Search PubMed⌕ Search

Biomedical subjects

Glyn W Humphreys

Publications and source records attributed to Glyn W Humphreys.

At least 19 recordsLinked to original sources
PubMed2006

Ventral extra-striate cortical areas are required for optimal orientation averaging.

We examined the ability of a previously well-studied patient with visual agnosia to compute the average orientation of elements in visual displays. In a structural MRI study, we show that the lesion is likely to involve a variety of ventral extra-striate areas, including V2, V3 and V4; however, the lesion does not extend dorsally. Subsequently we show that some ability to compute average orientation is spared, though there are limitations on the ability to scale the averaging process as a function of the numbers of elements. The results suggest that some aspects of orientation averaging can be accomplished in spared regions of V1 but flexible averaging requires ventral extra-striate cortex.

Aged, 80 and over↗
PubMed2006

A computational model of visual marking using an inter-connected network of spiking neurons: the spiking search over time & space model (sSoTS).

In the real world, visual information is selected over time as well as space, when we prioritise new stimuli for attention. Watson and Humphreys [Watson, D., Humphreys, G.W., 1997. Visual marking: prioritizing selection for new objects by top-down attentional inhibition of old objects. Psychological Review 104, 90-122] presented evidence that new information in search tasks is prioritised by (amongst other processes) active ignoring of old items - a process they termed visual marking. In this paper we present, for the first time, an explicit computational model of visual marking using biologically plausible activation functions. The "spiking search over time and space" model (sSoTS) incorporates different synaptic components (NMDA, AMPA, GABA) and a frequency adaptation mechanism based on [Ca(2+)] sensitive K(+) current. This frequency adaptation current can act as a mechanism that suppresses the previously attended items. We show that, when coupled with a process of active inhibition applied to old items, frequency adaptation leads to old items being de-prioritised (and new items prioritised) across time in search. Furthermore, the time course of these processes mimics the time course of the preview effect in human search. The results indicate that the sSoTS model can provide a biologically plausible account of human search over time as well as space.

Action Potentials↗
PubMed2006

Exploring the functional and anatomical bases of mirror-image and anatomical imitation: the role of the frontal lobes.

Humans are the most imitative species on earth, but how imitation is accomplished and which areas of the brain are directly involved in different kinds of imitation is still under debate. One view is that imitation entails representing observed behaviours as a set of hierarchically organised goals, which subsequently drive the construction of an action pattern [Bekkering, H., Wohlschläger, A., & Gattis, M. (2000). Imitation of gestures in children is goal-directed. The Quarterly Journal of Experimental Psychology, 53, 153-164; Wohlschläger, A., Gattis, M., & Bekkering, H. (2003). Action generation and action perception in imitation: An instance of the ideomotor principle. Philosophical Transactions of the Royal Society of London, 358, 501-515]. On this view, when working memory resources are limited, only the goals at the top-end of the hierarchy will be accurately reproduced. In the present study, neurologically intact participants and patients with frontal and non-frontal lesions were asked to make imitative responses that were either mirror-image (e.g., the observer's right side corresponding to the model's left side) or anatomically (e.g., the observer's right side corresponding to the model's right side) matching. Experiment 1 confirmed that individuals with brain damage, though globally impaired compared with neurologically intact controls, nevertheless followed the same goal hierarchy. However, there was a selective deficit in performing anatomical imitation for the frontal group. Experiment 2 demonstrated that the problem for frontal patients stemmed from an impaired ability to remember and reproduce incompatible stimulus-response mappings, which is fundamental for the selection of the appropriate frame of reference during anatomical imitation.

Adult↗
PubMed2006

When "happy" means "sad": neuropsychological evidence for the right prefrontal cortex contribution to executive semantic processing.

The contribution of the left inferior prefrontal cortex in semantic processing has been widely investigated in the last decade. Converging evidence from functional imaging studies shows that this region is involved in the "executive" or "controlled" aspects of semantic processing. In this study, we report a single case study of a patient, PW, with damage to the right prefrontal and temporal cortices following stroke. PW showed a problem in executive control of semantic processing, where he could not easily override automatic but irrelevant semantic processing. This case, thus, shows the necessary role of the right inferior prefrontal cortex in executive semantic processing. Compared to tasks previously used in the literature, our tasks placed higher demands on executive semantic processing. We suggest that the right inferior prefrontal cortex is recruited when the demands on executive semantic processing are particularly high.

Aged↗
PubMed2006

Testing the domain-specificity of a theory of mind deficit in brain-injured patients: evidence for consistent performance on non-verbal, "reality-unknown" false belief and false photograph tasks.

To test the domain-specificity of "theory of mind" abilities we compared the performance of a case-series of 11 brain-lesioned patients on a recently developed test of false belief reasoning () and on a matched false photograph task, which did not require belief reasoning and which addressed problems with existing false photograph methods. A strikingly similar pattern of performance was shown across the false belief and false photograph tests. Patients who were selectively impaired on false belief tasks were also impaired on false photograph tasks; patients spared on false belief tasks also showed preserved performance with false photographs. In some cases the impairment on false belief and false photograph tasks coincided with good performance on control tasks matched for executive demands. We discuss whether the patients have a domain-specific deficit in reasoning about representations common to both false belief and false photograph tasks.

Adult↗
PubMed2006

Opposite biases in salience-based selection for the left and right posterior parietal cortex.

Visual selection is determined in part by the saliency of stimuli. We assessed the brain mechanisms determining attentional responses to saliency. Repetitive transcranial magnetic stimulation (rTMS) was applied to the left and right posterior parietal cortices (PPC) immediately before participants were asked to identify a compound letter. rTMS to the right PPC disrupted the guidance of attention toward salient stimuli, whereas rTMS to the left PPC affected the ability to bias selection away from salient stimuli. We conclude that right and left PPC have opposite roles in biasing selection to and from salient stimuli in the environment.

Attention↗
PubMed2006

Comparing neural correlates of configural processing in faces and objects: an ERP study of the Thatcher illusion.

In the Thatcher illusion, a face with inverted eyes and mouth looks abnormal when upright but not when inverted. Behavioral studies have shown that thatcherization of an upright face disrupts perceptual processing of the local configuration. We recorded high-density EEG from normal observers to study ERP correlates of the illusion during the perception of faces and nonface objects, to determine whether inversion and thatcherization affect similar neural mechanisms. Observers viewed faces and houses in four conditions (upright vs. inverted, and normal vs. thatcherized) while detecting an oddball category (chairs). Thatcherization delayed the N170 component over occipito-temporal cortex to faces, but not to houses. This modulation matched the illusion as it was larger for upright than inverted faces. The P1 over medial occipital regions was delayed by face inversion but unaffected by thatcherization. Finally, face thatcherization delayed P2 over occipito-temporal but not over parietal regions, while inversion affected P2 across categories. All effects involving thatcherization were face-specific. These results indicate that effects of face inversion and feature inversion (in thatcherized faces) can be distinguished on a functional as well as neural level, and that they affect configural processing of faces in different time windows.

Adult↗
PubMed2006

Seeing the content of the mind: enhanced awareness through working memory in patients with visual extinction.

In the present study, we demonstrate that parietal patients with visual extinction show enhanced awareness when there is a match between the current contents of their working memory and the stimuli presented in the visual field. This effect cannot be attributed to automatic bottom-up priming from the presentation of a memory cue, because extinction was reduced only when patients committed the cue to memory, and not when primes were viewed passively or merely identified. The results suggest that reentrant processes, from working memory, modulate awareness.

Aged↗
PubMed2006

Dividing the mind: the necessary role of the frontal lobes in separating memory from search.

Working memory plays a crucial role in the control of visual selection. Previous research has shown that attentional deployment can be biased to objects in an array matching the contents of working memory. Here, we examined the role of the frontal cortex in determining the interaction between working memory and attention. At the start of each trial, participants memorized an object cue that could contain either the target or a distracter, when the object reappeared in the subsequent search array. Relative to age-matched controls, patients suffering from damage to the frontal lobes showed a stronger effect of the memory stimulus on search. Interestingly, there was an effect of frontal damage on the mean latencies to fixate targets but the effect of memory validity on the number of first saccades to the target, and their time of initiation, was similar across the groups. The results suggest that, following the earliest deployment of attention, frontal lobe structures are involved in separating relevant target from irrelevant (object cue) information, when both are held in memory.

Adult↗
PubMed2006

Compensatory strategies in processing facial emotions: evidence from prosopagnosia.

We report data on the processing of facial emotion in a prosopagnosic patient (H.J.A.). H.J.A. was relatively accurate at discriminating happy from angry upright faces, but he performed at chance when the faces were inverted. Furthermore, with upright faces there was no configural interference effect on emotion judgements, when face parts expressing different emotions were aligned to express a new emergent emotion. We propose that H.J.A.'s emotion judgements relied on local rather than on configural information, and this local information was disrupted by inversion. A compensatory strategy, based on processing local face parts, can be sufficient to process at least some facial emotions.

Aged↗
PubMed2006

Configural information in gender categorisation.

The role of configural information in gender categorisation was studied by aligning the top half of one face with the bottom half of another. The two faces had the same or different genders. Experiment 1 shows that participants were slower and made more errors in categorising the gender in either half of these composite faces when the two faces had a different gender, relative to control conditions where the two faces were nonaligned or had the same gender. This result parallels the composite effect for face recognition (Young et al, 1987 Perception 16 747-759) and facial-expression recognition (Calder et al, 2000 Journal of Experimental Psychology: Human Perception and Performance 26 527-551). Similarly to responses to face identity and expression, the composite effect on gender discrimination was disrupted by inverting the faces (experiment 2). Both experiments also show that the composite paradigm is sensitive to general contextual interference in gender categorisation.

Adolescent↗
PubMed2006

The use of memorised verbal scripts in the rehabilitation of action disorganisation syndrome.

We report data on the rehabilitation of action disorganisation syndrome (ADS). Although prior attempts to rehabilitate everyday actions in our patient, FK, had proved unsuccessful, we report positive results from using a verbalisation strategy. FK was taught a poem based on the steps involved in making a cup of tea. Following training, the everyday action was performed more successfully than prior to training, with the order of the actions in particular being improved when the poem was applied. In addition, there was evidence of error monitoring being carried out contingent on the verbalisation strategy. Across training sessions FK also became more likely to apply the poem and to perform the actions without prompting. However, there was relatively weak training effects across sessions, and the beneficial effects did not transfer to new tasks or to the same task with a different key object. The utility of the approach for severe cases of ADS is discussed.

Adult↗
PubMed2006

An onset advantage without a preview benefit: neuropsychological evidence separating onset and preview effects in search.

Visual search is facilitated if half the distractors are presented as a preview prior to the presentation of the target and second set of distractors--the preview benefit [Watson, D. G., & Humphreys, G. W. Visual marking: Prioritizing selection for new objects by top-down attentional inhibition of old objects. Psychological Review, 104, 90-122, 1997]. On one account, the preview advantage is due to automatic capture of attention by the onsets in the second, search display [Donk, M., & Theeuwes, J. Visual marking beside the mark: Prioritizing selection by abrupt onsets. Perception & Psychophysics, 93, 891-900, 2001]. We provide a neuropsychological test of this assertion. We examined onset capture and preview benefits in search in a group of neuropsychological patients with unilateral parietal damage. We demonstrate a normal pattern of performance when patients detected targets defined by onsets relative to those defined by offsets, irrespective of whether the onset target fell contra- or ipsilateral to the lesion. In contrast, there was a normal preview benefit in search only for ipsilesional targets, and preview search was impaired in the contralesional field. The data demonstrate that the preview benefit can dissociate from the onset advantage in search, and that onsets remain strongly weighted for attention even in the contralesional field of patients with parietal lesions.

Aged↗
PubMed2006

Object-based inhibitory priming in preview search: evidence from the "top-up" procedure.

Visual search in a conjunction task can be facilitated if half the distractor items are previewed prior to the other half of the distractors and the target item. Here, we investigate the nature of this preview by using a top-up procedure, which presents an initial preview followed by a secondary preview after a period of time (the offset period). In Experiment 1, we demonstrate that increasing the time of the offset period decreases search efficiency. If the offset period is increased to 2 sec, the previewed items are searched to a greater extent than when the offset period is 450 msec. This holds even when the old items remain in the same positions across presentations and when they differ in color from new search stimuli. However, when the offset intervals are reduced, the preview can be discounted from search even when the old items change locations between exposures (Experiment 2) and when they are not distinguished from search displays by their color (Experiment 3). The last result occurs only as long as the preview items can be grouped in terms of form. When the preview stimuli are heterogeneous, they are no longer discounted from search if their locations change across the offset period (Experiment 4). We interpret the data in terms of object-based priming, which enables the repeated form of the old distractors to be filtered more easily from search.

Adolescent↗
PubMed2006

Is it impossible to inhibit isoluminant items, or does it simply take longer? Evidence from preview search.

Visual search can be facilitated when participants receive a preview of half the distractors (the preview benefit in search; Watson & Humphreys, 1997). Donk and Theeuwes (2001) have argued that preview-based benefits are abolished if the display items are isoluminant to a background. This is consistent with the preview benefit being due to onset capture by the new stimuli. In contrast, the present experiments challenge this suggestion and show that preview benefits can occur under isoluminant conditions, providing that they are given enough time to occur. In Experiment 1, we showed that a preview benefit can occur even with isoluminant stimuli, provided that the old items are previewed for a sufficient time. In Experiment 2, we tested and rejected the idea that this advantage is due to low-level sensory fatigue for the preview stimuli. These findings indicate that the preview effect is not caused solely by onset capture.

Adolescent↗
PubMed2006

Dissociating the effects of similarity, salience, and top-down processes in search for linearly separable size targets.

In two experiments, we explored the role of foreknowledge on visual search for targets defined along the size continuum. Targets were of large, medium, or small size and of high or low similarity relative to the distractors. In Experiment 1, we compared search for known and unknown singleton feature targets as a function of their size and similarity to the distractors. When distractor similarity was high, target foreknowledge benefited targets at the end of the size continuum (i.e., large and small) relatively more than targets of medium size. In Experiment 2, participants were given foreknowledge of what the target was not The beneficial effect of foreknowledge for endpoint targets was reduced. The data indicate the role of top-down templates in search, which can be "tuned" more effectively for targets at the ends of feature dimensions.

Adult↗
PubMed2006

On the relations between implicit and explicit spatial binding: evidence from Balint's syndrome.

We report two experiments examining whether factors that affect binding in explicit report tasks also affect implicit binding in a patient with Balint's syndrome, G.K. Replicating prior studies, we showed that there could be implicit binding of visual features in a patient with Balint's syndrome who was at chance at explicitly discriminating the relations between the features. Nevertheless, when color-form relationships were coded, we showed that G.K.'s implicit binding, as well as his explicit report of binding relationships, was affected by the spatial distance between the stimuli. Similarly, we demonstrated effects of grouping on both implicit and explicit binding of the spatial relations between shapes. We interpret the qualitative similarities between implicit and explicit binding as indicating that they reflect a single process in which binding relations, initially established preattentively, are later consolidated by attention. This later process, of attentional consolidation, is disrupted in Balint's syndrome.

Aged↗