Temporal aspects of orientation pooling using visual noise stimuli

Previous psychophysical studies have shown that oriented signals are pooled over space to support better perceptual performance. We have investigated whether oriented signals may also be pooled over time. Using an orientation-in-noise paradigm, the threshold to discriminate large orientation differences was measured as the minimum amount of signal required to discriminate an oriented stimulus from unoriented noise. Discrimination thresholds were better with dynamic stimuli, containing multiple independent samples over time, than with static stimuli presented for an equal duration. Thresholds for dynamic stimuli showed gradual improvement from very brief (12 ms) to remarkably long presentation times (>4 s). Spatial integration of orientation signals is very efficient and can be understood in terms of a model based on signal detection theory, with performance limited by early and late stages of intrinsic noise. The nature of temporal integration is different, however, and is more consistent with probability summation of the outputs from low-level orientation detectors operating at a very brief timescale but whose outputs can be combined over a very long duration to yield better perceptual performance.