The spatial representation of numbers: evidence from neglect and pseudoneglect

The aim of the present paper is to provide an overview of the evidence that links spatial representation with representation of number magnitude. This aim is achieved by reviewing the literature concerning the number interval bisection task in patients with left hemispatial neglect and in healthy participants (pseudoneglect). Phenomena like the Spatial Numerical Association of Response Codes (SNARC) effect and the shifts of covert spatial attention caused by number processing are thought to support the notion that number magnitude is represented along a spatially organized mental number line. However, the evidence provided by chronometric studies is not univocal and is open to alternative, non-spatial interpretations. In contrast, neuropsychological studies have offered convincing evidence that humans indeed represent numbers on a mental number line oriented from left to right. Neglect patients systematically misplace the midpoint of a numerical interval they are asked to bisect (e.g., they say that 〈5〉 is halfway between 〈2〉 and 〈6〉) and their mistakes closely resemble the typical pattern found in bisection of true visual lines. The presence of dissociations between impaired explicit knowledge and spared implicit knowledge supports the notion that neglect produces a deficit in accessing an intact mental number line, rather than a distortion in the representation of that line. Other results show that the existence of a strong spatial connotation constitutes a specific property of number representations rather than a general characteristic of all ordered sequences.

Features, objects, action: The cognitive neuropsychology of visual object processing, 1984-2004

We review evidence on the cognitive neuropsychology of visual object processing, from 1984-2004, dividing the work according to whether it deals with the analysis of visual features, objects, or the relations between object processing and action. Research across this period has led to (1) a more detailed analysis of disorders of feature processing and feature binding, (2) a finer-grained understanding of disorders of object recognition, how these disorders can change over time, and their relations to visual imagery, and (3) new accounts of the relations between vision and action. Cognitive neuropsychological studies have played a key part in furthering our understanding of the functional nature of object processing in the brain.     

Neural basis of redundancy effects in visual object categorization

Bilateral redundant information often leads to improved detection performance compared to only unilateral input. This experiment investigates redundancy effects at the level of object processing in behavioral and functional imaging data. Subjects received different combinations of categorical information presented to both visual hemifields simultaneously. For pictures of faces as well as for pictures of buildings, response latencies were faster when two pictures from the same object category were presented compared to two pictures from different categories. This behavioral advantage was accompanied by increased activation in the respective object-selective areas, i.e. in the Fusiform Face Area (FFA) for pictures of faces and in the Parahippocampal Place Areas (PPAs) for pictures of buildings. These results suggest an involvement of visual object-selective areas in the behavioral redundancy gain.     

Long-term effects of prism adaptation in chronic visual neglect: A single case study

We investigated the effects of long-term training using prism adaptation on a patient with chronic neglect. A positive effect of prism adaptation was apparent on tests of visuo-spatial processing (cancellation, bisection, and grasping), but there was no benefit for neglect on the detection of errors on the contralesional side of words or on the detection the left side of chimeric faces. Across training sessions, the benefit of adaptation on immediate performance decreased, but it increased across sessions and within sessions when performance was tested up to 90 minutes after adaptation. The beneficial effect was maintained up to 1 year posttraining. In later sessions there were also increased negative after-effects following prism adaptation, as training progressed. There was no improvement on tests of mathematical cognition, which comprised an independent deficit in this patient. The data suggest that prolonged prism training can induce long-term adaptive spatial realignment of visuo-motor mappings, ameliorating some aspects of neglect. We discuss the implications for the rehabilitation of neglect and for understanding the neglect syndrome more generally.     

Long-term effects of prism adaptation in chronic visual neglect: A single case study

We investigated the effects of long-term training using prism adaptation on a patient with chronic neglect. A positive effect of prism adaptation was apparent on tests of visuo-spatial processing (cancellation, bisection, and grasping), but there was no benefit for neglect on the detection of errors on the contralesional side of words or on the detection the left side of chimeric faces. Across training sessions, the benefit of adaptation on immediate performance decreased, but it increased across sessions and within sessions when performance was tested up to 90 minutes after adaptation. The beneficial effect was maintained up to 1 year posttraining. In later sessions there were also increased negative after-effects following prism adaptation, as training progressed. There was no improvement on tests of mathematical cognition, which comprised an independent deficit in this patient. The data suggest that prolonged prism training can induce long-term adaptive spatial realignment of visuo-motor mappings, ameliorating some aspects of neglect. We discuss the implications for the rehabilitation of neglect and for understanding the neglect syndrome more generally.     

Distinct neural mechanisms for repetition effects of visual objects

Repetition of visually common objects was examined in relation to prior intentional learning and memory status using a delayed match-to-sample task in humans. Both response time and two temporally separate event-related potential (ERP) components indexed repetition. The early repetition effect ( approximately 200-550 ms) evoked more ERP responses for repeated visual objects, and was diminished by prior intentional learning (old/new) or being maintained in working memory (targets/distracters). In contrast, the late repetition effect (after approximately 550 ms) evoked reduced ERP activation for repeated items, and was not affected by prior learning or working memory status. Our source localization results indicate that the late and posterior repetition effect in visual cortex is consistent with repetition suppression results reported in monkey physiology and human fMRI studies. Meanwhile, the early and anterior repetition effect, in temporal pole and frontal cortices, is modulated by explicit memory mechanisms.     

The effect of category learning on visual attention and visual representation

Subordinate‐level category learning recruits neural resources associated with perceptual expertise, including the N250 component of the ERP, a posterolateral negative wave maximal between 230 and 330 ms. The N250 is a relatively late visual ERP and could plausibly be driven by attention to the features of categorized objects. Indeed, it has a latency and scalp distribution similar to the selection negativity (SN), an ERP component long known to be sensitive to attentional selection of target features. To clarify sensitivity of the N250 to attention and to more generally investigate the effect of category learning on attentional modulation of learned features, we independently manipulated subordinate‐level category learning and target detection in a speeded paradigm designed to optimally elicit the SN and accompanying frontal selection positivity (FSP). Participants first practiced categorizing a set of artificial animal stimuli and then performed a speeded target detection task on trained and untrained stimuli while ERPs were recorded. SN and FSP were roughly linearly related to the number of target features in the stimulus. Trained stimuli elicited a significantly larger N250 than untrained stimuli. The SN and N250 effects were additive, with all levels of target similarity equally affected by training, and had different time courses. Training had little effect on the FSP. The results suggest that (a) the N250 and SN have different sources, and (b) at the very least, the learning‐induced N250 indexes a different attentional subprocess from the target‐induced SN and could be driven by a different cognitive process altogether.     

Exploiting object constancy: effects of active exploration and shape morphing on similarity judgments of novel objects

Humans are experts at shape processing. This expertise has been learned and fine tuned by actively manipulating and perceiving thousands of objects during development. Therefore, shape processing possesses an active component and a perceptual component. Here, we investigate both components in six experiments in which participants view and/or interact with novel, parametrically defined 3D objects using a touch-screen interface. For probing shape processing, we use a similarity rating task. In Experiments 1–3, we show that active manipulation leads to a better perceptual reconstruction of the physical parameter space than judging rotating objects, or passively viewing someone else’s exploration pattern. In Experiment 4, we exploit object constancy—the fact that the visual system assumes that objects do not change their identity during manipulation. We show that slow morphing of an object during active manipulation systematically biases similarity ratings—despite the participants being unaware of the morphing. Experiments 5 and 6 investigate the time course of integrating shape information by restricting the morphing to the first and second half of the trial only. Interestingly, the results indicate that participants do not seem to integrate shape information beyond 5 s of exploration time. Finally, Experiment 7 uses a secondary task that suggests that the previous results are not simply due to lack of attention during the later parts of the trial. In summary, our results demonstrate the advantage of active manipulation for shape processing and indicate a continued, perceptual integration of complex shape information within a time window of a few seconds during object interactions.     

The development of visual object recognition in school-age children

This study documents the age-dependent development of visual object recognition abilities in 115 children aged 6 to 11 years, using a battery of neuropsychological tests based on Marr's model (Efron test, Warrington's Figure-Ground Test, Street Completion Test, Poppelreuter-Ghent Test, a selection of stimuli from the Birmingham Object Recognition Battery, a series of color photographs of objects presented from unusual perspectives or illuminated in unusual ways). The results suggest a maturation of complex visual perceptual abilities, possibly related to the development of the cerebral processes involved in object recognition, and could be the starting point for future investigations of these skills in impaired populations.     

The role of action representations in visual object recognition

It is typically assumed that perception for action and object recognition are subserved by functionally and neuroanatomically distinct processing streams in the brain. However, recent evidence challenges this classical view and suggests an interaction between both visual processing streams. While previous studies showed an influence of object perception on action-related tasks, we investigated whether action representations facilitate visual object recognition. In order to address this question, two briefly displayed masked objects were sequentially presented, either affording congruent or incongruent motor interactions. We found superior naming accuracy for object pairs with congruent as compared to incongruent motor interactions (Experiment 1). This action priming effect indicates that action representations can facilitate object recognition. We further investigated the nature of the representations underlying this action priming effect. The effect was absent when the prime stimulus was presented as a word (Experiment 2). Thus, the action priming effect seems to rely on action representations specified by visual object information. Our findings suggest that processes of object-directed action influence object recognition.     

Levels of representation,co-ordinate frames,and unilateral neglect

Abstract

We describe the performance of a brain-damaged subject, NG, who made reading errors only on the right half of words. This problem persisted even when the subject had demonstrated accurate recognition of die letters in a stimulus through naming all the letters. Furthermore, the spatially determined reading impairment was unaffected by topographic transformations of stimuli: identical performance was obtained for stimuli presented in horizontal, vertical, and mirror-reversed form. The same pattern of errors was also obtained in all forms of spelling tasks: written spelling, oral spelling, and backward oral spelling. The performance of the subject is interpreted in the context of a multi-stage model of the word recognition process. It is concluded that the locus of the deficit responsible for NG's reading impairment is at a stage of processing where word-centred grapheme representations are computed. The spatially determined pattern of performance reported for NG, as well as other patterns observed for other brain-damaged subjects, are interpreted as providing support for the proposed multi-stage model of word recognition. The more general implications of the reported results for models of visual processing and attention are also considered.     

The relationship between visual object exploration and action processing in schizophrenia

Introduction. Abnormalities in visual exploration and action processing are widely reported in schizophrenia. The aim of this study was to investigate whether object exploration (in order to recognise an action or the object) modulates visuomotor behaviour differently in schizophrenic patients and controls.

Methods. Visual scan paths were monitored in 36 patients and 36 controls. Participants performed three tasks, in which they were asked to either (1) name the object (the object-naming task), (2) picture themselves interacting with the object and then name the action (the action-naming task), or (3) explore the object (the free-viewing task).

Results. Patients explored objects less than controls did. Controls explored the part needed to identify an object in the object-naming task and the whole object in the action-naming and free-viewing tasks. In contrast, the patients maintained their gaze on the “identity” part of the object in all three tasks.

Conclusion. Our results were consistent with the literature findings on impaired action processing in schizophrenia but also extend the known impairment to implicit action processing when the subject is visually exploring an object. We discuss our results in terms of motivation, the effect of dopamine on eye movement, attentional capture, and frontal lobe dysfunction.     

Preattentive and attentive visual search in individuals with hemispatial neglect

Preattentive and attentive visual processing was examined in patients with hemispatial neglect, hemispatial neglect with hemianopia, and control participants. In the preattentive search task, targets possessed a unique feature that was not shared by distractors. In the attentive search task, targets lacked a feature that was present in the distractors. Preattentive search was normal in 3 neglect patients with cortical lesions but not in 2 neglect patients with hemianopia. A 4th neglect patient without hemianopia with a subcortical infarct abnormally used serial search mechanisms in the preattentive task. Neglect patients were characteristically impaired in the contralesional field in the attentive search task. This study demonstrates preserved explicit detection of visual features in cases of hemispatial neglect.     

Spatial object representation and its use in planning eye movements

The eye movements we make to look at objects require that the spatial information contained in the objects image on the retina be used to generate a motor command. This process is known as sensorimotor transformation and has been generally addressed using simple point targets. Here, we investigate the sensorimotor transformation involved in planning double saccade sequences directed at one or two objects. Using both visually guided saccades toward stationary objects and objects subjected to intrasaccadic displacements, and memory-guided saccades, we found that the coordinate transformations required to program the second saccade were different for saccades aimed at a new target object and saccades that scanned the same object. While saccades aimed at a new object were updated on the basis of the actual eye position, those that scanned the same object were performed with a fixed amplitude, irrespective of the actual eye position. Our findings demonstrate that different abstract representations of space are used in sensory-to-motor transformations, depending on what action is planned on the objects.     

The role of semantic, orthographic, and phonological prime information in unilateral visual neglect

The present study examined what type of lexical information could be extracted from the patient's neglected field. Patients (N =32) and age-matched controls (N =32) were required to name a foveally presented word after a parafoveal prime was presented either to the left or right visual field. Primes were identical to the target (KITE-KITE), related to the target at the level of semantics (DOG-CAT), orthography (LEMON-DEMON), phonology (ACHE-LAKE), or both orthography and phonology (HIDE-RIDE). Only when the target was semantically related to the prime were consistently reliable priming effects found in the patients, even in the patients' neglected visual field. These results demonstrate that semantic information can be extracted from the patients' neglected field.