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G W Humphreys

Publications and source records attributed to G W Humphreys.

16 recordsLinked to original sources
PubMed1992

The computation of perceptual structure from collinearity and closure: normality and pathology.

Three experiments tested how structural information affects the processing of fragmented forms, using evidence from both normal subjects and an agnosic patient. The stimuli were either (a) outline drawings of objects; (b) "well" or "poorly" structured fragmented forms derived from outline drawings (Experiments 1 and 2) or (c) "well" or "poorly" structured fragmented forms derived from outline drawings of meaningless forms (Experiment 3). "Well" and "poorly" structured forms varied on the properties of collinearity and closure of the elements. Subjects decided if two simultaneously presented forms had the same orientation or were mirror-reversed. Three levels of discriminability were examined: (a) for "symmetrical" forms the decision had to be based on the detection of a small feature appearing on the same or the opposite side in the two forms; (b) for "asymmetrical" forms the decision had to be based on a large part located on the same or the opposite side of each object; and (c) for "oblique" forms, matching could be based on the global orientation of the shapes. Normal subjects performed equivalently on outline drawings and "well" structured fragmented forms and worse on "poorly" structured forms. This effect was strongest on "asymmetrical" stimuli, both with meaningful (Experiments 1 and 2) and meaningless forms (Experiment 3). This indicates that the segmentation of forms into parts, for judgements with "asymmetrical" stimuli, is affected by the structural properties of collinearity and closure. For the agnosic patient, performance was similar for "well" and "poorly" structured forms, and there was better performance in the "oblique" condition. This suggests that the patient could use global shape information, but that this coexists with a selective deficit in computing local perceptual structure, based on collinearity and closure between form elements. The implications of the results for understanding form perception and the clinical use of fragmented forms, are discussed.

Adult↗
PubMed1992

Global shape cannot be attended without object identification.

Whether the global shape of objects can be processed without accessing semantic or identity information was tested. Ss judged which of 2 fragmented forms had the same global shape as a reference stimulus. Matching stimuli could be physically identical, semantically related, or unrelated. The reference stimulus and nonmatching (distractor) form could be semantically related or unrelated. Similarity effects in the related condition were unconfounded with matches nameable and nonnameable forms. For nameable forms, related matching forms facilitated performance; a related distractor disrupted performance. Semantic interference was eliminated when nameable distractors were replaced with nonnameable partners; semantic similarity effects on matching were eliminated with a nonnameable reference stimulus and with inverted targets and distractors. Access to information concerning global shape does not normally occur without object identification.

Adult↗
PubMed1992

Lesioning a connectionist model of visual search: selective effects on distractor grouping.

A connectionist model of visual search is presented, in which search is determined by patterns of grouping between distractors and between the target and the distractors. Grouping is based on conjunctive relations between simple form elements (corners, line-end terminators), and uses principles of similarity and spatial proximity. In its normal, 'unlesioned' state, the model simulates the search data generated by human subjects when they search for simple form conjunctions amongst either homogeneous or heterogeneous distractors (Humphreys & Muller, in press). In this paper, the performance of the model is examined after the model is subject to various types of 'lesion'. 'Lesioning' is produced either by increasing the internal noise on the activation functions governing the interactions between processing units, or by eliminating processing units from different loci in the model. Increasing the internal noise within the model, or lesioning units within 'high-level' processing stages, can generate the selective effects on search found in a detailed single case study of an agnosic patient (Humphreys, Riddoch, Quinlan, Price & Donnelly, this volume): Namely, there is selective disruption of search for a form conjunction amongst homogeneous distractors relative to search for the same target amongst heterogeneous distractors. This selective effect can be attributed to the 'lesion' disrupting grouping between distractors, which normally facilitates search with homogeneous distractors. The simulations demonstrate that (1) search processes can be disrupted by adding internal noise to search functions, (2) that there can be selective effects on grouping processes, and (3) that these selective effects can be associated with different types of lesion to different stages in the model. The implications of the simulations for understanding visual processing impairments in agnosia are discussed.

Agnosia↗
PubMed1992

Parallel pattern processing and visual agnosia.

A series of visual search experiments are reported examining pattern processing in a visual agnosic patient. We examined search for targets defined by: (I) the combination of their features relative to homogeneous distractors; (2) the combination of their features relative to heterogeneous distractors; and (3) a single feature difference relative to the distractors (their orientation). Normal subjects demonstrate evidence of spatially parallel search when combined-feature targets are detected against homogeneous distractors, and when targets are defined by a salient feature difference. There are non-linear effects of the number of distractors present, and absent responses can be as fast as present. In contrast, search times for combined-feature targets amongst heterogeneous distractors increase linearly with display size, with the slope for absent responses about twice that for present. The contrast between search for combined-feature targets amongst homogeneous and heterogeneous distractors can be attributed to the effects of grouping between distractors and between distractors and targets (Duncan & Humphreys, 1989, 1992; Humphreys & Muller, in press). Grouping between homogeneous distractors facilitates search. An agnosic patient, HJA, showed normal search functions for single-feature targets and for combined-feature targets amongst heterogeneous distractors. However, he was impaired at search for combined-feature targets amongst homogeneous distractors. This suggests that HJA is selectively impaired at grouping conjunctions of form features. The relations between HJA's agnosia and his problem in the parallel grouping of form conjunctions are discussed, as are the implications of the work for understanding normal vision.

Adult↗
PubMed1991

Luminance-increment detection: capacity-limited or not?

Three experiments investigated whether spatial cuing influences luminance-increment detection accuracy. Ss saw multiple-target displays and responded yes or no to 4 locations, including cued position. To test whether cuing effects are due to the load on visual short-term memory from the number of locations, Experiments 1 and 2 presented displays with 4 or 8 relevant locations. Experiment 1 used peripheral cues; Experiment 2 used central cues. Significant cuing effects were less marked with 4- than 8-location displays. Cuing effects were largest with multiple targets, but a small reliable effect remained even with single targets. Experiment 3 replicated the single-target effect with predominantly multiple- and single-target displays. A capacity-limited selection account is developed for these findings and their implications are discussed for separate central and peripheral cuing mechanisms and the locus of spatial cuing effects.

Adult↗
PubMed1991

Parallel computation of primitive shape descriptions.

This article reports evidence that parallel search functions can occur when heterogeneous conjunction elements configure into a single object description. Experiment 1 demonstrates parallel search for form conjunctions presented in displays of heterogeneous nontargets when closure and good continuation between display elements are exhibited. When closure and good continuation are absent, reaction times increase linearly with set size (Experiment 2). Experiment 3 demonstrates a halfway stage between these two extremes, in which displays have closure but not good continuation. Finally, selection of object descriptions is shown to be temporally limited to one object at a time (Experiment 4). The data are incompatible with both local feature-based and distractor similarity-based accounts of visual search; however, they are compatible with there being an important role for preattentive feature combinations forming primitive object descriptions. Parallel search functions can occur when heterogeneous conjunctions configure into a single object description.

Adult↗
PubMed1990

Orthographic processing in visual word identification.

A series of experiments is reported examining orthographic priming effects between briefly presented pairs of letter strings. The experiments investigate the effects of the number and position of letters shared by primes and targets, and the effects of prime-target length. Priming effects increase nonlinearly as a function of both the number and the position of shared letters, and they are dependent on the positions of letters relative to both the end positions in the string and to the identities of their nearest neighbours. There is little effect of absolute string length on priming. These priming effects can be distinguished from intrusion errors where letters from primes are reported in response to targets. An account of orthographic processing is outlined which attributes priming to cooperative interactions between coarse relative-position coded letter cluster representations activated by primes and targets. The implications of the findings for understanding other effects in word recognition and reading are discussed.

Adult↗
PubMed1989

Grouping processes in visual search: effects with single- and combined-feature targets.

We report evidence for spatially parallel visual search for targets defined by combinations of form elements in visual search. In Section 1, we show that flat search functions occur for combined-form targets when distractor forms are homogeneous and can be grouped together, thus segmenting the target from the distractors. Introducing heterogeneous distractors lessens distractor grouping and can produce serial search. These results cannot be easily attributed to subjects' use of local feature information to discriminate targets. Instead, they suggest that grouping can operate at a level at which combined form information is represented. In Section 2 we show that these grouping effects are spatially scaled by the size of the stimuli. In Section 3 we show that heterogeneity does not prevent flat search functions when the target has a unique defining feature. The data are interpreted in terms of a hierarchical processing system involving both devoted single-feature and combined-feature (junction) maps. Grouping processes can operate at both the single-feature and the combined-form levels. Selection in visual search remains confined to one object description at a time, but this description can be at various spatial scales, including that at the level of grouped forms.

Attention↗
PubMed1989

Varieties of object constancy.

Three experiments are described in which two pictures of isolated man-made objects were presented in succession. The subjects' task was to decide, as rapidly as possible, whether the two pictured objects had the same name. With a stimulus-onset asynchrony (SOA) of above 200 msec two types of facilitation were observed: (1) the response latency was reduced if the pictures showed the same object, even though seen from different viewpoints (object benefit); (2) decision time was reduced further if the pictures showed the same object from the same angle of view (viewpoint benefit). These facilitation effects were not affected by projecting the pictures to different retinal locations. Significant benefits of both types were also obtained when the projected images differed in size. However, in these circumstances there was a small but significant performance decrement in matching two similar views of a single object, but not if the views were different. Conversely, the object benefit, but not the viewpoint benefit, was reduced when the SOA was only 100 msec. The data suggest the existence of (at least) two different visual codes, one non-retinotopic but viewer-centred, the other object-centred.

Adolescent↗
PubMed1989

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.

Adult↗
PubMed1988

Priming effects between two-dimensional shapes.

The role of reference frames in shape perception was examined by investigating priming effects between two-dimensional shapes. Ambiguous shapes and shapes with an unambiguous axis were used, and primes and targets could be orientationally transformed so that for ambiguous shapes, different axes aligned with the vertical. Reliable priming effects were established that were dependent on the structural similarity of primes and targets, the informativeness of the prime, and the interval between the stimuli. Most interestingly, with short interstimulus intervals, differential priming effects were found between ambiguous shapes and unambiguous shapes when an orientational transformation aligned different axes in the ambiguous shape with the vertical. The result was interpreted as demonstrating that priming was determined by the similarity of the frame-based descriptions of primes and targets. The finding occurred irrespective of the informativeness of the prime, suggesting that it reflects obligatory processes in shape perception. Further data showed that under some circumstances prespecifying the orientation of a shape also facilitated its processing; this provided direct evidence that shapes are represented relative to a perceptual reference frame.

Adult↗
PubMed1987

A case of integrative visual agnosia.

A single case study of a patient with visual agnosia is presented. The patient had a marked impairment in visual object recognition along with good tactile object identification and a preserved ability to copy. Detailed investigations demonstrated impaired perceptual processes, with the patient's identification strongly affected by duration of stimulus exposure and by using overlapping figures. However, his stored knowledge of objects was shown to be intact. The results demonstrate that agnosia may be determined by a specific deficit in integrating form information; and that the input description for visual object recognition, disrupted in this patient, is functionally separate from stored object descriptions, which are intact. The implications of the results for understanding visual agnosia and for theories of normal visual object recognition are discussed.

Agnosia↗
PubMed1978

The use of category information in perception.

Three tachistoscopic experiments are reported in which presentation of the target stimulus in a letter/digit categorization task was preceded by a briefly exposed priming stimulus (letter or digit). The primer was subject to backward masking from either the target or a pattern mask, and observers were unaware of its occurrence. With a primer duration of 25 ms, when masking was presumed to be at a central level, performance deteriorated when the two items were from different categories. This inhibition effect was reduced when the characters were physically similar. In contrast, there was little evidence of facilitated processing when primers validly cued targets. At shorter primer durations, when masking is presumed to be peripheral in origin, between-condition differences were less marked. An interpretation in terms of an active model of information processing, with utilization of both categorical and physical information extracted from the primer, is proposed.

Adult↗