Visage stimulus generator

Stimuli were displayed on a Nokia MultiGraph 445X monitor using a ViSaGe stimulus generator (Cambridge Research Systems Ltd., Kent, UK) running in 14-bit mode. The monitor was gamma corrected using a photometer, and had a mean luminance of 60cd/m² and a refresh rate of 120Hz.
Stimulus elements were single cycles of a horizontal sine-wave grating with a spatial frequency of 4c/deg. The elements were curtailed in the horizontal direction by a half-wave rectified sine-wave of half the carrier frequency6 . The square grid-textures were of various sizes, and various densities (always with regular spacing between elements), which we report as mark:space ratios. High contrast examples are shown in Fig. 1a.
To compensate for the decline in sensitivity across the visual field we adjusted the local contrasts of our stimuli as a function of eccentricity. This normalization was based on measurements from a previous study16 (link) that were used to produce an inverse ‘attenuation surface’ (see Fig. 1b for an example) by which the stimuli were multiplied. This normalization procedure was performed independently for each observer. A high contrast example of the result of this normalization is shown in Fig. 1c. This general approach has been validated in a separate study using grating stimuli9 (link).

Visual stimuli were generated with a ViSaGe stimulus generator (Cambridge Research Systems, Cambridge, UK) and displayed on a calibrated CRT monitor (Clinton monoray, 100 Hz non-interlaced refresh rate, 1024 × 768 pixels, 57 cd/m² mean luminance). Viewing distances were 30 cm for mice and 57 cm for cats [17 (link)] and 114 cm for macaques [14 (link)]. For each recorded cell, the preferred temporal and spatial frequency (TF, SF), orientation, location and size of the RF were determined with drifting gratings at 100% contrast. Michelson contrast = [(Lum_max − Lum_min)/Lum_max + Lum_min)] × 100, where Lum_max and Lum_min are the maximum and minimum grating luminance.
Stimuli were drifting sinusoidal gratings with optimal TF, SF and orientation presented in a circular aperture the size of the RF. Receptive fields were within 2–5° eccentricity in monkeys and up to 7° eccentricity in cats and mice. Drifting gratings with contrast of 0%–100% were presented in pseudorandom order interleaved with 1 second blank periods (mean luminance). Gratings were presented for 3 seconds: first and last 0.5 seconds stationary; drifting in between.