J. Toby Mordkoff

Dividing attention between color and shape: Evidence of coactivation

When attention is divided between spatially distinct objects, the time-to detect a target decreases when two or more targets are present. This redundancy gain can be accounted for by an interactive race model (Mordkoff & Yantis, 1991) in which separate decisions are made about each object, but environmental contingencies among the objects can influence decision times. In the present study, we examined whether the model also accounts for performance when attention must be divided between stimulus attributes other than spatial location. Subjects made target-present responses when displays included a prespecified color, a prespecified letter, or both target features. The data violated the predictions of all separate-activations models, including the interactive race model. Two control experiments ruled out an alternative account based on task complexity. We conclude that coactivation occurs when target attributes from two separable dimensions are simultaneously present, but not when target attributes come from the same dimension. A modular hybrid of race and coactivation models is tentatively proposed.

Detecting the onset of the lateralized readiness potential: A comparison of available methods and procedures.

Studies that measure the onset of the lateralized readiness potential (LRP) could well provide researchers with important new data concerning the information-processing locus of experimental effects of interest. However, detecting the onset of the LRP has proved difficult. The present study used computer simulations involving both human and artificial data, and both stimulus- and response-locked effects, to compare a wide variety of techniques for detecting and estimating differences in the onset latency of the LRP. Across the two sets of simulations, different techniques were found to be the most accurate and reliable for the analysis of stimulus- and response-locked data. On the basis of these results, it is recommended that regression-based methods be used to analyze most LRP data.

Anticipatory postural adjustments under simple and choice reaction time conditions

Two different schemes of the central organization of anticipatory postural adjustments (APAs) have been proposed: one of them assumes that postural and focal components of an action are results of a single control process (single-process hypothesis) while the other one suggests that these two components result from two relatively independent control processes (dual-process hypothesis). To distinguish between the two hypotheses, we investigated changes in the relative timing of the postural and focal components under self-paced, simple reaction time (SRT) and choice reaction conditions (CRT). Standing subjects performed one of four small arm movements resulting in a standard postural perturbation (dropping a load). APAs were quantified as changes in the background muscle activity as well as shifts of the center of pressure. APAs occurred at a larger delay prior to the focal movement under the self-paced condition than under the SRT condition. Under the CRT condition, actual RTs were longer than under SRT, but APAs were more similar to those under self-paced conditions. A negative correlation between the reaction time and APA onset was found. The findings demonstrate that the focal and APA components of an action can be decoupled, thus supporting the dual-process hypothesis. Changes in APAs with action suggest a possibility of modifications of a function that transforms two parallel control signals into a single command to the focal and postural muscles, based on postural requirements and behavioral constraints.

Detecting conjunctions of color and form in parallel

Certain theories of visual attention assume that at least one processing stage must be serial when the target of search is defined as the conjunction of two or more separable features. To explain why conjunction-search response times do not always form linearly increasing functions of display size, recent versions of this general model have posited the existence of an early parallel process that guides the serial stage toward display elements that are likely targets. Other models have relaxed the seriality assumption, allowing for a limited number of parallel decisions. In the three experiments reported here, a redundant-target detection task was used with conjunctively defined targets and display sizes of two (Experiment 1), one or two (Experiment 2), and six (Experiment 3). In all three experiments, strong evidence for parallel processing was observed. The implications for models of elementary visual processes are discussed.

Observation: Three reasons to avoid having half of the trials be congruent in a four-alternative forced-choice experiment on sequential modulation

Sequential modulation is the finding that the sizes of several selective-attention phenomena—namely, the Simon, flanker, and Stroop effects—are larger following congruent trials than following incongruent trials. In order to rule out relatively uninteresting explanations of sequential modulation that are based on a variety of stimulus- and response-repetition confounds, a four-alternative forced choice task must be used, such that all trials with any kind of repetition can be omitted from the analysis. When a four-alternative task is used, the question arises as to whether to have the proportions of congruent and incongruent trials be set by chance (and, therefore, be 25% congruent and 75% incongruent) or to raise the proportion of congruent trials to 50%, so that it matches the proportion of incongruent trials. In this observation, it is argued that raising the proportion of congruent trials to 50% should not be done. For theoretical, practical, and empirical reasons, having half of the trials be congruent in a four-alternative task aimed at providing unambiguous evidence of sequential modulation should be avoided.

The path of least persistence: object status mediates visual updating.

On what basis does the visual system use recently sampled information to update existing representations of the world? One possibility is that representations are updated through an image-based point-for-point replacement process. An alternative possibility is that representations are updated on the basis of perceptually organized units that reflect objects in the scene rather than locations within the visual field. We report a new effect involving a modulation of visible persistence that seems to support this alternative possibility. In particular, we show that a moving stimulus leaves a visible trace of itself when it undergoes an abrupt and transient change in size but does not do so when the stimulus does not change. Further we show that this effect is substantially reduced when a scene-based reason for the abrupt change in size is provided (i.e., the object is shown to be passing behind an occluding surface that has a very small window in it through which the stimulus shows briefly). We suggest that the visible persistence in the face of change reflects a disruption of the normal updating process which is object-based and disrupted because of the discontinuity of the object. Providing a scene-based reason for the discontinuity allows the object representation to be maintained, and thus does not result in a disruption of the updating process.

Redundancy gains and coactivation with two different, targets: The problem of target preferences and the effects of display frequency

When a visual display contains two targets, both of which require the same response, reaction times (RTs) are faster than when only one target appears. This effect has previously been obtained regardless of whether the redundant targets are the same or different in shape, and in at least one set of two-target experiments, the redundancy gains have been larger for different targets (Grice & Reed, 1992). Experiments with two different targets have also revealed violations of the race-model inequality, suggesting that redundant targets coactivate the response (Miller, 1982). The present paper reexamines both of these findings, because both appear to be inconsistent with the interactive race model (Mordkoff & Yantis, 1991). Experiment 1 shows that the race-model inequality is not violated when the experimental design is free of biased contingencies; Experiment 1 also provides evidence that target preferences may artifactually produce the RT advantage fordifferent- oversame-target trials. Experiment 2, however, shows that the race-model inequality is violated when the frequencies of single- and redundant-target displays are equated (without introducing any biased contingencies), implying that the interactive race model cannot account for the results of experiments involving more than one type of target. Alternative loci for coactivation are briefly discussed.

Contingent Capture at a very short SOA: Evidence against Rapid Disengagement

Salient cues that do not contain the targets defining attribute do not cause a spatial cueing effect in response time when presented 150 ms before the target. This can be explained by Contingent Capture (under which incongruent cues do not capture attention) and also by Rapid Disengagement (under which salient but incongruent cues cause only brief capture). To discriminate between these models, a very short SOA of 35ms was employed in the standard paradigm. As is true for longer SOAs, there was no evidence of attentional capture by salient but incongruent cues. Thus, the evidence to date favours Contingent Capture over Rapid Disengagement.

Gastric myoelectrical and autonomic cardiac reactivity to laboratory stressors

We evaluated the effects of two laboratory stressors (speech preparation and isometric handgrip) on gastric myoelectrical and autonomic cardiac activity, and the extent to which autonomic responses to these stressors and somatization predict reports of motion sickness during exposure to a rotating optokinetic drum. Both stressors prompted a decrease in preejection period (PEP) and respiratory sinus arrhythmia (RSA), and an increase in a dysrhythmic pattern of gastric myoelectrical activity, termed gastric tachyarrhythmia. Stressor-induced decreases in RSA and higher somatization scores predicted increased reports of motion sickness during drum rotation. These results demonstrate that laboratory stressors concurrently affect gastric myoelectrical activity and autonomic control of the heart, and that stressor-induced decreases in RSA and higher levels of somatization predict motion sickness susceptibility.

Feature integration without visual attention: evidence from the correlated flankers task.

It is widely assumed that the separable features of visual objects, such as their colors and shapes, require attention to be integrated. However, the evidence in favor of this claim comes from experiments in which the colors and shapes of objects would have to be integrated and then also subjected to an arbitrary, instruction-based, stimulus-response (S-R) translation in order to have an observable effect. This raises the possibility that attention is not required for feature integration, per se, but is only required when color-shape conjunctions must undergo an arbitrary S-R translation. The present study conducted a more specific test and found strong evidence in favor of feature integration in the absence of attention. The implications of these results are discussed.