To make judgements upon unknown sensory stimulus based on prior knowledge is a basic decision process, called perceptual decision-making. This is an essential ability both for higher cognition and for survival, and is also a key feature of biological intelligence. One of the higher association region of the mammalian cerebral cortex, the posterior parietal cortex (PPC), has been long thought to play an important role in information processing during decision-making, as well as in many other cognitive functions, however, whether PPC activity is necessary for decision-making, and its exact contributions remain unclear. Recent studies showed a lack of behavioral effect following inactivation of PPC, leading to a widespread debate over PPC’s real function, and casting a shadow over decades of research on the neural mechanisms of decision-making based on correlations between PPC activity and behavior. It has therefore become an urgent issue to resolve in the field of systems neuroscience.  

　　A recent study published in Nature Neuroscience from Dr. XU Ninglong’s lab at the Institute of Neuroscience, Chinese Academy of Sciences, now provided new insights into the functional role of PPC in decision-making. In this study, ZHONG et al. obtained compelling evidence to show that PPC indeed plays an essential role in decision-making under critical conditions that were largely ignored in previous studies. The authors employed a novel behavioral technique in mice combined with precise manipulation and monitoring of neuronal activity in PPC, using optogenetics, chemogenetics and in vivo two-photon imaging. The authors found that when neuronal activity in PPC was temporarily inhibited, animals’ ability to judge whether a new sensory stimulus belongs to one or the other learned categories was markedly impaired. Furthermore, based on the neuronal activity recorded from large populations of neurons in PPC using in vivo two-photon imaging technique, the authors proposed that PPC not only represents learned categories, but also use this information to guide perceptual judgments on future sensory stimuli. 

　　In the real world, when facing an unknown sensory stimulus, we can only properly respond to it after we have determined which category in our knowledge base this stimulus belongs to. For example, when you see the face of an approaching person on an empty street, before you can properly react, you need to quickly determine whether this face belongs to one of your acquaintances, to a friendly stranger, or to a hostile stranger that may represent some sort of threats. Such decision-making process, namely, stimulus categorization, happens all the time in our daily life, as well as in animals’ living in natural environment, and is therefore essential for cognition and survival. On the other hand, this process may become no longer necessary when you are confronted with a familiar sensory item, to which you already learned how to react. You can simply choose a proper action based on your memory without necessarily going through a decision-making process.

　　Following this contemplation, the investigators at the Institute of Neuroscience hypothesized that PPC may play an important role when making decisions to category unknown sensory stimuli, but may not be required when the subject has already learned how to respond to the sensory stimuli. This may be why many previous studies did not find any behavioral effect following PPC inactivation, in that the sensory stimuli used in those experiments were already used for task training, and the experimental animals already learned how to respond to all the stimuli. To test the idea, the investigators decided to redesign the decision-making task using laboratory mouse to capture the critical elements of decision-making process, and to test the causal role of PPC during the new behavioral task. The results showed that PPC activity is indeed required for decision-making, but only when the decisions were made for newly presented tone stimuli. When the same new tone stimuli were used to train the mice for two more days, PPC activity was no longer required for the animals to make correct choices. These results not only provide key evidence supporting the essential role of PPC in decision-making, but also provide an explanation to the negative results in previous studies, and therefore may resolve the widespread debate on PPC’s role in decision-making. Importantly, these findings provide a new angle to study the neural mechanisms underlying decision-making by underscoring its dynamic nature due to learning.

　　These findings have been summarized as a research article entitled “Causal contributions of parietal cortex to perceptual decision-making during stimulus categorization”, with graduate students ZHONG Lin and ZHANG Yuan as the equal contributing first authors, and Dr. XU Ninglong as the corresponding author, published online in Nature Neuroscience April 29, 2019. This study was supported by Key Research Program of Frontier Sciences, CAS (grant No. QYZDB-SSW-SMC045); National Natural Science Foundation of China (grant No. 31571081); The Strategic Priority Research Program of Chinese Academy of Sciences (grant No. XDB32010000); National Key R&D Program of China (grant No. 2017YFA0103900 / 2017YFA0103901); Shanghai Municipal Science and Technology Major Project (grant No. 2018SHZDZX05 ).