monocular stimulation with patterned light induced a marked shift of ocular dominance toward the stimulated eye, when the light stimulus was paired with electric activation of either the mesencephalic reticular formation or of the medial thalamus. Moreover, a substantial fraction of cells acquired mature receptive fields. No such changes occurred with light or electric stimulation alone. It is concluded that central core projections which modulate cortical excitability gate experience-dependent modifications of connections in the kitten visual cortex.

Animals↗
PubMed1982

Binocular deprivation can erase the effects of preceding monocular or binocular vision in kitten cortex.

Kittens were given visual experience through one or both eyes for two weeks around the peak of the sensitive period. Subsequently they were binocularly deprived for at least one year. This period of pattern deprivation erased completely the effects of the preceding temporary experience. Ocular dominance distribution, orientation selectivity and response quality of the cortical units resembled those obtained from kittens which are contour-deprived throughout their early postnatal development. This suggests that the effect of visual experience is not to engrave irreversibly certain features of the early visual world, but to adapt the cortex continuously and in an integrative fashion to features which are prominent throughout the sensitive period.

Animals↗
PubMed1982

Central gating of developmental plasticity in kitten visual cortex.

1. In nine 4-week-old, dark-reared kittens we sutured one eye closed and rotated the other surgically. The kittens then grew up in a normally lighted animal colony with adequate room to play.2. For about two weeks after surgery their visual-motor co-ordination did not differ from that of kittens with conventional monocular deprivation; then severe disturbance of visually guided behaviour became progressively more apparent until, after another two to three weeks, all the kittens stopped responding to most visual stimuli entirely. At that point their behaviour in an unfamiliar environment closely resembled that of binocularly deprived cats exposed to light for the first time.3. Four weeks (n = 3) and 6 months (n = 6) after surgery, we examined the visual cortex with single-unit recordings, and with evoked potentials elicited by electrical stimuli and patterned lights. We obtained the single-unit recordings from 586 neurones of the striate cortex in both hemispheres, both ipsi- and contralateral to the deprived eye.4. The single-unit recordings and the evoked potentials showed a clear relation between the kitten's abnormal visual behaviour and the functioning of the striate cortex. Only about half the normal percentage of cells responded to light, and most of those which did react had abnormal receptive field properties: they responded only sluggishly even when the light stimuli were aligned optimally.5. We also evoked cortical potentials with phase alternating square wave gratings of variable contrast and spatial frequency. The amplitude of the potentials indicated that contrast-sensitivity was reduced at all spatial frequencies.6. In the kittens tested 4 weeks after surgery, ocular dominance had shifted toward the open rotated eye but this shift was considerably less pronounced than in control kittens monocularly deprived for a comparable period of time.7. In the kittens tested 6 months after surgery fewer cells than normal were binocular; ocular dominance had not shifted towards the open eye.8. Numerous control experiments indicated that these abnormalities did not result from transitory immobilization of the eye alone nor from lesions of the retina or of the optic nerve.We infer that a central mechanism prevents the inappropriate signals from the rotated eye from influencing the consolidation of central pathways.

Animals↗
PubMed1982

Evidence for long-term functional plasticity in the visual cortex of adult cats.

1. Vision was investigated with behavioural and electrophysiological techniques in three groups of cats: (a) two normally raised kittens in which one eye was rotated at an age of 3 months, (b) three adult cats in which one eye had been rotated and the other closed 6 months prior to recording, (c) two adult cats in which first one eye had been rotated and the other closed and subsequently, after one year, the rotated eye had been closed and the normal eye re-opened. The latter two cats were investigated 6 and 12 months after reverse suture, respectively. All adult cats were at least 2 years old when operated on for the first time.2. Behavioural analysis revealed that the kittens of the first group no longer used the rotated eye for fixation, visuo-motor behaviour being impaired when tested through this eye. Binocularity was found to be disrupted to nearly the same extent as in kittens made strabismic at the beginning of the critical period. In addition, ocular dominance was shifted towards the normal eye.3. The adult cats in the second group developed a virtually complete neglect of the visual modality subsequent to a period of severely disturbed visuo-motor behaviour.4. These behavioural abnormalities were associated with clear alterations in the functional state of striate cortex. Only 47% of the recorded cells could be driven with light, the majority of these reactive neurones yielding only sluggish responses to optimally aligned stimuli. The ocular dominance distribution showed a significant reduction of binocular cells but gave no indication of a shift in ocular dominance towards either of the two eyes. Moreover, contrast sensitivity as assessed with pattern-evoked potentials was significantly reduced.5. The remaining two animals that were reverse sutured after the visual neglect had developed showed complete behavioural recovery when tested through the re-opened normal eye. However, this recovery was not instantaneous and occurred only after the cats had been forced to use their visual sense.6. Behavioural recovery was paralleled by an increase of cortical reactivity to normal levels and by a marked increase in binocularity. This gain increase of excitatory transmission was, however, selective for neurones dominated by the normal eye, leading to a bias in ocular dominance towards this eye.7. The observed modifications in the functional state of striate cortex indicate that reversible changes in the gain of excitatory transmission can still occur beyond the end of the classical critical period. These long-lasting changes in synaptic efficiency appear to follow the rules postulated by Hebb for adaptive synaptic connexions.

Animals↗
PubMed1982

The role of attention in developmental plasticity.

The development of suppression amblyopia in strabismic humans suggests that experience-dependent maturation of sensory functions is gated by attentional mechanisms. Evidence from animal experiments is in favour of this interpretation. Retinal signals support the development of normal receptive fields in the visual cortex of kittens only when the kittens are alert and attend to these signals. Lesion and stimulation studies in kittens suggest that ascending projections from the mesencephalic reticular formation and from the medial thalamic nuclei contribute to the gating of developmental plasticity. It is proposed that this gating is achieved through modulatory control of dendritic depolarization. Disfacilitation of those cortical neurones, which relay activity from the deviated eye could thus effectively block experience-dependent modifications in the respective circuits and lead to the amblyopic deficits.

Amblyopia↗
PubMed1981

Acidophil stem cell adenoma of the human pituitary: clinicopathologic analysis of 15 cases.

In material of 347 surgically removed pituitary adenomas, 15 tumors (4.3%) were diagnosed as acidophil stem cell adenomas. These are immature neoplasms, assumed to derive from the common progenitor of growth hormone and prolactin cells, and usually containing both hormones by the immunoperoxidase technique. Clinically, they are regularly associated with hyperprolactinemia. Some patients may exhibit physical stigmata of acromegaly without biochemical evidence of the disease ("fugitive acromegaly"). The entity is also characterized by (1) relatively short clinical history; (2) large (grade III--IV), locally invasive adenoma, and (3) relatively low hormonal activity. By electron microscopy, these tumors are unicellular with immature cytoplasm, exhibiting some features of adenomatous growth hormone and prolactin with immature cytoplasm, exhibiting some features of adenomatous growth hormone and prolactin cells and frequently mitochondrial abnormalities as well. They are more aggressive than the well-differentiated adenomas of the "acidophil" cell line--a fact to be considered in postoperative management.

Adenoma, Acidophil↗
PubMed1981

Topographic organization of orientation columns in the cat visual cortex. A deoxyglucose study.

Three-dimensional reconstructions of the orientation column system were obtained from the visual cortex of four cats using the deoxyglucose technique. One cat had normal visual experience, one was monocularly and two had selective experience with vertical and horizontal contours, respectively. In areas 17 and 18 orientation columns form a remarkably regular system of equally spaced parallel bands whose trajectory is orthogonal to the borderline between areas 17 and 18. This topographic organization is resistant to manipulations of early visual experience.

Animals↗
PubMed1981

Restriction of visual experience to a single orientation affects the organization of orientation columns in cat visual cortex. A study with deoxyglucose.

In six dark reared, 4-weak-old kittens visual experience was restricted to contours of a single orientation, horizontal or vertical, using cylindrical lenses. Subsequently, the deoxyglucose method was used to determine whether these artificial raising conditions had affected the development of orientation columns in the visual cortex. After application of the deoxyglucose pulse one hemifield was stimulated with vertical, the other with horizontal contours. Thus, from interhemispheric comparison, changes in columnar systems corresponding to experienced and inexperienced orientations could be determined. The following results were obtained: (1) Irrespective of the restrictions in visual experience, orientation columns develop in areas 17, 18, 19 and in the visual areas of the posterior suprasylvian sulcus. (2) Within area 17, spacing between columns encoding the same orientations is remarkably regular (1 mm), is not influenced by selective experience and shows only slight interindividual variation. (3) In non-striate areas the spacing of columns is less regular and the spatial frequency of the periodicity is lower. (4) The modifiability of this columnar pattern by selective experience is small within the granular layer of striate cortex but substantial in non-granular layers: Within layer IV columns whose preference corresponds to the experienced orientation are wider and more active than those encoding the orthogonal orientation but the columnar grid remains basically unaltered. Outside layer IV the columnar system is maintained only for columns encoding the experienced orientations. The deprived columns by contrast frequently fail to extend into non-granular layers and remain confined to the vicinity of layer IV. (5) These modifications in the columnar arrangement are more pronounced in striate cortex than in nonstriate visual areas and, within the former, more conspicuous in the central than in the peripheral representation of the visual field. It is concluded that within layer IV the blue print for the system of orientation columns is determined by genetic instructions: first order cells in layer IV develop orientation selectivity irrespective of experience whereby the preference for a particular orientation is predetermined by the position in the columnar grid. Dependent on experience is, however, the expansion of the columnar system from layer IV into non-granular layers. It is argued that all distortions following selective rearing can be accounted for by competitive interactions between intracortical pathways, the mechanisms being identical to those established for competitive processes in the domain of ocular dominance columns. It is proposed that such experience dependent modifiability of connections between first and second order cells is a necessary prerequisite for the development of orientation selectivity in cells with large and complex receptive fields.

Animals↗
PubMed1981

The effects of early visual experience on the cat's visual cortex and their possible explanation by Hebb synapses.

1. Kittens were dark-reared until 4-6 weeks old, and then for another 4-7 weeks with various combinations of cylindrical lenses, monocular occlusion, and normal vision. 2. Single unit recordings from 816 neurones of the visual cortex (area 17) were obtained after the end of exposure. Clear-cut effects on the distributions of the neurones' ocular dominance and orientation preference were found yielding close correlations with the rearing conditions. 3. It was confirmed that most cortical neurones prefer vertical stimulus orientations when experience is restricted to vertical contours in both eyes. It was further confirmed that, if the experienced orientations are different in the two eyes, each eye dominates over those neurones whose orientation preference corresponds to the orientation this eye has experienced. 4. When one eye is covered while the other sees only contours of one orientation, the ocular dominance distribution of cortical neurones shows a bias towards the open eye. Neurones dominated by this eye prefer orientations corresponding to the experienced range. Neurones preferring other orientations are shared between both eyes. 5. When vision is unimpaired in one eye and restricted to vertical contours in the other, binocularity is common among neurones preferring vertical orientations. Neurones with orientation preferences off the vertical are mainly monocular and dominated by the eye with unrestricted vision. 6. When normal monocular vision of one eye precedes restricted monocular vision of the other eye, only a few binocular units are encountered. Reversal of the initial effects of monocular experience is found only in neurones preferring the orientation that has been experienced by the newly opened eye. The other neurones remain dominated by the originally open eye. Thus, complementary distributions of orientation preferences are found for the two eyes. 7. A good correlation was found between the amount of orientational experience as determined by the number of orientations exposed and the number of normally tuned neurones. Conversely, the number of neurones responding to all orientations decreases with increasing amount of experience.

Animals↗