Direction Discrimination of Sinusoidal Gratings under Transient and Steady Adapting Conditions

BARRAZA J.F.; COLOMBO E.M.

Forty Lauderdale, Florida, USA

Congreso; ARVO Annual Meeting; 2000

ARVO

It is well known that the loss of sensitivity in the fovea produced by the sudden onset of a conditioning field was significantly greater than the effect from a steady conditioning field of the same luminance (Graham N. & Hood D. (1992). Vis Res, 32, 1373-1393). The purpose of this study is to investigate the lower threshold of motion under conditions of transient adaptation using suprathreshold drifting gratings of different spatial frequency and using glare as a conditioning field. We compared direction of motion discrimination performance obtained under steady and transient situations. Stimuli were sinusoidal luminance gratings presented in a circular patch of 4 deg. The contrast of the pattern (25% peak value) was controlled by a gaussian function of time with a standard deviation of 150 mseg. The experiment was carried out using spatial frequencies of 1 cpd, 2 cpd, 4 cpd, 6 cpd and 8 cpd.  Glare was generated using an incandescent lamp located 10º away from the line sight, and was onset 50 msec before the presentation of the stimulus. The glare illuminance at the middle of the eyes was 60 lx. Two observers took part in this experiment. A 2AFC paradigm using the method of constant stimuli was used to measure the psychometric functions (50 observations per point) relating performance to temporal frequency. The Observer’s task was to discriminate the interval at which the grating had moved to the left. Results show that LTM, expressed as a temporal frequency, increased in an approximately linear fashion as a function of spatial frequency, for the two adapting situations. It means that threshold occurs at a constant velocity. Velocity threshold obtained under transient adapting condition is greater than that obtained under steady conditions. The reduction of retinal contrast in the glare situation can not explain our results because it kept above the limit in the dependency of LTM on contrast (2%) (Nakayama K. & Silverman G. (1985). J. Opt. Soc. Am, A/2, 267-274), along the whole stimulus presentation. However, under transient adapting conditions, the effect can be extended to greater values of contrast. We think that contrast is encoded differently by motion detectors in transient conditions.