Neural activation associated with corrective saccades during tasks with fixation, pursuit and saccades

Corrective saccades are small eye movements that redirect gaze whenever the actual eye position differs from the desired eye position. In contrast to various forms of saccades including pro-saccades, recentering-saccades or memory guided saccades, corrective saccades have been widely neglected so far. The fMRI correlates of corrective saccades were studied that spontaneously occurred during fixation, pursuit or saccadic tasks. Eyetracking was performed during the fMRI data acquisition with a fiber-optic device. Using a combined block and event-related design, we isolated the cortical activations associated with visually guided fixation, pursuit or saccadic tasks and compared these to the activation associated with the occurrence of corrective saccades. Neuronal activations in anterior inferior cingulate, bilateral middle and inferior frontal gyri, bilateral insula and cerebellum are most likely specifically associated with corrective saccades. Additionally, overlapping activations with the established pro-saccade and, to a lesser extent, pursuit network were present. The presented results imply that corrective saccades represent a potential systematic confound in eye-movement studies, in particular because the frequency of spontaneously occurring corrective saccades significantly differed between fixation, pursuit and pro-saccade

Optic Flow Stimuli in and Near the Visual Field Centre: A Group fMRI Study of Motion Sensitive Regions

Motion stimuli in one visual hemifield activate human primary visual areas of the contralateral side, but suppress activity of the corresponding ipsilateral regions. While hemifield motion is rare in everyday life, motion in both hemifields occurs regularly whenever we move. Consequently, during motion primary visual regions should simultaneously receive excitatory and inhibitory inputs. A comparison of primary and higher visual cortex activations induced by bilateral and unilateral motion stimuli is missing up to now. Many motion studies focused on the MT+ complex in the parieto-occipito-temporal cortex. In single human subjects MT+ has been subdivided in area MT, which was activated by motion stimuli in the contralateral visual field, and area MST, which responded to motion in both the contra- and ipsilateral field. In this study we investigated the cortical activation when excitatory and inhibitory inputs interfere with each other in primary visual regions and we present for the first time group results of the MT+ subregions, allowing for comparisons with the group results of other motion processing studies. Using functional magnetic resonance imaging (fMRI), we investigated whole brain activations in a large group of healthy humans by applying optic flow stimuli in and near the visual field centre and performed a second level analysis. Primary visual areas were activated exclusively by motion in the contralateral field but to our surprise not by central flow fields. Inhibitory inputs to primary visual regions appear to cancel simultaneously occurring excitatory inputs during central flow field stimulation. Within MT+ we identified two subregions. Putative area MST (pMST) was activated by ipsi- and contralateral stimulation and located in the anterior part of MT+. The second subregion was located in the more posterior part of MT+ (putative area MT, pMT)

Cortical control of smooth pursuit eye movements

In this PhD project, the functions of cortical regions that control smooth pursuit eye movements (SPEM) and visual attention were investigated. Combining behavioural (eye movement) measurements and functional magnetic resonance imaging (fMRI) the cortical areas participating in the processing of visual information and motor information were investigated. Overlapping cortical Blood Oxygen Level Dependency (BOLD) activations might indicate where the transformation of visual input information into a motor output response takes place. Furthermore, the influence of visual attention on these mechanisms was studied. In a third step, the location and function of subregions of the motion sensitive MT+ complex – which plays a crucial role in the control of SPEM – was explored in more detail. In the final experiment, functional differences between regions of the SPEM network during the processing of visual motion by varying the amount of coherently moving target dots were investigated. In the first study it was shown that visual information processing takes place in the posterior parietal cortex (PPC) and MT+ and that oculomotor output processing takes place in the frontal eye fields (FEF), the supplementary eye fields (SEF), the cingulate gyrus and precuneus in addition to the above mentioned areas. Possible transformation sites were found in MT+ and within the PPC. In the second study it was shown that processing of visual attention during SPEM is fully integrated in the SPEM network, but certain aspects of the control of attention like the dissociation of attention from gaze are especially processed in the PPC. Furthermore it was shown that the ‘premotor theory’ of Rizzolatti (1984) is also valid for SPEM. In the third study two subregions of the motion sensitive MT+ complex, MST (medial superior temporal) and MT (middle temporal), were identified on group level. In contrast to monkey studies in the current study the eccentricity of the flow field relative to the midline played a minor role for the location of the MT+ subregions. These results question the assumed size of MT receptive fields in humans. The fourth study revealed that the visual input signal is modulated by retinal information whereas the oculomotor output is modulated by the eye movement signal or a mixture of visual and oculomotor information. Integration of visual and oculomotor information seems to take place in MST and visual areas V7/LOP. Processing of differential motion of eye and background appears to take place in the PPC. Surprisingly PPC hardly reacted if eye and background moved in phase. Primary visual area V1 probably receives eye movement signals. Its functional connections and exact functional role need further investigation

Neural activation associated with corrective saccades during tasks with fixation, pursuit and saccades

Corrective saccades are small eye movements that redirect gaze whenever the actual eye position differs from the desired eye position. In contrast to various forms of saccades including pro-saccades, recentering-saccades or memory guided saccades, corrective saccades have been widely neglected so far. The fMRI correlates of corrective saccades were studied that spontaneously occurred during fixation, pursuit or saccadic tasks. Eyetracking was performed during the fMRI data acquisition with a fiber-optic device. Using a combined block and event-related design, we isolated the cortical activations associated with visually guided fixation, pursuit or saccadic tasks and compared these to the activation associated with the occurrence of corrective saccades. Neuronal activations in anterior inferior cingulate, bilateral middle and inferior frontal gyri, bilateral insula and cerebellum are most likely specifically associated with corrective saccades. Additionally, overlapping activations with the established pro-saccade and, to a lesser extent, pursuit network were present. The presented results imply that corrective saccades represent a potential systematic confound in eye-movement studies, in particular because the frequency of spontaneously occurring corrective saccades significantly differed between fixation, pursuit and pro-saccades

Activation locations of main contrasts (stimulation (Stim.) vs. rest).

<p>Coordinates show the local maximum of an activated voxel cluster in MNI space; BA = Brodmann Area; fR = supposed functional region (V1/V2 means located in V1 or V2); T = T-value of maximum activated voxel, CS = cluster size, Mid. Temp. Gyr. = Middle Temporal Gyrus, Mid. Occ. Gyr. = Middle Occipital Gyrus, Inf. Par. Lob = Inferior Parietal Lobule.</p

Flatmaps of (A) the left and (B) the right hemisphere of the human PALS Atlas.

<p>Functional data overlaid on the flattened template brain. Functional data are RGB coded, intensity scaled to arbitrary values between 0–255. Blue, bilateral flow stimulus activations; red, unilateral flow stimulus - ipsilateral activations; green, unilateral flow stimulus - contralateral activations. Mixed colors show overlay of activations. T-threshold = 6. Insets show enlarged sections of the MT+ complex. For ease of interpretation known human visual areas are outlined in blue, taken from human PALS atlas <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004043#pone.0004043-vanEssen3" target="_blank">[26]</a>, and a lateral view on the slightly inflated 3D PALS template is given.</p

Eye movement data.

<p>(A) Original eye position trace of one subject in a rest and consecutive stimulation period. Leftward eye movements are shown as negative values. Small saccades as well as blinks (large positive excursions) are present in both rest and stimulation periods. (B) Saccadic frequency of n = 18 subjects in the rest and stimulation conditions.</p

Schematic drawing of stimuli and corresponding cortical activations.

<p>First row: (A) Bilateral flow stimulus, (B) Unilateral flow stimulus in the right visual hemifield, (C) Unilateral flow stimulus in the left visual hemifield. 2–4^th row (glassbrains; cortical activation contrasts calculated vs. corresponding rest conditions) (A): bilateral flow stimulus leads to activation in MT+ and additional occipito-parietal activation in both hemispheres. (B): unilateral flow in right visual hemifield leads to stronger activation in the left MT+ area, weaker activation in the right MT+ area (pMST) and additional activation in primary visual areas of the left hemisphere. (C): unilateral flow in the left visual hemifield leads to stronger activation in the right MT+ area, very weak activation in the left MT+ area (pMST) and activation in primary visual areas of the right hemisphere. Shown activations are FWE corrected, n = 18. Glassbrain presentation of data in axial coronal and sagittal planes (for maximum t-values compare <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004043#pone-0004043-t001" target="_blank">table 1</a>).</p

Radiocarbon-dated multiproxy record of Holocene lacustrine sediments from Laguna Azul, southern Patagonia (Argentina)

Multiproxy investigations, including XRF core-scanning, sedimentology, stable isotopes, pollen and diatoms, of lacustrine sediments from Laguna Azul (52 °S) document a superior climatic control on environmental conditions: Position and strength of Southern Hemispheric Westerlies (SHW) have overprinted the lake's ontogeny. SHW influenced local hydroclimatic conditions on millennial to centennial timescales, which impact on water-column stratification as well as on lake-level fluctuations with feedbacks on lakeshore erosion, algal communities, trophic conditions, methanogenesis and authigenic mineral formation. Via the link between SHW and regional hydrology, our high-resolution environmental reconstruction documents hydroclimatic variability during the Holocene, which compares well with other South American records. We detected a cool and wet period from 11,600-10,100 cal. BP followed by an extended Early Holocene dry period (10,100 and 8300 cal. BP) with ectogenic meromixis and high salinity. From 8300 until c. 4000 cal. BP the influence of the SHW weakened resulting in less arid conditions and a deep freshwater lake. Since 4000 cal. BP, regional temperature decreased accompanied by intensification of SHW reaching its full strength at 3000 cal. BP. Superimposed on these multi-millennial SHW fluctuations, Laguna Azul additionally documents a centennial variability during the Late Holocene with dry spells centered around 3700, 2200, 1000 cal. BP and in the 20th century. Although less arid periods are evident between these dry spells, the only pronounced moist period is representative for the "Little Ice Age" (1460-1740 cal. BP). During the last two centuries, human impact slightly obscures the climatic signal. With this study, we introduce a new and high-resolution dataset of hydroclimatic variability from southeastern Patagonia, documenting a multi-millennial variability of SHW for the Holocene overprinted by higher frequency (centennial) hydrologic variability for the last ca. 4000 years