Glyn W. Humphreys

Cascade processes in picture identification

Abstract The naming of pictures is typically thought to require sequential access to stored structural knowledge about objects, to semantic knowledge, and to a stored phonological description. Access to these different types of knowledge may constitute discrete processing stages; alternatively, it may be that information is transmitted continuously (in cascade) from one type of description to the next. The discrete stage and the cascade accounts make different predictions about the effects of structural and semantic similarity between objects on picture naming. The discrete stage account maintains that the effects of structural similarity should be confined to the process of accessing an objects structural description, and the effects of semantic similarity should be confined to the process of accessing semantic knowledge. The cascade account predicts that the effect of both variables may be passed on to subsequent processing stages. We present evidence drawn from both normal observers and from a patient...

Competitive brain activity in visual attention.

Visual attention can be considered from the perspective of distributed brain activity engendered by visual input. We propose that visual objects compete for representation in multiple brain systems, sensory and motor, cortical and subcortical. Competition is integrated, however, such that multiple systems converge, working on the different properties and action implications of a selected object. Top-down priming biases competition towards objects relevant to current behaviour. Recent single-unit studies have shown widespread suppression of ignored-object representations in extrastriate cortex, and patterns of spatial and nonspatial priming by task relevance. Human and monkey lesion studies have demonstrated the strong integration tendency of different spatial and nonspatial systems, also revealed in recent studies of normal behaviour. In many cases, no unitary brain system may be responsible for unitary cognitive events such as attention. Such events may emerge as distinct systems converge to work on common cognitive problems.

Visual object processing in optic aphasia: A case of semantic access agnosia

Abstract A single case study is reported of a patient with a naming disorder specific to visually presented stimuli. The patient was often able to gesture correctly to objects he could not name, and he showed intact access to structural knowledge of objects. Further examination revealed an impairment in accessing semantic knowledge about objects, which was most marked when the patient had to discriminate between objects which were visually as well as semantically similar. It is suggested that the patients naming deficit is due to an impairment in accessing semantic information from vision, following intact access to stored structural knowledge. Correct gestures may be contingent on access to the system specifying structural knowledge. The data are interpreted in terms of a model of visual object identification in which access to semantic information, from the system specifying structural knowledge, is held to operate in cascade.

Segregating semantic from phonological processes during reading

A number of previous functional neuroimaging studies have linked activation of the left inferior frontal gyms with semantic processing, yet damage to the frontal lobes does not critically impair semantic knowledge. This study distinguishes between semantic knowledge and the strategic processes required to make verbal decisions. Using positron emission tomography (PET), we identify the neural correlates of semantic knowledge by contrasting semantic decision on visually presented words to phonological decision on the same words. Both tasks involve identical stimuli and a verbal decision on central lingual codes (semantics and phonology), but the explicit task demands directed attention either to meaning or to the segmentation of phonology. Relative to the phonological task, the semantic task was associated with activations in left extrasylvian temporal cortex with the highest activity in the left temporal pole and a posterior region of the left middle temporal cortex (BA 39) close to the angular gyrus. The reverse contrast showed increased activity in both supramarginal gyri, the left precentral sulcus, and the cuneus with a trend toward enhanced activation in the inferior frontal cortex. These results fit well with neuropsychological evidence, associating semantic knowledge with the extrasylvian left temporal cortex and the segmentation of phonology with the perisylvian cortex.

The effect of cueing on unilateral neglect

Heilman and Valenstein recently failed to reduce unilateral neglect, assessed by a line bisection task, by cueing patients to attend to their neglected field. Cueing was accomplished by placing letters at both ends of the line and instructing subjects to identify either the right or left hand letter prior to bisecting the line. The present experiments tested whether this failure to improve neglect occurred because patients were presented with competing stimuli in their neglected and non-neglected fields. Five patients with unilateral neglect and hemianopia took part in two experiments. The results showed a marked decrease in neglect when subjects were cued and forced to report stimuli in their neglected field. This occurred even when there was a competing stimulus in the non-neglected field. However, in the absence of forced report requirements, patients oriented to stimuli in the non-neglected field. The results are interpreted as a failure of patients with unilateral neglect to orient automatically to the side of space contralateral to the lesion, though processes governing the conscious orienting of attention are intact.

Early, Involuntary Top-Down Guidance of Attention From Working Memory

Four experiments explored the interrelations between working memory, attention, and eye movements. Observers had to identify a tilted line amongst vertical distractors. Each line was surrounded by a colored shape that could be precued by a matching item held in memory. Relative to a neutral baseline, in which no shapes matched the memory item, search was more efficient when the memory cue matched the shape containing the target, and it was less efficient when the cued stimulus contained a distractor. Cuing affected the shortest reaction times and the first saccade in search. The effect occurred even when the memory cue was always invalid but not when the cue did not have to be held in memory. There was also no evidence for priming effects between consecutive trials. The results suggest that there can be early, involuntary top-down directing of attention to a stimulus matching the contents of working memory.

Beyond the search surface: visual search and attentional engagement.

Treisman (1991) described a series of visual search studies testing feature integration theory against an alternative (Duncan & Humphreys, 1989) in which feature and conjunction search are basically similar. Here the latter account is noted to have 2 distinct levels: (a) a summary of search findings in terms of stimulus similarities, and (b) a theory of how visual attention is brought to bear on relevant objects. Working at the 1st level, Treisman found that even when similarities were calibrated and controlled, conjunction search was much harder than feature search. The theory, however, can only really be tested at the 2nd level, because the 1st is an approximation. An account of the findings is developed at the 2nd level, based on the 2 processes of input-template matching and spreading suppression. New data show that, when both of these factors are controlled, feature and conjunction search are equally difficult. Possibilities for unification of the alternative views are considered.

The Use of Abstract Graphemic Information in Lexical Access

Three experiments investigated the nature of the information required for the lexical access of visual words. A four-field masking procedure was used, in which the presentation of consecutive prime and target letter strings was preceded and followed by presentations of a pattern mask. This procedure prevented subjects from identifying, and thus intentionally using, prime information. Experiment I extablished the existence of a semantic priming effect on target identification, demonstrating the lexical access of primes under these conditions. It also showed a word repetition effect independent of letter case. Experiment II tested whether this repetition effect was due to the activation of graphemic or phonemic information. The graphemic and phonemic similarity of primes and targets was varied. No evidence for phonemic priming was found, although a graphemic priming effect, independent of the physical similarity of the stimuli, was obtained. Finally Experiment III demonstrated that, irrespective of whether the prime was a word or a nonword, graphemic priming was equally effective. In both Experiments II and III, however, the word repetition effect was stronger than the graphemic priming effect. It is argued that facilitation from graphemic priming was due to the prime activating a target representation coded for abstract (non-visual) graphemic features, such as letter identities. The extra facilitation from same identity priming was attributed to semantic as well as graphemic activation. The implications of these results for models of word recognition are discussed.

The effects of surface detail on object categorization and naming.

Three experiments are reported examining the effects of surface colour and brightness/texture gradients (photographic detail) on object classification and naming. Objects were drawn from classes with either structurally similar or structurally dissimilar exemplars. In Experiment 1a, object naming was facilitated by both congruent surface colour and photographic detail, with the effects of these two variables combining under-additively. In addition incongruent colour disrupted naming accuracy. These effects tended to be larger on objects from structurally similar classes than on objects from structurally dissimilar classes. Experiment 1b examined superordinate classification. There were again advantages due to congruent colour and photographic detail on responses to objects from both structurally similar and structurally dissimilar classes. Incongruent colour disrupted classification accuracy on structurally distinct but not structurally similar items. For structurally similar items, the advantages of congruent surface attributes on classification were smaller than on naming, but this was not the case for structurally dissimilar items. Experiment 2 examined subordinate classification of structurally similar objects. Now effects of congruent and incongruent colour, but not of photographic detail, were found. Experiment 3 showed that congruent and incongruent colour effects occur only when the colours occupy the internal surfaces of objects. The results suggest that surface details can affect object recognition and naming, depending upon: (1) the degree to which objects must be differentiated for a correct response to be made, and (2) the nature of the rate-limiting process determining performance.

Systematic analysis of deficits in visual attention.

A variety of impairments in visual attention can follow damage to the brain. The authors develop systematic methods for analyzing such impairments in terms of C. Bundesens (1990) Theory of Visual Attention and apply these in a group of 9 patients with parietal lobe lesions and variable spatial neglect. In whole report, patients report letters from brief, vertical arrays in left or right visual field. The results show substantial, largely bilateral impairments in processing capacity, implying a major nonlateralized aspect to neglect. In partial report, arrays contain 1 or 2 letters in red and/or green. The task is to report only those letters in a specified target color. In addition to the expected bias against left-sided letters, patients show striking, bilateral preservation of top-down control, or attentional priority for targets. The results show how differentiation of attentional impairments can be informed by a theory of normal function.