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

Many psychophysical studies suggest that target depth and direction during reaches are processed independently, but the neurophysiological support to this view is so far limited. Here, we investigated the representation of reach depth and direction by single neurons in area V6A of the medial posterior parietal cortex (PPC) of macaques, while a fixation-to-reach task in 3-dimensional (3D) space was performed. We found that, in a substantial percentage of V6A neurons, depth and direction signals jointly influenced fixation, planning, and arm movement-related activity. While target depth and direction were equally encoded during fixation, depth tuning became stronger during arm movement planning, execution, and target holding. The spatial tuning of fixation activity was often maintained across epochs, and depth tuning persisted more than directional tuning across epochs. These findings support for the first time the existence of a common neural substrate for the encoding of target depth and direction during reaches in the PPC. Present results also highlight the presence of several types of V6A cells that process independently or jointly signals about eye position and arm movement planning and execution in order to control reaches in 3D space. A conceptual framework for the processing of depth and direction for reaching is proposed.

The frames of reference used by neurons in posterior parietal cortex (PPC) to encode spatial locations during arm reaching movements is a debated topic in modern neurophysiology. Traditionally, target location, encoded in retinocentric reference frame (RF) in caudal PPC, was assumed to be serially transformed to body-centered and then hand-centered coordinates rostrally. However, recent studies suggest that these transformations occur within a single area. The caudal PPC area V6A has been shown to represent reach targets in eye-centered, body-centered, and a combination of both RFs, but the presence of hand-centered coding has not been yet investigated. To examine this issue, 141 single neurons were recorded from V6A in 2 Macaca fascicularis monkeys while they performed a foveated reaching task in darkness. The targets were presented at different distances and lateralities from the body and were reached from initial hand positions located at different depths. Most V6A cells used body-centered, or mixed body- and hand-centered coordinates. Only a few neurons used pure hand-centered coordinates, thus clearly distinguishing V6A from nearby PPC regions. Our findings support the view of a gradual RF transformation in PPC and also highlight the impact of mixed frames of reference.

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