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Dalila Achoui, Axel Cleeremans; The fine-grained sub-millisecond objective and subjective psychophysics of stimulus representation. Journal of Vision 2018;18(10):1160. doi: 10.1167/18.10.1160.
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A custom designed LCD tachistoscope enabled us to display stimuli in the sub-millisecond range with very high precision. These extremely brief presentation times allow us, for the first time, to zoom in on the psychophysics between two subsequent levels of stimulus representation. Moreover, a precision of ~5 microseconds allows us to individually adjust and control the level of representation at which the stimulus is being perceived by each specific participant. We were interested in both the objective and subjective psychophysics between representational levels. Particularly the question of whether subjective perception shows a similar psychophysical curve as objective performance or whether it increases with a step-like function between each hierarchical level of stimulus representation. We first obtained the perceptual thresholds for a low-level detection task (absent vs. present) and a higher-level discrimination task (object A vs. B) over a range of 300 - 1125 microseconds. We then used the detection threshold in a subjective perception task in which participants indicated in which of two time intervals the stimulus was most visible. Crucially, the two intervals different slightly in duration but were both centered around their individual threshold. Importantly, by setting the threshold individually we were able to ensure that participants perceived the stimulus only at the low-level representation ("An object is present" ), but were not able to perceive any other stimulus features. The average perceptual thresholds were as low as 550 microseconds for detection, and 790 microseconds for discrimination. Participants performed above chance on the subjective task, suggesting that, independently of the level of representation, subjective perception improves with stimulus duration. Our tachistoscope allowed us to use such brief stimulus durations that we were able to obtain distinct psychophysical curves for two subsequent levels in the visual hierarchy. Moreover, we showed that subjective perception increases independently of level of representation.
Meeting abstract presented at VSS 2018
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