Trading Noise for Stimulus Duration in Orientation Judgments

1. Trading Noise for Stimulus Duration in Orientation Judgments Kei Kurosawa, Kristen Strong& Nestor Matthews Department of Psychology, Denison University, Granville OH 43023 USA Results Motivation Discussion Earlier studies showed that our ability to judge a subtle angular difference increases over the tens of milliseconds following stimulus presentation1, and that there may be greater neural noise at oblique (diagonal) axes than at cardinal (horizontal) axes2. Here we investigated the extent to which stimulus duration can be traded for orientation noise when judging angular differences. In Exp1, orientation sensitivity grew with decreases in noise bandwidth. At the greatest bandwidths, orientation sensitivity was poor and was comparable at the cardinal and oblique axes, although performance was reliably better than chance at each axis. The oblique effect (OE) emerged only as the noise bandwidth decreased. This is similar to previous work1 that showed that the OE emerges with increasing stimulus duration. Exp 2 revealed a trade-off between noise bandwidth and stimulus duration. Increases in noise bandwidth effectively canceled the enhancements in orientation sensitivity that occurred with increasing stimulus duration. In Exp 3, participants judged the bandwidth range rather than the mean orientation. The data indicated that an OE was absent at each bandwidth condition. The OE for range judgments was absent in EXP 3 even when the stimuli were identical to those that generated an OE for mean judgments (Exp 1). Because identical stimuli would create identical responses in the neurons that detect orientation, the OE must originate after orientation is detected in the visual system. Experiment 1 Experiment 2 Method Exp 1 – Participants foveally viewed two sequentially presented grating-movies with bulls-eye masks preceding and following each movie. The task was to judge the mean orientation of the second movie as clockwise or anti-clockwise to the first (figure below). With the movie duration fixed at 108 msec and a mean difference of ~3 deg between the two movies, the axis (cardinal or oblique) and the bandwidth of orientation noise (0 - 60 deg) were randomly varied across trials. Exp 2 – The procedure was the same as in Exp 1 except that we varied the movie duration (42 - 142 msec), and varied the noise bandwidth (0 – 60 deg) around a cardinal axis only. Exp 3 – The first movie on each trial had a randomly selected bandwidth (10 - 90 deg). The noise bandwidth of the second movie randomly increased or decreased by 10 – 50% relative to the first. Participants judged whether the second orientation-bandwidth was narrower or broader than the first. Experiment 3 The Bottom Line The findings suggest that orientation sensitivity arises from axis-specific noise reductions over time, with the oblique effect originating after the stage at which orientation is detected. References Matthews, Rojewski & Cox (2005) PMID 15929646. Heeley, Buchanan-Smith, Cromwell & Wright (1997) PMID 9068823. This poster can be viewed and downloaded at http://www.denison.edu/~matthewsn/noise05.html