PubMed Health⌕ Search

Biomedical subjects

M J Farah

Publications and source records attributed to M J Farah.

At least 19 recordsLinked to original sources
PubMed1998

Verb generation in patients with focal frontal lesions: a neuropsychological test of neuroimaging findings.

What are the neural bases of semantic memory? Traditional beliefs that the temporal lobes subserve the retrieval of semantic knowledge, arising from lesion studies, have been recently called into question by functional neuroimaging studies finding correlations between semantic retrieval and activity in left prefrontal cortex. Has neuroimaging taught us something new about the neural bases of cognition that older methods could not reveal or has it merely identified brain activity that is correlated with but not causally related to the process of semantic retrieval? We examined the ability of patients with focal frontal lesions to perform a task commonly used in neuroimaging experiments, the generation of semantically appropriate action words for concrete nouns, and found evidence of the necessity of the left inferior frontal gyrus for certain components of the verb generation task. Notably, these components did not include semantic retrieval per se.

Adult↗
PubMed1998

The neural development and organization of letter recognition: evidence from functional neuroimaging, computational modeling, and behavioral studies.

Although much of the brain's functional organization is genetically predetermined, it appears that some noninnate functions can come to depend on dedicated and segregated neural tissue. In this paper, we describe a series of experiments that have investigated the neural development and organization of one such noninnate function: letter recognition. Functional neuroimaging demonstrates that letter and digit recognition depend on different neural substrates in some literate adults. How could the processing of two stimulus categories that are distinguished solely by cultural conventions become segregated in the brain? One possibility is that correlation-based learning in the brain leads to a spatial organization in cortex that reflects the temporal and spatial clustering of letters with letters in the environment. Simulations confirm that environmental co-occurrence does indeed lead to spatial localization in a neural network that uses correlation-based learning. Furthermore, behavioral studies confirm one critical prediction of this co-occurrence hypothesis, namely, that subjects exposed to a visual environment in which letters and digits occur together rather than separately (postal workers who process letters and digits together in Canadian postal codes) do indeed show less behavioral evidence for segregated letter and digit processing.

Adult↗
PubMed1998

What is "special" about face perception?

There is growing evidence that face recognition is "special" but less certainty concerning the way in which it is special. The authors review and compare previous proposals and their own more recent hypothesis, that faces are recognized "holistically" (i.e., using relatively less part decomposition than other types of objects). This hypothesis, which can account for a variety of data from experiments on face memory, was tested with 4 new experiments on face perception. A selective attention paradigm and a masking paradigm were used to compare the perception of faces with the perception of inverted faces, words, and houses. Evidence was found of relatively less part-based shape representation for faces. The literatures on machine vision and single unit recording in monkey temporal cortex are also reviewed for converging evidence on face representation. The neuropsychological literature is reviewed for-evidence on the question of whether face representation differs in degree or kind from the representation of other types of objects.

Face↗
PubMed1998

Is the category-specific deficit for living things spurious?

Some neurological patients make more errors naming living than nonliving things. This is true even when the difficulty of naming living and nonliving things, as measured by the performance of normal subjects, is taken into account. Gaffan and Keywood (1993) argued that this apparently selective deficit for living things is spurious and offered an alternative explanation of it in terms of the effect of noise on regression analyses. We report Monte Carlo simulations showing that, in two patients, the living things deficit cannot be accounted for this in this year. These simulations are more consistent with the claim that the recognition of living things depends on specialized mechanisms.

Artifacts↗
PubMed1997

Role of left inferior prefrontal cortex in retrieval of semantic knowledge: a reevaluation.

A number of neuroimaging findings have been interpreted as evidence that the left inferior frontal gyrus (IFG) subserves retrieval of semantic knowledge. We provide a fundamentally different interpretation, that it is not retrieval of semantic knowledge per se that is associated with left IFG activity but rather selection of information among competing alternatives from semantic memory. Selection demands were varied across three semantic tasks in a single group of subjects. Functional magnetic resonance imaging signal in overlapping regions of left IFG was dependent on selection demands in all three tasks. In addition, the degree of semantic processing was varied independently of selection demands in one of the tasks. The absence of left IFG activity for this comparison counters the argument that the effects of selection can be attributed solely to variations in degree of semantic retrieval. Our findings suggest that it is selection, not retrieval, of semantic knowledge that drives activity in the left IFG.

Humans↗
PubMed1997

Effects of bromocriptine on human subjects depend on working memory capacity.

Pharmacological manipulation of brain dopamine concentration affects visuospatial working memory in humans and in animals, the latter effects localized to the prefrontal cortex. However, the effects of dopamine agonists on humans are poorly understood. We hypothesized that bromocriptine would have an effect on cognitive functions associated with the prefrontal cortex via its effects on cortical dopamine receptors and on subcortical receptors in areas that project to the neocortex. We found that the effect of bromocriptine on young normal subjects depended on the subjects' working memory capacity. High-capacity subjects performed more poorly on the drug, while low-capacity subjects improved. These results demonstrate an empirical link between a dopamine-mediated working memory system and higher cognitive function in humans.

Association Learning↗
PubMed1997

A simple common contexts explanation for the development of abstract letter identities.

Abstract letter identities (ALIs) are an early representation in visual word recognition that are specific to written language. They do not reflect visual or phonological features, but rather encode the identities of letters independent of case, font, sound, and so forth. How could the visual system come to develop such a representation? We propose that because many letters look similar regardless of case, font, and other characteristics, these provide common contexts for visually dissimilar uppercase and lowercase forms of other letters (e.g., e between k and y in key and E in the visually similar context K-Y). Assuming that the distribution of words' relative frequencies is comparable in upper- and lowercase (that just as key is more frequent than pew, KEY is more frequent than PEW), these common contexts will also be similarly distributed in the two cases. We show how this statistical regularity could lead Hebbian learning to produce ALIs in a competitive architecture. We present a self-organizing artificial neural network that illustrates this idea and produces ALIs when presented with the most frequent words from a beginning reading corpus, as well as with artificial input.

Form Perception↗
PubMed1997

A functional MRI study of mental image generation.

The neural substrates of mental image generation were investigated with functional MRI. Subjects listened to words under two different instructional conditions: to generate visual mental images of the words' referents, or to simply listen to each word and wait for the next word. Analyses were performed which directly compared the regional brain activity during each condition, with the goal of discovering whether mental image generation engages modality-specific visual areas, whether it engages primary visual cortex, and whether it recruits the left hemisphere to a greater extent than the right. Results revealed that visual association cortex, and not primary visual cortex, was engaged during the mental image generation condition. Left inferior temporal lobe (Brodmann's area 37) was the most reliably and robustly activated area across subjects, had activity which extended superiorly into occipital association cortex (area 19). The results of this experiment support the hypothesis that visual mental imagery is a function of visual association cortex, and that image generation is asymmetrically localized to the left.

Adolescent↗
PubMed1997

Consciousness of perception after brain damage.

Disturbances of visual perception after brain damage provide clues to understanding consciousness and the brain. In this article we review six visual disorders in which perception and consciousness are dissociated as a result of brain damage: blindsight, implicit shape perception in apperceptive visual agnosia, covert recognition of faces in prosopagnosia, unconscious perception in neglect and extinction, implicit reading in pure alexia, and implicit object recognition in associative visual agnosia. We consider these six disorders from the standpoint of three main schools of thought concerning consciousness and the brain, namely a localized system for consciousness, consciousness as a state of integration, and consciousness as a property of graded representation. The findings suggest that these syndromes do not share a single mechanism and that it is conceivable that more than one explanation will be necessary both within and across syndromes. We conclude on the basis of the current evidence that it is unlikely that any single brain system is necessary for conscious awareness of perception that does not play a role in perception as well.

Agnosia↗
PubMed1997

Is visual image segmentation a bottom-up or an interactive process?

Visual image segmentation is the process by which the visual system groups features that are part of a single shape. Is image segmentation a bottom-up or an interactive process? In Experiments 1 and 2, we presented subjects with two overlapping shapes and asked them to determine whether two probed locations were on the same shape or on different shapes. The availability of top-down support was manipulated by presenting either upright or rotated letters. Subjects were fastest to respond when the shapes corresponded to familiar shapes--the upright letters. In Experiment 3, we used a variant of this segmentation task to rule out the possibility that subjects performed same/different judgments after segmentation and recognition of both letters. Finally, in Experiment 4, we ruled out the possibility that the advantage for upright letters was merely due to faster recognition of upright letters relative to rotated letters. The results suggested that the previous effects were not due to faster recognition of upright letters; stimulus familiarity influenced segmentation per se. The results are discussed in terms of an interactive model of visual image segmentation.

Adult↗
PubMed1996

Mental rotation may underlie apparent object-based neglect.

Previous investigations have suggested that object-based neglect may reflect an impairment in attentional allocation that occurs relative to the intrinsic left and right of objects. We report a patient with apparent "object-based" neglect of 90 degrees rotated stimuli for whom the pattern of neglect was a function of task strategy. When the patient was instructed to visualize the rotated stimuli as if they were upright, i.e. mentally rotate them, he showed apparent "object-based" neglect to the left of the principal axes of the stimuli. In contrast, when instructed to refrain from mental rotation, neglect was apparent only with respect to his left, but not the left of the stimuli. Thus, the apparent "object-based" neglect of this patient may be attributed to a process of mental rotation of objects to upright, and subsequent neglect in viewer-centered or environment-centered coordinates. These data suggest a mechanism whereby object-based and viewer/environment-centered reference frames may be aligned, thereby causing viewer/environment-centered neglect to appear as if object-based.

Aged↗
PubMed1996

A category-specific naming impairment after temporal lobectomy.

Unilateral temporal lobectomy patients and normal control subjects were tested in a speeded naming task with pictures of living and nonliving things that were equated for name frequency, familiarity, and visual complexity. Although right temporal lobectomy patients and normal subjects performed equally well with the living relative to nonliving things, left temporal lobectomy patients were disproportionately impaired at naming nonliving things. This result has several implications: First, it supports the existence of category-specific naming impairments. In particular, it undermines the proposal that living-nonliving dissociations are artifactual, resulting from the greater difficulty of living things. Second, it demonstrates an asymmetry in the neural representation of nonliving things, in favor of the left hemisphere. Third, it casts doubt on the hypothesis that the anterior temporal cortices are convergence zones that are particularly necessary for the naming of living things.

Adolescent↗
PubMed1996

Is face recognition 'special'? Evidence from neuropsychology.

Is face recognition 'special,' in the sense of relying on functionally and anatomically distinct mechanisms from those required for other kinds of pattern recognition? A number of different neuropsychological dissociations involving recognition and learning of faces and nonface objects are reviewed. In addition, studies of the nature of shape representation in normal face and object recognition are reviewed. The evidence from brain-damaged and normal subjects suggests that face recognition is, indeed, 'special,' and provides some clues to the functional differences between face and object recognition.

Adult↗
PubMed1996

The semantic memory impairment of Alzheimer's disease: category-specific?

We addressed the question of whether Alzheimer's Disease (AD) causes a selective impairment for knowledge of living things. Although we replicated a previous finding that AD subjects name pictures of living things less accurately than pictures of nonliving things, we also failed to observe this selective impairment when we used two new stimulus sets, which more tightly controlled the overall naming difficulty of the living and nonliving items. We conclude that, whereas some individuals may have bona fide selective impairments in semantic memory as a result of herpes simplex encephalitis or head injury, AD does not generally give rise to selective impairments in knowledge of living things.

Aged↗
PubMed1996

Tactile agnosia. Underlying impairment and implications for normal tactile object recognition.

In a series of experimental investigations of a subject with a unilateral impairment of tactile object recognition without impaired tactile sensation, several issues were addressed. First, is tactile agnosia secondary to a general impairment of spatial cognition? On tests of spatial ability, including those directed at the same spatial integration process assumed to be taxed by tactile object recognition, the subject performed well, implying a more specific impairment of high level, modality specific tactile perception. Secondly, within the realm of high level tactile perception, is there a distinction between the ability to derive shape ('what') and spatial ('where') information? Our testing showed an impairment confined to shape perception. Thirdly, what aspects of shape perception are impaired in tactile agnosia? Our results indicate that despite accurate encoding of metric length and normal manual exploration strategies, the ability tactually to perceive objects with the impaired hand, deteriorated as the complexity of shape increased. In addition, asymmetrical performance was not found for other body surfaces (e.g. her feet). Our results suggest that tactile shape perception can be disrupted independent of general spatial ability, tactile spatial ability, manual shape exploration, or even the precise perception of metric length in the tactile modality.

Agnosia↗
PubMed1995

Brain localization for arbitrary stimulus categories: a simple account based on Hebbian learning.

A central theme of cognitive neuroscience is that different parts of the brain perform different functions. Recent evidence from neuropsychology suggests that even the processing of arbitrary stimulus categories that are defined solely by cultural conventions (e.g., letters versus digits) can become spatially segregated in the cerebral cortex. How could the processing of stimulus categories that are not innate and that have no inherent structural differences become segregated? We propose that the temporal clustering of stimuli from a given category interacts with Hebbian learning to lead to functional localization. Neural network simulations bear out this hypothesis.

Brain↗