Characteristics of covert and overt visual orienting: Evidence from attentional and oculomotor capture

Five visual search experiments found oculomotor and attentional capture consistent with predictions of contingent orienting, contrary to claims that oculomotor capture is purely stimulus driven. Separate saccade and attend-only conditions contained a color target appearing either singly, with an onset or color distractor, or both. In singleton mode, onsets produced oculomotor and attentional capture. In feature mode, capture was absent or greatly reduced, providing evidence for top-down modulation of both types of capture. Although attentional capture by color abstractors was present throughout, oculomotor capture by color occurred only when accompanied by transient change, providing evidence for a dissociation between oculomotor and attentional capture. Oculomotor and attentional capture appear to be mediated by top-down attentional control settings, but transient change may be necessary for oculomotor capture. ((c) 2003 APA, all rights reserved)     

Involuntary attentional capture by abrupt onsets.

The extent to which brief abrupt-onset visual stimuli involuntarily capture spatial attention was examined in five experiments. The paradigm used was intended to maximize the opportunity and incentive for subjects to ignore abrupt-onset distractor stimuli in nontarget locations. Subjects made a speeded two-choice response to a target letter appearing in one of four boxes. An abrupt-onset visual stimulus, easily discriminable from the target, was flashed briefly prior to the presentation of a target. In separate blocks, the flash stimulus marked the box in which the target would subsequently appear (SAME), a different box (DIFF), fixation (CENTER), or all four boxes (ALL). Prior to each block, subjects were informed of the flash-target relationship. In all five experiments, response time was elevated in the DIFF, CENTER, and ALL conditions. The interference effect was larger for the DIFF condition and persisted for longer flash-target SOAs. These results suggest that, under appropriate conditions, spatial attention can be involuntarily drawn to abrupt-onset events despite the intention of subjects' to ignore them.     

Involuntary covert orienting is contingent on attentional control settings.

Four experiments tested a new hypothesis that involuntary attention shifts are contingent on the relationship between the properties of the eliciting event and the properties required for task performance. In a variant of the spatial cuing paradigm, the relation between cue property and the property useful in locating the target was systematically manipulated. In Experiment 1, invalid abrupt-onset precues produced costs for targets characterized by an abrupt onset but not for targets characterized by a discontinuity in color. In Experiment 2, invalid color precues produced greater costs for color targets than for abrupt-onset targets. Experiment 3 provided converging evidence for this pattern. Experiment 4 investigated the boundary conditions and time course for attention shifts elicited by color discontinuities. The results of these experiments suggest that attention capture is contingent on attentional control settings induced by task demands.     

A dual-task investigation of automaticity in visual word processing

An analysis of activation models of visual word processing suggests that frequency-sensitive forms of lexical processing should proceed normally while unattended. This hypothesis was tested by having participants perform a speeded pitch discrimination task followed by lexical decisions or word naming. As the stimulus onset asynchrony between the tasks was reduced, lexical-decision and naming latencies increased dramatically. Word-frequency effects were additive with the increase, indicating that frequency-sensitive processing was subject to postponement while attention was devoted to the other task. Either (a) the same neural hardware shares responsibility for lexical processing and central stages of choice reaction time task processing and cannot perform both computations simultaneously, or (b) lexical processing is blocked in order to optimize performance on the pitch discrimination task. Either way, word processing is not as automatic as activation models suggest.     

Human variable ratio performance

The generality of the mathematical principles of reinforcement (MPR) was tested with humans. In Experiment 1A, participants’ mouse clicks were reinforced according to a series of variable-ratio (VR) values. In one condition, a situated task was used (searching for treasure on a map presented via a PC monitor); in the other, abstract version of the task, participants clicked on a white screen. Under the “map” condition, response rates increased with increasing ratio value before decreasing with further ratio increases; under the “no-map” condition, response rates decreased linearly from a peak at the smallest ratio. In Experiment 1B, the pattern of responding was confirmed under the “map” conditions using a different set of ratio values. In Experiment 2, reinforcer magnitude was manipulated using the “map” context. Response rates did not differ significantly when large rather than small cash reinforcers were delivered according to a VR 30 but were significantly higher when large rather than small cash reinforcers were delivered according to a VR 120. Together these experiments offer qualified support for extending MPR to the behavior of human participants.     

The spider does not always win the fight for attention: Disengagement from threat is modulated by goal set

Stimulus-driven preferential attention to threat can be modulated by goal-driven attention. However, it remains unclear how this goal-driven modulation affects specific attentional components implied in threat interference. We hypothesise that goal-driven modulation most strongly impacts delayed disengagement from threat. A spatial cueing task was used that disentangles delayed disengagement from attentional capture by tightly manipulating the locus of attention at the time of target onset. Different top-down goals were induced by instructing participants to identify bird/fish targets (Experiment 1) or spider/cat targets (Experiment 2) among animal non-targets. Delayed disengagement from a non-target spider was observed only when the spider was part of the target set, not when it was task-irrelevant. This corroborates evidence that threat stimuli do not necessarily override goal-driven attentional control and that extended processing of threatening distractors is not obligatory.     

What determines saccade timing in sequences of coordinated eye and hand movements?

In action sequences, the eyes are generally fixated ahead of the stimulus being responded to, overlapping the processing of adjacent stimuli. What determines when the saccade to the next item is initiated and what processing is overlapped remain controversial. Saccade initiation has been linked directly to the difficulty of the underlying processing on the fixated item or to a rhythmic timer whose period is adjusted to the difficulty of the context. We compared the effects of item and context difficulty when subjects made speeded choice responses to five visual stimuli arrayed linearly. Item difficulty was manipulated by varying stimulus-response compatibility, creating contexts (sequences) whose average item difficulty differed. Results showed that saccade initiation time reflected item difficulty, not context difficulty, with evidence that saccade initiation was delayed until central processing was completed. We discuss the implications of these results for the control of saccades and processing in action sequences.