Distribution of Activity Across the Monkey Cerebral Cortical Surface, Thalamus and Midbrain during Rapid, Visually Guided Saccades

Salience maps in parietal cortex: imaging and computational modeling

Models of spatial attention are often based on the concept of a salience map. In computational cognitive neuroscience, such maps are implemented as a collection of nodes with self-excitation and lateral inhibition between all nodes (competitive interaction map). Here, we test some critical predictions of this idea. We argued that task demands, more precisely the level of attention required, can top-down modulate the level of lateral inhibition in a salience map, and thus induce different activation functions. We first show that a model with a high lateral inhibition parameter generates a monotonous activation curve as a function of set size similar to that typically observed in the literature (e.g. Todd and Marois, 2004). Next, we show that a competitive interaction map with medium lateral inhibition leads to a Lambda-shaped activation curve when set sizes increase. This prediction is confirmed in an fMRI experiment with medium attention demands where a similar Lambda-shaped activation curve is found in a posterior superior parietal area that was proposed to house a salience map (Todd and Marois, 2004). Finally, we show that a qualitatively different V-shaped activation curve is predicted with a very low inhibition parameter. An fMRI experiment with low attentional demands revealed this V-shaped activation curve in the same region. These findings provide critical support for the existence of a salience map based on competitive interactions in posterior superior parietal cortex, and suggest that its parameters (in particular, lateral inhibition) can be modulated in a top down manner dependent on task demands.

Functional connectivity of the macaque posterior parahippocampal cortex

Neuroimaging experiments in humans suggest that regions in parietal cortex and along the posterior midline are functionally connected to the medial temporal lobe and are active during memory retrieval. It is unknown whether macaques have a similar network. We examined functional connectivity in isoflurane-anesthetized macaques to identify a network associated with posterior parahippocampal cortex (PPHC). Functional connectivity was observed between the PPHC and retrosplenial, posterior cingulate, superior temporal gyrus, and inferior parietal cortex. PPHC correlations were distinct from regions in parietal and temporal cortex activated by an oculomotor task. Comparison of macaque and human PPHC correlations revealed similarities that suggest the temporal-parietal region identified in the macaque may share a common lineage with human Brodmann area 39, a region thought to be involved in recollection. These results suggest that macaques and humans may have homologous PPHC-parietal pathways. By specifying the location of the putative macaque homologue in parietal cortex, we provide a target for future physiological exploration of this area's role in mnemonic or alternative processes.

Functional connectivity between the thalamus and visual cortex under eyes closed and eyes open conditions: a resting-state fMRI study

The thalamus and visual cortex are two key components associated with the alpha power of electroencephalography. However, their functional relationship remains to be elucidated. Here, we employ resting-state functional MRI to investigate the temporal correlations of spontaneous fluctuations between the thalamus [the whole thalamus and its three largest nuclei (bilateral mediodorsal, ventrolateral and pulvinar nuclei)] and visual cortex under both eyes open and eyes closed conditions. The whole thalamus show negative correlations with the visual cortex and positive correlations with its contralateral counterpart in eyes closed condition, but which are significantly decreased in eyes open condition, consistent with previous findings of electroencephalography desynchronization during eyes open resting state. Furthermore, we find that bilateral thalamic mediodorsal nuclei and bilateral ventrolateral nuclei have remarkably similar connectivity maps, and resemble to those of the whole thalamus, suggesting their crucial contributions to the thalamus-visual correlations. The bilateral pulvinar nuclei are found to show distinct functional connectivity patterns, compatible with previous findings of the asymmetry of anatomical and functional organization in the nuclei. Our data provides evidence for the associations of intrinsic spontaneous neuronal activity between the thalamus and visual cortex under different resting conditions, which might have implications on the understanding of the generation and modulation of the alpha rhythm.

Neuronal substrates of gaze following in monkeys

Human and non-human primates follow the gaze of their respective conspecific to identify objects of common interest. Whereas humans rely on eye-gaze for such purposes, monkeys preferentially use head-gaze information. Functional magnetic resonance imaging (fMRI) studies have delineated an area in the human superior temporal sulcus (STS), which is specifically activated when subjects actively follow the eye-gaze of others. Similarly, using fMRI, we have identified an analogous region in the monkey's middle STS responding to gaze following. Hence, although humans and monkeys might rely on different directional cues guiding their attention, they seem to deploy a similar and possibly homologous cortical area to follow the gaze of a conspecific. Our results support the idea that the eyes developed a new social function in human evolution, most likely to support cooperative mutual social interactions building on a phylogenetically old STS module for the processing of head cues.

Cortical connections of the visuomotor parietooccipital area V6Ad of the macaque monkey

Area V6A, a functionally defined region in the anterior bank of the parietooccipital sulcus, has been subdivided into dorsal and ventral cytoarchitectonic fields (V6Ad and V6Av). The aim of this study was to define the cortical connections of the cytoarchitectonic field V6Ad. Retrograde and bidirectional neuronal tracers were injected into the dorsal part of the anterior bank of parietooccipital sulcus of seven macaque monkeys (Macaca fascicularis). The limits of injection sites were compared with those of cytoarchitectonic fields. The major connections of V6Ad were with areas of the superior parietal lobule. The densest labeling was observed in the medial intraparietal area (MIP). Areas PEc, PGm, and V6Av were also strongly connected. Labeled cells were found in medial parietal area 31; in cingulate area 23; in the anterior (AIP), ventral (VIP), and lateral (LIP) intraparietal areas; in the inferior parietal lobule (fields Opt and PG); and in the medial superior temporal area (MST). In the frontal lobe, the main projection originated from F2, although labeled cells were also found in F7 and area 46. Preliminary results obtained from injections in nearby areas PEc and V6Av revealed connections different from those of V6Ad. In agreement with functional data, the strong connections with areas where arm-reaching activity is represented suggest that V6Ad is part of a parietofrontal circuit involved in the control of prehension, and connections with AIP specifically support an involvement in the control of grasping. Connections with areas LIP and Opt are likely related to the oculomotor activities observed in V6Ad.

Parietal regions processing visual 3D shape extracted from disparity

Three-dimensional (3D) shape is important for the visual control of grasping and manipulation. We used fMRI to study the processing of 3D shape extracted from disparity in human parietal cortex. Subjects stereoscopically viewed random-line stimuli portraying a 3D structure, a 2D structure in multiple depth planes or a 2D structure in the fixation plane. Subtracting the second from the first condition yields depth-structure sensitive regions and subtracting the third from the second position-in-depth sensitive regions. Two anterior intraparietal sulcus (IPS) regions, the dorsal IPS medial (DIPSM) and the dorsal IPS anterior (DIPSA) regions, were sensitive to depth structure and not to position in depth, while a posterior IPS region, the ventral IPS (VIPS) region, had a mixed sensitivity. All three IPS regions were also sensitive to 2D shape, indicating that they carry full 3D shape information. Finally DIPSM, but not DIPSA was sensitive to a saccade-related task. These results underscore the importance of anterior IPS regions in the processing of 3D shape, in agreement with their proximity to grasping-related regions. Moreover, comparison with the results of Durand, J.B., Nelissen, K., Joly, O., Wardak, C., Todd, J.T., Norman, J.F., Janssen, P., Vanduffel, W., Orban, G.A., 2007. Anterior Regions of Monkey Parietal Cortex Process Visual 3D Shape. Neuron 55, 493-505 obtained in the monkey indicates that DIPSA and DIPSM may represent human homologues for the posterior part of AIP and the adjoining part of LIP respectively.

BOLD fMRI activation for anti-saccades in nonhuman primates

Most of our knowledge about the functional organization of the nonhuman primate brain has come from single neuron recordings, whereas functional magnetic resonance imaging (fMRI) has rapidly become the method of choice for the study of the human brain. In some cases these two methods have resulted in conflicting models of frontal lobe function. Based on the finding that the frontal eye fields (FEF) exhibit a higher blood-oxygenation-level dependent (BOLD) activation for anti-saccades compared with pro-saccades, it has been proposed that this area is more involved in voluntary than automatic saccade generation. This model has been questioned by the finding of decreased single neuron activity in FEF for anti-compared with pro-saccades in monkeys. To reconcile these findings, we employed fMRI to compare BOLD activation between anti-saccades and pro-saccades in monkeys. FEF and a number of other cortical and subcortical areas showed an increased activation for anti-saccades. The results indicate that previous contrary findings between single neuron recordings and fMRI were due to differences between these techniques and were not related to differences between the two primate species.

Connectivity of the primate superior colliculus mapped by concurrent microstimulation and event-related FMRI

Background

Neuroanatomical studies investigating the connectivity of brain areas have heretofore employed procedures in which chemical or viral tracers are injected into an area of interest, and connected areas are subsequently identified using histological techniques. Such experiments require the sacrifice of the animals and do not allow for subsequent electrophysiological studies in the same subjects, rendering a direct investigation of the functional properties of anatomically identified areas impossible.

Methodology/Principal Findings

Here, we used a combination of microstimulation and fMRI in an anesthetized monkey preparation to study the connectivity of the superior colliculus (SC). Microstimulation of the SC resulted in changes in the blood oxygenation level-dependent (BOLD) signals in the SC and in several cortical and subcortical areas consistent with the known connectivity of the SC in primates.

Conclusions/Significance

These findings demonstrates that the concurrent use of microstimulation and fMRI can be used to identify brain networks for further electrophysiological or fMRI investigation.

Countering postural posteffects following prolonged exposure to whole-body vibration: a sensorimotor treatment

Postural stability of bulldozer operators after a day of work is investigated. When operators are no longer exposed to whole-body vibration (WBV) generated by their vehicle, their sensorimotor coordination and body representation remain altered. A sensorimotor treatment based on a set of customized voluntary movements is tested to counter and prevent potential post-work accidents due to prolonged exposure to WBV. This treatment includes muscle stretching, joint rotations, and plantar pressures, all known to minimize the deleterious effects of prolonged exposure to mechanical vibrations. The postural stability of participants (drivers; N = 12) was assessed via the area of an ellipse computed from the X and Y displacements of the center-of-pressure (CoP) in the horizontal plane when they executed a simple balance task before driving, after driving, and after driving and having performed the sensorimotor treatment. An ancillary experiment is also reported in which a group of non-driver participants (N = 12) performed the same postural task three times during the same day but without exposure to WBV or the sensorimotor treatment. Prolonged exposure to WBV significantly increased postural instability in bulldozer drivers after they operated their vehicle compared to prior to their day of work. The sensorimotor treatment allowed postural stability to return to a level that was not significantly different from that before driving. The results reveal that (1) the postural system remains perturbed after prolonged exposure to WBV due to operating a bulldozer and (2) treatment immediately after driving provides a "sensorimotor recalibration" and a significant decrease in WBV-induced postural instability. If confirmed in different contexts, the postural re-stabilizing effect of the sensorimotor treatment would constitute a simple, rapid, inexpensive, and efficient means to prevent post-work accidents due to balance-related issues.

The frontal eye field as a prediction map

Predictive processes are widespread in the motor and sensory areas of the primate brain. They enable rapid computations despite processing delays and assist in resolving noisy, ambiguous input. Here we propose that the frontal eye field, a cortical area devoted to sensorimotor aspects of eye movement control, implements a prediction map of the postsaccadic visual scene for the purpose of constructing a stable percept despite saccadic eye movements.

Anterior regions of monkey parietal cortex process visual 3D shape

The intraparietal cortex is involved in the control of visually guided actions, like reach-to-grasp movements, which require extracting the 3D shape and position of objects from 2D retinal images. Using fMRI in behaving monkeys, we investigated the role of the intraparietal cortex in processing stereoscopic information for recovering the depth structure and the position in depth of objects. We found that while several areas (CIP, LIP, and AIP on the lateral bank; PIP and MIP on the medial bank) are activated by stereoscopic stimuli, AIP and an adjoining portion of LIP are sensitive only to depth structure. Furthermore, only these two regions are sensitive to both the depth structure and the 2D shape of small objects. These results indicate that extracting 3D spatial information from stereo involves several intraparietal areas, among which AIP and anterior LIP are more specifically engaged in extracting the 3D shape of objects.

Intrinsic functional architecture in the anaesthetized monkey brain

The traditional approach to studying brain function is to measure physiological responses to controlled sensory, motor and cognitive paradigms. However, most of the brain's energy consumption is devoted to ongoing metabolic activity not clearly associated with any particular stimulus or behaviour. Functional magnetic resonance imaging studies in humans aimed at understanding this ongoing activity have shown that spontaneous fluctuations of the blood-oxygen-level-dependent signal occur continuously in the resting state. In humans, these fluctuations are temporally coherent within widely distributed cortical systems that recapitulate the functional architecture of responses evoked by experimentally administered tasks. Here, we show that the same phenomenon is present in anaesthetized monkeys even at anaesthetic levels known to induce profound loss of consciousness. We specifically demonstrate coherent spontaneous fluctuations within three well known systems (oculomotor, somatomotor and visual) and the 'default' system, a set of brain regions thought by some to support uniquely human capabilities. Our results indicate that coherent system fluctuations probably reflect an evolutionarily conserved aspect of brain functional organization that transcends levels of consciousness.

Topographic Maps in Human Frontal Cortex Revealed in Memory-Guided Saccade and Spatial Working-Memory Tasks

We used fMRI at 3 Tesla and improved spatial resolution (2 × 2 × 2 mm3) to investigate topographic organization in human frontal cortex using memory-guided response tasks performed at 8 or 12 peripheral locations arranged clockwise around a central fixation point. The tasks required the location of a peripheral target to be remembered for several seconds after which the subjects either made a saccade to the remembered location (memory-guided saccade task) or judged whether a test stimulus appeared in the same or a slightly different location by button press (spatial working-memory task). With these tasks, we found two topographic maps in each hemisphere, one in the superior branch of precentral cortex and caudalmost part of the superior frontal sulcus, in the region of the human frontal eye field, and a second in the inferior branch of precentral cortex and caudalmost part of the inferior frontal sulcus, both of which greatly overlapped with activations evoked by visually guided saccades. In each map, activated voxels coded for saccade directions and memorized locations predominantly in the contralateral hemifield with neighboring saccade directions and memorized locations represented in adjacent locations of the map. Particular saccade directions or memorized locations were often represented in multiple locations of the map. The topographic activation patterns showed individual variability from subject to subject but were reproducible within subjects. Notably, only saccade-related activation, but no topographic organization, was found in the region of the human supplementary eye field in dorsomedial prefrontal cortex. Together these results show that topographic organization can be revealed outside sensory cortical areas using more complex behavioral tasks.

Saccade-related information in the superior temporal motion complex: quantitative functional mapping in the monkey

Although the role of the motion complex [cortical areas middle temporal (V5/MT), medial superior temporal (MST), and fundus of the superior temporal (FST)] in visual motion and smooth-pursuit eye movements is well understood, little is known about its involvement in rapid eye movements (saccades). To address this issue, we used the quantitative 14C-deoxyglucose method to obtain functional maps of the cerebral cortex lying in the superior temporal sulcus of rhesus monkeys executing saccades to visual targets and saccades to memorized targets in complete darkness. Fixational effects were observed in MT-foveal, FST, the anterior part of V4-transitional (V4t), and temporal-occipital areas. Saccades to memorized targets activated areas V5/MT, MST, and V4t, which were also activated for saccades to visual targets. Regions activated in the light and in the dark overlapped extensively. In addition, saccades to visual targets activated areas FST and the intermediate part of the polysensory temporal-parietal-occipital area. Cortical activity related to visually guided saccades could be explained, at least in part, by visual motion. Because only oculomotor signals can account for the equally robust activations induced by memory saccades in complete darkness, we suggest that areas V5/MT, MST, and V4t receive and/or process saccade-related oculomotor information.

Exploring the neural basis of cognition: multi-modal links between human fMRI and macaque neurophysiology

Although functional magnetic resonance imaging (fMRI) with sophisticated behavioral paradigms has enabled the investigation of increasingly higher-level cognitive functions in humans, these studies seem to lose touch with neurophysiological studies in macaque monkeys. The application of fMRI and other MRI-based techniques to macaque brains allows studies in the two species to be linked. fMRI in human and macaque subjects using equivalent cognitive tasks enables direct comparisons of the functional brain architecture, even for high-level cognitive functions. Combinations of functional or structural MRI and microelectrode techniques provide ways to explore functional brain networks at multiple spatiotemporal scales. These approaches would illuminate the neurophysiological underpinnings of human cognitive functions by integrating human functional neuroimaging with macaque single-unit recordings.