Biomedical subjects

G W Humphreys

Publications and source records attributed to G W Humphreys.

At least 19 recordsLinked to original sources

PubMed2006

Ideational action impairments in Alzheimer's disease.

We report data from a group of patients with mild Alzheimer's disease on a range of tasks requiring either stored semantic knowledge about objects (e.g., naming object use) or the execution of action to objects (e.g., miming and using objects). We found that the patients were impaired at miming in response to objects, even when they could describe the object's function. On the other hand, copying gestures was not impaired relative to naming gestures, indicating that an ideomotor deficit in action execution, per se, was unlikely to explain the impairments in object use. We suggest instead that the patients had an impairment in stored motor programmes for action, over and above their deficits in semantic knowledge. Despite this, the patients were better at using than at miming to objects, consistent with the view that proprioceptive input (when using objects) can directly constrain selection of the appropriate motor programme for action.

Aged↗

PubMed2005

Case mixing and the right parietal cortex: evidence from rTMS.

We investigated the necessary role of the right parietal lobe in visual word recognition using transcranial magnetic stimulation (TMS). TMS was applied to the right posterior parietal lobe and to a control area as participants read aloud words presented either in lower case or in mIxEd-cAsE. The words were presented either with unlimited duration and high contrast (Experiment 1), or with brief presentation and low-contrast (Experiments 2 and 3). In all three experiments, TMS over the parietal area disrupted reading, and in Experiments 2 and 3 this effect was most pronounced for mIxEd-cAsE words. This suggests that the right parietal lobe mediates the recognition of words in unfamiliar formats.

Adult↗

PubMed2005

Don't think twice, it's all right--contralesional dependency for bimanual prehension movements.

In bimanual coordination when moving the hands to two separate objects, subjects tend to initiate and terminate the movements together, even when the targets are at different distances or are of a different size. Additionally, each hand tends to scale its grasp independently to the object to be grasped. Here, we report the performance of a patient, who had previously shown signs of motor neglect, on two experiments investigating coupling and independence in bimanual coordination. The patient showed relatively normal bimanual behaviour for the transport phase of prehension when objects were placed at different distances (Experiment 1), but abnormal behaviour for the grasp component when objects were of different sizes (Experiment 2). Moreover, the contralesional limb demonstrated a dependency of grasp that was related to the object grasped by the ipsilesional limb. We discuss the possible underlying mechanisms of this dependency in relation to competitive motor programming and attentional bias. The results also reinforce the view that the transport and grasp components of prehension are distinct processes.

Aged↗

PubMed2005

The effect of target foreknowledge on visual search for categorically separable orientation targets.

This research note assesses the role of target foreknowledge in visual search for categorically defined orientation targets, as first described by Wolfe et al. [Wolfe, J. M., Friedman-Hill, S. R., Stewart, M. I., & O'Connell, K. M. (1992). The role of categorisation in visual search for orientation. Journal of Experimental Psychology: Human Perception and Performance, 18, 34-49]. We compared search with known versus unknown (respond to the odd item) targets. An RT advantage for categorical search only emerged with known targets. The evidence points to an important role for top-down processes in search for categorically--defined orientation targets.

Adolescent↗

PubMed2004

Knowing your nose better than your thumb: measures of over-grasp reveal that face-parts are special for grasping.

Typically, when a grasping response is made, the hand opens wider than the target object. We show that this "over-grasp" response is reduced when we reach to parts of our own face, relative to when we reach to other body parts or to neutral objects. This is not due to reaching to different parts of body space, as over-grasp responses are indifferent to whether or not other body parts or neutral objects are placed close to the face. It is also not due to differences in perceptual knowledge of the size of the target object. We conclude instead that the familiarity of face parts influences the grasping response directly. Subsequent experiments demonstrate that the movement representation determining any effect is not based on a torso-centred frame, and not abstracted from the specific hand used for grasping. We discuss the implications of the results for understanding and measuring motor representations for familiar actions.

Adolescent↗

PubMed2003

Separating distractor rejection and target detection in posterior parietal cortex--an event-related fMRI study of visual marking.

Successful survival in a competitive world requires the employment of efficient procedures for selecting new in preference to old information. Recent behavioral studies have shown that efficient selection is dependent not only on properties of new stimuli but also on an intentional bias that we can introduce against old stimuli. Event-related analysis of functional magnetic resonance imaging data from a task involving visual search across time as well as space indicates that the superior parietal lobule is specifically involved in processes leading to the efficient segmentation of old from new items, whereas the temporoparietal junction area and the ascending limb of the right intraparietal sulcus are involved in the detection of salient new items and in response preparation. The study provides evidence for the functional segregration of brain regions within the posterior parietal lobe.

Adult↗

PubMed2002

Brain activity related to integrative processes in visual object recognition: bottom-up integration and the modulatory influence of stored knowledge.

We report evidence from a PET activation study that the inferior occipital gyri (likely to include area V2) and the posterior parts of the fusiform and inferior temporal gyri are involved in the integration of visual elements into perceptual wholes (single objects). Of these areas, the fusiform and inferior temporal gyri were more activated by tasks with recognizable stimuli than by tasks with unrecognizable stimuli. We propose that the posterior parts of the fusiform and inferior temporal gyri, compared with the inferior occipital gyri, are involved in higher level integration, due to the involvement of re-entrant activation from stored structural knowledge. Evidence in favor of this interpretation comes from the additional finding that activation of the anterior part of the left fusiform gyrus and a more anterior part of the right inferior temporal gyrus, areas previously associated with access to stored structural knowledge, was found with recognizable stimuli, but not with unrecognizable stimuli. This latter finding also indicates: (i) that subjects may not refrain from (automatically) identifying objects even if they only have to attend to the objects' global shape, and (ii) that perceptual and memorial processes can be dissociated on both functional and anatomical grounds. No evidence was obtained for the involvement of the parietal lobes in the integration of single objects.

Adult↗

PubMed2001

Facilitation of visual search at new positions: a behavioral and ERP study of new object capture.

Visual search was studied in two consecutive displays. Display 1 items changed identity whilst retaining their positions when the additional items appeared in Display 2. In the New condition, the target appeared at one of the new positions, whereas in the Old condition, the target appeared at one of the old positions. Responses were faster and accuracy increased in the New condition. Event-related brain potentials revealed an Old-New difference 400 ms after Display 2 onset for the smaller set size, suggesting that subjects had a holistic impression that the target was absent at a new position. A posteriorly distributed processing difference between both conditions was manifest at around 1200 ms, suggesting a bias for search at new positions.

Adult↗

PubMed2001

The effects of case mixing on word recognition: evidence from a PET study.

The early stages of visual word recognition were investigated by scanning participants using PET as they took part in implicit and explicit reading tasks with visually disrupted stimuli. CaSe MiXiNg has been shown in behavioral studies to increase reaction times (RTs) in naming and other word recognition tasks. In this study, we found that during both an implicit (feature detection) task and an explicit word-naming task, mixed-case words compared to same-case words produced increased activation in an area of the right parietal cortex previously associated with visual attention. No effect of case was found in this area for pseudowords or consonant strings. Further, lowering the contrast of the stimuli slowed RTs as much as case mixing, but did not lead to the same increase in right parietal activation. No significant effect of case mixing was observed in left-hemisphere language areas. The results suggest that reading mixed-case words requires increased attentional processing. However, later word recognition processes may be relatively unaffected by the disruption in presentation.

Attention↗

PubMed2001

Cognitive neuropsychology and functional brain imaging: implications for functional and anatomical models of cognition.

We discuss the relations between functional imaging and cognitive neuropsychological research. We begin by elaborating on some of the problems of traditional neuropsychological research, which attempted to provide accounts of cognitive performance at a neural as well as at a functional level of description. The difficulties in making neural-level arguments from neuropsychological data include: problems of associated deficits, problems due to interactive effects between brain regions, problems with analyses based on behavioural syndromes, problems due to the influence of compensatory strategies, and problems in separating damaged from disconnected representations. We discuss how cognitive neuropsychology by-passed many of these problems by emphasising functional rather than neural-level theories, though problems with inferences at the neural-level remain. We then consider the contribution that functional imaging can make to cognitive neuropsychology. Using evidence drawn from studies of language, object recognition and visual attention, we argue that functional imaging complements cognitive neuropsychology by: (i) not being reliant on accidents of nature and by enabling effects of lesions on 'distant' neural areas to be measured, (ii) revealing the brain systems necessary and sufficient for a given task, (iii) providing tests of neural-level models of cognition, and by (iv) providing novel evidence on the mechanisms of functional recovery in patients. In addition to this, imaging studies can contribute directly to functional-level theories, by providing converging evidence on the neural locus of cognition--knowing 'where' can allow new inferences about 'how' a given task is performed.

Brain↗

PubMed2001

An experimental analysis of unintentional lower limb action.

We provide an experimental analysis of the factors that determine unintentional lower limb action in a patient with cortico-basal degeneration. We show a dissociation between lower limb movements in a novel task and the ability to remember task instructions. In addition, there were effects of the pre-potency of the learned response to the stimulus, with different effects emerging with familiar and unfamiliar stimuli.

Basal Ganglia Diseases↗

PubMed2001

Detection by action: neuropsychological evidence for action-defined templates in search.

How do we detect a target in a cluttered environment? Here we present neuropsychological evidence that detection can be based on the action afforded by a target. A patient showing symptoms of unilateral neglect following damage to the right fronto-temporal-parietal region was slow and sometimes unable to find targets when they were defined by their name or even by a salient visual property (such as their color). In contrast, he was relatively efficient at finding a target defined by the action it afforded. Two other patients with neglect showed an opposite pattern; they were better at finding a target defined by its name. The data suggest that affordances can be effective even when a brain lesion limits the use of other properties in search tasks. The findings give evidence for a direct pragmatic route from vision to action in the brain.

Agnosia↗

PubMed2001

Spatially parallel processing of within-dimension conjunctions.

Within-dimension conjunction search for red-green targets amongst red-blue, and blue-green, nontargets is extremely inefficient (Wolfe et al, 1990 Journal of Experimental Psychology: Human Perception and Performance 16 879-892). We tested whether pairs of red-green conjunction targets can nevertheless be processed spatially in parallel. Participants made speeded detection responses whenever a red-green target was present. Across trials where a second identical target was present, the distribution of detection times was compatible with the assumption that targets were processed in parallel (Miller, 1982 Cognitive Psychology 14 247-279). We show that this was not an artifact of response-competition or feature-based processing. We suggest that within-dimension conjunctions can be processed spatially in parallel. Visual search for such items may be inefficient owing to within-dimension grouping between items.

Adult↗

PubMed2001

Neuropsychological evidence for case-specific reading: multi-letter units in visual word recognition.

We describe a patient (GK) who shows symptoms associated with Balint's syndrome and attentional dyslexia. GK was able to read words, but not nonwords. He also made many misidentification and mislocation errors when reporting letters in words, suggesting that his word-naming ability did not depend upon preserved position-coded, letter identification. We show that GK was able to read lower-case words better than upper-case words, but upper-case abbreviations better than lower-case abbreviations. Spacing the letters in abbreviations disrupted identification, as did mixing the case of letters within words. These data cannot be explained in terms of letter-based reading or preserved holistic word recognition. We propose that GK was sensitive to the visual familiarity of adjacent letter forms.

Adult↗

PubMed2001

Separating effects of orthographic similarity and contour summation in the identification of masked letter strings.

We separated effects of contour summation and orthographic similarity under masking conditions, by comparing identification with simultaneous and sequentially presented letter strings, which either did or did not overlap spatially. With overlapping simultaneous stimuli, performance was better for strings with similar contours than for strings with the same letters (the orthographic similarity condition). This suggests that contour summation effects were strongest in the condition where stimuli had similar contours. With sequential presentations, performance in the similar contour and the orthographically similar conditions was equated when the stimuli were overlapping. However, effects of contour summation decreased when prime and target letters were spatially displaced, whereas performance in the orthographically similar condition was maintained. We conclude that effects of orthographic similarity can be distinguished from effects of contour summation, under masking conditions.

Female↗

PubMed2001

Driving attention with the top down: the relative contribution of target templates to the linear separability effect in the size dimension.

Bauer, Jolicoeur, and Cowan (1996a, 1996b, 1998) have shown that visual search for a target among distractors is apparently serial if the target is nonlinearly separable from the distractors in a particular feature space (e.g., color or size). In contrast, if the target is linearly separable from the distractors, search is relatively easy and seemingly spatially parallel. We examined the contribution of top-down knowledge of the target to the linear separability effect on search. Two visual search experiments were conducted using small, medium, or large circles as targets. In the first experiment, participants could use knowledge of the target to guide search, whereas, in the second, the target was unknown on each trial. Search for a medium (nonlinearly separable) target among small or large distractors benefited least from knowledge of the target as compared with search for a small or large target. Thus, the linear separability effect can be determined in part by use of top-down knowledge to facilitate the detection of targets at the ends of a continuum defining the stimuli.

Adolescent↗

PubMed2001

Hierarchies, similarity, and interactivity in object recognition: "category-specific" neuropsychological deficits.

Category-specific impairments of object recognition and naming are among the most intriguing disorders in neuropsychology, affecting the retrieval of knowledge about either living or nonliving things. They can give us insight into the nature of our representations of objects: Have we evolved different neural systems for recognizing different categories of object? What kinds of knowledge are important for recognizing particular objects? How does visual similarity within a category influence object recognition and representation? What is the nature of our semantic knowledge about different objects? We review the evidence on category-specific impairments, arguing that deficits even for one class of object (e.g., living things) cannot be accounted for in terms of a single information processing disorder across all patients; problems arise at contrasting loci in different patients. The same apparent pattern of impairment can be produced by damage to different loci. According to a new processing framework for object recognition and naming, the hierarchical interactive theory (HIT), we have a hierarchy of highly interactive stored representations. HIT explains the variety of patients in terms of (1) lesions at different levels of processing and (2) different forms of stored knowledge used both for particular tasks and for particular categories of object.

Brain↗

PubMed2000

Differential effects of word length and visual contrast in the fusiform and lingual gyri during reading.

Previous studies have shown differential responses in the fusiform and lingual gyri during reading and suggested that the former is engaged in processing local features of visual stimuli and the latter is engaged in global shape processing. We used positron emission tomography in order to investigate how these regions are modulated by two common variables in reading: word length (three, six and nine letters) and perceptive similarity to the background (high and low contrast). Increasing both word length and visual contrast had a positive monotonic effect on activation in the bilateral fusiform. However, in the lingual gyrus, activation increased with increasing word length but decreased with increasing contrast. On the basis of previous studies, we suggest that (i) increasing word length increases the demands on both local feature and global shape processing, but (ii) increasing visual contrast increases the demands on local feature processing while decreasing the demands on global shape processing.

Brain Mapping↗