Eye-hand coordination during reaching. II. An analysis of the relationships between visuomanual signals in parietal cortex and parieto-frontal association projections

The relationships between the distribution of visuomanual signals in pariet al cortex and that of parieto-frontal projections are the subject of the pr esent study. Single cell recording was performed in areas PEc and V6A, wher e different anatomical tracers were also injected. The monkeys performed a variety of behavioral tasks, aimed at studying the visual and motor propert ies of parietal cells, as well as the potential combination of retinal-, ey e- and hand-related signals on cell activity. The activity of most cells wa s related to the direction of movement and the active position of the hand. Many of these reach-related cells were influenced by eye position informat ion. Fewer cells displayed relationships to saccadic eye movements. The act ivity of most neurons related to a combination of both hand and eye signals . Many cells were also modulated during preparation for hand movement. Ligh t-dark differences of activity were common and interpreted as related to th e sight and monitoring of hand motion and/or position in the visual field. Most cells studied were very sensitive to moving visual stimuli and also re sponded to optic flow stimulation. Visual receptive fields were generally l arge and extended to the periphery of the visual field. For most neurons, t he orientation of the preferred directions computed across different epochs and tasks conditions clustered within a limited sector of space, the field of global tuning. This can be regarded as an ideal frame to combine spatia lly congruent eye- and hand-related information for different forms of visu omanual behavior. All these properties were common to both PEc and V6A. Ret inal, eye- and hand-related activity types, as well as parieto-frontal asso ciation calls, were distributed in a periodic fashion across the tangential domain of areas PEc and V6A. These functional and anatomical distributions were characterized and compared through a spectral and coherency analysis, which revealed the existence of a selective 'match' between activity types and parieto-frontal connections. This match depended on where each individ ual efferent projection was addressed. The results of the present and of th e companion study can be relevant for a re-interpretation of optic ataxia a s the consequence of the breakdown of the combination of retinal-, eye and hand-related directional signals within the global tuning fields of parieta l neurons.