Biomedical subjects
M J Farah
Publications and source records attributed to M J Farah.
Face perception and within-category discrimination in prosopagnosia.
Prosopagnosics are impaired at face recognition, but unimpaired, or relatively less impaired, at common object recognition. It has been suggested that this dissociation results simply from the greater difficulty of face recognition compared to object recognition, or from the greater need to discriminate visually similar members of a single category in face recognition compared to object recognition. We tested these hypotheses using the performance of normal subjects in an 'old/new' recognition paradigm to establish the true relative difficulty of face and object recognition, and required both normal subjects and a prosopagnosic subject to discriminate both faces and visually similar exemplars of nonface object categories. In two different experiments, the prosopagnosic patient performed disproportionately poorly with faces. These results disconfirm the hypotheses described above, and imply that prosopagnosia is an impairment of a specialized form of visual recognition that is necessary for face recognition and is not necessary, or less necessary, for the recognition of common objects.
The inverted face inversion effect in prosopagnosia: evidence for mandatory, face-specific perceptual mechanisms.
Does the human visual system contain a specialized system for face recognition, not used for the recognition of other objects? This question was addressed using the "face inversion effect" which refers to the loss of our normal proficiency at face perception when faces are inverted. We found that a prosopagnosic subject paradoxically performed better at matching inverted faces than upright faces, the opposite of the normal "face inversion effect". The fact that his impairment was most pronounced with the stimuli for which normal subjects show the greatest proficiency in face perception provides evidence of a neurologically localized module for upright face recognition in humans. An additional implication of these data is that specialized systems may control behavior even when they are malfunctioning and therefore maladeaptive, consistent with the mandatory operation of such systems according to the "modularity" hypothesis of the cognitive architecture.
The psychological reality of the body schema: a test with normal participants.
Neuropsychological dissociations suggest the existence of a body schema, a representation of the spatial relations among body parts, not used for other spatial stimuli. Four experiments verify the psychological reality of the body schema in normal participants. In Experiments 1 and 2, proprioceptive information concerning one's own body position influences visual perception of others' body positions. Contrary to expectations, facilitation is observed rather than interference in the dual-performance task. Experiment 3 eliminates the possibility that the effect is due to a particular mnemonic strategy. In Experiment 4, this effect is shown to be specific to the perception of bodies, as opposed to other complex 3-dimensional forms.
What causes the face inversion effect?
What is it about the way faces are represented by the visual system that makes them so much harder to recognize when inverted? The authors tested the hypothesis that the "face inversion" effect results from the use of holistic shape representations. This suggests that the susceptibility of nonface patterns to inversion should be a function of their degree of part decomposition. In Experiment 1 this was tested and confirmed with dot patterns in which the degree of part decomposition was manipulated by grouping and segregation on the basis of dot color. The hypothesis also predicted that the face inversion effect can be eliminated with face stimuli if participants are induced to recognize the faces in terms of their component parts. In Experiment 2 this was tested and confirmed with whole, intact faces, in which the degree of part decomposition was manipulated by allowing participants to study them, initially, in either whole, intact versions or versions with parts presented separately.
Beyond 'pet' methodologies to converging evidence.
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Perception and awareness after brain damage.
Dissociations between perception and awareness of perception are of interest for what they can tell us about the neural correlates of awareness. Recent research in this field has focussed on perception-awareness dissociations in the syndromes of blindsight, covert recognition in two kinds of visual agnosia, and visual neglect.
Associative visual agnosia and alexia without prosopagnosia.
Disagreement over the neuroanatomical substrate of associative visual agnosia encompasses such basic issues as: (1) the necessity for bilateral lesions; (2) the intrahemispheric locus of damage; and (3) the roles of disconnection versus cortical damage. We examined three patients whose associative visual agnosia encompassed objects and printed words but spared faces. CAT scans revealed unilateral dominant occipitotemporal strokes. CAT scans of four previously reported cases with this same profile of associative agnosia were obtained. Dominant parahippocampal, fusiform and lingual gyri were the most extensively damaged cortical regions surveyed and were involved in all cases. Of white matter tracts surveyed, only temporal white matter including inferior longitudinal fasciculus was severely and universally involved. Splenium of the corpus callosum was frequently but not always involved. We conclude there is a form of associative visual agnosia with agnosia for objects and printed words but sparing face recognition which has a characteristic unilateral neuropathology. Damage or disconnection of dominant parahippocampal, fusiform and lingual gyri is the necessary and sufficient lesion.
Does visual attention select objects or locations?
Much research supports location-based attentional selection, but J. Duncan (1984) presented data favoring object-based selection in a shape discrimination task. Does attention select objects or locations? We confirmed that Duncan's task elicits selection from spatially invariant object representations rather than from a grouped location-based representation. We next asked whether this finding was due to location-based filtering; the results again supported object-based selection. Finally, we demonstrated that when Duncan's objects were used in a cued detection task the results were consistent with location-based selection. These results suggest that there may not be a single attention mechanism, consistent with Duncan's original claim that object-based and location-based attentional selection are not mutually exclusive. Rather, attentional limitations may depend on the type of stimulus representation used in performing a given task.
A unified account of cognitive impairments following frontal lobe damage: the role of working memory in complex, organized behavior.
A computer model is presented that performs 4 tasks sometimes impaired by frontal damage: motor sequencing, the Stroop task, the Wisconsin Card Sorting Test, and a context memory task. In each task, patterns of performance typical of frontal-damaged patients are shown to result from the same type of damage to the model, specifically, weakening of associations among elements in working memory. The simulation shows how a single underlying type of damage could result in impairments on a variety of seemingly distinct tasks. Furthermore, the hypothesized damage affects the processing components that carry out the task rather than a distinct central executive.
Dissociated overt and covert recognition as an emergent property of a lesioned neural network.
Covert recognition of faces in prosopagnosia, in which patients cannot overtly recognize faces but nevertheless manifest recognition when tested in certain indirect ways, has been interpreted as the functioning of an intact visual face recognition system deprived of access to other brain systems necessary for consciousness. The authors propose an alternative hypothesis: that the visual face recognition system is damaged but not obliterated in these patients and that damaged neural networks will manifest their residual knowledge in just the kinds of tasks used to measure covert recognition. To test this, a simple model of face recognition is lesioned in the parts of the model corresponding to visual processing. The model demonstrates covert recognition in 3 qualitatively different tasks. Implications for the nature of prosopagnosia, and for other types of dissociations between conscious and unconscious perception, are discussed.
Parts and wholes in face recognition.
Are faces recognized using more holistic representations than other types of stimuli? Taking holistic representation to mean representation without an internal part structure, we interpret the available evidence on this issue and then design new empirical tests. Based on previous research, we reasoned that if a portion of an object corresponds to an explicitly represented part in a hierarchical visual representation, then when that portion is presented in isolation it will be identified relatively more easily than if it did not have the status of an explicitly represented part. The hypothesis that face recognition is holistic therefore predicts that a part of a face will be disproportionately more easily recognized in the whole face than as an isolated part, relative to recognition of the parts and wholes of other kinds of stimuli. This prediction was borne out in three experiments: subjects were more accurate at identifying the parts of faces, presented in the whole object, than they were at identifying the same part presented in isolation, even though both parts and wholes were tested in a forced-choice format and the whole faces differed only by one part. In contrast, three other types of stimuli--scrambled faces, inverted faces, and houses--did not show this advantage for part identification in whole object recognition.
Semantically-bounded anomia: implications for the neural implementation of naming.
We describe a patient whose anomia is disproportionately severe for fruits and vegetables when familiarity and name frequency are taken into account. His fruit and vegetable naming impairment was evident in a variety of different tasks. In contrast, he retained good general knowledge of fruits and vegetables, and he could access their names when given a phonemic cue. We discuss the phenomenon of semantically-bounded anomia in relation to the issues of local vs distributed representation, the existence of semantic "maps" in the brain, and the implementation of arbitrary associations in neural networks.
Agnosia.
Object recognition can break down in a variety of ways after brain damage. The resulting different forms of agnosia provide us with useful constraints on theories of normal object recognition. Recent studies suggest a division of labor for the recognition of different types of stimuli (common objects, words, faces, direction of eye gaze, spatial relations among parts of the human body), a high degree of interactivity in the processes underlying object recognition, and the possibility that recognition and awareness of recognition may be neurally distinct.
Visual angle of the mind's eye before and after unilateral occipital lobectomy.
Do mental images occur in a spatially mapped (i.e., analog, or array-format) representational medium? Kosslyn's (1978) method was used to measure the visual angle of "the mind's eye" to estimate the extent of the imagery medium before and after unilateral occipital lobectomy. It was found that the overall size of the largest possible image was reduced following the surgery. In addition, only the horizontal extent, and not the vertical extent, of the imagery medium was reduced. Finally, it was determined that the S understood the tasks, was not aware of our predictions, and was unaffected by a strong demand characteristic in a different imagery task. These results are consistent with the hypothesis that imagery occurs in a spatially mapped representational medium dependent on occipital cortex.
Can recognition of living things be selectively impaired?
Brain damage sometimes seems to impair recognition of living things, despite relatively preserved recognition of nonliving things. The most straightforward interpretation of this dissociation is that the recognition of living things depends on some specialized mechanisms that are not needed for the recognition of nonliving things. However, there are alternative interpretations of the dissociation in terms of the greater complexity or inter-item similarity of living things, or the more specific, within-category identifications that are usually required for living things. Surprisingly, the relevant tests to discriminate among these rival hypotheses have never been performed. We took the factors of visual complexity, inter-item similarity, specificity of identification, as well as others, into account in analyzing the visual recognition performance of two head-injured visual agnosic patients. In each case we found that recognition of living things was still disproportionately impaired when the effects of the other factors were accounted for.
Unconscious perception of "extinguished" visual stimuli: reassessing the evidence.
When parietal-damaged patients fail to report a contralesional stimulus because of extinction, is this because the stimulus is not perceived, or because it is perceived but cannot reach conscious awareness? VOLPE et al. [10] reported an intriguing study that seemed to locate the problem at least partly in the transfer of information to conscious awareness. They showed patients with extinction pairs of stimuli, one in each hemifield. Although patients were predictably poor at reporting the identity of the contralesional stimulus, they were able to make accurate same/different judgements comparing the two stimuli. This was interpreted as evidence that both stimuli were perceived. In the present paper, we point out that the dissociation between identification and same/different matching could also be due to the possibility that less visual information about the contralesional stimulus is necessary to make a same/different judgement than to identify the stimulus, and that chance performance is considerably higher in the first than in the second type of task. In Experiment 1, we verified this by degrading one side of a stimulus display and "replicating" the dissociation with normal subjects. We also equated the amount of visual information needed for the two tasks by yoking the stimulus pairs on "different" trials of the same/different matching task with the choice pairs on a forced choice identification task. Under these conditions, the dissociation vanished. In Experiment 2, we administered these tasks to three parietal-damaged patients with extinction. When the original method was used, same/different matching was better than identification of the contralesional stimulus. With the forced choice identification method, the dissociation again vanished.
Second-order relational properties and the inversion effect: testing a theory of face perception.
Recognition of faces is more severely impaired by inversion than is recognition of other types of objects. This was originally interpreted as evidence for the existence of special face-recognition mechanisms. Recently, Diamond and Carey (1986) attributed the inversion effect to the use of second-order relational properties that are important for, but not unique to, face recognition. According to their hypothesis, face recognition differs from the recognition of most other objects in its dependence on second-order relational properties. This hypothesis was tested in two experiments by comparing the effects of inversion on the identification of dot patterns that differed in the extent to which they required the encoding of second-order relational properties. Identification of the second-order relational patterns was not more disrupted by inversion than was identification of first-order patterns. These results fail to support the hypothesis that second-order relational properties are responsible for the inversion effect.