Alan Allport

Cue-based preparation and stimulus-based priming of tasks in task switching

In this study, we investigated the interaction of three different sources of task activation in precued task switching. We distinguished (1) intentional, cue-based task activation from two other, involuntary sources of activation: (2) persisting activation from the preceding task and (3) stimulus-based task activation elicited by the task stimulus itself. We assumed that cue-based task activation increases as a function of cue—stimulus interval (CSI) and that task activation from the preceding trial decays as a function of response—stimulus interval. Stimulus-based task activation is thought to be due to involuntary retrieval of stimulus-associated tasks. We manipulated stimulus-based task activation by mapping each of the stimuli consistently to only one or the other of the two tasks. After practice, we reversed this mapping in order to test the effects of item-specific stimulus—task association. The mapping reversal resulted in increased reaction times and increased task shift costs. These stimulus-based priming effects were markedly reduced with a long CSI, relative to a short CSI, suggesting that stimulus-based priming shows up in performance principally when competition between tasks is high and that cue-based task activation reduces task competition. In contrast, lengthening the response—cue interval (decay time) reduced shift costs but did not reduce the stimulus-based priming effect. The data are consistent with separable stimulus-related and response-related components of task activation. Further theoretical implications of these findings are discussed.

Interaction of Task Readiness and Automatic Retrieval in Task Switching: Negative Priming and Competitor Priming

When subjects switch between tasks, performance is slower after a task switch than after a task repetition, even when preparation time is long. We report two experiments that support the idea that a large part of these residual task shift costs can be due to stimulus-cued retrieval of previous task episodes. We demonstrate that there are two different factors at work: (1) facilitation of response to the current distractor stimulus, appropriate to the previously relevant, competing task (competitor priming), and (2) impaired processing of previously suppressed responses (negative priming). Negative priming was contingent on the size of the stimulus set, suggesting that distractor suppression comes into effect only if the distractors are highly activated. Importantly, both types of interference interacted with task readiness: Whereas in the nondominant task (picture naming), switch and nonswitch trials were equally affected, the dominant task (word reading) showed priming effects on switch trials only. Thus, the retrieval of previous processing episodes has a selective impact on situations in which task competition is high.

Levels of processing for visual stimuli in an "extinguished" field.

Volpe et al. (Nature 282, 722, 1979 [19]) described an experimental study of four patients with parietal tumours who were able to judge whether two simultaneous stimuli were identical or different, even when they were unable to name the stimulus contralateral to their brain injury. We report the case of another patient, E.M., in whom we have investigated this phenomenon further. E.M. had undergone a right temporal lobectomy to prevent recurrent seizures. She could correctly name photographs of objects presented in isolation to either the left or right visual field, at 150 msec exposure (although she was impaired for single objects on the left at 10 msec exposures). She was able to judge correctly whether two simultaneous objects on the left and right had the same or different names, even though she was often unable to name the object on the left. These judgements remained above chance when same-name pairs of stimuli showed the same object but seen from two different viewpoints, or even when they showed visually dissimilar exemplars of the same name category. This implies that the patient based her same-different judgements on categorical information about the pair of objects, even though she was often unable to name the contralateral object.

Semantic generalization of stimulus-task bindings

People find it difficult to switch between two tasks, even if they have time to prepare—the so-called residual task shift cost. We studied a switch of tasks from picture naming to word reading, using picture-word Stroop stimuli. Consistent with previous findings, we demonstrate that a large part of the observed task shift cost was due to priming from prior stimulus-response episodes, in which the current task stimulus was encountered in a competing task. We further show that this task-priming effect generalizes to semantically related stimuli, which opens the possibility that most or all of these residual shift costs reflect some sort of generalized proactive interference from previous stimulus-task episodes.

Independent reference frames in human spatial memory: Body-centered and environmentcentered coding in near and far space

Our apparently seamless experience of the spatial environment seems to be derived from information coded across a variety of spatial reference frameworks, each tied to the metric of a different sensory or motor system. A fundamental distinction is that between body-centered and environmentcentered reference frameworks. This study reports the first clear evidence of a behavioral dissociation between body-centered and environment-centered coding in human adults. Subjects, seated in a rotating chair with closed eyes, were required to point to remembered, auditorily presented target locations. The subjects were rotated between the presentation and recall of targets. Targets were held stationary with respect to either body-centered or environment-centered spatial coordinates. Prior to recall, subjects were required to point to a series of prelearned distractor locations, which also remained fixed with reference either to the subject’s body or to the stationary environment. Memory for the target locations was selectively impaired when distractor locations were specified within the same spatial reference frame as the target, regardless of whether target and distractor locations were near to or distant from the subjects. In contrast, distractor locations specified in a different reference frame from that of the target had either little or no effect on memory for target locations.

Coding of Far and Near Space in Neglect Patients

Far (extrapersonal) and near (peripersonal) spaces are behaviorally defined as the space outside arm-reaching distance and the space within arm-reaching distance. Animal and human studies have shown that this behavioral distinction corresponds in the brain to a composite neural architecture for space representation. In this paper we discuss how the activation of the neural correlates of far and near space can be modulated by the use of tools that change the effective spatial relationship between the agents body and the target object. When subjects reach for a far object with a tool, it is possible to show that far space is remapped as near. We shall also argue that space remapping may not occur when far space is reached by walking instead of using a tool.

Attention and lexical decomposition in chinese word recognition: Conjunctions of form and position guide selective attention

Abstract A series of experiments on skilled readers of Chinese demonstrated selective attention to different orthographic components within compound, single-character words, as a function of different reading tasks (pronunciation vs. meaning). Chinese orthography provides a number of contrasts with alphabetic writing systems. The majority (over 90%) of Chinese single-character words are compound, consisting of a lexical radical (LR) and one or more other constituents. which together form the “non-radical component” (NR). The NR component as a whole specifies the syllabic pronunciation of the whole word (except in the case of “irregular” words). In contrast, the LR specifies (an aspect of) its meaning. Thus in order to take advantage of these regularities in pronunciation tasks, the skilled reader should attend selectively to the NR component. To do so, however, the reader must first locate the LR, which can occur in practically any relative location (e.g. top, bottom, left, or right of the character), in ...

SHIFTING ATTENTION IN A RAPID VISUAL SEARCH PARADIGM

A method is introduced for studying shifts of attention in semantic space, testing 56 subjects in four experiments on a semantic monitoring task based on rapid, serial, visually presented (RSVP) word-sequences. Following a cue to shift attention, accuracy of semantic monitoring drops abruptly to a low level, then gradually recovers to reach preshift levels over successive stimuli in the RSVP sequence. Using this method, we compared two kinds of criterion-shifts, one requiring a set-reversal (‘reversal shifts’), the other involving a shift between orthogonally defined categories (‘orthogonal shifts’); no differences were found. We have also examined the difference in a shift between two different processing domains (semantic vs typographic) compared with a shift of criterion within the same processing domain. The results showed no differences for within- vs between-domain shifts. Finally, we studied the time-course of a semantic attention shift. Execution of a semantic shift did not follow an internally controlled time-course but was a direct function of the rate of stimulus presentation. No evidence was found for the operation of a ‘supervisory attentional system’ independent of external stimulus triggering.

Symbolically speaking: Communicating with blissymbols in aphasia

Abstract Two severely aphasic patients were taught logographic symbols as an alternative medium of communication. The question of interest was whether logographic symbol communication would provide a superior medium relative to their residual natural language abilities. The natural language abilities of each patient were investigated in three different transcoding tasks—oral reading, writing to dictation and aural-visual matching—and the cognitive processes available to each patient in these tasks were identified. The learning of logographic symbols mirrored each patients natural language performance and was consistent with the use of the spared cognitive processes identified as available for natural language processing. It was concluded that logographic symbols provided no communicative advantage to these patients compared with their processing of alphabetically written language.