Decoding of path-guided apparent motion from neural ensembles in posterior parietal cortex

We compared quantitatively the psychometric capacity of human subjects to detect path-guided apparent motion (PAM) and the accuracy of cell ensembles in area 7a to code the same type of stimuli. Nine human subjects performed a detection task of PAM. They were instructed to indicate with a key-press whether they perceived a circularly moving object when five stimuli were flashed successively at the vertices of a regular pentagon. The stimuli were presented along a low contrast circular path with one of 33 speeds (150-600 degrees /s). The average psychometric curve revealed that the threshold for PAM detection was 314 degrees /s. The minimum and maximum thresholds for individual subjects were 277 degrees and 378 degrees /s, respectively. In addition, the activity of cells in area 7a that were modulated by the stimulus position in real or apparent motion was used in a multivariate linear regression analysis to recover the stimulus position over time. Real stimulus motion was decoded successfully from neural ensemble activity at all speeds. In contrast, the decoding of PAM was poor at low stimulus speeds but improved markedly above 300 degrees /s: in fact, this was very close to the threshold above for human subjects to perceive continuous stimulus motion in this condition. These results suggest that the posterior parietal cortex is part of a high-level system that is directly involved in the dynamic representation of complex motion.