Optical Imaging of Visual Cortex in Unanaesthetised Rat

Abstract

Abstract: : Purpose: A recent study developed methodology for optical imaging of somatosensory cortex in unanaesthetised rat (Berwick et al., 2001). Here, we investigate whether this technique could be extended to assess visual cortical function in rat. Methods: Over a period of 2-3 weeks prior to surgery, animals were habituated to a body harness that would be used to secure the animal during the period of optical imaging. After habituation, animals were anaesthetised and the skull overlying right visual cortex was thinned to translucency. A stainless steel well was placed over the craniotomy to allow the attachment of a medical endoscope that was used to illuminate the visual cortex and collect the remitted light. Attached to the endoscope was a 12-bit CCD camera for collection of the image data. To localise the region of activity single wavelength optical imaging was used. The cortex was illuminated with broadband red light (624nm). Horizontal and vertical drifting gratings from 0.05 to 0.8 cycles/degree were presented to the left eye via a computer screen at a distance of 20cm. Data were averaged over 30 trials. A spectrograph was attached to the camera and the system was positioned so that the slit overlaid the centre of the activated region. Optical imaging spectroscopy was performed to further examine the hemodynamic response to stimulation (blood oxygenation and volume). Results: Stimulation produced an early increase in the concentration of deoxyhemoglobin followed by a decrease below baseline. Blood volume and oxyhemoglobin were monophasic and simply increased following stimulation. The most robust responses were elicited by gratings at 0.05c/d. However, responses to gratings of spatial frequencies as high as 0.8c/d could be reliably detected. Conclusion: This technique offers a possible method for the longitudinal assessment of both visual and cortical function without the complication of anaesthetic influence.

Keywords: 432 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 621 visual cortex • 620 visual acuity