Human photopic ON- and OFF-ERG responses elicited by square wave and sawtooth stimuli

In this study, the on- and off-responses elicited by luminance square wave and sawtooth stimuli at different temporal frequencies and contrasts are described quantitatively. Adding on- and off-responses reveals response asymmetries. Full-field stimuli were produced using a Ganzfeld bowl with arrays of light-emitting diodes (LEDs) as light sources. ERG responses were recorded from six normal subjects. The amplitudes and implicit times of components of the on- and off-responses and the additions were analyzed. The amplitudes of the on-, off- and addition components elicited by square wave stimuli did not depend on temporal frequency with the exception of the late negative components, which decreased with increasing temporal frequency up to 4 Hz. The amplitudes of all components elicited by sawtooth stimuli, except the P-on, decreased with increasing temporal frequency. The amplitude of all components elicited by square wave and sawtooth stimuli were positively correlated with stimulus contrast. The implicit times of the on-components to square wave stimuli and all response components to sawtooth stimuli decreased with increasing temporal frequency. Contrast had an effect on the implicit times of the N-on, P-on, P-off and P-add components elicited by square wave stimuli and the N-on, P-on, P-add and LN-on components elicited by sawtooth stimuli. The asymmetries between on- and off-responses can possibly be used to reveal inner retinal contributions and may therefore be interesting in detecting glaucomatous changes...

Asymmetries in the contributions of On- and Off-mechanisms to the ERG signal

ERG responses were recorded to rapid-on and rapid-off L- and M-cone isolating sawtooth stimuli of different cone contrasts. In addition, the responses were recorded to simultaneous in-phase stimulation of the L- and M-cones at equal cone contrast. Linear responses to mirror imaged rapid-on and rapid-off sawtooth stimuli are also mirror imaged. By adding on- and off-responses, linear response components will cancel and nonlinearities will remain. Because nonlinearities that occur at a certain stage of visual processing will influence subsequent stages, linear response components will probably have an outer retinal origin and nonlinearities probably originate mainly in the inner (post-receptoral) retina...

Lateral interactions in visual perception of temporal signals: cortical and subcortical components

The aim of this work was to isolate and investigate subcortical and cortical lateral interactions involved in flicker perception. We quantified the perceived flicker strength (PFS) in the center of a test stimulus which was simultaneously modulated with a surround stimulus (50 percent Michelson contrast in both stimuli). Subjects were requested to adjust the modulation depth of a separate matching stimulus that was physically identical to the center of the test stimulus but without the surround. Using LCD goggles, synchronized to the frame rate of a CRT screen, the center and surround could be presented monoptically or dichoptically. In the monoptic condition, center-surround interactions can have both subcortical and cortical origins. In the dichoptic condition, center-surround interactions cannot occur in the retina and the LGN, therefore isolating a cortical mechanism. Results revealed both a strong monoptic (subcortical plus cortical) lateral interaction and a weaker dichoptic (cortical) lateral interaction. Subtraction of the dichoptic from the monoptic data revealed a subcortical mechanism of the lateral interaction. While the modulation of the cortical PFS component showed a low-pass temporal-frequency tuning, the modulation of the subcortical PFS component was maximal at 6 Hz. These findings are consistent with two separate temporal channels influencing the monoptic PFS, each with distinct lateral interactions strength and frequency tuning characteristics. We conclude that both subcortical and cortical lateral interactions modulate flicker perception.