Free Will or Not: Causality Preserved but Access to Motor Decisions Obscured

Empirical studies having addressed the free-will issue suffer from controversial methodologies and interpretations. We present a new paradigm involving a synchronization task where the time interval to synchronize with is randomly within and without subject's synchronization capabilities and ask subjects to retrospectively evaluate (Q1) which of the two occurred or (Q2) whether their motor response had had been reactive/speeded or delayed. Contrary to the non-FW view, Q1-judgments correlate with the actual duration of the synchronization interval rather than with subjects's motor response latencies. Instead of postidictively reshuffling their judgment to make it match the outcome of their actions, subjects preserve the causal chain of events having entailed them. When answering Q2 subjects' judgments also correlate with the synchronization interval, proof that they cannot decide on the intentionality of their actions. Hence the present results reveal the intrinsic duality of the free-will concept

Visual perceptual learning of task-irrelevant feature of the stimulus: preliminary results

During pre- and post-tests participants performed a 3-dot Vernier task and a 3-dot bisection task. During training participants performed a luminance discrimination task on the same 3-dot stimulus. The task-irrelevant feature of the stimulus manipulated during training was the position of the middle dot. Our results suggest that perceptual learning can occur as a result of mere exposure to a subthreshold and task-irrelevant feature of the stimulus. The present findings add new evidence in support of task-irrelevant PL, which seems to occur not only when two different stimuli are used during test and training, but also when the same stimulus is used in both stages

No correlations between visual illusion strength

In cognition, audition and somatosensation, performance correlates strongly between different tasks suggesting the existence of common factors. Surprisingly, this does not hold true for vision. For example, Vernier acuity and Gabor detection correlate very weakly (r2 = 0.003). Here, we show similar results for visual illusions. 143 participants, aged from 8 to 81, adjusted the strength of six illusions. Correlations were very low and mostly non-significant. For example, the correlation between the Ebbinghaus and the Ponzo illusion was r2 = 0.08, i.e., the two illusions have only 8% of variance in common. Results for males and females did not differ. Illusion magnitude decreased with age for the Ebbinghaus, Ponzo, and Tilt illusion. Our null results are supported by good test-retest reliability and a Bayesian analysis. We suggest that, contrary to cognition, audition and somatosensation, there is no general factor for vision

Linking perceptual learning with identical stimuli to imagery perceptual learning

Perceptual learning is usually thought to be exclusively driven by the stimuli presented during training (and the underlying synaptic learning rules). In some way, we are slaves of our visual experiences. However, learning can occur even when no stimuli are presented at all. For example, Gabor contrast detection improves when only a blank screen is presented and observers are asked to imagine Gabor patches. Likewise, performance improves when observers are asked to imagine the non-existing central line of a bisection stimulus to be offset either to the right or left. Hence, performance can improve without stimulus presentation. As shown in the auditory domain, performance can also improve when the very same stimulus is presented in all learning trials and observers were asked to discriminate differences which do not exist (observers were not told about the set up). Classic models of perceptual learning cannot handle these situations since they need proper stimulus presentation, i.e., variance in the stimuli, such as a left vs. right offset in the bisection stimulus. Here, we show that perceptual learning with identical stimuli occurs in the visual domain, too. Second, we linked the two paradigms by telling observers that only the very same bisection stimulus was presented in all trials and asked them to imagine the central line to be offset either to the left or right. As in imagery learning, performance improved

Individual differences in the Muller-Lyer and Ponzo illusions are stable across different contexts

Vision scientists have attempted to classify visual illusions according to certain aspects, such as brightness or spatial features. For example, Piaget proposed that visual illusion magnitudes either decrease or increase with age. Subsequently, it was suggested that illusions are segregated according to their context: real-world contexts enhance and abstract contexts inhibit illusion magnitudes with age. We tested the effects of context on the Muller-Lyer and Ponzo illusions with a standard condition (no additional context), a line-drawing perspective condition, and a real-world perspective condition. A mixed-effects model analysis, based on data from 76 observers with ages ranging from 6 to 66 years, did not reveal any significant interaction between context and age. Although we found strong intra-illusion correlations for both illusions, we found only weak inter-illusion correlations, suggesting that the structure underlying these two spatial illusions includes several specific factors

Sex-related differences in vision are heterogeneous

Despite well-established sex differences for cognition, audition, and somatosensation, few studies have investigated whether there are also sex differences in visual perception. We report the results of fifteen perceptual measures (such as visual acuity, visual backward masking, contrast detection threshold or motion detection) for a cohort of over 800 participants. On six of the fifteen tests, males significantly outperformed females. On no test did females significantly outperform males. Given this heterogeneity of the sex effects, it is unlikely that the sex differences are due to any single mechanism. A practical consequence of the results is that it is important to control for sex in vision research, and that findings of sex differences for cognitive measures using visually based tasks should confirm that their results cannot be explained by baseline sex differences in visual perception

Hyper-specificity of perceptual learning

Perceptual learning is the ability to improve perception through practice. Perceptual learning is usually specific for the task and the stimulus features trained with. For example, training orientation discrimination with vertically oriented stimuli does not improve performance when the same stimuli are rotated by 90°. Often, perceptual learning is assumed to occur within the early sensory areas or when mapping sensory evidence onto decisions. Motor responses, involved in all perceptual learning experiments, are thought to play no role in the learning process. Here, we show that this is not true by providing evidence that perceptual learning is specific for the type of motor response. Interestingly, this specificity can be overcome by the use of a double training protocol. In a second project, we show that, unlike in the somatosensory and auditory system, there is surprisingly no common factor in vision. Finally, we challenge classic models of perceptual learning by showing that conditions under which perceptual learning should not occur as predicted can nevertheless occur. Taken together, our results indicate that the complexity of perceptual learning mechanisms goes much beyond classic models and current theories of perceptual learning

No transfer from visual to visuomotor perceptual learning and vice versa

Perceptual learning is usually very specific. For example, performance improves when observers train with a vertical bisection stimulus. However, this improvement does not transfer to a horizontal bisection stimulus. Here, we show that perceptual learning is even specific for the response mode. Observers trained with vertical bisection stimuli comprising two outer and a central line. In the visuomotor learning condition, observers could adjust the center line by means of a computer mouse. Observers placed the center line in the middle between the two outer lines with a small displacement towards either the left or right outer line indicated by a cue randomly chosen. We asked observers to adjust the smallest displacement they could perceive. In the classical, visual learning condition, observers indicated by push button press if the center line was closer to the left or right outer line. Performance improved strongly in both conditions. However, improvements in either condition did not transfer to the other condition as shown by pre- and post-measurements, i.e., visuomotor learning did not improve visual performance and visual learning did not improve visuomotor performance. Hence, perceptual learning is specific to the response mode used during training- even though the visual stimuli are almost identical