Neural responses in motor cortex and area 7a to real and apparent motion
The neural activity in area 7a and the arm area of motor cortex was recorded while real or path-guided apparent motion stimuli were presented to behaving monkeys in the absence of a motor response. A smooth stimulus motion was produced in the real motion condition, whereas in the apparent motion condition five stimuli were flashed successively at the vertices of a regular pentagon. The stimuli moved along a low contrast circular path with one of five speeds (180-540 deg/s). We found strong neural responses to real and apparent motion in area 7a and motor cortex. In the motor cortex, a substantial population of neurons showed a selective response to real moving stimuli in the absence of a motor response. This activity was modulated in some cases by the stimulus speed, and some of the neurons showed a response during a particular part of the circular trajectory of the stimulus; the preferred stimulus angular locations were evenly distributed across this neuronal ensemble. It is likely that these neural signals are continuously available to the motor cortex in order to generate responses that demand immediate action. In area 7a, two overlapping populations of neurons were observed. The first comprised cells the activity of which was tuned to the angular location of a circularly moving stimulus in the real motion condition. These cells also responded to apparent motion at high stimulus speeds. A visual receptive field analysis showed that the angular tuning in most of the area 7a neurons did not depend on the spatial location of the stimulus in relation to their receptive field. The second population was selective to apparent moving stimuli and showed a periodic entrainment of activation with the period of the inter-stimulus interval of the flashing dots. Both the angular location and the inter-stimulus interval neural signals can be used to generate precise behavioral responses towards real or apparent moving stimuli.