Research code: 9990
Ethics code: IR.IUMS.REC.1399.1281

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1- Shahid Beheshti University
2- Institute for Research in Fundamental Sciences
3- University of Tehran
4- Shahid Rajaee Teacher Training University
Abstract:   (424 Views)
Introduction. Perceptual decision-making studies have widely focused on simple stimuli with uniform distribution of spatial information say Random Dot Motion to address the accumulation of sensory evidence resulting in a decision. However, the real-world stimuli are mostly complex containing a non-uniform distribution of spatial information. Hence perceptual decision-making process in presence of natural stimuli has remained unknown because of the complexity in controlling this information over time. Recently, a novel framework has been introduced to broaden perceptual decision-making studies into multi-spatial stimuli. Taking advantage of this framework, we aim to quantitatively investigate the weight of spatial information in making a binary decision using a face discrimination task with controlled fluctuating local features (eyes, nose, and mouth) over time. We hypothesize that the perfect accumulator assumption might explain the integration of spatial information.
Methods. Having developed a customized algorithm, neutral faces have been geometrically morphed such that -100%,+100%, and 0% represent the first, the second, and the middle face of the morph stream respectively. The trials were designed in a way that stimuli either have three informative features (eyes, mouth, nose simultaneously) or only one informative feature (eyes, mouth, nose individually) fluctuating in time. Seven human subjects (24-39 years old, four women) participate in this study with normal or corrected-to-normal vision. All procedures were approved by the ethics committee of the Iran University of Medical Sciences. Stimuli were created and controlled in MATLAB and presented using routines from the psychophysics Toolbox extension. After passing the train phases the participants were invited to the main phase of the experiment. They were instructed to maintain their gaze on the red fixation point in the center of the screen and report if the stimuli displayed on the side of the fixation point were more similar to the first prototype or the second one. Each participant completed a total of 1568 trials in eight blocks of 196 trials.
Results. Logistic regression analysis reveals that the subjects are not biased toward any of the choices. The reaction time decreases and the probability of correct answers increases with the stimulus strength (morph%). The design of the experiment allows us to compute psychophysical kernels for 0% morph trials for which the positive amplitudes of the psychophysical kernels indicate that the decisions are made based on the integration of spatial information yet with unequal weight. The average of psychophysical kernels amplitude shows that the presence of eye information intensifies the extraction of information from nose and mouth in consistency with the reliability of the information. To quantify the contribution of each feature in the decision, we have used logistic regression. This shows only linear coefficients have a significant effect rather than multiplicative terms relating to the interaction of features. Having calculated the evidence contained in each one-informative feature trial and using the perfect accumulator assumption, we compute the expected accuracy to predict the accuracy in three-informative feature trials. We show that by considering the effective weight of each feature we are able to extend the perfect accumulator assumption to explain the experimental accuracy for three-informative feature trials.
Conclusion. Findings suggest that the mechanism of perceptual decision-making in presence of natural stimuli can be quantitatively explained by an extension of perfect accumulator assumption. In fact, considering the effective weight of each feature, the perfect accumulator assumption provides an explanation for such a decision-making process which might address the holistic effect quantitatively. Finally, taking advantage of perceptual decision-making findings can enhance our understanding of natural stimuli processing in the brain.
Type of Study: Research |
Received: 2021/03/16 | Accepted: 2021/05/17

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