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

A Gestalt in primary visual cortex? : applying neurophysiological methods to capture a psychophysical phenomenon

"The whole is more than the sum of its parts." This idea has been brought forward by psychologists such as Max Wertheimer who formulated Gestalt laws that describe our perception. One law is that of collinearity: elements that correspond in their local orientation to their global axis of alignment form a collinear line, compared to a noncollinear line where local and global orientations are orthogonal. Psychophysical studies revealed a perceptual advantage for collinear over non-collinear stimulus context. It was suggested that this behavioral finding could be related to underlying neuronal mechanisms already in the primary visual cortex (V1). Studies have shown that neurons in V1 are linked according to a common fate: cells responding to collinearly aligned contours are predominantly interconnected by anisotropic long-range lateral connections. In the cat, the same holds true for visual interhemispheric connections. In the present study we aimed to test how the perceptual advantage of a collinear line is reflected in the anatomical properties within or between the two primary visual cortices. We applied two neurophysiological methods, electrode and optical recording, and reversibly deactivated the topographically corresponding contralateral region by cooling in eight anesthetized cats. In electrophysiology experiments our results revealed that influences by stimulus context significantly depend on a unit’s orientation preference. Vertical preferring units had on average a higher spike rate for collinear over non-collinear context. Horizontal preferring units showed the opposite result. Optical imaging experiments confirmed these findings for cortical areas assigned to vertical orientation preference. Further, when deactivating the contralateral region the spike rate for horizontal preferring units in the intact hemisphere significantly decreased in response to a collinear stimulus context. Most of the optical imaging experiments revealed a decrease in cortical activity in response to either stimulus context crossing the vertical midline. In conclusion, our results support the notion that modulating influences from stimulus context can be quite variable. We suggest that the kind of influence may depend on a cell’s orientation preference. The perceptual advantage of a collinear line as one of the Gestalt laws proposes is not uniformly represented in the activity of individual cells in V1. However, it is likely that the combined activity of many V1 neurons serves to activate neurons further up the processing stream which eventually leads to the perceptual phenomenon."Das Ganze ist mehr als die Summe seiner Teile." Dieser Ausspruch wurde besonders durch Gestaltpsychologen wie Max Wertheimer geprägt. Es wurden Gestaltgesetze formuliert, welche unsere Wahrnehmung beschreiben. Eines dieser Gesetze betrifft die Kollinearität: Objekte, die in ihrer lokalen Orientierung mit der globalen Achse ihrer Anordnung übereinstimmen, bilden eine kollineare Linie. Diese wird im Gegensatz zu einer nicht-kollinearen Linie, bei der die lokale und globale Orientierung orthogonal zueinander sind, als "bessere" Gestalt wahrgenommen (Wertheimer, 1923). Psychophysische Studien konnten diesen Wahrnehmungsvorteil einer kollinearen Linie bestätigen. Ein gängiger Stimulus zur experimentellen Testung ist ein Gabor Element, ein sinusoidales Streifenmuster gewichtet durch eine Gausfunktion. Versuchspersonen konnten eine Linie kollinear angeordneter Gabors umgeben von zufällig angeordneten Gabors schneller wieder erkennen, als wenn die Elemente nicht-kollinear angeordnet waren (Field & Hess, 1993). Weiterhin konnte gezeigt werden, dass ein zentraler Gabor schon bei sehr geringem Kontrast erkannt wird, wenn dieser von anderen gleich orientierten Elementen entlang der kollinearen Achse umgeben war, nicht jedoch, wenn die Elemente orthogonal zu ihrer lokalen Orientierung angeordnet waren (Polat & Sagi, 1994). Die Forscher hatten außerdem zeigen können, dass die Elemente optimaler weise 3 λ (lies: Lambda) von einander entfernt sein sollten, ein Wert der die sinusoidale Wellenlänge des Streifenmusters beschreibt (Polat & Sagi, 1993). Den Forschern gemein war der Versuch, ihre Erkenntnisse auf Verhaltensebene neuronalen Grundlagen zuordnen zu können. Es wurde festgestellt, dass neuronale Zellen im primären visuellen Kortex (V1) bereits eine entscheidende Rolle für den beobachteten Wahrnehmungsvorteil kollinearer Linien spielen könnten. ..

Imaging plasticity and structure of cortical maps in cat and mouse visual cortex

The study reported in the first part of this thesis utilized optical imaging of intrinsic signals to visualize changes in orientation maps in cat visual cortex induced by pairing a visual stimulus with an intracortical electrical stimulation. We found that the direction of plasticity within orientation maps depends critically on the relative timing between visual and electrical stimulation on a millisecond time scale: a shift in orientation preference towards the paired orientation was observed if the cortex was first visually and then electrically stimulated. In contrast, the cortical response to the paired orientation was diminished if the electrical preceded the visual cortical stimulation. Spike-time-dependent plasticity has been observed in single cell studies; however, our results demonstrate an analogous effect at the systems level in the live animal. Thus, timing-dependent plasticity needs to be incorporated into our conception of cortical map development. While the pairing paradigm induced pronounced shifts in orientation preference, the general setup of the orientation preference map remained unaltered. In order to unravel potential factors contributing to this overall stability, we determined the distribution of plasticity across the cortical surface. We found that pinwheel centers, points were domains of all orientation meet, exhibited less plasticity than other regions of the orientation map. The resistance of pinwheel centers to changes in orientation preference may support maintenance of the general structure of the orientation map. The study that forms the second part employs optical imaging to visualize the retinotopy in mouse visual cortex. We were able to resolve the pattern of retinotopic activity with high precision and reliability in the primary visual cortex (area 17). Functional imaging of the position, size and shape of area 17 corresponded exactly to the location of this area in stained histological sections. The imaged maps were also confirmed with electrophysiological recordings. The retinotopic structure of area 17 showed very low inter-animal variability, thus allowing averaging maps across animals and therefore statistical analysis. These averaged maps greatly facilitated the identification of at least four extrastriate visual areas. In addition, we detected decreases in the intrinsic signal below baseline with a shape and location reminiscent of lateral inhibition. This decrease of the intrinsic signal was shown to be correlated with a decrease in neuronal firing rate below baseline. Both studies were facilitated by the development of a signal analysis technique (part III), which improves the quality of optical imaging data. Intrinsic signal fluctuations originating from blood vessels were minimized based on their correlation with the actual superficial blood vessel pattern. These fluctuation components were then extracted from images obtained during sensory stimulation. This method increases the reproducibility of functional maps from cat, rat, and mouse visual cortex significantly and might also be applied to high resolution imaging using voltage sensitve dyes or functional magnetic resonance

An Updated Midline Rule: Visual Callosal Connections Anticipate Shape and Motion in Ongoing Activity across the Hemispheres

It is generally thought that callosal connections (CCs) in primary visual cortices serve to unify the visual scenery parted in two at the vertical midline (VM). Here, we present evidence that this applies also to visual features that do not cross yet but might cross the VM in the future. During reversible deactivation of the contralateral visual cortex in cats, we observed that ipsilaterally recorded neurons close to the border between areas 17 and 18 receive selective excitatory callosal input on both ongoing and evoked activity. In detail, neurons responding well to a vertical Gabor patch moving away from the deactivated hemifield decreased prestimulus and stimulus-driven activity much more than those preferring motion toward the cooled hemifield. Further, activity of neurons responding to horizontal lines decreased more than the response to vertical lines. Embedding a single Gabor into a collinear line context selectively stabilized responses, especially when the context was limited to the intact hemifield. These findings indicate that CCs interconnect not only neurons coding for similar orientations but also for similar directions of motion. We conclude that CCs anticipate stimulus features that are potentially relevant for both hemifields (i.e., coherent motion but also collinear shape) because already prestimulus activity and activity to stimuli not crossing the VM revealed feature specificity. Finally, we hypothesize that intrinsic and callosal networks processing different orientations and directions are anisotropic close to the VM facilitating perceptual grouping along likely future motion or (shape) trajectories before the visual stimulus arrives

Colour and spatial pattern discrimination in human vision

Imperial Users onl

Probing cortical excitability with transcranial magnetic stimulation

This thesis, consisting of seven original publications (I-VII), explored the technical and neurophysiological plausibility of combining neuro-navigated transcranial magnetic stimulation (nTMS) with neuroimaging techniques such as multichannel electroencephalography (EEG) and magnetoencephalography (MEG). This work has focused on the interaction between the current state of neuronal activity at the targeted cortical network and the effects of TMS. We took an interactive approach, including a correlation betweeen cortical (EEG, MEG) vs. peripheral electromyographic (EMG) measurements. TMS-evoked EEG responses were used as probes for current functional state of the cortex during the processing of sensory stimuli and the preparation/execution of different motor activities. Contrary to standard indirect approaches utilizing peripheral EMG measures, our study directly demonstrated graded excitability in contra- and ipsilateral hemispheres during the preparation/execution of unilateral movements. The obtained data suggest that the specific balance of interhemispehric excitability is tailored for the optimal performance of unilateral movement by preventing not only mirror movements through decreased excitability of ipsilateral hemispehre, but also via pre-emptive background tonic inhibition of this hemisphere. The utility of the TMS-EEG combination was further demonstrated by providing direct evidence for cortical involvement in short-latency afferent inhibition. We found a linear correlation between the attenuation of TMS-evoked EEG responses and the attenuation of muscle responses, thus revealing how changes in cortical neuronal activity are related to changes on the periphery. The clinical feasibility of the TMS-MEG combination was demonstrated by showing that delivering trains of TMS pulses to the motor cortex of Parkinson's patients successfully modulated the spontaneous beta-range oscillations measured with MEG over the rolandic cortical regions, suggesting probable alteration of the cortico-thalamo-basal ganglia networks. The present thesis demonstrates that the spatial accuracy of localizing primary motor representational areas with both MEG and nTMS in superior to electrical cortical stimulation via subdural grids. Furthermore, this work demonstrates very high reproducibility of TMS-evoked EEG deflections after repeated stimulation of both the primary motor and prefrontal cortices. This suggests new standards in preoperative clinical workup and a wide range studies with test-retest design. Thus, this thesis provides a new methodological and technical framework for measuring the time-resolved functional connectivity and causality of activation in the observed neural networks of human cerebral cortex

ENCODING OF SALTATORY TACTILE VELOCITY IN THE ADULT OROFACIAL SOMATOSENSORY SYSTEM

Processing dynamic tactile inputs is a key function of somatosensory systems. Spatial velocity encoding mechanisms by the nervous system are important for skilled movement production and may play a role in recovery of motor function following neurological insult. Little is known about tactile velocity encoding in trigeminal networks associated with mechanosensory inputs to the face, or the consequences of movement. High resolution functional magnetic resonance imaging (fMRI) was used to investigate the neural substrates of velocity encoding in the human orofacial somatosensory system during unilateral saltatory pneumotactile inputs to perioral hairy skin in 20 healthy adults. A custom multichannel, scalable pneumotactile array consisting of 7 TAC-Cells was used to present 5 stimulus conditions: 5 cm/s, 25 cm/s, 65 cm/s, ALL-ON synchronous activation, and ALL-OFF. The spatial organization of cerebral and cerebellar blood oxygen level-dependent (BOLD) response as a function of stimulus velocity was analyzed using general linear modeling (GLM) of pooled group fMRI signal data. The sequential saltatory inputs to the lower face produced localized, predominantly contralateral BOLD responses in primary somatosensory (SI), secondary somatosensory (SII), primary motor (MI), supplemental motor area (SMA), posterior parietal cortices (PPC), and insula, whose spatial organization and intensity were highly dependent on velocity. Additionally, ipsilateral sensorimotor, insular and cerebellar BOLD responses were prominent during the lowest velocity (5 cm/s). Advisor: Steven M. Barlo