effects of brightness and dragutin ivanec & veseljka rebić department of psychology faculty of...
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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
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.
Conventional explanation
• Mechanisms of lateral inhibition based on the center-surround receptive field properties of retinal ganglion cells are responsible for perceiving different lightness.
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.
Aim of the study
• To examine validity of conventional explanation, using simple reaction time (SRT) as a measure of retinal processing of stimuli intensity.
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.
Relation between int. of stimulus and SRT
200
250
300
350
400
450
500
550
Stimulus intensity
SRTms
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.
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
Results Black background - 3 stimuli
Stimulus cd/m2 0,01 24 130
Backg. cd/m2 0 0 0
210,3
174,5
177,5
150
160
170
180
190
200
210
220
1 2 3
stimulus type
SR
T m
s
results tentative expected trend
Black background - 6 stimuli (narrow range of stimulus intensities)
Stimulus cd/m2 0,4 6 14 42 61 86
backg. cd/m2 0 0 0 0 0 0
225
164
184182
175
166
150
160
170
180
190
200
210
220
230
1 2 3 4 5 6
SR
T m
s
results tentative expected trend
The same stimuli – diff. backg. Simultaneous contrast situation (3 stimuli)
Stimulus cd/m2 70 70 70
Backg. cd/m2 130 24 0
171,1
181,4179,4
150
160
170
180
190
200
1 2 3
SR
T m
sresults tentative expected trend
6 stimuli of the same luminance on different backgroud luminnace
Simultaneous contrast situation (lower intensities)
Stimulus cd/m2 26 26 26 26 26 26
backg. cd/m2 87 61 42 14 6 0,4
200,1
190,3
195,4
199,7
211,6
206,5
150
160
170
180
190
200
210
220
1 2 3 4 5 6
SR
T m
s
results tentative expected trend
Different int. of stimuli on the same (gray) background
Stimulus cd/m2 0,4 6 14 42 61 86
backg. cd/m2 26 26 26 26 26 26
187,4
189,8189,5189,6192,1
185,1
150
160
170
180
190
200
210
220
1 2 3 4 5 6
SRT
ms
results tentative expected trend
Stimulus cd/m2 0 130
backg.
cd/m2130 0
Max. contrast
184,4
191,8
160
170
180
190
200
1 2
SR
T m
sresults tentative expected trend
3 stimuli – the same ratio
Stimulus cd/m2 0,55 27 109
Backg. cd/m2 0,39 21 87
188,9
213,8
189,4
160
170
180
190
200
210
220
1 2 3
SR
T m
s
results tentative expected trend
6 stimuli the same ratio
Stimulus cd/m2 6 8 25 35 61 86
backg. cd/m2 8 6 35 25 86 61
207,24
198,61
196,16
189,13
194,27
191,8
180
190
200
210
220
1 2 3 4 5 6
SR
T m
sresults tentative expected trend
Conclusion
• Need for further research- More experimental situations- Better control (adaptation, afterimages)- Judgement data (normal and short presentation of diff.
luminance)