LOW LUMINANCE CONTRAST SENSITIVITY - EFFECTS OF TRAINING ON PSYCHOPHYSICAL AND OPTOKINETIC NYSTAGMUS THRESHOLDS IN MAN

We compared psychophysical contrast sensitivity function (psi-CSF) and optokinetic contrast sensitivity function (OKN-CSF) in man, for the c ombination of three spatial and three temporal frequencies. psi-CSF wa s defined as the inverse of the contrast threshold, that is the contra st value of a sinusoidal grating for which a subject was able to ident ify the width of a drifting grating. OKN-CSF was defined as the invers e of the contrast value of the grating which triggered an involuntary optokinetic nystagmus. In highly experienced subjects, OKN-CSF was ove rall higher than psi-CSF. More precisely, differences between both con trast sensitivity functions occurred mainly in the low spatio-temporal frequency range (below 4 c/deg and 9 Hz). In naive subjects, psi-CSF reached the level of OKN-CSF after two consecutive test sessions. OKN- CSF did not change with training. Similarly, high spatio-temporal freq uency psychophysical thresholds did not change with training and, more over, approximated OKN-CSF thresholds. Low spatio-temporal frequency p sychophysical sensitivity was initially lower than corresponding OKN-C SF sensitivity; however, after only two training sessions, the two fun ctions were indistinguishable due to a selective increase in psychophy sical low spatio-temporal frequency sensitivity.