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S. Plainis, N.R. A. Parry, P. Sapountzis, I.J. Murray, I.G. Pallikaris; The Effect of Contrast Adaptation on Visual Reaction Times (RTs): Spatial–Frequency and Orientation Tuning . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5353.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the bandwidth characteristics of spatial and orientational selective channels in the human visual system by showing the effects of high contrast adaptation to visual reaction times (RTs).
Simple RTs were measured to the onset of a gaussian spot (s.d. = 1.0°) displayed on a Sony GDM F–520 CRT monitor (mean luminance of 30 cd/m2, frame rate of 120 Hz) by means of a VSG2/5 stimulus generator card. Three subjects were tested. The subjects were adapted for 10 sec to a grating of 100% contrast (C) to a wide range of orientations (0, 2, 5, 10, 22.5, 45, 90 deg angles either side to the testing grating) and spatial frequencies (0.5, 1, 2 octaves difference either side to the testing grating) combinations. A single session was devoted to one orientation and one spatial frequency. In all conditions the contrast of the test grating was 4x above the average unadapted threshold, while its spatial frequency was 4 c/deg and its orientation 0 or 90 deg. Averages of 20 reaction times for the adapted and unadapted conditions were compared.
The effect of contrast adaptation on RTs was maximum (RTs were increased by ∼ 100 to 150ms) when the test and adapting grating were of the same orientation and spatial frequency, being less pronounced the more remote is the adapting grating in orientation or spatial frequency from the test. Using the linear RT vs. 1/C functions (see Plainis and Murray, 2000) the increase in RTs was converted to percentage of elevated contrast thresholds. The effects of adaptation are explained by a single mechanism model: exponential functions fitted to the data revealed the bandwidth characteristics of orientation / spatial frequency tuning.
Visual RTs form supra–threshold behavioural responses that could lead to indicative values of selectivity tuning in orientation and spatial frequencies, using contrast adaptation paradigms. They offer the advantage over threshold settings in terms of accuracy in timing, which is very important due to the time course of contrast adaptation.
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