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Ignacio Serrano-Pedraza, Vicente Sierra-Vázquez; Perceptual prevalence of first-order information in letter identification showed using visual chimeras. Journal of Vision 2012;12(9):1055. doi: 10.1167/12.9.1055.
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© ARVO (1962-2015); The Authors (2016-present)
Two parallel visual mechanisms have been proposed for spatial processing: a linear one devoted to processing luminance and the detection of fine-grain details (first-order mechanism), and an independent linear-nonlinear-linear mechanism devoted to processing the modulation of contrast, frequency, or orientation (second-order mechanism). The perceptual contribution of each mechanism in the identification of natural images can be shown by means of AM-PM visual chimeras (Sierra-Vázquez & Serrano-Pedraza, 2011, Perception, 40). A visual chimera is a synthetic image that has the first-order structure (phase-modulated or PM carrier) of one natural image and the local contrast or envelope (amplitude-modulated or AM component) of another image in a particular spatial-frequency band. In order to synthesize it, the PM carrier and the envelope were computed using the Riesz transform and the associated monogenic signal (the counterpart of the analytic signal in which the Riesz transform replaces the Hilbert transform). In this work we used a letter identification task. We used two visual chimeras as stimuli: a chimera with the envelope of the letter B and the PM carrier of the letter S, and vice versa. We manipulated the PM carrier, changing its contrast from 0.01 to 0.64 and masking it with broadband white noise of fixed energy. Psychometric functions and reaction times (RTs) from the identification task showed that at high PM-carrier contrast, the first-order information dominated and the subject identified the letter from which the PM carrier was taken, with RTs of about 400 msec. However, when the PM-carrier contrast was low, the first-order structure was absolutely masked and the subjects identified the letter from the information present in the envelope, with slower RTs (550 msec). Our results show that when first- and second-order information is available, our visual system shows a consistent perceptual prevalence of the fine-structure information in letter identification.
Meeting abstract presented at VSS 2012
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