Two-photon imaging of neuronal populations in the primary visual cortex representation of the overhead visual field

Rodents have a large binocular field of view that extends from the snout to over the animals head. Recent experiments have shown that rodents have a strong, innate, evasive behavior evoked exclusively by stimuli presented above them. However, little is known about the functional properties of cortical neurons that represent the overhead visual field. Here we describe a method for allowing direct optical recording from populations of neurons representing the overhead visual field. Firstly, the conventional microscope objective has been replaced with a periscope coupled to a miniature objective to facilitate placement of a stimulus monitor above the rat’s head. Secondly, we developed a method for presentation of visual stimuli on the OLED display of a tablet running the Android OS, and a camera-based method for calibrating the position of the stimulus display in relation to the animals head. Using this setup, we recorded in rats the activity of neurons in the representation of the overhead visual field of the primary visual cortex in response to a range of stimuli. Neurons were labeled with the calcium indicator OGB-1 with counterstaining of astrocytes using sulforhodamine 101. Stimuli were either an expanding or contracting looming dot, or a moving dot that moved at constant speed along multiple trajectories to cover all positions within the display. In both stimulus types, differing sets of foreground/background luminance were used. Preliminary results show that 19 of the neurons responded with clear and reproducible transients to the looming dot stimulus, and 30 were responsive to moving dot stimuli. The response profiles of neurons to different stimulus types and parameters were further analyzed in detail and compared between cortical areas and receptive field properties established for this cortical region