1. Effects of brightness and Dragutin Ivanec & Veseljka Rebić Department of Psychology Faculty of Humanities and Social Sciences, University of Zagreb background-stimulus contrast variations on simple reaction time

2. The enigma of lightness contrast Two patches of equal photometric intensity are usually perceived as being differently light when placed on backgrounds that have different luminances.

3. Conventional explanation Mechanisms of lateral inhibition based on the center-surround receptive field properties of retinal ganglion cells are responsible for perceiving different lightness.

4. Reasons for doubting this conventional explanation There are several examples where only perceptual organization (for example - belongingness) can make lightness contrast effect disappear. If retinal processes are substantial, such factors should not be relevant.

5. Aim of the study To examine validity of conventional explanation, using simple reaction time (SRT) as a measure of retinal processing of stimuli intensity.

6. Simple reaction time as an indicator or retinal processes: basic logic A considerable number of research showed that a larger proportion of visual simple reaction time variance is caused by stimulus intensity: the stronger stimulus intensity – the shorter reaction time. Large proportion of that relationship is due to the latency of sensation – i.e. perceived intensity of stimulation. Processes on retina are mostly responsible for such results.

7. Assumption If perceived lightness is under the influence of different intensity level in lightness contrast effect (caused by retinal processes, i.e. lateral inhibition), then SRT could be associated to this perceived lightness.

8. M E T H O D O L O G Y Seven experiments have been done Participants (well trained in SRT task, had to react as fast as possible (press the button) on appearing stimulus (patch 12 x 12 cm on 17'' CRT of computer screen (Visual angle 11°). Experimental situations were different variations of lightness contrast. X X

9. Results Black background - 3 stimuli Stimulus cd/m 2 0,0124130 Backg. cd/m 2 000

10. Black background - 6 stimuli (narrow range of stimulus intensities) Stimulus cd/m2 0,4614426186 backg. cd/m2 000000

11. The same stimuli – diff. backg. Simultaneous contrast situation (3 stimuli) Stimulus cd/m 2 70 Backg. cd/m 2 130240

12. 6 stimuli of the same luminance on different backgroud luminnace Simultaneous contrast situation (lower intensities) Stimulus cd/m2 26 backg. cd/m2 8761421460,4

13. Different int. of stimuli on the same (gray) background Stimulus cd/m2 0,4614426186 backg. cd/m2 26

14. Stimulus cd/m2 0130 backg. cd/m2 1300 Max. contrast

15. 3 stimuli – the same ratio Stimulus cd/m 2 0,5527109 Backg. cd/m 2 0,392187

16. 6 stimuli the same ratio Stimulus cd/m2 6825356186 backg. cd/m2 8635258661

17. Conclusion Need for further research -More experimental situations -Better control ( adaptation, afterimages ) -Judgement data ( normal and short presentation of diff. luminance )