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Agnieszka Nowik, Piotr Ja'kowski; Effect of stimulus brightness on LRP latency and RT. Journal of Vision 2007;7(9):380. doi: https://doi.org/10.1167/7.9.380.
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Van der Molen and Keuss (1979, 1981) reported U-shaped relationship between reaction time (RT) and loudness in difficult tasks requiring choice responses. This effect was recently replicated by Ja'kowski and Włodarczyk (2006) for visual stimuli. In the current study, we used ERP to investigate the locus of this paradoxical elongation of RTs for extremely bright stimuli. The visual stimuli were a black capital letter in the centre presented against a white square. The luminance of the square was manipulated. We tested participants whose task was to make either a simple responses or choice responses according to letter identity. Results: The RT-luminance relationship was monotonic for simple responses and U-shaped for choice responses, with RTs up to 50 ms longer for the brightest as compared to moderately intense stimuli. Notably, LRP-R was independent of stimulus intensity for both tasks. However, S-LRP latency changed with brightness similarly to RTs. That is, it decreased gradually with intensity for the simple task and it showed elongation with very bright intensities for the choice task. These results support Van der Molen and Keuss' proposal that it is the response selection stage that is affected by very strong stimuli. Conclusions: our study clearly indicates that response selection is influenced by intensity changes, resulting in a U-shaped relationship between RT and intensity when the task needs choice.
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