Adaptation to a direction-dependent visuomotor gain in the young and elderly

Consistent with the widely accepted notion of separate specification of movement amplitude and direction, it has been argued that there is also a categorical difference between adaptation to novel visuomotor rotations and to novel visuomotor gains. In line with this view, ageing seems to affect rotation and gain adaptation differently in that age-related impairments are consistently found for the former, but not for the latter. In this study we ask whether the contrasting findings could also be ascribed to differences in the level of difficulty of gain and rotation adaptation tasks, respectively. In order to increase the difficulty of gain adaptation, younger and older participants had to adapt to a direction-dependent gain transformation. Results revealed direction-dependent adaptation in both groups. More importantly, we replicated the typical findings of age-related impairments of adaptation, but not of aftereffects, that were previously only reported for rotation adaptation. Younger participants also showed superior explicit knowledge regarding the novel visuomotor mapping as compared to the older participants. We show that this knowledge was used by younger participants to selectively augment adaptive shifts. Finally, our findings suggest that the difficulty of the novel visuomotor transformation and, related to this, the involvement of explicit knowledge in adaptation is critical for age-related changes to show up, but not the type of adaptation task, rotation and gain adaptation, respectively.     

Redundancy gain for categorical targets depends on display configuration and duration

Redundancy gain is an improvement in speeded target detection when the number of targets associated with a single response is increased within a single display. The effect has been clearly demonstrated with specific targets, but it is not clear if it occurs in categorization tasks with non-identical targets. The current study tested the effect of target redundancy on speed and accuracy in a go/no-go categorization task. Targets were digits tilted 45° to the left, and were displayed in unilateral, bilateral, or central displays for either 1500?ms or 100?ms. Redundancy gain only occurred for brief targets displayed bilaterally in the upper visual field. The results indicate that redundancy gain is possible for categorization tasks with some bilateral configurations, supporting a role for interhemispheric processing in redundancy gain. Additionally, the results may indicate that processing strategies mask redundancy gain when participants can view targets for a long period of time.     

Difference in sensorimotor adaptation to horizontal and vertical mirror distortions during ballistic arm movements

When learning a novel motor task, the sensorimotor system must develop new strategies to efficiently control the limb(s) involved, and this adaptation appears to be developed through the construction of a behavioral map known as an 'internal model'. A common method to uncover the mechanisms of adaptation and reorganization processes is to expose the system to new environmental conditions, typically by introducing visual or mechanical distortions. The present study investigated the adaptation mechanisms of the human sensorimotor system to horizontal and vertical mirror distortions (HMD and VMD) during the execution of fast goal-directed arm movements. Mirror distortions (MDs) were created by means of virtual visual feedback on a computer screen while the movement was executed on a graphics tablet. Twenty healthy adult participants were recruited and assigned to one of two groups of 10 people each. Tests were divided in two subsequent blocks of five trials. The first block consisted of trials with no mirror distortion (NMD), while the second block was recorded when exposing one group to HMD and the other to VMD. Both MDs resulted in kinematic changes: during the tests with the MDs the participants did not reach the performance level found at the NMD test. Motor performance during HMD appeared to be globally better than during VMD and the adaptation process to VMD appeared to be slower than to HMD, but data interpretation was hampered by large within-participant and between-participant variability. In-depth analyses of the data revealed that most of the motor performance information was contained in the direction of movement. The data supported the idea that the internal model for HMD was already partially built.     

Global VOR gain adaptation during near fixation to foveal targets

Long-term rotational vestibulo-ocular (VOR) adaptation occurs during systematic dysmetria between visual and vestibular afferents, adjusting eye-rotation angular velocity to re-establish retinal stability of the visual field. Due to translational motion of the eyes during head rotation, VOR gain is higher when fixating near objects. The current study measures VOR in humans before and after 6 min of exposure to a foveal near-target during sinusoidal whole-body rotation at 0.45 Hz. All of six participants showed post-exposure increases in open-loop VOR gain after fixating near targets, demonstrating a mean modulation increase of open-loop VOR gain from 0.86 before adaptation to 1.2 after adaptation. We discuss a number of theoretical and applied implications.     

Controlled and automatic processing during mental rotation

In two experiments, 9- and 10-year-olds and adults were tested on a mental rotation task in which they judged whether stimuli presented in different orientations were letters or mirror-images of letters. The mental rotation task was performed alone on 48 trials and concurrently with a memory task on 48 additional trials. The concurrent memory task in Experiment 1 was recalling digits; in Experiment 2, recalling positions in a matrix. The key result was that the slope of the function relating response time to stimulus orientation was the same when the mental rotation task was performed alone and when performed concurrently with the memory task. This result is interpreted as showing that mental rotation is an automatic process for both children and adults.     

Fearful expressions gain preferential access to awareness during continuous flash suppression

Rapid evaluation of ecologically relevant stimuli may lead to their preferential access to awareness. Continuous flash suppression allows assessment of affective processing under conditions in which stimuli have been rendered invisible due to the strongly suppressive nature of dynamic noise relative to static images. The authors investigated whether fearful expressions emerge from suppression into awareness more quickly than images of neutral or happy expressions. Fearful faces were consistently detected faster than neutral or happy faces. Responses to inverted faces were slower than those to upright faces but showed the same effect of emotional expression, suggesting that some key feature or features in the inverted faces remained salient. When using stimuli solely representing the eyes, a similar bias for detecting fear emerged, implicating the importance of information from the eyes in the preconscious processing of fear expressions.     

Pregenital fixation and perceptual distortions to conflict-laden material.

Three standard questionnaire measures of oral and anal fixation were administered to 142 students. On the basis of their scores 26 students were selected and shown tachistoscopic exposures of pictures designed to activate oral or anal conflicts. Five subjects in each group had extremely high scores on the corresponding measure of fixation, five had extremely low scores, and 16 had intermediate scores. It was hypothesised that individuals who were fixated at one or other of the psychosexual stages would show stereotyped perceptual distortions to repeated dim exposures of the corresponding picture. Highly fixated subjects showed significantly more deviant responses than did subjects who showed low levels of fixation at the same stage. A form of Q-factor analysis indicated that subjects who were fixated at the oral sadistic or anal stages showed different patterns of perceptual distortions to the anal picture than those who were not fixated. These results were interpreted as supporting the link between defenses against anal eroticism and the anal character.     

Eye movements and display change detection during reading

In the boundary change paradigm (Rayner, 1975), when a reader's eyes cross an invisible boundary location, a preview word is replaced by a target word. Readers are generally unaware of such changes due to saccadic suppression. However, some readers detect changes on a few trials and a small percentage of them detect many changes. Two experiments are reported in which we combined eye movement data with signal detection analyses to investigate display change detection. On each trial, readers had to indicate if they saw a display change in addition to reading for meaning. On half the trials the display change occurred during the saccade (immediate condition); on the other half, it was slowed by 15-25 ms (delay condition) to increase the likelihood that a change would be detected. Sentences were presented in an alternating case fashion allowing us to investigate the influence of both letter identity and case. In the immediate condition, change detection was higher when letters changed than when case changed corroborating findings that word processing utilizes abstract (case independent) letter identities. However, in the delay condition (where d' was much higher than the immediate condition), detection was equal for letter and case changes. The results of both experiments indicate that sensitivity to display changes was related to how close the eyes were to the invalid preview on the fixation prior to the display change, as well as the timing of the completion of this change relative to the start of the post-change fixation.     

Superego distortions and self-attack

Some people demand an unachievable level of self-perfection, and experience extremely harsh self-criticism when they inevitably fall short of this impossibly high standard. As a result of early trauma or neglect, patients who have internalized part-objects that are particularly responsive to aggression also appear to lack supportive introjects which could normally serve to counterbalance innate aggressivity. The resultant superego malformations are overly harsh and charged with self-destructive intent. Self-attack may range from guilt-ridden self-loathing to physical assault on the body. Suicide is viewed as restorative and survivable. Treatment interventions include superego analysis as a way to identify internalized self-attack, disentangle organized self-destructive elements and support counter-balancing forces for affect tolerance.     

Adaptation to context

Argument theorists often stress the idea of adaptation to context as an alternative to seeing argument as linked propositions. But adaptation is not a clear idea. It is in fact a complicated puzzle. Though many aspects of this puzzle are obscure, one clear conclusion is that the question-answer pair is not a good way to conceptualize adaptation to situation.     

Studies on sensory feedback: III the effects of display gain on tracking performance

Instrumentation is described which permits study of the effects of different forms of visual feedback display on the patterns of fine movement obtained from the extended human index finger when the subject is attempting to keep his finger at a fixed point in space. The task is a compensatory tracking task in which the only source of input to the system is the subject's own finger movement. The effects of increasing the gain (or amplification) of a proportional error signal on the pattern of finger movement was studied. Gains of 1, 2, 4, 10, 20 and 40 were studied with a group of 24 subjects. Increasing the gain of a proportional error signal resulted in a marked improvement in the ability of subjects to maintain their extended finger at a fixed point in space. As the gain of the error signal was increased, the subject's high-amplitude, low frequency errors were reduced, and there was a progressive appearance of high-frequency activity of low-amplitude, more accurately centred about the reference position in space. A total off-target area measure (integrated absolute error) showed marked decrease in scores as the amplification of the error signal was increased from 1 through 10. Beyond this gain there was no appreciable additional improvement in motor control, however no degradation of control was noted to characterize the group performance.

Exploratory studies were undertaken to permit comparison of the effects of increasing the gain of a proportional visual display with the effects of increasing the gain of non-proportional visual and auditory displays. An increase in the dominant-energy frequency was noted as the error signal gain was increased, independent of whether a proportional visual, or non-proportional visual or auditory display was used. This observation suggests that common mechanisms mediate the processing of the gain parameters of feedback displays, in some measure independent of the display form or the sensory modality used for presentation.     

Adaptation to field displacement during head movement unrelated to the constancy of visual direction

Adaptation to vertical field displacements dependent on head turning about a vertical axis was demonstrated under two conditions, rapid training with 100 head movements and 1-h-long training with continuous head turning. The effect of rapid training was measured with the slant estimation method. Adaptation after the longer training was ascertained by comparing the uncertainty ranges for apparent target immobility before and after the adaptation period. Adaptation to field displacements in directions parallel to the plane of the head rotation obtained under corresponding conditions was also measured and found to be somewhat greater than adaptation to vertical field displacements. The result of work by Wallach and Frey that adaptation to field displacement in the direction with the head rotation is greater than to displacement against it was corroborated. While the previous result had, been obtained with rapid adaptation and with the slant estimation method, we confirmed it with 1-h training and by measuring the uncertainty ranges before and after the adaptation period.     

Concurrent adaptation to opposite visual distortions: impairment and cue

The present study compared single and dual adaptation to visuomotor rotations in different cueing conditions. Participants adapted either to a constant rotation or to opposing rotations (dual adaptation) applied in an alternating order. In Experiment 1, visual and corresponding postural cues were provided to indicate different rotation directions. In Experiment 2, either a visual or a postural cue was available. In all cueing conditions, substantial dual adaptation was observed, although it was attenuated in comparison to single adaptation. Analysis of switching costs determined as the performance difference between the last trial before and the first trial after the change of rotation direction suggested substantial advantage of the visual cue compared to the postural cue, which was in line with previous findings demonstrating the dominance of visual sense in movement representation and control.     

The flash-lag effect during illusory chopstick rotation

In the 'flash-lag' effect, a static object that is briefly flashed next to a moving object appears to lag behind the moving object. A flash was put up next to an intersection that appeared to be moving clockwise along a circular path but was actually moving counterclockwise [the chopstick illusion; Anstis, 1990, in AI and the Eye Eds A Blake, T Troscianko (London: John Wiley) pp 105 117; 2003, in Levels of Perception Eds L Harris, M Jenkin (New York: Springer) pp 90 93]. As a result, the flash appeared displaced clockwise. This was appropriate to the physical, not the subjective, direction of rotation, and it suggests that the flash-lag illusion occurs early in the visual system, before motion signals are parsed into moving objects.     

Spatial distortions induced by multiple visual landmarks: how local distortions combine to produce complex distortion patterns

Visual landmarks introduce systematic distortions into spatial short-term memory for single target positions, the exact form of the distortion depending on the spatial layout of the landmarks. In two experiments, we investigated how the combined effect of two landmarks can be predicted from the effects of individual landmarks. Participants used a mouse cursor to reproduce the positions of briefly presented targets in the context of one, the other, or both landmarks. We found that distortions near a landmark are independent of whether another landmark is present, so that remembered space is partitioned into regions dominated by single landmarks. Interestingly, the display midpoint behaves like a "virtual landmark," with its own pattern of distortion. Results are inconsistent with current models of spatial memory distortions but suggest that attentional processes can lead to enhanced fidelity of salient regions in topographical neural networks while also introducing some spatial biases.