Biomedical subjects

G W Humphreys

Publications and source records attributed to G W Humphreys.

At least 37 records · Page 2Linked to original sources

PubMed2000

Failure to identify the target does not prevent the attentional blink effect.

The attentional blink has been attributed to capacity limitations at a central level of processing. We tested whether failure to identify the target would eliminate the blink. Two agnostic patients were presented with streams of letters, which they were able to identify, and streams of pictures, which they were unable to identify. The dual-task involved identification of a target and detection of a probe. With letters the duration of the blink was equivalent to that of the control subjects. A prolonged blink was observed in both patients for pictures irrespective of whether the target was identified. This result indicates that failure to identify the target does nevertheless mobilize attentional resources sufficiently to prevent detection of a second target stimulus.

Aged↗

PubMed2000

Automatic object identification: an fMRI study.

Boucart and Humphreys reported an automatic access to object identity when observers attend to a physical property of the form of an object (e.g. the orientation) but not to its colour. We sought evidence for automatic identification in a brain imaging study using fMRI. In an orientation decision task participants decided whether a picture was vertical or horizontal. In the colour decision task participants decided if a picture was blue or green. Activation of areas 18-19 was found for both color and orientation. Activation of the temporal area 37 occurred more frequently in the orientation than in the colour decision task. This result suggests that automatic identification activates the same brain area as overt processing of semantic information.

Adult↗

PubMed2000

The role of semantic knowledge and working memory in everyday tasks.

We present a single case study of a patient, HG, who was severely impaired on routine everyday tasks, such as cleaning his teeth and preparing a cup of tea. We used the Action Coding System developed by Schwartz et al. (1991) to provide quantitative and qualitative measures of his performance in a number of experimental manipulations: (a) with task-congruent objects only, (b) with task-congruent objects and semantic distractors, (c) with a set of written commands to follow, (d) when he was given one command at a time, (e) when he was shown how the task should be performed before starting himself, and (f) when the task was divided into smaller subgoals. In general, the majority of HG's errors were step omissions, perseverations, sequence errors, and semantic errors. These semantic errors are particularly interesting since HG was able to name, gesture to, and define all the objects when they were presented in isolation or in task-congruent arrays. We suggest that semantic errors may arise for a number of reasons: (1) impaired access from semantic memory to a network representing action schema, (2) degradation of stored schema, and (3) behavior that is abnormally driven by the goal, by preceding actions, or by salient objects rather than by an appropriate association between these elements in working memory.

Aged↗

PubMed2000

One more cup of coffee for the road: object-action assemblies, response blocking and response capture after frontal lobe damage.

We report data from a patient, FK, who manifested 'utilisation' behaviour after bilateral lesions of the medial frontal and anterior temporal lobes. In tasks requiring actions to be made with the hand compatible with the orientation of a cup, FK was able to ignore irrelevant distractors that fell in the path of action to the target. However, errors were made when the distractors were other cups. These errors reflected the hand selected on the basis of the properties of the target, suggesting that distractors competed to control the action after selection of the target. When similar in kind to the target, distractors sometimes captured the action. However, distractors that were associated with an action related to the target blocked the usual hand response. These results suggest a complex interlocking of attention and action when grasping responses are made to a target amongst distractors. There is initial selection of the target, but distractors on the path of a reach can then compete for control of the action. Distractors can be inhibited if they do not match the properties of the specified target, and there can also be inhibition of concurrently activated responses. We show that the magnitude of the inhibition is proportional to the strength of learned object-action associations.

Adult↗

PubMed2000

Task-specific effects of orientation information: neuropsychological evidence.

The deficits underlying orientation agnosia in a patient (MB) with a right fronto-temporo-parietal lesion were examined. Like similar patients in the literature, MB was impaired at discriminating whether objects were upright or not and, in copying, she tended to re-represent stimuli as upright. In addition, MB failed to show the normal effects of rotation on object identification; her naming of objects rotated 45 degrees from upright was no slower than her naming of upright items. Effects of the degree of rotation did emerge, however, when she had to perform a matching task that required mental rotation. The evidence suggests that orientation may be coded in several ways (e.g. separately between objects and relative to the viewer), and that brain-damage can selectively affect the use of some but not all types of orientation information.

Agnosia↗

PubMed2000

Axis-based grouping reduces visual extinction.

We examined the effects on extinction of grouping by collinearity of edges and grouping by alignment of internal axes of shapes, in a patient (GK) with simultanagnosia following bilateral parietal brain damage. GK's visual extinction was reduced when items (equilateral triangles and angles) could be grouped by base alignment (i.e., collinearity) or by axis alignment, relative to a condition in which items were ungrouped. These grouping effects disappeared when inter-item spacing was increased, though factors such as display symmetry remained constant. Overall, the results suggest that, under some conditions, grouping by alignment of axes of symmetry can have an equal beneficial effect on visual extinction as edge-based grouping; thus, in the extinguished field, there is derivation of axis-based representations from the contours present.

Agnosia↗

PubMed2000

Coding space within but not between objects: evidence from Balint's syndrome.

The ability to make spatial judgements was examined in a patient demonstrating poor perception of multiple objects following bilateral parietal lesions, under conditions in which the presence of the stimuli to which judgements were made could be detected. The tasks required judgements of spatial length or the position of coloured parts of stimuli. We manipulated the degree to which two uprights in a display could be encoded into a single perceptual object using either stored knowledge or bottom-up cues based on 2D or 3D image relations. Performance was dependent on the presence of both bottom-up grouping and familiarity. However, connectedness in the image was not sufficient to benefit performance, when stimuli were separate objects in 3D space. This deficit in spatial judgements, arising following detection of the relevant stimulus elements, is attributed to an impairment in coding the spatial relations between separate perceptual objects. This deficit could be overcome if stimuli could be grouped in 3D, using bottom-up cues and top-down knowledge.

Agnosia↗

PubMed2000

Fractionating the binding process: neuropsychological evidence distinguishing binding of form from binding of surface features.

We present neuropsychological evidence demonstrating that the binding of form elements into shapes dissociates from the binding of surface detail to shape. Data are reported from a patient with bilateral parietal lesions, GK, who manifests left-side visual extinction along with many illusory conjunctions when asked to discriminate both surface and form information about stimuli. We show that there are effects of grouping on both extinction and illusory conjunctions when the tasks require report of object shape. In contrast, illusory conjunctions involving surface and form information were unaffected by grouping based on shape. In addition, grouping was stronger when forms were presented within the same hemifield than when they appeared in different hemifields, whilst illusory conjunctions of form and colour occurred equally often within and across hemifields. These results support a two-stage account of visual binding: form elements are first bound together locally into shapes, and this is followed by a second stage of binding in which shapes are integrated with surface details. The second but not the first stage of binding is impaired in this patient.

Color Perception↗

PubMed2000

The combined effects of plane disorientation and foreshortening on picture naming: one manipulation or two?

Objects disoriented in plane away from the upright and objects rotated in depth producing foreshortening are harder to identify than canonical views. In Experiments 1 and 2, participants named pictures of familiar objects. There was no interaction between plane and depth rotation effects on initial presentation or after practice. Experiment 3 was a dual-task psychological refractory period study. Participants classified a high-low tone with a speeded keypress and then named a canonical, plane-rotated, or foreshortened view of an object. Naming was slower when the picture was presented 50 ms after the tone compared with 800 ms after the tone. Plane rotation effects were reduced (but not eliminated) at the short tone-picture stimulus onset asynchrony, but foreshortening effects were not reduced. The results implicate an early, prebottleneck locus for some processes compensating for plane rotation and a subsequent bottleneck or postbottleneck locus for compensation for foreshortening.

Adult↗

PubMed2000

A search asymmetry reversed by figure-ground assignment.

We report evidence demonstrating that a search asymmetry favoring concave over convex targets can be reversed by altering the figure-ground assignment of edges in shapes. Visual search for a concave target among convex distractors is faster than search for a convex target among concave distractors (a search asymmetry). By using shapes with ambiguous local figure-ground relations, we demonstrated that search can be efficient (with search slopes around 10 ms/item) or inefficient (with search slopes around 30-40 ms/item) with the same stimuli, depending on whether edges are assigned to concave or convex "figures." This assignment process can operate in a top-down manner, according to the task set. The results suggest that attention is allocated to spatial regions following the computation of figure-ground relations in parallel across the elements present. This computation can also be modulated by top-down processes.

Attention↗

PubMed2000

3-D constraints on spatially parallel shape perception.

We report evidence from three sets of experiments dealing with spatially parallel grouping of parts in single objects. A first set of experiments demonstrates that parts can be encoded in a spatially parallel manner in three-dimensional (3-D) objects, while there is a serial selection of parts across objects. A second set of experiments further shows that grouping in 3-D is less affected by eliminating collinearity between the parts of objects than grouping in two dimensions, suggesting that 3-D constraints operate directly on visual grouping. A final pair of experiments demonstrates that rotating the elements in the plane, to make a physically unstable 3-D object, disrupts the benefit found with 3-D stimuli when collinearity is eliminated. The evidence indicates that there is rapid and spatially parallel encoding of 3-D object descriptions in vision.

Attention↗

PubMed2000

Visual marking: evidence for inhibition using a probe-dot detection paradigm.

Watson and Humphreys (1997, 1998) have recently demonstrated that new objects can be prioritized for visual attentional processing by the top-down attentional inhibition of old objects already in the field, a mechanism they called visual marking. The experiments reported here show that the detection of a dim probe dot is impaired when it falls at the location of an old object (Experiments 1 and 3) but that this occurs only in conditions in which it is advantageous for subjects to mark (inhibit) old objects (Experiment 2). These results further support previous work showing that visual marking is based on the inhibition of the locations of old objects and that visual marking can be flexibly applied (or withheld), depending on the goals of current behavior.

Adult↗

PubMed1999

From objects to names: a cognitive neuroscience approach.

To name an object, we need both to recognize it and to access the associated phonological form, and phonological retrieval itself may be constrained by aspects of the visual recognition process. This paper reviews evidence for such constraints, drawing on data from experimental psychology, neuropsychology, functional imaging, and computational modelling. Data on picture identification in normal observers demonstrate that the speed of name retrieval processes differs for natural objects and artifacts, due at least in part to differences in visual similarity between exemplars within these categories. Also, effects of variables on early and late stages of object identification combine in an interactive rather than an additive manner, consistent with object processing stages operating in a continuous rather than a discrete manner. Neuropsychological evidence supports this proposal, demonstrating that subtle perceptual deficits can produce naming problems, even when there is good access to associated semantic knowledge. Functional activation studies further show increased activity in visual processing areas when conditions stress object naming relative to the recognition of familiar object structures. These studies indicate that object naming is based on a series of continuous processing stages and that naming involves increased visual processing relative to recognition tasks. The data can be modelled within an interactive activation and competition framework.

Adult↗

PubMed1999

Pointing and grasping in unilateral visual neglect: effect of on-line visual feedback in grasping.

Three experiments are reported examining judgements of the centre of a stick in a patient with unilateral neglect after right hemisphere damage. Replicating previous data [35, 37], judgements showed more evidence of neglect when pointing rather than when a grasp response was used (Experiment 1), particularly when pointing preceded grasp (Experiment 2). Neglect also increased for longer sticks and when sticks fell in the patient's left hemispace; the effects of stick length and hemispace were additive with those of response (point vs grasp). Experiment 3 showed that the advantage for grasp over pointing responses occurred only when performance was guided by on-line visual feedback, and it emerged only during the end part of the reach trajectory. The results are discussed in relation to the role of visual feedback in movement control.

Cerebral Infarction↗

PubMed1999

Parallel and competitive processes in hierarchical analysis: perceptual grouping and encoding of closure.

The role of perceptual grouping and the encoding of closure of local elements in the processing of hierarchical patterns was studied. Experiments 1 and 2 showed a global advantage over the local level for 2 tasks involving the discrimination of orientation and closure, but there was a local advantage for the closure discrimination task relative to the orientation discrimination task. Experiment 3 showed a local precedence effect for the closure discrimination task when local element grouping was weakened by embedding the stimuli from Experiment 1 in a background made up of cross patterns. Experiments 4A and 4B found that dissimilarity of closure between the local elements of hierarchical stimuli and the background figures could facilitate the grouping of closed local elements and enhanced the perception of global structure. Experiment 5 showed that the advantage for detecting the closure of local elements in hierarchical analysis also held under divided- and selective-attention conditions. Results are consistent with the idea that grouping between local elements takes place in parallel and competes with the computation of closure of local elements in determining the selection between global and local levels of hierarchical patterns for response.

Adult↗

PubMed1999

Memories are made of this: the effects of time on stored visual knowledge in a case of visual agnosia.

We report the effects of the passage of time on the longterm visual knowledge for objects in a patient with visual agnosia (H.J.A.). The naming of real objects was found to have improved, although this was not associated with any change in H.J.A.'s basic perceptual abilities which were stable over a 16-year period. The improvement in object naming was attributed to better use of non-contour-based visual information (such as surface detail and depth cues). In addition, we demonstrate a deterioration in H.J.A.'s long-term memory for the visual properties of objects, and argue that this has occurred as a result of his having impaired perceptual input. The deterioration was only apparent in drawing from memory and in the verbal descriptions of items; with forced-choice testing, H.J.A. operated at ceiling; we propose that current tests of visual imagery may not be sufficiently sensitive to detect subtle impairments of visual memory. Our findings can be taken to indicate that perceptual and memorial processes are not functionally independent, but are linked in an interactive manner.

Aged↗

PubMed1999

Uniform connectedness and classical Gestalt principles of perceptual grouping.

We assessed whether uniform connectedness (UC; Palmer & Rock, 1994) operates prior to effects reflecting classical principles of grouping: proximity and similarity. In Experiments 1 and 2, reaction times to discriminate global letters (H vs. E), made up of small circles, were recorded. The small circles were respectively grouped by proximity, similarity of shapes, and by UC. The discrimination of stimuli grouped by similarity was slower than those grouped by proximity, and it was speeded up by the addition of UC. However, the discrimination of stimuli grouped by proximity was unaffected by connecting the local elements. In Experiment 3, similar results occurred in a task requiring discrimination of the orientation of grouped elements, except that the discrimination of stimuli grouped by UC was faster than that of those grouped by weak proximity. Experiment 4 further showed that subjects could respond to letters composed of discriminably separate local elements as fast as to those without separated local elements. The results suggest that grouping by similarity of shapes is perceived slower than grouping by UC, but grouping by proximity can be as fast and efficient as that by UC.

Adult↗

PubMed1999

Interactions between perceptual organization based on Gestalt laws and those based on hierarchical processing.

Our recent research with compound stimuli (Han, Humphreys, & Chen, 1999a) suggests that grouping between local elements facilitates the perception of global structure, whereas encoding closure in local elements enhances their segmentation. The present study presents further evidence supporting this assertion. Experiment 1 first developed a new paradigm in which grouping between local elements was manipulated. Subjects responded to the orientations of perceptual groups consisting of local arrows or triangles embedded in background crosses. Responses to the orientations of the groups were slowed as a function of the increased contrast of the crosses, indicating that the strength of grouping between local arrows or triangles was gradually weakened by increasing the contrast of the crosses. Using a similar paradigm, Experiments 2 and 3 investigated the role of Gestalt factors in hierarchical analysis. Global arrows or triangles made up of local arrows, or triangles were embedded in background crosses. Subjects responded to global or local stimuli in terms of orientation or closure. Increasing the contrast of the background crosses produced stronger effects on global responses than on local responses and resulted in elimination of the global precedence effect and emerging of a local precedence effect, which was stronger for closure discrimination than for orientation discrimination. These results provide new evidence supporting our previous claim about the role of Gestalt factors in hierarchical analysis.

Adult↗