No exception from Bayes' rule: The presence and absence of the range effect for saccades explained

Sensorimotor serial dependencies in head movements

On average, we redirect our gaze with a frequency at about 3 Hz. In real life, gaze shifts consist of eye and head movements. Much research has focused on how the accuracy of eye movements is monitored and calibrated. By contrast, little is known about how head movements remain accurate. I wondered whether serial dependencies between artificially induced errors in head movement targeting and the immediately following head movement might recalibrate movement accuracy. I also asked whether head movement targeting errors would influence visual localization. To this end, participants wore a head mounted display and performed head movements to targets, which were displaced as soon as the start of the head movement was detected. I found that target displacements influenced head movement amplitudes in the same trial, indicating that participants could adjust their movement online to reach the new target location. However, I also found serial dependencies between the target displacement in trial n-1 and head movements amplitudes in the following trial n. I did not find serial dependencies between target displacements and visuomotor localization. The results reveal that serial dependencies recalibrate head movement accuracy.

Central tendency biases must be accounted for to consistently capture Bayesian cue combination in continuous response data

Observers in perceptual tasks are often reported to combine multiple sensory cues in a weighted average that improves precision—in some studies, approaching statistically optimal (Bayesian) weighting, but in others departing from optimality, or not benefitting from combined cues at all. To correctly conclude which combination rules observers use, it is crucial to have accurate measures of their sensory precision and cue weighting. Here, we present a new approach for accurately recovering these parameters in perceptual tasks with continuous responses. Continuous responses have many advantages, but are susceptible to a central tendency bias, where responses are biased towards the central stimulus value. We show that such biases lead to inaccuracies in estimating both precision gains and cue weightings, two key measures used to assess sensory cue combination. We introduce a method that estimates sensory precision by regressing continuous responses on targets and dividing the variance of the residuals by the squared slope of the regression line, “correcting-out” the error introduced by the central bias and increasing statistical power. We also suggest a complementary analysis that recovers the sensory cue weights. Using both simulations and empirical data, we show that the proposed methods can accurately estimate sensory precision and cue weightings in the presence of central tendency biases. We conclude that central tendency biases should be (and can easily be) accounted for to consistently capture Bayesian cue combination in continuous response data.

Do readers use character information when programming return-sweep saccades?

Reading saccades that occur within a single line of text are guided by the size of letters. However, readers occasionally need to make longer saccades (known as return-sweeps) that take their eyes from the end of one line of text to the beginning of the next. In this study, we tested whether return-sweep saccades are also guided by font size information and whether this guidance depends on visual acuity of the return-sweep target area. To do this, we manipulated the font size of letters (0.29 vs 0.39° per character) and the length of the first line of text (16 vs 26°). The larger font resulted in return-sweeps that landed further to the right of the line start and in a reduction of under-sweeps compared to the smaller font. This suggests that font size information is used when programming return-sweeps. Return-sweeps in the longer line condition landed further to the right of the line start and the proportion of under-sweeps increased compared to the short line condition. This likely reflects an increase in saccadic undershoot error with the increase in intended saccade size. Critically, there was no interaction between font size and line length. This suggests that when programming return-sweeps, the use of font size information does not depend on visual acuity at the saccade target. Instead, it appears that readers rely on global typographic properties of the text in order to maintain an optimal number of characters to the left of their first fixation on a new line.

A summary statistical representation influences perceptions but not visually or memory-guided grasping

A statistical summary representation (SSR) is a phenomenon wherein a target property (e.g., size) is encoded based on the average of the stimulus-set to which it belongs. An SSR has been demonstrated in obligatory judgment tasks; however, to our knowledge no work has examined whether it influences grasps to 3D targets. Here, participants completed a method of adjustment task, and visually and memory-guided grasps in conditions wherein differently sized 3D targets (widths: 20, 30 and 40 mm; height and depth = 10 mm) were presented with equal frequency (i.e., control) and when the smallest (i.e., 20-mm: small-target) and largest (i.e., 40-mm: large-target) targets were presented five times as often as the other targets in the stimulus-set. In the method of adjustment task, responses for the small- and large-target weighting conditions were smaller and larger than the control condition, respectively. In other words, an SSR biased perceptions in the direction of the most frequently presented target in the stimulus-set - a result consistent with the view that perceptions are supported by relative visual information laid down by the ventral visual pathway. In contrast, grip apertures were refractory to target-weighting and was a finding independent of the presence (i.e., visually guided) or absence (i.e., memory-guided) of visual feedback. Furthermore, two one-sided tests showed that peak grip apertures for the different target weighting conditions were within an equivalence boundary. Accordingly, an SSR does not influence 3D grasps and is a finding adding to a growing literature reporting that actions are supported by the absolute visuomotor networks of the dorsal visual pathway.