Fourteen adult cats were used for this study. Anesthesia was initiated with ketamine (15mg/kg) and xylazine (1.5 mg/kg) i.m., and maintained with isofluorane (typically 0.6-1.5% in 70/30 NO/O) delivered through a tracheal cannula. Cats were paralyzed with a cocktail of gallamine triethiodide (3.6 mg/hour) and tubocurarine chloride (0.15 mg/hour), and artificially respired to maintain end-tidal CO at a partial pressure of mm Hg. EEG and heart rate were continuously monitored to ensure adequate anesthesia. After a craniotomy to expose visual cortex, the recording chamber was centered at Horsley-Clark coordinate P2, filled with silicone oil and sealed with a quartz plate. The location of area 17 was confirmed by histology in previous experiments .
Techniques for intrinsic signal imaging were briefly as follows. Images were obtained from a depth of m under 610nm light, using a tandem-lens macroscope arrangement giving an overall magnification of 75 pixels/mm. Images were collected using a Bischke CCD-5024N video camera at a 655x480 resolution without binning, and digitized by an Imager 2001 system (Optical Imaging Inc.) and a 486-66 PC with a Matrox graphics board. Frames were summed between 1.3 and 4.0 sec after onset of stimulus motion, corresponding to the time of maximum signal as determined by our previous experiments. Data was analyzed using programs written by the authors (L.T., C.R.) using C++ (Borland) or IDL (Research Systems Inc. Boulder, CO). Gratings (spatial frequency 1.0 cyc/, drifting at 1.5 /sec) were shown on a 17 inch monitor positioned 28.5 cm in front of the animal. Grating contrast was 99%; neutral grey intensity was 6.0 cd/.
Single-units were recorded at the conclusion of some imaging experiments using electrodes of impedance (Micro Probe Inc., Hagerstown, MD). Receptive fields were plotted by collecting spike responses to short moving bars and measuring the minimum response field. All stimuli were shown to the contralateral eye only. Eye position was checked regularly during the course of the experiment by using a reversing ophthalmoscope to project an image of the retinal vasculature onto the monitor.
Gratings of 4 or 8 orientations per stimulus type (``center'', ``surround'' or ``full-field'') were shown in an interleaved fashion 40-80 times over a period of 2-6 hours. The size of the ``center'' stimulus (and the complementary ``surround'') was systematically varied in early experiments and adjusted to cover the aggregate classical receptive field of neurons within the center representation (see figure 6). Maps of orientation preference were obtained by dividing two maps of orthogonal stimulus orientations. To obtain composite maps, the vector average of the signal was computed pixel by pixel, and the resulting map displayed in ``polar'' form using color to code the vector angle and the intensity of each color to code the vector magnitude (see ).