M. Jane Riddoch

Visual object processing in optic aphasia: A case of semantic access agnosia

Abstract A single case study is reported of a patient with a naming disorder specific to visually presented stimuli. The patient was often able to gesture correctly to objects he could not name, and he showed intact access to structural knowledge of objects. Further examination revealed an impairment in accessing semantic knowledge about objects, which was most marked when the patient had to discriminate between objects which were visually as well as semantically similar. It is suggested that the patients naming deficit is due to an impairment in accessing semantic information from vision, following intact access to stored structural knowledge. Correct gestures may be contingent on access to the system specifying structural knowledge. The data are interpreted in terms of a model of visual object identification in which access to semantic information, from the system specifying structural knowledge, is held to operate in cascade.

The effect of cueing on unilateral neglect

Heilman and Valenstein recently failed to reduce unilateral neglect, assessed by a line bisection task, by cueing patients to attend to their neglected field. Cueing was accomplished by placing letters at both ends of the line and instructing subjects to identify either the right or left hand letter prior to bisecting the line. The present experiments tested whether this failure to improve neglect occurred because patients were presented with competing stimuli in their neglected and non-neglected fields. Five patients with unilateral neglect and hemianopia took part in two experiments. The results showed a marked decrease in neglect when subjects were cued and forced to report stimuli in their neglected field. This occurred even when there was a competing stimulus in the non-neglected field. However, in the absence of forced report requirements, patients oriented to stimuli in the non-neglected field. The results are interpreted as a failure of patients with unilateral neglect to orient automatically to the side of space contralateral to the lesion, though processes governing the conscious orienting of attention are intact.

Expression is computed separately from facial identity, and it is computed separately for moving and static faces: Neuropsychological evidence

We report data contrasting the processing of facial identity from static photographs, and facial expression from static and moving images, in two patients with face processing impairments. One patient is markedly impaired at perceiving facial identity and he is poor at discriminating facial expression and gender from static photographs of faces. In contrast, he performs normally when required to make judgements of facial expression and gender to faces depicted by sets of moving light dots. The second patient is relatively good at perceiving facial identity, but poor at judging emotional expression from both static and moving faces. The data are consistent with the existence of separate processes for encoding face identity and facial expression, and, furthermore, indicate the separate encoding of expression from moving and static images.

Visual object processing : a cognitive neuropsychological approach

Recently there have been considerable advances made in understanding the kinds of processes which underlie our ability to recognize and name visually presented objects. These advances have come from experimental psychology, from studies of computer vision and from studies of the effects of brain damage on object recognition and naming. This book draws together research from these different approaches to visual object processing in an attempt to relate evidence from disorders of object processing to models of normal performance, and how such models can in turn help us understand object processing disorders.

Attention to within-object and between-object spatial representations: multiple sites for visual selection

Abstract Our ability both to recognise visually presented objects and to interact with them appropriately depends upon detailed interaction between visual attentional and visual recognition systems. We report data from two neuropsychological cases indicating that the allocation of attention in vision is determined by at least two forms of spatial representation (within-object and between-object spatial coding). In both cases there was unilateral neglect of one side of visual displays if items in the displays were coded as parts of a single perceptual object, and neglect of the opposite side if items were coded as separate perceptual objects. In addition, attentional cueing effects to either the left or right side were specific to the form of representation involved. These cases demonstrate that there is parallel coding of visual displays into within- and between-object spatial representations. Further, visual selection operates independently on the two forms of visual representation. The data support an a...

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.

Perceptual and action systems in unilateral visual neglect

In this chapter we review three different accounts of unilateral neglect: one maintaining that neglect is due to early visual processing deficits; one maintaining that neglect is due to a disorder of an internal representation of space; and one maintaining that neglect is due to a disorder of visual attention. An attentional view of neglect is elaborated in which neglect is attributed to a breakdown in the processes enabling stimul on the contralateral side of space to a lesion to “capture” visual attention. This attentional account predicts that contralateral stimuli may vary according to the ease with which they “capture” the attention of neglect patients. Further, even when “attentional capture” does not occur, attention may nevertheless be consciously directed to the neglected side. Data supporting this position are reported from visual search tasks where targets are defined either by single feature differences relative to distractors or by some combination of local feature information, and where neglect patients are either free to adopt their own search strategy or they are cued to the neglected side. Neglect was more marked in the combined-feature searches than the single-feature searches, and it tended to be reduced by spatial cueing. Implications for understanding unilateral visual neglect and for understanding the operation of normal visual attention are discussed.

Evidence from unilateral visual neglect

Abstract We report the case of a patient, JR, who manifests left neglect in reading single words but right neglect in copying and drawing tasks. We show that left neglect is not confined to reading, but is also found in picture naming and in line-bisection tasks. In each of these tasks, JR neglected the right side of multiple stimulus displays. We show that neglect is manifest on the left or right side as a function of whether visual stimuli are encoded as parts of a single perceptual object or as separate perceptual objects. JRs left neglect in reading words can be switched to right neglect by having him read aloud all the letters in the string. He also showed left neglect in a search task in which the visual elements configure into a coherent object, but right neglect in displays using similar elements which did not so configure. JRs case demonstrates that the visual system employs separate representations to encode the spatial relations between the parts of single objects (within-object spatial codin...

Routes to action: Evidence from apraxia

Abstract We report the case of a right-handed patient (CD), with a unilateral lesion of the left parietal lobe, who was unimanually apraxic to visually presented objects. In particular, CD was unable to initiate learned gestures to seen objects with his right hand, even though he was able to perform the same gestures with his left hand (and was often able to perform right-hand gestures subsequently). CD could perform right-handed gestures when the objects were not present and he was given their names, and his performance with seen objects improved when he was given pairs of objects. From CDs performance we argue that gestures to visually presented objects are normally based on co-operation between stored knowledge about the class of gesture to use, and directly computed visual representations that provide the spatial co-ordinates for action. We suggest that CDs problem is due to his having an impaired “route” from vision to action that selectively impairs right-hand actions for him.

Are faces special? A case of pure prosopagnosia

The ability to recognize individual faces is of crucial social importance for humans and evolutionarily necessary for survival. Consequently, faces may be “special” stimuli, for which we have developed unique modular perceptual and recognition processes. Some of the strongest evidence for face processing being modular comes from cases of prosopagnosia, where patients are unable to recognize faces whilst retaining the ability to recognize other objects. Here we present the case of an acquired prosopagnosic whose poor recognition was linked to a perceptual impairment in face processing. Despite this, she had intact object recognition, even at a subordinate level. She also showed a normal ability to learn and to generalize learning of nonfacial exemplars differing in the nature and arrangement of their parts, along with impaired learning and generalization of facial exemplars. The case provides evidence for modular perceptual processes for faces.