Asymmetry and similarity phenomena in backwards masking experiments suggest reentrant processing

Abstract

Non-linear effects can be observed in fast backwards-masking psychophysics experiments. Such experiments show that: 1) when targets and masks are similar they require longer processing times 2) patterns with unique features are processed quicker, displaying an asymmetry 3) these properties can also determine whether masking functions follow monotonic or U-shape curves. Hypotheses regarding the structure of neural processing involved in rapid image presentations range from pure feedforward models to models with lateral connections and top-down reentrant feedback. Although the prevailing interpretation has been that for very fast image presentations under effective masking, a feedforward model is adequate, there is now growing evidence against this possibility. It remains unclear if the earliest signals of object detection in the brain are connected with object identification or more general statistical properties of the signal and 150 ms may be enough time for limited feedback processing. Furthermore it is possible that simultaneous processing of both the target and mask continues until the person responds. We construct and test a set of hypotheses for simultaneous and serial processing of the target and mask while evaluating the consequences of different cortical dynamics on speed of sight psychophysics experiments. We demonstrate that models whose dynamics are essentially feedforward HMAX-like cannot easily account for effects of differential masking and asymmetries in image sequences, because they are not well-suited for simultaneous processing. We then show that a simple model with top-down feedback naturally accounts for these properties, suggesting that even for very short presentation times reentrant connections play an important role in visual perception in humans.