Spots, annuli, and sine wave gratings were created using a stimulator based on a digital light projector (
Packer et al., 2001). These patterns were relayed by an optical system to the camera port of the microscope and imaged on the retina by a microscope objective.
H1 horizontal cell receptive fields were characterized by measuring their responses to drifting sinusoidal gratings as a function of spatial frequency, to flashing spots as a function of spot diameter, and to flashing annuli as a function of annulus inner diameter. The receptive field center was found by moving a small flickering spot of light across the retina to the location of maximum response. Temporal frequency was 2, 4, or 10 Hz. Stimulus contrast was nominally 5, 10, 25, 50, or 100%. Ideally, a 100% contrast grating would have 100% contrast at all spatial frequencies. In fact, a reduction in the contrast of grating stimuli at higher spatial frequencies was introduced by the visual stimulator (
Packer et al., 2001). This was corrected by multiplying the response by the inverse of the contrast sensitivity function of the stimulator at that spatial frequency. The validity of this correction, which depends on linear contrast response by the H1 cell, was verified experimentally. Unless otherwise stated, stimuli were modulated around a mid-photopic luminance of ∼1,000 trolands (167 cd/m
2, 6 mm pupil) to maintain a stable state of adaptation. The relative strengths of the L and M cone inputs to many cells were measured with a stimulus (
Dacey, Diller, et al., 2000) that varied the ratio of L and M cone contrasts over a wide range from pure L cone contrast through equal L and M cone contrasts to pure M cone contrast. A few cells were also tested with a stimulus designed to stimulate L, M, and S cones in isolation.
The intracellular voltage response to a stimulus was amplified (Axoprobe-1A; Axon Instruments, Foster City, CA), digitized (NBIO16 installed in a Macintosh computer; National Instruments, Austin, TX) at a sampling rate of up to 10 kHz, and averaged over multiple stimulus cycles. The amplitude and phase of the response at the temporal frequency of stimulus modulation were calculated using a digital Fourier transform.