Biomedical subjects

A K Engel

Publications and source records attributed to A K Engel.

10 recordsLinked to original sources

PubMed1992

Temporal coding in the visual cortex: new vistas on integration in the nervous system.

Although our knowledge of the cellular components of the cortex is accumulating rapidly, we are still largely ignorant about how distributed neuronal activity can be integrated to contribute to unified perception and behaviour. In the visual system, it is still unresolved how responses of feature-detecting neurons can be bound into representations of perceptual objects. Recent crosscorrelation studies show that visual cortical neurons synchronize their responses depending on how coherent features are in the visual field. These results support the hypothesis that temporal correlation of neuronal discharges may serve to bind distributed neuronal activity into unique representations. Furthermore, these studies indicate that neuronal responses with an oscillatory temporal structure may be particularly advantageous as carrier signals for such a temporal coding mechanism. Based on these recent findings, it is suggested here that binding of neuronal activity by a temporal code may provide a solution to the problem of integration in distributed neuronal networks.

Animals↗

PubMed1992

Synchronization of oscillatory neuronal responses in cat striate cortex: temporal properties.

Previously, we have demonstrated that a subpopulation of visual cortical neurons exhibit oscillatory responses to their preferred stimuli at a frequency near 50 Hz (Gray & Singer, 1989). These responses can selectively synchronize over large distances of cortex in a stimulus-specific manner (Gray et al., 1989; Engel et al., 1990 alpha). Here we report the results of a new analysis which reveals the fine temporal structure inherent in these interactions. We utilized pairs of recordings of the local field potential (LFP) activity from area 17 in the anesthetized cat which met two criteria. The LFP was correlated with the underlying unit activity at each site and the recording sites were at least 5 mm apart in cortex. A moving-window technique was applied to compute cross correlograms on 100-ms epochs of data repeated at intervals of 30 ms for a period of 3 s during each direction of stimulus movement. A statistical test was devised to determine the significance of detected correlations. In this way we were able to determine the magnitude, phase difference, frequency, and duration of correlated oscillations as a function of time. The results demonstrate that (1) the duration of synchrony is variable and lasts from 100-900 ms; (2) the phase differences between and the frequencies of synchronized responses are also variable within and between events and range from +3 to -3 ms and 40-60 Hz, respectively; and (3) multiple correlation events often occur within a single stimulus period. These results demonstrate a high degree of dynamic variability and a rapid onset and offset of synchrony among interacting populations of neurons which is consistent with the requirements of a mechanism for feature integration.

Animals↗

PubMed1991

Direct physiological evidence for scene segmentation by temporal coding.

Theoretical studies have suggested that scene segmentation may be accomplished by a temporal coding mechanism using synchronization of neuronal responses. Here we report a direct experimental test of this hypothesis. Neuronal responses were recorded simultaneously from two to four sites with overlapping receptive fields in cat visual cortex. Correlation analysis revealed that all cells synchronized their responses irrespective of their orientation preference when they were activated by a single light bar. However, when stimulated with two superimposed light bars of different orientations, the same cells segregated into distinct assemblies according to their orientation preferences. Within each of these assemblies responses were synchronized, but correlation was absent between the two assemblies. These results are compatible with the hypothesis that responses to individual objects in a scene are distinguished by synchrony, whereas responses to different objects show no temporal correlation, thus allowing for the segmentation of superimposed stimuli. We conclude that stimulus-specific synchronization of spatially distributed neuronal responses may provide a physiological mechanism for scene segmentation.

Animals↗

PubMed1991

Synchronization of oscillatory neuronal responses between striate and extrastriate visual cortical areas of the cat.

Recent studies have shown that neurons in area 17 of cat visual cortex display oscillatory responses which can synchronize across spatially separate orientation columns. Here, we demonstrate that unit responses recorded from the posteromedial lateral suprasylvian area, a visual association area specialized for the analysis of motion, also exhibit an oscillatory temporal structure. Cross-correlation analysis of unit responses reveals that cells in area 17 and the posteromedial lateral suprasylvian area can oscillate synchronously. Moreover, we find that the interareal synchronization is sensitive to features of the visual stimuli, such as spatial continuity and coherence of motion. These results support the hypothesis that synchronous neuronal oscillations may serve to establish relationships between features processed in different areas of visual cortex.

Animals↗

PubMed1991

Interhemispheric synchronization of oscillatory neuronal responses in cat visual cortex.

Neurons in area 17 of cat visual cortex display oscillatory responses that can synchronize across spatially separate columns in a stimulus-specific way. Response synchronization has now been shown to occur also between neurons in area 17 of the right and left cerebral hemispheres. This synchronization was abolished by section of the corpus callosum. Thus, the response synchronization is mediated by corticocortical connections. These data are compatible with the hypothesis that temporal synchrony of neuronal discharges serves to bind features within and between the visual hemifields.

Animals↗

PubMed1989

Oscillatory responses in cat visual cortex exhibit inter-columnar synchronization which reflects global stimulus properties.

A fundamental step in visual pattern recognition is the establishment of relations between spatially separate features. Recently, we have shown that neurons in the cat visual cortex have oscillatory responses in the range 40-60 Hz (refs 1, 2) which occur in synchrony for cells in a functional column and are tightly correlated with a local oscillatory field potential. This led us to hypothesize that the synchronization of oscillatory responses of spatially distributed, feature selective cells might be a way to establish relations between features in different parts of the visual field. In support of this hypothesis, we demonstrate here that neurons in spatially separate columns can synchronize their oscillatory responses. The synchronization has, on average, no phase difference, depends on the spatial separation and the orientation preference of the cells and is influenced by global stimulus properties.

Animals↗

PubMed1989

Postnatal development of vimentin-immunoreactive radial glial cells in the primary visual cortex of the cat.

In kitten area 17 vimentin-like immunoreactivity is expressed in radial glial fibres up to one month postnatally, i.e. the time for which neuronal migration continues. During this time fibre density gradually decreases. A subpopulation of these fibres also contains S-100 protein and glial fibrillary acidic protein. However, these latter antigens disappear earlier than vimentin. In addition, vimentin immunoreactivity can be observed in astroglial cells of the white matter between the second and fifth postnatal week. Many of these cells resemble mature astrocytes but partially they have an intermediate morphology suggesting the possibility that they originated from radial glia. Such displaced radial glial cells' are also positive for S-100 protein both in the cortex and white matter. Thus it is conceivable that at least part of the astrocytes of mature cat visual cortex originate from vimentin- or S-100-immunoreactive radial glia.

Animals↗

PubMed1988

Neuronal surface changes in the dorsal vagal motor nucleus of the guinea pig in response to axotomy.

Ultrastructural changes occurring in the dorsal motor nucleus of the vagus of the guinea pig after nerve transection were investigated. Two neuronal populations could be distinguished. Large neurons corresponding to the vagal motoneurons showed chromatolysis. They were found to develop complex changes in cell surface, which appeared either as a folding up and formation of flaplike processes or as invagination of adjacent neuronal or glial elements. Large processes often covered part of the plasmalemma and formed stacks of several neuronal lamellae. Smaller processes were mostly seen to extend into the neuropil, where they intermingled and adopted a budlike shape. These changes occurred in the cell somata within the first week after axotomy. The dendrites were affected after a short delay. The changes persisted for several months in most of the neurons, including the ones that showed signs of recovery from chromatolysis. The newly formed cellular extensions had a growth-cone-like internal structure, containing numerous smooth-surfaced vesicles or cisternae, a feltwork of filamentous material, dense-cored vesicles, and occasionally free polyribosomes. These surface changes did not occur in the second neuronal cell type of this nucleus, which had a smaller perikaryon characterized by a scanty cytoplasm. These cells did not show a retrograde degeneration and thus are probably interneurons. Acetylcholinesterase was used as a cytochemical marker of neuronal membranes. Surprisingly, the vagal motoneurons did not show a loss of enzymatic activity after nerve transection. Rather, a redistribution seemed to occur with intensified staining of the plasmalemma. The newly formed processes were consistently found to be acetylcholinesterase positive. It is suggested that the morphological changes observed correspond to an as-yet-unobserved growth process in the adult central nervous system, which involves perikarya and dendrites of regenerating guinea pig vagal motoneurons.

Acetylcholinesterase↗

PubMed1988

Axonal transport of 16S acetylcholinesterase is increased in regenerating peripheral nerve in guinea-pig, but not in rat.

The axonal transport of the molecular forms of acetylcholinesterase was investigated in regenerating facial nerves of guinea-pig and rat. Four forms were separated by velocity sedimentation corresponding to 16S (A12), 10S (G4), 6S (G2) and 4S (G1) acetylcholinesterase. They displayed species-specific changes, which are in good accordance with those previously found in the neuronal perikarya. In the rat, axonal transport decreased for all forms. In the guinea-pig, however, the molecular forms showed differential changes. Whereas after transection, the nerve content of 10S acetylcholinesterase decreased, 16S activity was considerably increased. Anterograde transport of 16S acetylcholinesterase was found to be enhanced, whilst transport of the 10S from decreased. The two lighter forms showed only minor changes. Similar results were obtained for the guinea-pig sciatic nerve. Changes in the localization of acetylcholinesterase activity were investigated by electron microscopical cytochemistry. In the normal facial nerve of both species, activity was located intra-axonally in tubular membraneous structures and on the outer surface of the axonal membrane. In the regenerating facial nerve of the rat, intra-axonal as well as axolemmal activity decreased. Axonal sprouts at the end of the proximal nerve stump showed no activity. In the guinea-pig, however, activity of the axonal membrane increased. This was especially prominent on the surface of axonal sprouts. Strong activity was found also in the extracellular space between the sprouting axons and in the endoneurial space filled by collagen fibres. Biochemical analysis of this region revealed that the histochemical activity was mainly due to the A12 form. Thus it was concluded that, in the guinea-pig, axonal sprouts represent a target for axonally transported A12 acetylcholinesterase, which may also be secreted to extracellular sites.

Acetylcholinesterase↗

PubMed1986

Changes of acetylcholinesterase molecular forms in regenerating motor neurons.

Axotomy-induced changes of the molecular forms of acetylcholinesterase in the facial nucleus of the rat and guinea pig were investigated. Evidence is presented that facial motoneurons of the guinea pig are capable of synthesizing considerable amounts of 16S acetylcholinesterase, and furthermore that acetylcholinesterase isoenzymes show species differences in their response to axon transection. Three isoenzymes could be separated by velocity sedimentation, which correspond to G1 (4S), G4 (10S) and A12 (16S) acetylcholinesterase. After axotomy, G4 activity was decreased in both species by 40% 2-3 weeks after nerve transection. In the rat, G1 was even further depressed, whereas in guinea pig facial nucleus G1 showed only a slight change. A12 displayed a clear species difference: in the rat, it was decreased to 60% of control 5 days after axotomy. In guinea pig, however, A12 increased dramatically to values of 400-500% of the unoperated control, and maintained elevated levels even 120 days after operation. This result does not agree with the decrease of transmitter metabolism in regenerating nerves and provides support to the hypothesis that acetylcholinesterase in regenerating nerves may have functions different from transmitter hydrolysis.

Acetylcholinesterase↗