Biomedical subjects

Kerstin E Schmidt

Publications and source records attributed to Kerstin E Schmidt.

8 recordsLinked to original sources

PubMed2006

Pattern motion selectivity in population responses of area 18.

It is commonly believed that the complexity of visual stimuli represented by individual neurons increases towards higher cortical areas. However, even in early visual areas an individual neuron's response is influenced by stimuli presented outside its classical receptive field. Thus, it has been proven difficult to characterize the coding of complex stimuli at the level of single neurons. We therefore investigated population responses using optical imaging in cat area 18 to complex stimuli, plaids. Plaid stimuli are composed of two superimposed gratings moving in different directions. They may be perceived as either two separate surfaces or as a global pattern moving in intermediate direction to the components' direction of motion. We found that in addition to activity maps representing the individual components' motion, plaid stimuli produced activity distributions matching the predictions from a pattern-motion model in central area 18. Thereby, relative component- and pattern-like modulations followed the degree of psychophysical pattern bias in the stimulus. Thus, our results strongly indicate that area 18 exhibits a substantial response to pattern-motion signals at the population level suggesting the presence of intrinsic or extrinsic mechanisms that allow for integration of motion responses from far outside the classical receptive field.

Animals↗

PubMed2005

Hemodynamic signals correlate tightly with synchronized gamma oscillations.

Functional imaging methods monitor neural activity by measuring hemodynamic signals. These are more closely related to local field potentials (LFPs) than to action potentials. We simultaneously recorded electrical and hemodynamic responses in the cat visual cortex. Increasing stimulus strength enhanced spiking activity, high-frequency LFP oscillations, and hemodynamic responses. With constant stimulus intensity, the hemodynamic response fluctuated; these fluctuations were only loosely related to action potential frequency but tightly correlated to the power of LFP oscillations in the gamma range. These oscillations increase with the synchrony of synaptic events, which suggests a close correlation between hemodynamic responses and neuronal synchronization.

Action Potentials↗

PubMed2003

Processing deficits in primary visual cortex of amblyopic cats.

Early esotropic squint frequently results in permanent visual deficits in one eye, referred to as strabismic amblyopia. The neurophysiological substrate corresponding to these deficits is still a matter of investigation. Electrophysiological evidence is available for disturbed neuronal interactions in both V1 and higher cortical areas. In this study, we investigated the modulation of responses in cat V1 to gratings at different orientations and spatial frequencies (SFs; 0.1-2.0 cycles/degrees) with optical imaging of intrinsic signals. Maps evoked by both eyes were well modulated at most spatial frequencies. The layout of the maps resembled that of normal cats, and iso-orientation domains tended to cross adjacent ocular dominance borders preferentially at right angles. Visually evoked potentials (VEPs) were recorded at SFs ranging from 0.1 to 3.5 cycles/degrees and revealed a consistently weaker eye for the majority of squinting cats. At each SF, interocular differences in VEP amplitudes corresponded well with differences in orientation response and selectivity in the maps. At 0.7-1.3 cycles/ degrees, population orientation selectivity was significantly lower for the weaker eye in cats with VEP differences compared with those with no VEP amplitude differences. In addition, the cutoff SF, above which gratings no longer induced orientation maps, was lower for the weaker eye (> or =1.0 cycles/degrees). These data reveal a close correlation between the loss of visual acuity in amblyopia as assessed by VEPs and the modulation of neuronal activation as seen by optical imaging of intrinsic signals. Furthermore, the results indicate that amblyopia is associated with altered intracortical processing already in V1.

Amblyopia↗

PubMed2003

Striatal activation during blepharospasm revealed by fMRI.

OBJECTIVE: To investigate brain areas involved in the initiation and execution of eyelid spasm in patients with benign essential blepharospasm. METHODS: The authors used fMRI and correlated the blood oxygenation level-dependent (BOLD) signal with epochs of frequent eyelid spasm in six patients and with epochs of voluntary eye blinks in four healthy subjects. RESULTS: Spasm epochs were accompanied by activation in a subregion of the putamen in all patients, whereas voluntary blinking in healthy subjects was not. Other areas of activation common to patients and healthy subjects included frontal and parietal operculum, supplementary motor area, primary sensorimotor cortex, various visual areas, and the cerebellum. CONCLUSIONS: The striatum may be involved in the initiation or execution of eyelid spasm. Future studies, possibly including electromyography (EMG) during fMRI, are needed to detect the sequence and role of other concomitantly activated areas.

Adult↗

PubMed2003

Postnatal growth and column spacing in cat primary visual cortex.

The primary visual cortex (area 17) of cats continues to grow substantially during early postnatal development. To assess the implications of this growth for the organization of visual cortical maps, we analysed both postnatal growth of area 17 and the spacing of ocular dominance columns in the same animals using 2-deoxyglucose autoradiography to label cortical activation patterns. Quantitative analyses of area size and column spacing were performed on flat-mount sections of the cortical hemispheres. Our analysis of the surface area revealed an average increase of the size of area 17 between the third and tenth postnatal weeks by about 51%. About 75% of this increase occurred during the third and sixth postnatal weeks (size increase of 37%). However, the distance between adjacent columns did not exhibit a similar increase but rather remained constant during the same postnatal period. Since cortical growth is not accompanied by an increased spacing of ocular dominance columns, new functional modules must somehow be added during the first postnatal weeks to occupy the enlarging cortical sheet. Possible mechanisms underlying the formation of new modules are discussed.

Age Factors↗

PubMed2002

The role of feedback in shaping neural representations in cat visual cortex.

In the primary visual cortex, neurons with similar response preferences are grouped into domains forming continuous maps of stimulus orientation and direction of movement. These properties are widely believed to result from the combination of ascending and lateral interactions in the visual system. We have tested this view by examining the influence of deactivating feedback signals descending from the visuoparietal cortex on the emergence of these response properties and representations in cat area 18. We thermally deactivated the dominant motion-processing region of the visuoparietal cortex and used optical and electrophysiological methods to assay neural activity evoked in area 18 by stimulation with moving gratings and fields of coherently moving randomly distributed dots. Feedback deactivation decreased signal strength in both orientation and direction maps and virtually abolished the global layout of direction maps, whereas the basic structure of the orientation maps was preserved. These findings could be accounted for by a selective silencing of highly direction-selective neurons and by the redirection of preferences of less selective neurons. Our data suggest that signals fed back from the visuoparietal cortex strongly contribute to the emergence of direction selectivity in early visual areas. Thus we propose that higher cortical areas have significant influence over fundamental neuronal properties as they emerge in lower areas.

Animals↗

PubMed2002

Two-dimensional analysis of the spacing of ocular dominance columns in normally raised and strabismic kittens.

In the primary visual cortex of cats, ferrets and macaque monkeys, the thalamocortical afferents conveying signals from the two eyes terminate in alternating regions of layer IV known as ocular dominance columns. Previous experiments have indicated that the periodicity of these columns can be influenced by visual experience: compared to normally raised animals both strabismic cats and cats raised with alternating monocular exposure displayed an increased spacing of adjacent ocular dominance columns in the primary visual cortex (area 17). However, recently it was shown that the formation of ocular dominance columns begins much earlier than previously supposed, indicating that early visual experience might only have a limited influence on the development of the spatial pattern of ocular dominance columns. We therefore visualized the complete pattern of ocular dominance columns in area 17 of normally raised and strabismic kittens during early postnatal development (age 3-6 weeks), particularly focussing on littermates. In addition, we used a previously developed spatial analysis (period statistics) to quantify columnar spacing two-dimensionally. We observed a pronounced interindividual variability in both normally raised and strabismic animals, with column spacings ranging from 783 to 1362 microm. In contrast to previous reports, there were no significant differences in columnar periodicity between normally raised and strabismic cats. These data indicate that rearing has less influence on column spacing while the interindividual variability is much greater than previously supposed, suggesting that genetic differences have an influence on column spacing.

Animals↗

PubMed2002

Spatial analysis of ocular dominance patterns in monocularly deprived cats.

Monocular deprivation during a critical period in postnatal development leads to a shift in functional and anatomical ocular dominance at the expense of the deprived eye. We analyzed the complete two-dimensional pattern of [(3)H]proline-labeled afferents in primary visual cortex (area 17) of cats monocularly deprived of vision at eye opening. Substantial shrinkage of deprived eye territory in favor of the normal eye extended into optic disc and monocular segment representations. However, small domains of deprived eye afferents were distributed evenly over the entire visual field representation. Interestingly, normal and deprived eye afferents overlapped extensively in the ipsilateral and in the peripheral contralateral visual field representation of the deprived eye, so that ipsi- and contralateral ocular dominance patterns are not at all complementary. We suggest that this could be the result of both an earlier maturation of crossed versus uncrossed visual pathways and of a maturational gradient within area 17 leading to a lower vulnerability of the central visual field representation to monocular deprivation. Quantitative analysis, using a triangulation algorithm which confirmed previously described larger spacing of adjacent ocular dominance columns in strabismic cats, revealed no difference in spacing of ocular dominance domains in area 17 between monocularly deprived and normals cats. In addition, column spacing was very similar in the same animal and in littermates, indicating that the genetic influence on columnar layout is stronger than previously assumed.

Age Factors↗