In the posterior parietal cortex signals from different modalities, such as visual, tactile, auditory, vestibular and proprioceptive inputs, are combined to create an abstract representation of space that can be used to plan and control body movements. Neurons in many areas of the posterior parietal cortex are known to respond to optic flow stimuli. Several papers show that the optic flow is a substantial source of visual information to control and plan self-motion and to avoid obstacles during locomotion. Many studies have shown that the analysis of visual motion has the largest expression in the inferior parietal lobule and in the medial superior temporal area, in which the heading perception is believed to have place, because its neurons are selectively activated by the position of the focus of expansion with respect to the fovea. Our recent studies performed in the dorsal portion of the superior parietal lobule showed a neuronal responsiveness to moving visual stimuli especially in the caudal part of area PE, the area PEc. This region receives the visual input from areas V6 and V6a and directly projects to the premotor area F2, that is considered an important link between visual signals relevant for body position and motor acts. So we were interested to study if heading information from visual cues might contribute to this control system. We expected a specific sensitivity to radial optic flow, possibly with eccentric focus of expansion, along the occipito-frontal pathways linking primary visual cortex to motor areas. We believed that a possible candidate would be area PEc.

PEc neurons were found to encode optic flow stimuli, thus supplying self-movement information to the fronto-parietal cortical network. The extracellular activity of single neurons was recorded in three hemispheres of two male java monkeys (M. fascicularis) trained in a fixation task while optic flow stimuli were presented. Recordings were carried out from both the top surface and the mesial bank of the caudal part of the superior parietal lobule. The stimuli were produced by random dot simulating planar motion, radial expansion and radial contraction. A one-way ANOVA performed across the mean firing rates in response to optic flow with focus of expansion in different positions showed that a great deal of PEc neurons are specifically activated (p<0.05) by radial optic flow, with selectivity for the position of the focus of expansion with respect to the fovea. Eccentric positions of focus of expansion were preferred. Almost all neurons showed opponent excitatory/inhibitory responses to expanding/contracting visual fields. The background planar motion elicited very weak responses.

The optic flow responsiveness is not entirely explained by the local motion sensitivity of PEc neurons, suggesting that global and local visual responses could serve different mechanisms in the integration of visuo-motor signals to prepare body movements. It seems that area PEc contributes to visually-derived self-motion signals, since its neurons represent forward and backward body movements. Furthermore, the selectivity for the position of the focus of expansion with respect to the fovea could provide the system with visual information about heading. This information, together with that from local motion, could be suitable for behavioral conditions in which self-motion and object motion interact, such as intercepting or avoiding visual targets during locomotion. We believe that these results are very important to support a novel role of the superior parietal lobule, classically know as a somato-sensory association cortex, in the integration of visuo-motor signal to plan and control self-motion.

Supported by Italian Ministry of University and Scientific Research (MURST).