Recency memory effects in Macaques during sequential delayed match-to-sample task with visual noise

Visual object recognition requires both visual sensory information and memory, and its mechanisms are often studied using old-world monkeys. Wittig et al. (2014, 2016) reported that Rhesus monkeys and humans seem to adopt different strategies in a short-term visual memory task. The Rhesus monkeys seemed to rely on recency of stimulus repetition, whereas humans relied on specific memorization. We conducted experiments using a sequential delayed match-to-sample task with random dot visual noise using Rhesus and Japanese monkeys and found that recency effect was observed in both species. There were differences in the noise effect on behavioral performances across species.

Contrast thresholds reveal different visual masking functions in humans and praying mantises

Recently, we showed a novel property of the Hassenstein–Reichardt detector, namely that insect motion detection can be masked by ‘undetectable’ noise, i.e. visual noise presented at spatial frequencies at which coherently moving gratings do not elicit a response (Tarawneh et al., 2017). That study compared the responses of human and insect motion detectors using different ways of quantifying masking (contrast threshold in humans and masking tuning function in insects). In addition, some adjustments in experimental procedure, such as presenting the stimulus at a short viewing distance, were necessary to elicit a response in insects. These differences offer alternative explanations for the observed difference between human and insect responses to visual motion noise. Here, we report the results of new masking experiments in which we test whether differences in experimental paradigm and stimulus presentation between humans and insects can account for the undetectable noise effect reported earlier. We obtained contrast thresholds at two signal and two noise frequencies in both humans and praying mantises (Sphodromantis lineola), and compared contrast threshold differences when noise has the same versus different spatial frequency as the signal. Furthermore, we investigated whether differences in viewing geometry had any qualitative impact on the results. Consistent with our earlier finding, differences in contrast threshold show that visual noise masks much more effectively when presented at signal spatial frequency in humans (compared to a lower or higher spatial frequency), while in insects, noise is roughly equivalently effective when presented at either the signal spatial frequency or lower (compared to a higher spatial frequency). The characteristic difference between human and insect responses was unaffected by correcting for the stimulus distortion caused by short viewing distances in insects. These findings constitute stronger evidence that the undetectable noise effect reported earlier is a genuine difference between human and insect motion processing, and not an artefact caused by differences in experimental paradigms.

Summary: We investigate alternative explanations for a reported difference between human and insect responses to masked moving gratings, and demonstrate that it is a genuine feature of their visual systems.