Exp Brain Res (1998) 123:172±189  Springer-Verlag 1998 Alexandra Battaglia Mayer ´ Stefano Ferraina Barbara Marconi ´ James B. Bullis Francesco Lacquaniti ´ Yves Burnod ´ Pierre Baraduc Roberto Caminiti Early motor influences on visuomotor transformations for reaching: a positive image of optic ataxia A. Battaglia Mayer ´ S. Ferraina ´ B. Marconi ´ J.B. Bullis R. Caminiti ( ) ) Istituto di Fisiologia umana, Università di Roma ªla Sapienzaº, Piazzale Aldo Moro 5, I-00185 Rome, Italy e-mail: caminitir@axrma.uniroma1.it, Tel.: +39-6-4991-0967, Fax: +39-6-4991-0942 F. Lacquaniti Dipartimento di Medicina sperimentale, Università di Roma ªTor Vergataº, I-00179 Rome, Italy F. Lacquaniti IRCCS S. Lucia, CNR, I-00179 Rome, Italy Y. Burnod ´ P. Baraduc Inserm Creare U. 483, UPMC, F-75005 Paris, France Abstract Coding of reaching in the cerebral cortex is based on the operation of distributed populations of pari- etal and frontal neurons, whose main functional character- istics reside in their combinatorial power, i.e., in the ca- pacity for combining different information related to the spatial aspects of reaching. The tangential distribution of reach-related neurons endowed with different function- al properties changes gradually in the cortex and defines, in the parieto-frontal network, trends of functional proper- ties. These visual-to-somatic gradients imply the exis- tence of cortical regions of functional overlaps, i.e., of combinatorial domains, where the integration of different reach-related signals occurs. Studies of early coding of reaching in the mesial parietal areas show how somato- motor information, such as that related to arm posture and movement, influences neuronal activity in the very early stages of the visuomotor transformation underlying the composition of the motor command and is not added ªdownstreamº in the frontal cortex. This influence is probably due to re-entrant signals traveling through fron- to-parietal-association connections. Together with the gradient architecture of the network and the reciprocity of cortico-cortical connections, this implies that coding of reaching cannot be regarded as a top-down, serial se- quence of coordinate transformation, each performed by a given cortical area, but as a recursive process, where different signals are progressively matched and further elaborated locally, due to intrinsic cortical connections. This model of reaching is also supported by psychophys- ical studies stressing the parallel processing of the differ- ent relevant parameters and the ªhybridº nature of the ref- erence frame where they are combined. The theoretical frame presented here can also offer a background for a new interpretation of a well-known visuomotor disorder, due to superior parietal lesions, i.e., optic ataxia. More than a disconnection syndrome, this can now be interpret- ed as the consequence of the breakdown of the operations occurring in the combinatorial domains of the superior pa- rietal segment of the parieto-frontal network. Key words Reaching ´ Fronto-parietal network ´ Combinatorial domain ´ Visuomotor transformations ´ Optic ataxia Coding of reaching occurs within a distributed parieto-frontal network Cortical coding of reaching occurs within a distributed network, including different parietal and frontal areas (Fig. 1), heavily interconnected by reciprocal sets of asso- ciation connections (Fig. 2). Areas of interest are V6 and V6A in the rostral bank of the parieto-occipital sulcus; ar- ea 7m, SMA (supplementary motor area, F3), pre-SMA (F6), and some cingulate motor areas in the medial wall of the hemisphere; areas MIP (medial intraparietal), PE, and PEa in the superior parietal lobule (SPL); areas 7a and LIP (lateral intraparietal) in the inferior parietal lob- ule; primary motor cortex (M1, F1), dorsolateral premotor cortex (PMd, F2 and F7), and dorsomedial premotor cor- tex in the frontal lobe. In the last 15 years, the study of the dynamic properties of neurons in behaving monkeys performing a variety of arm-reaching tasks and the analysis of the cortico-cortical connectivity in these areas have provided new insights on the basic operations performed within this network. We will briefly illustrate only those anatomical and functional aspects of the network that we believe are relevant for in-