Kerstin E. Schmidt

The Perceptual Grouping Criterion of Colinearity is Reflected by Anisotropies of Connections in the Primary Visual Cortex

An important step in the processing of visual patterns is the segmentation of the retinal image. Neuronal responses evoked by the contours of individual objects need to be identified and associated for further joint processing. These grouping operations are based on a number of Gestalt criteria. Here we report that connections in the visual cortex of the cat exhibit a highly significant anisotropy, preferentially linking neurons activated by contours that have similar orientation and are aligned colinearly. These anatomical data suggest a close relation between the perceptual grouping criterion of colinearity and the topology of tangential intracortical connections. We propose that tangential intracortical connections support perceptual grouping by modulating the saliency of distributed cortical responses in a context‐dependent way. The present data are compatible with the hypothesis that the criteria for this grouping operation are determined by the architecture of the tangential connections.

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.

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.

The layout of orientation and ocular dominance domains in area 17 of strabismic cats.

In the primary visual cortex of strabismic cats, the elimination of correlated activity between the two eyes enhances the segregation of the geniculocortical afferents into alternating ocular dominance domains. In addition, both tangential intracortical fibres and neuronal synchronization are severely reduced between neurons activated by different eyes. Consequently, ocular dominance columns belonging to different eyes are functionally rather independent. We wondered whether this would also affect the organization of orientation preference maps. To this end, we visualized the functional architecture of area 17 of strabismic cats with both optical imaging based on intrinsic signals and double labelling of orientation and ocular dominance columns with [14C]2‐deoxyglucose and [3H]proline. As expected, monocular iso‐orientation domains had a patchy appearance and differed for the two eyes, leading to a clear segregation of the ocular dominance domains. Comparison of ‘angle maps’ revealed that orientation domains exhibit a pinwheel organization as in normally reared cats. Interestingly, the map of orientation preferences did not show any breaks at the borders between ocular dominance columns: iso‐orientation domains were continuous across these borders. In addition, iso‐orientation contours tended to cross the borders of adjacent ocular dominance columns at right angles. These data suggest that the basic relations between the layout of orientation maps and ocular dominance columns are not disturbed by artificial decorrelation of binocular input. Therefore in cat area 17, the orientation map does not seem to be modified by experience‐dependent changes of thalamic input connections. This suggests the possibility that use‐dependent rearrangement of geniculocortical afferents into ocular dominance columns is due to Hebbian modifications whereby postsynaptic responsivity is constrained by the scaffold of the orientation map.

Cretaceous African life captured in amber.

Amber is of great paleontological importance because it preserves a diverse array of organisms and associated remains from different habitats in and close to the amber-producing forests. Therefore, the discovery of amber inclusions is important not only for tracing the evolutionary history of lineages with otherwise poor fossil records, but also for elucidating the composition, diversity, and ecology of terrestrial paleoecosystems. Here, we report a unique find of African amber with inclusions, from the Cretaceous of Ethiopia. Ancient arthropods belonging to the ants, wasps, thrips, zorapterans, and spiders are the earliest African records of these ecologically important groups and constitute significant discoveries providing insight into the temporal and geographical origins of these lineages. Together with diverse microscopic inclusions, these findings reveal the interactions of plants, fungi and arthropods during an epoch of major change in terrestrial ecosystems, which was caused by the initial radiation of the angiosperms. Because of its age, paleogeographic location and the exceptional preservation of the inclusions, this fossil resin broadens our understanding of the ecology of Cretaceous woodlands.

Specificity of Neuronal Responses in Primary Visual Cortex Is Modulated by Interhemispheric CorticoCortical Input

Within the visual cortex, it has been proposed that interhemispheric interactions serve to re-establish the continuity of the visual field across its vertical meridian (VM) by mechanisms similar to those used by intrinsic connections within a hemisphere. However, other specific functions of transcallosal projections have also been proposed, including contributing to disparity tuning and depth perception. Here, we consider whether interhemispheric connections modulate specific response properties, orientation and direction selectivity, of neurons in areas 17 and 18 of the ferret by combining reversible thermal deactivation in one hemisphere with optical imaging of intrinsic signals and single-cell electrophysiology in the other hemisphere. We found interhemispheric influences on both the strength and specificity of the responses to stimulus orientation and direction of motion, predominantly at the VM. However, neurons and domains preferring cardinal contours, in particular vertical contours, seem to receive stronger interhemispheric input than others. This finding is compatible with interhemispheric connections being involved in horizontal disparity tuning. In conclusion, our results support the view that interhemispheric interactions mainly perform integrative functions similar to those of connections intrinsic to one hemisphere.

Matching the modules: Cortical maps and long‐range intrinsic connections in visual cortex during development

Visual cortical neurons exhibit a high degree of response selectivity and are grouped into small columns according to their response preferences. The columns are located at regularly spaced intervals covering the whole cortical representation of the visual field with a modular system of feature-selective neurons. The selectivity of these cells and their modular arrangement is thought to emerge from interactions in the network of specific intracortical and thalamocortical connections. Understanding the ontogenesis of this complex structure and contributions of intrinsic and extrinsic, experience-dependent mechanisms during cortical development can provide new insights into the way the visual cortex processes information about the environment. Available data about the development of connections and response properties in the visual cortex suggest that maturation proceeds in two distinct steps. In the first phase, mechanisms inherent to the cortex establish a crude framework of interconnected neural modules which exhibit the basic but still immature traits of the adult state. Relevant mechanisms in this phase are assumed to consist of molecular cues and patterns of spontaneous neural activity in cortical and corticothalamic interconnections. In a second phase, the primordial layout becomes refined under the control of visual experience establishing a fine-tuned network of connections and mature response properties.

A novel interhemispheric interaction: modulation of neuronal cooperativity in the visual areas.

Background The cortical representation of the visual field is split along the vertical midline, with the left and the right hemi-fields projecting to separate hemispheres. Connections between the visual areas of the two hemispheres are abundant near the representation of the visual midline. It was suggested that they re-establish the functional continuity of the visual field by controlling the dynamics of the responses in the two hemispheres. Methods/Principal Findings To understand if and how the interactions between the two hemispheres participate in processing visual stimuli, the synchronization of responses to identical or different moving gratings in the two hemi-fields were studied in anesthetized ferrets. The responses were recorded by multiple electrodes in the primary visual areas and the synchronization of local field potentials across the electrodes were analyzed with a recent method derived from dynamical system theory. Inactivating the visual areas of one hemisphere modulated the synchronization of the stimulus-driven activity in the other hemisphere. The modulation was stimulus-specific and was consistent with the fine morphology of callosal axons in particular with the spatio-temporal pattern of activity that axonal geometry can generate. Conclusions/Significance These findings describe a new kind of interaction between the cerebral hemispheres and highlight the role of axonal geometry in modulating aspects of cortical dynamics responsible for stimulus detection and/or categorization.

Multiplicative Mechanism of Lateral Interactions Revealed by Controlling Interhemispheric Input

Long-range horizontal connections are thought to modulate the responsiveness of neurons by supplying contextual information. A special type of long-range connections are interhemispheric projections, linking the 2 cerebral hemispheres. To investigate the action of those projections in a causal approach, we recorded in cat primary visual cortex while deactivating corresponding regions on the contralateral hemisphere. Interestingly, the action of callosal projections turned out to depend on the local and global composition of the stimulus: Full-field stimulation with gratings revealed moderate rate decreases (modulation index -0.24) and some significant increases (+0.21), whereas with lesser salient random dot textures, much more neurons were affected and reacted with pronounced rate decreases (-0.4). However, orientation and direction selectivity of those neurons were only slightly influenced by callosal input. This invariance could be achieved by scaling responses multiplicatively. Indeed, we could quantify the action of callosal input as a multiplicative scaling of responses, but additive scaling also occurred, especially for grating stimulation. We conclude that the quantitative action of long-range horizontal connections is by no means fixed but depends on how the network is driven by an external stimulus. Qualitatively, those connections seem to adjust the response gain of neurons, thereby preserving their selectivity.

Long-Range Intrinsic Connections in Cat Primary Visual Cortex

Publisher Summary This chapter focuses on the detailed layout of long-range horizontal connections in the primary visual cortex of cats, their topographic relation to functional cortical maps, learning and activity-induced modifiability, and possible functions. The majority of long-range intrinsic connections of excitatory cortical neurons in the primary visual cortex of cats link neurons with similar response properties such as orientation preference and possibly also direction preference. The selectivity of the long-range connections may depend on the relative position of a neuron within a functional map such as projections extending from pinwheel centers are less orientation selective than projections originating from linear zones of an orientation preference map. Both the selectivity and maximal lateral extent of long-range connections depend on the layer. There is extensive cross-talk between different orientation domains and clear differences in the layout of long-range excitatory and inhibitory connections. Many different functions have been ascribed to long-range intrinsic connections. In particular, there is growing evidence that they are important for perceptual integration. The outgrowth of new connections, previously thought to be restricted to so-called “critical periods” in early life, also occurs in the adult brain and most probably mediates long-term long-range representational plasticity in the cortex.