Derick Valadao

Static versus dynamic judgments of spatial extent

Research exploring how scanning affects judgments of spatial extent has produced conflicting results. We conducted four experiments on line bisection judgments measuring ocular and pointing behavior, with line length, position, speed, acceleration, and direction of scanning manipulated. Ocular and pointing judgments produced distinct patterns. For static judgments (i.e., no scanning), the eyes were sensitive to position and line length with pointing much less sensitive to these factors. For dynamic judgments (i.e., scanning the line), bisection biases were influenced by the speed of scanning but not acceleration, while both ocular and pointing results varied with scan direction. We suggest that static and dynamic probes of spatial judgments are different. Furthermore, the substantial differences seen between static and dynamic bisection suggest the two invoke different neural processes for computing spatial extent for ocular and pointing judgments.

Functional MRI of dynamic judgments of spatial extent

Judgments of spatial relationships are often made when the object or observer is moving. Behaviourally, there is evidence that these ‘dynamic’ judgments of spatial extent differ from static judgments. For example, in one of the simplest probes of spatial extent—the line bisection task—the typically observed leftward bisection bias of about 1% of line length is increased considerably after left-to-right scanning. Here we used fMRI to examine whether or not different brain regions would be involved in static and dynamic judgments of spatial extent. Dynamic (i.e., scan the line prior to bisecting) relative to static ocular bisections (i.e., line bisection by fixation) produced activations in the cuneus and precuneus bilaterally, with reduced activation relative to static judgments observed in the supramarginal gyrus bilaterally. Dynamic bisections relative to a control condition (i.e., scan a line and then saccade to a transection mark) produced activations in the precuneus/superior parietal lobe bilaterally and left cerebellum. Only marginal evidence was found for different activations due to the initial scan direction of the line. These results highlight the fact that dynamic judgments of spatial extent use distinct brain regions from those employed to make static judgments, and the same mechanism is employed independent of scan direction. It may be the case that velocity processing and time estimates are integrated primarily in the cuneus and precuneus to produce estimates of spatial extent under dynamic scanning conditions.

Examining the influence of ‘noise’ on judgements of spatial extent

The line bisection task—commonly used as a clinical measure of unilateral neglect—requires participants to place a mark on a horizontal line to indicate where they think centre is. In general, results suggest that the allocation of attention mediates bisection. In addition, previous research shows that participants rarely explore the endpoints of lines, suggesting that peripheral visual information informs bisection. Here, we examined bisection performance under conditions in which differing levels of ‘noise’ were introduced to the line to examine the hypothesis that the fidelity and symmetry of peripheral information would inform performance. Contrary to our expectations, results showed that symmetrically introducing noise to the line biased bisection further leftward compared to a ‘no-noise’ condition. Furthermore, asymmetrical noise increased leftward bisection errors primarily when lines were presented in left space or when the greater amount of noise was on the left half of the line. These results indicate that the fidelity of peripheral visual information mediates bisection behaviour that is already biased leftwards probably due to right hemisphere attentional mechanisms.

Examining the influence of working memory on updating mental models

The ability to accurately build and update mental representations of our environment depends on our ability to integrate information over a variety of time scales and detect changes in the regularity of events. As such, the cognitive mechanisms that support model building and updating are likely to interact with those involved in working memory (WM). To examine this, we performed three experiments that manipulated WM demands concurrently with the need to attend to regularities in other stimulus properties (i.e., location and shape). That is, participants completed a prediction task while simultaneously performing an n-back WM task with either no load or a moderate load. The distribution of target locations (Experiment 1) or shapes (Experiments 2 and 3) included some level of probabilistic regularity, which, unbeknown to participants, changed abruptly within each block. Moderate WM load hampered the ability to benefit from target regularities and to adapt to changes in those regularities (i.e., the prediction task). This was most pronounced when both prediction and WM requirements shared the same target feature. Our results show that representational updating depends on free WM resources in a domain-specific fashion.

Rejecting Outliers: Surprising Changes Do Not Always Improve Belief Updating.

An important human skill is the ability to update one’s beliefs when they are no longer supported by the environment. Current models of dynamic decision-making suggest that more unexpected, or “surprising,” events lead to quicker belief updating. The current article tests the ubiquity of the notion that surprising environmental changes are always positively related to updating. Using a novel task based on the game Plinko, we tracked participants’ beliefs as they learned distributions of ball drops. At an unannounced point during the task, the distribution of ball drops changed and we computed how surprising these changes were relative to participants’ beliefs and compared how this surprise factor influenced their ability to update their beliefs to reflect the change. We found that, consistent with current models, there were some situations in which belief updating was positively related to the surprise of a change. However, we also found a situation in which highly surprising changes were negatively related to updating—situations where participants tended to update less with increasingly surprising changes. This negative relationship seems due to participants’ treating highly surprising events as “outliers” and choosing not to integrate them in their current beliefs. Our results provide a novel and more nuanced representation of the relationship between surprise and updating that should be considered in models of dynamic decision-making.