Complex, but flexible neural encoding of arm movements in the macaque parietal cortex

A central role in interacting with the environment is played by higher cortical areas, in which neurons are often sensitive to multiple features of the external world. This type of encoding is termed ‘mixed selectivity’ and it greatly expands the complexity of neural representations, making the neural code more reliable and flexible. Past studies demonstrated that the medial posterior parietal area V6A in the macaque is tuned by many aspects of reaching movements, thus it is likely to contain ‘mixed selectivity’ neurons. Here, we used Generalized Linear Models (GLMs) to simultaneously evaluate the contribution of several factors upon V6A cells during a fix-to-reach task and directly test this hypothesis. The activity of 181 V6A neurons has been recorded from 2 Macaca fascicularis during a foveated delayed reaching task performed towards 9 different targets in the darkness. During the first step of the fitting procedure, LASSO optimization was used to remove from the model the regressors with a negligible effect. Then many models were estimated and subsequently compared, leading to the definition of a ‘functional fingerprint’ representative of the properties of each unit. The analysis of the ‘functional fingerprints’ highlighted the virtual absence of units strictly selective for only one factor, revealing that most cells are characterized by ‘mixed selectivity’. The population resulted ‘category free’, with no evidence for any functional segregation. We also found that the spatial tuning of the population reached a peak during the movement phase, but overall a motor code is maintained along the entire task. Finally, the previous spiking history of each cell resulted extremely relevant to explain the neural modulations for most neurons (62%). In conclusion, the presence of ‘mixed selectivity’ in area V6A, highly expected but never directly tested before, supports the growing emphasis on the importance of neurons exhibiting complex neural behaviours.