Katinka van der Kooij

Inspecting Visual Mental Images: Can People "See" Implicit Properties as Easily in Imagery and Perception?

Can people “see” previously unnoticed properties in objects that they visualize, or are they locked into the organization of the pattern that was encoded during perception? To answer this question, we first asked a group to describe letters of the alphabet and found that some properties (such as the presence of a diagonal line) are often mentioned, whereas others (such as symmetry) are rarely if ever mentioned. Then we showed not only that other participants could correctly detect both kinds of properties in visualized letters, but also that the relative differences in the ease of detecting these two types of properties are highly similar in perception (when the letters are actually visible) and imagery (when the letters are merely visualized). These findings provide support for the view that images can be reinterpreted in ways much like what occurs during perception and speak to the wider issue of the long-standing debate about the format of mental images.

Visuomotor adaptation: how forgetting keeps us conservative

Even when provided with feedback after every movement, adaptation levels off before biases are completely removed. Incomplete adaptation has recently been attributed to forgetting: the adaptation is already partially forgotten by the time the next movement is made. Here we test whether this idea is correct. If so, the final level of adaptation is determined by a balance between learning and forgetting. Because we learn from perceived errors, scaling these errors by a magnification factor has the same effect as subjects increasing the amount by which they learn from each error. In contrast, there is no reason to expect scaling the errors to affect forgetting. The magnification factor should therefore influence the balance between learning and forgetting, and thereby the final level of adaptation. We found that adaptation was indeed more complete for larger magnification factors. This supports the idea that incomplete adaptation is caused by part of what has been learnt quickly being forgotten.

Uncertainty reveals surround modulation of shape

Noisy estimations of shape can be partially resolved by incorporating relevant information from the context. The effect of surround stimuli on shape perception becomes clear in illusions of shape contrast and assimilation. In this study, we answer the question how a surround-induced bias depends on the reliability of shape signals. This way, we assess the processes by which an observer incorporates relevant data from the context into the shape estimate. We selectively added visual noise to the center and surround and compared a bias in shape perception with a control condition where no noise was added. In the conditions where shape and surround stimuli were well defined, we found a shape-contrast bias. When the surround stimuli were degraded, this contrast bias decreased. Most interestingly, when the central shape was degraded, an assimilation bias was observed. This bias was larger when the entire stimulus was degraded compared to when only the central shape was degraded. This suggests that shape contrast is the result of inference processes relying on local representations in early visual areas whereas assimilation is related to inference processes by global representations in higher visual areas.Noisy estimations of shape can be partially resolved by incorporating relevant information from the context. The effect of surround stimuli on shape perception becomes clear in illusions of shape contrast and assimilation. In this study, we answer the question how a surround-induced bias depends on the reliability of shape signals. This way, we assess the processes by which an observer incorporates relevant data from the context into the shape estimate. We selectively added visual noise to the center and surround and compared a bias in shape perception with a control condition where no noise was added. In the conditions where shape and surround stimuli were well defined, we found a shape-contrast bias. When the surround stimuli were degraded, this contrast bias decreased. Most interestingly, when the central shape was degraded, an assimilation bias was observed. This bias was larger when the entire stimulus was degraded compared to when only the central shape was degraded. This suggests that shape contrast is the result of inference processes relying on local representations in early visual areas whereas assimilation is related to inference processes by global representations in higher visual areas.

Shape contrast: a global mechanism?

We investigated whether a shape contrast bias is caused by local contrast enhancement or by a global mechanism. In a baseline condition, observers performed a shape discrimination task on an isolated hinged plane. But in the experimental conditions, five dihedral surfaces, of which we varied the dihedral angle distribution, were added on each side. Shape perception was influenced not only by the adjacent surface but also by the mean of the shape distribution in the extended surround. Thus, shape contrast is not locally determined and has to be understood from a global mechanism. We propose divisive normalization of shape signals as such a mechanism.We investigated whether a shape contrast bias is caused by local contrast enhancement or by a global mechanism. In a baseline condition, observers performed a shape discrimination task on an isolated hinged plane. But in the experimental conditions, five dihedral surfaces, of which we varied the dihedral angle distribution, were added on each side. Shape perception was influenced not only by the adjacent surface but also by the mean of the shape distribution in the extended surround. Thus, shape contrast is not locally determined and has to be understood from a global mechanism. We propose divisive normalization of shape signals as such a mechanism.

Reward abundance interferes with error-based learning in a visuomotor adaptation task

The brain rapidly adapts reaching movements to changing circumstances by using visual feedback about errors. Providing reward in addition to error feedback facilitates the adaptation but the underlying mechanism is unknown. Here, we investigate whether the proportion of trials rewarded (the ‘reward abundance’) influences how much participants adapt to their errors. We used a 3D multi-target pointing task in which reward alone is insufficient for motor adaptation. Participants (N = 423) performed the pointing task with feedback based on a shifted hand-position. On a proportion of trials we gave them rewarding feedback that their hand hit the target. Half of the participants only received this reward feedback. The other half also received feedback about endpoint errors. In different groups, we varied the proportion of trials that was rewarded. As expected, participants who received feedback about their errors did adapt, but participants who only received reward-feedback did not. Critically, participants who received abundant rewards adapted less to their errors than participants who received less reward. Thus, reward abundance negatively influences how much participants learn from their errors. Probably participants used a mechanism that relied more on the reward feedback when the reward was abundant. Because participants could not adapt to the reward, this interfered with adaptation to errors.

Reward-based motor adaptation can generalize across actions.

Recently it has been shown that rewarded variability can be used to adapt visuomotor behavior. However, its relevance seems limited because adaptation to binary rewards has been demonstrated only when the same movement is repeated throughout the experiment. We therefore investigated whether the adaptation is action-specific and whether the amount of exploration depends on spatial complexity. Participants pointed to 3−D visual targets without seeing their hand and could use only binary reward feedback to adapt their movements. We varied the number of target positions and the number of dimensions the feedback was based on. Because the feedback was based on a 5-cm rightward shifted hand position, adaptation was needed for good performance. The participants started naïve to the perturbation. If actions were made toward a single target position and the feedback was based on the lateral component of their response only, participants adapted completely within 200 trials. Having more than 1 target position or more than 1 dimension of performance resulted in considerably less adaptation but did not affect the exploration. Thus, reward-based adaptation can generalize across actions but is reduced by spatial complexity, whereas exploration is not affected by spatial complexity.

Perception of 3D Slant Out of the Box

Evidence for contextual effects is widespread in visual perception. Although this suggests that contextual effects are the result of an inherent property of the visual system, current explanations are limited to the domain in which they occur. In this paper we propose a more general mechanism of global influences on the perception of slant. We review empirical data and evaluate proposed explanations of contextual biases. By assessing not only a model about 3D slant perception but also evaluating more generic mechanisms of contextual modulation, we show that surround suppression of neural responses explains the major phenomena in the empirical data on contextual biases. Moreover, contextual biases may be part of a mechanism of grouping and segmentation.