In humans, several neuroimaging studies have demonstrated that passive viewing of optic flow stimuli activates higher-level motion areas, like V6 and the cingulate sulcus visual area (CSv). In macaque, there are few studies on the sensitivity of V6 and CSv to egomotion compatible optic flow. The only fMRI study on this issue revealed selectivity to egomotion compatible optic flow in macaque CSv but not in V6 (Cotterau et al. Cereb Cortex 27(1):330-343, 2017, but see Fan et al. J Neurosci. 35:16303-16314, 2015). Yet, it is unknown whether monkey visual motion areas MT + and V6 display any distinctive fMRI functional profile relative to the optic flow stimulation, as it is the case for the homologous human areas (Pitzalis et al., Cereb Cortex 20(2):411-424, 2010). Here, we described the sensitivity of the monkey brain to two motion stimuli (radial rings and flow fields) originally used in humans to functionally map the motion middle temporal area MT + (Tootell et al. J Neurosci 15: 3215-3230, 1995a; Nature 375:139-141, 1995b) and the motion medial parietal area V6 (Pitzalis et al. 2010), respectively. In both animals, we found regions responding only to optic flow or radial rings stimulation, and regions responding to both stimuli. A region in the parieto-occipital sulcus (likely including V6) was one of the most highly selective area for coherently moving fields of dots, further demonstrating the power of this type of stimulation to activate V6 in both humans and monkeys. We did not find any evidence that putative macaque CSv responds to Flow Fields.

In the superior parietal lobule (SPL), the anterior part (area PE) is known to process somatosensory information, while the caudalmost part (areas V6Av and V6) processes visual information. Here we studied the visual and somatosensory properties of the areas PEc and V6Ad located in between the somatosensory and visual domains of SPL. About 1500 neurons were extracellularly recorded in 19 hemispheres of 12 monkeys (Macaca fascicularis). Visual and somatosensory properties of single neurons were generally studied separately, while in a subpopulation of neurons, both the sensory properties were tested. Visual neurons were more represented in V6Ad and somatosensory neurons in PEc. The visual neurons of these two areas showed similar properties and represented a large part of the contralateral visual field, mostly the lower part. In contrast, somatosensory neurons showed remarkable differences. The arms were overrepresented in both the areas, but V6Ad represented only the upper limbs, whereas PEc both the upper and lower limbs. Interestingly, we found that in both the areas, bimodal visual–somatosensory cells represented the proximal part of the arms. We suggest that PEc is involved in locomotion and in the control of hand/foot interaction with the objects of the environment, while V6Ad is in the control of the object prehension specifically performed with the upper limbs. Neuroimaging and lesion studies from literature support a strict homology with humans.

Single cell recordings in the awake monkey have so far neglected the depth dimension of visuomotor transformations for reaching. In the few cases where depth has been taken into account, direction has been left apart. The coexistence of depth and direction information for reaching requires further studies, as usually our reaching movements in real life occurr by changes in the distance and laterality of our hand positions in peripersonal space. Here, we investigated how reach depth and reach direction interact at single cell level in two areas of superior parietal cortex (SPL): area V6A and PEc, located caudally in the SPL, in a region of the posterior parietal cortex medial to area MIP/PRR. Two macaque monkeys performed a fixation-to-reach task in 3-dimensional space, toward foveated targets located at different distances and lateralities. We analized the spatial tuning of about 200 neurons per area in several phases of this delay reaching task: target fixation, early and late delay period, movement and holding times. We found that depth and direction signals influenced jointly a large number of neurons in both areas in all epochs of the task considered, with PEc showing more independent processing of depth and direction, especially before the arm movement onset. In PEc, the effect of direction was more prevalent than depth before reaching execution while the reverse was true for depth. In V6A, depth and direction similarly influenced neural activity for the entire trial. These findings suggest the involvement of both areas in visuospatial and action representations in 3D peripersonal space, with a caudo-rostral trend from a joint processing of depth and direction signals for eye position and reach execution in V6A to an encoding of depth related mostly to arm movement in PEc. These data reflect a rostro-caudal trend similar to that observed in human fMRI studies. Meeting abstract presented at VSS 2015

Reaching movements in the real world have typically a direction and a depth component. Despite numerous behavioral studies, there is no consensus on whether reach coordinates are processed in separate or common visuomotor channels. Furthermore, the neural substrates of reach depth in parietal cortex have been ignored in most neurophysiological studies. In the medial posterior parietal area V6A, we recently demonstrated the strong presence of depth signals and the extensive convergence of depth and direction information on single neurons during all phases of a fixate-to-reach task in 3-dimensional (3D) space. Using the same task, in the present work we examined the processing of direction and depth information in area PEc of the caudal superior parietal lobule (SPL) in three Macaca fascicularis monkeys. Across the task, depth and direction had a similar, high incidence of modulatory effect. The effect of direction was stronger than depth during the initial fixation period. As the task progressed toward arm movement execution, depth tuning became more prominent than directional tuning and the number of cells modulated by both depth and direction increased significantly. Neurons tuned by depth showed a small bias for far peripersonal space. Cells with directional modulations were more frequently tuned toward contralateral spatial locations, but ipsilateral space was also represented. These findings, combined with results from neighboring areas V6A and PE, support a rostral-to-caudal gradient of overlapping representations for reach depth and direction in SPL. These findings also support a progressive change from visuospatial (vergence angle) to somatomotor representations of 3D space in SPL.

The posterior parietal cortex is involved in the visuomotor transformations occurring during arm-reaching movements. The medial posterior parietal area V6A has been shown to be implicated in reaching execution, but its role in reaching preparation has not been sufficiently investigated. Here, we addressed this issue exploring the neural correlates of reaching preparation in V6A. Neural activity of single cells during the instructed delay period of a foveated Reaching task was compared with the activity in the same delay period during a Detection task. In this latter task, animals fixated the target but, instead of performing an arm reaching movement, they responded with a button release to the go signal. Targets were allocated in different positions in 3-D space. We found three types of neurons: cells where delay activity was equally spatially tuned in the two tasks (Gaze cells), cells spatially tuned only during reaching preparation (Set cells), and cells influenced by both gaze and reaching preparation signals (Gaze/Set cells). In cells influenced by reaching preparation, the delay activity in the Reaching task could be higher or lower compared with the Detection task. All the Set cells and a minority of Gaze/Set cells were more active during reaching preparation. Most cells modulated by movement preparation were also modulated with a congruent spatial tuning during movement execution. Present results highlight the convergence of visuospatial information, reach planning and reach execution signals on V6A, and indicate that visuospatial processing and movement execution have a larger influence on V6A activity than the encoding of reach plans.

Posterior parietal cortex (PPC) in humans and monkeys contains many reach-related areas, and V6A is one of them. Although several fMRI studies in humans found reach planning activations in regions putatively homolog to monkey V6A (Galati et al., 2011; Gallivan et al., 2011; Bernier et al., 2012), the involvement of this area in encoding reach intentions has never been explicitly demonstrated. Here, we addressed this issue by comparing the activity of single cells during the instructed delay period in a reaching and a fixation task, performed in separate blocks. In these tasks, the animals were fixating targets arranged in 3D space and waited for a Go cue to reach (reaching task), or release a button (fixation task). We found different types of cells: cells modulated by the fixation of a target regardless of whether it was reached or not (25%), cells modulated only by reaching preparation (17%), and cells influenced by both signals (44%). Reach preparation cells often showed a congruent spatial tuning during movement execution. Most cells showed lower delay activity before reaches compared to before button releases, whereas a minority showed the opposite effect. The timing of excitation, but not that of inhibition, was time-locked to reach onset. We propose that inhibition could derive from changes of attention/alertness associated with preparing a reaching movement, whereas excitation reflects reach planning. In agreement with human fMRI studies, cells involved in reach planning are less in V6A than in nearby parietal reach region (PRR), suggesting common neuronal mechanisms for reach planning in monkey and human medial PPC. In V6A, reach planning signals coexist with target location and attentional signals. We propose that before reaches, while PRR encodes the intention to reach, V6A is more involved in processing various inputs to localize reach targets in the 3D space. Meeting abstract presented at VSS 2013

The superior parietal lobule (SPL) is strongly involved in organizing arm actions in space. Despite neuropsychological studies reported that patients with SPL lesions show deficits in reaching targets in depth, so far only few single cell recording studies addressed this issue. Cortical area V6A, located in the SPL of primates, carries signals related to the distance of targets from the eyes, and contains neurons with arm movement-related activity. Recent neurophysiological studies show that area V6A integrates, often at single cell level, vergence with gaze direction signals, thus encoding spatial location in 3D space (Hadjidimitrakis et al., Plos One 2011). The aim of present study was to examine the spatial encoding of single V6A neurons during fixation, preparation, and execution of reaches towards foveated visual targets in depth. Single unit activity was recorded in two macaque monkeys performing a visually-guided reaching task in darkness. Animals were required to fixate and reach targets (LEDs) placed at different positions and depth in 3D space. In the majority of cells, a significant effect of both target direction and depth was found in all time epochs. Spatial modulations were generally maintained from fixation through subsequent task epochs, till reaching execution. Spatial encoding was remarkably consistent across epochs, with common preferences evenly distributed in 3D space. Given the functional properties of V6A neurons, we suggest that this spatial information is integrated with somatosensory input coming from the upper limbs, to define and control the part of space that can be reached by the hands. These new findings demonstrate that, in an area involved in reaching and grasping (Fattori et al., Eur. J. Neurosci 2005; Fattori et al., J. Neurosci 2010), the encoding of action in 3D space is accomplished through the interaction of a variety of signals. Meeting abstract presented at VSS 2012