A longitudinal study of "intrusion-based reasoning" and posttraumatic stress disorder after exposure to a train disaster

Previously, we found that chronic PTSD relates to “intrusion-based reasoning” (IR), i.e. the tendency to interpret distressing intrusions themselves as evidence that danger is impending, regardless of objective danger information (Engelhard et al., Behav. Res. Ther. 39 (2001) 1139). This study was intended to elucidate the causal status of this relation. Twenty-nine residents of a Belgian town witnessed a train crash and were assessed for IR and PTSD symptoms within 1 month and were re-assessed for PTSD at 3.5 months. Fourteen control residents did not witness the crash and were also tested for IR. The IR paradigm involved rating the danger of brief scenarios in which objective danger and presence of intrusions about the crash were systematically varied. The directly exposed residents showed greater danger ratings to scenarios in which intrusions were included than did the controls. IR was strongly related to both acute and chronic PTSD symptoms. It did not significantly predict chronic PTSD symptoms after controlling for acute symptoms, although the partial correlation (r=0.26, p=0.09) was in the expected direction. The data suggest that IR is involved in the onset and maintenance of PTSD symptoms, but more clarity about causality awaits future larger and experimental studies.     

Sense of body and sense of action both contribute to self-recognition

Recognizing oneself, easy as it appears to be, seems at least to require awareness of one's body and one's actions. To investigate the contribution of these factors to self-recognition, we presented normal subjects with an image of both their own and the experimenter's hand. The hands could make the same, a different or no movement and could be displayed in various orientations. Subjects had to tell whether the indicated hand was theirs or not. The results showed that a congruence between visual signals and signals indicating the position of the body is one component on which self-recognition is based. Recognition of one's actions is another component. Subjects had most difficulty in recognizing their hand when movements were absent. When the two hands made different movements, subjects relied exclusively on the movement cue and recognition was almost perfect. Our findings are in line with pathological alterations in the sense of body and the sense of action.     

How a group goal may reduce social matching in group performance: shifts in standards for determining a fair contribution of effort

The authors investigated whether the presence of a specific group goal would reduce social matching (i.e., matching one's own performance to the performance expected from others) by serving as an alternative standard. As predicted, when there was no specific goal, the participants matched their own performance to the performance expected from other group members. When there was a specific group goal, the women no longer engaged in social matching, although that effect did not emerge among the men. Instead, the women's mean personal performance was close to the performance level representing an equal share of the group goal. Moreover, the participants' perceptions of a fair contribution mediated the performance of the men and the women, both in the presence and in the absence of a goal.     

Postural orientation: age-related changes in variability and time-to-boundary

The relation between age-specific postural instability and the detection of stability boundaries was examined. Balance control was investigated under different visual conditions (eyes open/closed) and postural orientations (forward/backward lean) while standing on a force platform. Dependent variables included center of pressure variability and the time-to-contact of the center of pressure with the stability boundaries around the feet (i.e., time-to-boundary). While leaning maximally, older individuals (ages 55-69) showed increased center of pressure variability compared to no lean, while younger subjects (ages 24-38) showed a decrease. These significant differences were found only in anterior-posterior direction. No significant age-specific differences were found between eyes open and eyes closed conditions. Time-to-boundary analysis revealed reduced spatio-temporal stability margins in older individuals in both anterior-posterior and medio-lateral directions. Time-to-boundary variability, however, was not significantly different between the groups in both medio-lateral and anterior-posterior direction. These results show the importance of boundary relevant center of pressure measures in the study of postural control, especially concerning the lateral instability often observed in older adults.     

Global visual completion of quasi-regular shapes

The topic of amodal completion has often been investigated by using partly occluded shapes that are regular. In research that has typically been done with displays such as these regular shapes, it has been shown that global aspects of a shape can determine completion. To see how robust these global influences in the completion process are, we investigated quasi-regular shapes, ie shapes with a certain overall regularity but not based on metrical identities. First, in experiment 1 participants had to complete quasi-regular shapes in a drawing task. Then, in experiment 2 the primed-matching paradigm was used. Results from both experiments provided evidence for global completions. In experiment 3 we found that multiple global completions can be primed, which, as a control experiment showed, cannot be explained by some inability of the visual system to see the difference between the different completions. These data support the notion that global influences on visual occlusion are apparent even when the partly occluded stimulus is outside the domain of regular shapes. Implications for a global approach are provided.     

Action-based and vision-based selection of input: Two sources of control

In the first part of this paper we review evidence suggesting that there exists a mechanism that selects input on the basis of its similarity to the required action. This response-based input selection differs from the more established space- and object-based input selection in that it is not constrained by the structure of the input. Our evidence suggests that the two-choice Stroop effect is caused by this response-based selection mechanism. By contrast, it is known that the flanker effect is determined by the space- and object- based selection mechanisms. We explore whether the conflict resolution of the Stroop and flanker tasks is different as well by embedding these two tasks in a PRP (Psychological Refractory Period) paradigm. We show that the Stroop and the PRP effects are additive whereas the flanker and the PRP effects are underadditive, suggesting that the processes in charge of the conflict resolution in the Stroop and the flanker tasks are indeed different. We discuss possible reasons for this difference, and discuss possible ways in which the response-based mechanism can be implemented in information processing models.     

When timing the mind one should also mind the timing: biases in the measurement of voluntary actions

Trevena and Miller (2002, this issue) provide further evidence that readiness potentials occur in the brain prior to the time that participants claim to have initiated a voluntary movement, a contention originally forwarded by Libet, Gleason, Wright, and Pearl (1983). In their examination of this issue, though, aspects of their data lead them to question whether their measurement of the initiation of a voluntary movement was accurate. The current article addresses this concern by providing a direct analysis of biases in this task. This was done by asking participants to make subjective timing decisions regarding a stimulus that could be measured objectively. Our findings suggest that their timing task was indeed biased such that participants' tend to report events as happening approximately 70 ms later than they actually happened. Implications for the original Libet et al. claims are discussed.     

Physical,neural, and mental timing

The conclusions drawn by Benjamin Libet from his work with colleagues on the timing of somatosensorial conscious experiences has met with a lot of praise and criticism. In this issue we find three examples of the latter. Here I attempt to place the divide between the two opponent camps in a broader perspective by analyzing the question of the relation between physical timing, neural timing, and experiential (mental) timing. The nervous system does a sophisticated job of recombining and recoding messages from the sensorial surfaces and if these processes are slighted in a theory, it might become necessary to postulate weird operations, including subjective back-referral. Neuroscientifically inspired theories are of necessity still based on guesses, extrapolations, and philosophically dubious manners of speech. They often assume some neural correlate of consciousness (NCC) as a part of the nervous system that transforms neural activity in reportable experiences. The majority of neuroscientists appear to assume that the NCC can compare and bind activity patterns only if they arrive simultaneously at the NCC. This leads to a search for synchrony or to theories in terms of the compensation of differences in neural delays (latencies). This is the main dimension of the Libet discussion. Examples from vision research, such as "temporal-binding-by-synchrony" and the "flash-lag" effect, are then used to illustrate these reasoning patterns in more detail. Alternatively one could assume symbolic representations of time and space (symbolic "tags") that are not coded in their own dimension (not time in time and space in space). Unless such tags are multiplexed with the quality message (tickle, color, or motion), one gets a binding problem for tags. One of the hidden aspects of the discussion between Libet and opponents appears to be the following. Is the NCC smarter than the rest of the nervous system, so that it can solve the problems of local sign (e.g., "where is the event"?) and timing (e.g., "when did it occur?" and "how long did it last?") on its own, or are these pieces of information coded symbolically early on in the system? A supersmart NCC appears to be the assumption of Libet's camp (which includes Descartes, but also mystics). The wish to distribute the smartness evenly across all stages of processing in the nervous system (smart recodings) appears to motivate the opponents. I argue that there are reasons to side with the latter group.