Visual stimuli were generated by the user-defined MATLAB (MathWorks, Natick, MA, USA) program based on Psychtoolbox-3 and displayed on a gamma-corrected cathode ray tube (CRT) monitor (Dell P1130, 1280 × 960 pixels, 100 Hz; Dell, Round Rock, TX, USA) placed at 28.5 cm in front of the eyes. To obtain cortical orientation preference maps, the cats were stimulated binocularly with full-screen square-wave drifting gratings. The presented gratings involved 8 orientations (0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, 157.5°) and two spatial frequencies (0.58 and 0.14 cycles/deg [cpd]), which moved back and forth at a speed of 2 cycles/s (cps). The contrast of gratings was 100% with mean luminance of 30 cd/m
2. The stimuli were presented continuously for 4 seconds, with an interstimulus interval (ISI) of 20 seconds using blank at mean luminance. Each block consisted of 8 different oriented gratings at the same spatial frequency followed by a blank trial appeared for 8 times. Totally, 16 to 20 blocks (one half for 0.58 cpd, the other half for 0.14 cpd) were completed in one experiment. The range of visual field elevation (from ∼20° above to 4° below the horizontal meridian) was much wider than that of azimuth (∼5° close to the vertical meridian) in our imaging regions.
27,28 Hence, we only used continuous-periodic stimuli horizontally oriented combined with a continuous acquisition paradigm to obtain the retinotopic elevation map.
29 The right eye of the cat was first stimulated with a horizontal bar (2° × 80°) drifting continuously in one direction for 48 cycles at a speed of 6°/s. Then, the stimulus bar drifted in the opposite direction for another 48 cycles.
A contact lens electrode (ERG-jet; CareFusion, Middleton, WI, USA) placed on the cornea surface of the right eye was used for TcES. Hydroxyethylcellulose gel (1.3%) was applied to protect the cornea, and keep good conductivity between the electrode and the cornea. A stainless steel needle inserted into the dorsal neck muscle ipsilateral to the stimulated eye was used as the return electrode. Biphasic charge-balanced rectangular cathode-first current pulses were generated by an isolated and programmable stimulating system (MS16; Tucker-Davis Technologies).
In optical imaging experiment, TcES was applied continuously for 2 seconds, with an ISI of 20 seconds in each trial. While changing electrical stimulation frequency, current intensity was set at 1.2 mA. To keep the total injected current charge constant, pulse width of the stimulation varied from 40 to 2 ms with stimulation frequency changing from 5 to 100 Hz. The following frequencies were used: 5, 10, 15, 20, 40, 60, 80, and 100 Hz, while the corresponding pulse widths per phase were: 40, 20, 13.3, 10, 5, 3.3, 2.5, and 2 ms. In the experiments studying effects of different stimulation pulse widths and current intensities, we kept stimulation frequency at 20 Hz, and used current intensities of 0.24, 0.36, 0.6, 0.84, 1.2, 1.68, 2.16, and 2.64 mA with 10 ms pulse width per phase, and pulse widths per phase of 2, 3, 5, 7, 10, 14, 18, and 22 ms with 1.2 mA intensity. Each block consisted of 8 different stimulus conditions followed by a blank trial repeated for 8 times. Totally, 16 to 20 blocks were finished for each experiment. In the electrophysiological experiment, shorter stimulation pulse width of 2 ms (frequency, 20 Hz; current intensity, 1.2 mA; stimulus duration, 2 seconds; ISI, 5 seconds) was used to reduce the artifact of electrical stimulation.