The effect of reversible inactivation of the caudal intraparietal area on 3D structure categorization

Previous monkey fMRI-studies have revealed an extensive network involved in processing disparity-defined three-dimensional (3D) shapes in both the dorsal and ventral visual stream, (Durand et al., 2007. Neuron 55, 493-505). In contrast to what was previously reported, however, we recently found that also the caudal intraparietal area (CIP) is activated by disparity-defined 3D curved surfaces (Van Dromme et al., 2010. Program No. 19.7. Neuroscience 2010. Online). Moreover, reversible inactivation of area CIP in one hemisphere during contrast-agent enhanced fMRI caused a significant reduction of the activations related to depth structure (the interaction between the factors disparity and depth order) in the anterior intraparietal area (AIP), the premotor area F5 and in the anterior part of the superior temporal sulcus (anterior TE) (Van Dromme et al., 2013. Program No. 161.80 Neuroscience 2010. Online). Therefore, area CIP may play an essential role as input stage of higher-order disparity information for both the dorsal and the ventral stream. However, the role of CIP in the perception of 3D curved shapes remains unclear. Therefore, we reversibly inactivated area CIP while the monkeys performed a 3D structure discrimination task. Two monkeys were trained to discriminate convex and concave curved surfaces by means of an eye movement. All stimuli were presented for 800 ms at the fixation point and at one of three positions in depth. Task difficulty was manipulated by changing the percentage of dots defining the surface (between 10 and 100% disparity coherence). The monkeys were trained until above chance performance was reached for the most difficult disparity coherence condition, i.e. at 10% disparity coherence. Inactivation sessions (N = 3 for monkey 1 and N = 4 for monkey 2) were alternated with sessions without inactivations (N = 6 and N = 8, respectively). In inactivation sessions we injected 4-6μl muscimol solution (10mg/ml) unilaterally in area CIP. Subsequently, the monkeys were tested with 4 different disparity coherence levels: 10%, 20%, 30% and 50%. In both monkeys, reversible inactivation of area CIP caused a significant reduction in 3D structure discrimination performance, with the most consistent effects in the 10 and 20% coherence conditions. However, even in the latter conditions the maximal reduction in discrimination performance was 6%. These results suggest that area CIP could play a role in the discrimination of 3D structure in disparity-defined curved surfaces.status: publishe