Browsing by Subject "näköaisti"

Objectives. Different biases induced by stimulus and response history have been identified in the field of perceptual psychology. Serial dependence is a bias in the perception of objects caused by previous stimuli. Perception can also be biased towards the average of previous stimuli. This is called central tendency bias. The interdependence of responses in a serial task is serial response bias. These history biases have mostly been studied separately despite their effects being similar. This thesis aims to unravel the independent effects of the biases on observers’ responses in a perceptual task and the effect of working memory delay on their magnitude. Methods. The biases were studied with two experiments. Eleven subjects took part in the first experiment which used colored patches with different hues as stimuli. The second experiment had 7 subjects and used grating patches differing in orientation. Subjects were presented with a continuous stream of reference and comparison stimuli. In each trial, they had to respond if the comparison stimulus, presented second, was bluer or greener or was oriented more to the left or the right than the reference stimulus, presented first. Inter-stimulus interval was either short or long in two blocked conditions. The independent effects of the history biases on responses were tested using a generalized linear mixed model with a probit link function. Results and conclusions. In the hue experiment, independent effects were found for all history biases. Previous stimuli attracted the perception of subsequent stimuli and this effect lasted for 12-16 seconds. Independently of this, the percept was also biased towards the average of previous stimuli. In addition, subjects tended to change their response between trials. Working memory delay had no observable effect on the magnitude of the biases. In the orientation experiment, results on serial dependence were mixed, likely due to more noise in the data. Central tendency bias was not observed. Response bias was dependent on the inter-stimulus interval in such a way that with a short delay, subjects were more likely to repeat their choices and with a long, delay they were more likely to alternate between choices. The results clarify the relationships between previously reported history biases. Neither central tendency bias nor serial response bias can explain serial dependence. Furthermore, the results suggest that serial dependence cannot originate solely from the inertia of responses.