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Biomedical subjects

Leslie G Ungerleider

Publications and source records attributed to Leslie G Ungerleider.

At least 19 recordsLinked to original sources
PubMed2006

Temporal dynamics of face repetition suppression.

Single-unit recordings and functional brain imaging studies have shown reduced neural responses to repeated stimuli in the visual cortex. Using MEG, we compared responses evoked by repetitions of neutral faces to those evoked by fearful faces, which were either task relevant (targets) or irrelevant (distracters). Faces evoked a bi-phasic response in extrastriate cortex, peaking at 160-185 ms and at 220-250 ms, with stronger responses to neutral faces at the earlier interval and stronger responses to fearful faces at the later interval. At both latencies, repetitions of neutral and fearful targets resulted in reduced amplitude of the MEG signal. Additionally, we found that the context in which targets were presented affected their processing: fearful distracters increased the responses evoked by both neutral and fearful targets. Our data indicate that valence enhancement and context effects can be detected in extrastriate visual cortex within 250 ms and that these processes likely reflect feedback from other regions.

Adult↗
PubMed2005

Tactile form and location processing in the human brain.

To elucidate the neural basis of the recognition of tactile form and location, we used functional MRI while subjects discriminated gratings delivered to the fingertip of either the right or left hand. Subjects were required to selectively attend to either grating orientation or grating location under identical stimulus conditions. Independent of the hand that was stimulated, grating orientation discrimination selectively activated the left intraparietal sulcus, whereas grating location discrimination selectively activated the right temporoparietal junction. Hence, hemispheric dominance appears to be an organizing principle for cortical processing of tactile form and location.

Adult↗
PubMed2005

Target visibility and visual awareness modulate amygdala responses to fearful faces.

The goals of the present study were twofold. First, we wished to investigate the neural correlates of aware and unaware emotional face perception after characterizing each subject's behavioral performance via signal detection theory methods. Second, we wished to investigate the extent to which amygdala responses to fearful faces depend on the physical characteristics of the stimulus independently of the percept. We show that amygdala responses depend on visual awareness. Under conditions in which subjects were not aware of fearful faces flashed for 33 ms, no differential activation was observed in the amygdala. On the other hand, differential activation was observed for 67 ms fearful targets that the subjects could reliably detect. When trials were divided into hits, misses, correct rejects, and false alarms, we show that target visibility is an important factor in determining amygdala responses to fearful faces. Taken together, our results further challenge the view that amygdala responses occur automatically.

Adaptation, Physiological↗
PubMed2005

Visual awareness and the detection of fearful faces.

A commonly held view is that emotional stimuli are processed independently of awareness. Here, the authors parametrically varied the duration of a fearful face target stimulus that was backward masked by a neutral face. The authors evaluated awareness by characterizing behavioral performance using receiver operating characteristic curves from signal detection theory. Their main finding was that no universal objective awareness threshold exists for fear perception. Although several subjects displayed a behavioral pattern consistent with previous reports (i.e., targets masked at 33 ms), a considerable percentage of their subjects (64%) were capable of reliably detecting 33-ms targets. Their findings suggest that considerable information is available even in briefly presented stimuli (possibly as short as 17 ms) to support masked fear detection.

Adult↗
PubMed2004

Impaired filtering of distracter stimuli by TE neurons following V4 and TEO lesions in macaques.

Directing attention to a behaviorally relevant visual stimulus can overcome the distracting effects of other nearby stimuli. Correspondingly, physiological studies indicate that attention serves to filter distracting stimuli from receptive fields (RFs) in several extrastriate areas. Moreover, a recent study demonstrated that lesions of extrastriate areas V4 and TEO produce impairments in attentional filtering. A critical remaining question concerns why lesions of ventral stream areas cause attentional filtering impairments. To address this question, we tested the effects of restricted area V4 and TEO lesions on both behavioral performance and the responses of downstream neurons in area TE. The lesions impaired behavioral discrimination thresholds and altered neuronal selectivity for target stimuli in the presence of distracters. With attention to the target, but in the absence of V4 and/or TEO inputs, TE neurons responded as though attentional inputs could no longer be used to filter distracters from their RFs. This presumably occurred because top-down attentional signals were no longer able to filter distracters from the RFs of the cells that provide TE with major input. Consistent with this interpretation, increasing the spatial separation between targets and distracters, such that they no longer fell within a typical V4 RF dimension, restored both behavioral performance and neuronal selectivity in the portion of TE RFs affected by the V4 lesion.

Animals↗
PubMed2004

Visual responses to targets and distracters by inferior temporal neurons after lesions of extrastriate areas V4 and TEO.

While lesions of visual areas V4 and TEO only modestly affect discrimination of isolated objects, they significantly impair the ability to selectively attend to an object surrounded by distracters. To test whether such deficits result from a loss of inputs to higher order areas, we recorded from area TE neurons after removing portions of V4 and TEO in a monkey. Responses to isolated targets in a lesion-affected visual quadrant were substantially preserved, indicating that TE still receives information even after removing a major source of input. Distracters increased or decreased the response to targets more in the lesion-affected than in the normal quadrant, supporting the idea that V4 and/or TEO are sites where top-down attentional inputs filter out distracting stimuli.

Analysis of Variance↗
PubMed2004

Repetition suppression of faces is modulated by emotion.

Single-unit recordings and functional brain imaging studies have shown reduced neural responses to repeated stimuli in the visual cortex. By using event-related functional MRI, we compared the activation evoked by repetitions of neutral and fearful faces, which were either task relevant (targets) or irrelevant (distracters). We found that within the inferior occipital gyri, lateral fusiform gyri, superior temporal sulci, amygdala, and the inferior frontal gyri/insula, targets evoked stronger responses than distracters and their repetition was associated with significantly reduced responses. Repetition suppression, as manifested by the difference in response amplitude between the first and third repetitions of a target, was stronger for fearful than neutral faces. Distracter faces, regardless of their repetition or valence, evoked negligible activation, indicating top-down attenuation of behaviorally irrelevant stimuli. Our findings demonstrate a three-way interaction between emotional valence, repetition, and task relevance and suggest that repetition suppression is influenced by high-level cognitive processes in the human brain.

Adult↗
PubMed2004

Neural correlates of change detection and change blindness in a working memory task.

Detecting changes in an ever-changing environment is highly advantageous, and this ability may be critical for survival. In the present study, we investigated the neural substrates of change detection in the context of a visual working memory task. Subjects maintained a sample visual stimulus in short-term memory for 6 s, and were asked to indicate whether a subsequent, test stimulus matched or did not match the original sample. To study change detection largely uncontaminated by attentional state, we compared correct change and correct no-change trials at test. Our results revealed that correctly detecting a change was associated with activation of a network comprising parietal and frontal brain regions, as well as activation of the pulvinar, cerebellum, and inferior temporal gyrus. Moreover, incorrectly reporting a change when none occurred led to a very similar pattern of activations. Finally, few regions were differentially activated by trials in which a change occurred but subjects failed to detect it (change blindness). Thus, brain activation was correlated with a subject's report of a change, instead of correlated with the physical change per se. We propose that frontal and parietal regions, possibly assisted by the cerebellum and the pulvinar, might be involved in controlling the deployment of attention to the location of a change, thereby allowing further processing of the visual stimulus. Visual processing areas, such as the inferior temporal gyrus, may be the recipients of top-down feedback from fronto-parietal regions that control the reactive deployment of attention, and thus exhibit increased activation when a change is reported (irrespective of whether it occurred or not). Whereas reporting that a change occurred, be it correctly or incorrectly, was associated with strong activation in fronto-parietal sites, change blindness appears to involve very limited territories.

Adaptation, Physiological↗
PubMed2004

Neuroimaging studies of attention and the processing of emotion-laden stimuli.

Because the processing capacity of the visual system is limited, selective attention to one part of the visual field comes at the cost of neglecting other parts. In this paper, we review evidence from single-cell studies in monkeys and functional magnetic resonance imaging (fMRI) studies in humans for neural competition and how competition is biased by attention. We suggest that, at the neural level, an important consequence of attention is to enhance the influence of behaviorally relevant stimuli at the expense of irrelevant ones, providing a mechanism for the filtering of distracting information in cluttered visual scenes. Psychophysical evidence suggests that processing outside the focus of attention is attenuated and may be even eliminated under some conditions. A major exception to the critical role of attention may be in the neural processing of emotion-laden stimuli, which are reported to be processed automatically, namely, without attention. Contrary to this prevailing view, in a recent study we found that all brain regions responding differentially to faces with emotional content, including the amygdala, did so only when sufficient resources were available to process those faces. After reviewing our findings, we discuss their implications, in particular (1) how emotional stimuli can bias competition for processing resources; (2) the source of the biasing signal for emotional stimuli; (3) how visual information reaches the amygdala; and finally (4) the relationship between attention and awareness.

Amygdala↗
PubMed2003

Posterior parietal cortex and the filtering of distractors.

Neural systems for visual processing can focus attention on behaviorally relevant objects, filtering out competing distractors. Neurophysiological studies in animals and brain imaging studies in humans suggest that such filtering depends on top-down inputs to extrastriate visual areas, originating in structures important for attentional control. To test whether the posterior parietal cortex may be a necessary source of signals that filter distractors, we measured the ability of a patient with bilateral parietal lesions to discriminate the features of a target surrounded by distractors of variable contrast. In the presence of distractors, the patient was impaired at discriminating both grating orientation and faces, and the magnitude of the impairment increased with distractor salience. These attentional deficits are remarkably similar to those caused by damage to monkey extrastriate regions V4 andor TEO, which are thought to be recipients of top-down attentional feedback. In contrast to the effects of V4 and TEO lesions, however, the parietal lesions impaired performance even with widely spaced targets and distractors, a finding consistent with the projections of parietal cortex to visual processing areas covering a wide range of receptive field sizes and eccentricities.

Aged↗
PubMed2003

Distinct contribution of the cortico-striatal and cortico-cerebellar systems to motor skill learning.

This review paper focuses on studies in healthy human subjects that examined the functional neuroanatomy and cerebral plasticity associated with the learning, consolidation and retention phases of motor skilled behaviors using modern brain imaging techniques. Evidence in support of a recent model proposed by Doyon and Ungerleider [Functional Anatomy of Motor Skill Learning. In: Squire LR, Schacter DL, editors. Neuropsychology of Memory. New York: Guilford Press, 2002.] is also discussed. The latter suggests that experience-dependent changes in the brain depend not only on the stage of learning, but also on whether subjects are required to learn a new sequence of movements (motor sequence learning) or learn to adapt to environmental perturbations (motor adaptation). This model proposes that the cortico-striatal and cortico-cerebellar systems contribute differentially to motor sequence learning and motor adaptation, respectively, and that this is most apparent during the slow learning phase (i.e. automatization) when subjects achieve asymptotic performance, as well as during reactivation of the new skilled behavior in the retention phase.

Animals↗
PubMed2003

Impairments in spatial generalization of visual skills after V4 and TEO lesions in macaques (Macaca mulatta).

The authors tested the spatial generalization of shape and color discriminations in 2 monkeys, in which 3 visual field quadrants were affected, respectively, by lesions in area V4, TEO, or both areas combined. The fourth quadrant served as a normal control. The monkeys were trained to discriminate stimuli presented in a standard location in each quadrant, followed by tests of discrimination performance in new locations in the same quadrant. In the quadrant affected by the V4 + TEO lesion, the authors found temporary but striking deficits in spatial generalization of shape and color discriminations over small distances, suggesting a contribution of areas V4 and TEO to short-range spatial generalization of visual skills.

Animals↗
PubMed2003

Generalized deficits in visual selective attention after V4 and TEO lesions in macaques.

To test the role of areas V4 and TEO in the attentional filtering of distracting information, we studied the effects of lesions in these areas, in monkeys discriminating target stimuli surrounded by irrelevant distracters. The lesions were restricted, such that a single visual field quadrant was affected by a V4 lesion alone, a TEO lesion alone, or a combined lesion in V4 and TEO, while one quadrant served as a normal control. The monkeys fixated a spot while discriminating the orientation, colour or motion of target stimuli presented extrafoveally in each quadrant. When the target was presented alone, discrimination deficits in the quadrants affected by the lesions were generally small. However, these deficits were substantially increased by surrounding the target with luminance, colour or motion distracters. The discrimination of target orientation was more impaired than the discrimination of target colour or motion, irrespective of distracter type. The discrimination of target motion was strongly affected only by motion distracters. The magnitude of the impairments increased with distracter strength and with the extent to which the distracters conveyed information conflicting with the target. Deficits in the quadrant affected by combined V4 and TEO lesions were twice as large as those in quadrants affected by V4 or TEO lesions alone. The results suggest that in the absence of V4 and TEO, information from both relevant and irrelevant stimuli is 'averaged' together across several different feature domains, impairing the discrimination of the relevant target features. The results suggest a broad role of V4 and TEO in visual selective attention.

Animals↗
PubMed2002

Experience-dependent changes in cerebellar contributions to motor sequence learning.

Studies in experimental animals and humans have stressed the role of the cerebellum in motor skill learning. Yet, the relative importance of the cerebellar cortex and deep nuclei, as well as the nature of the dynamic functional changes occurring between these and other motor-related structures during learning, remains in dispute. Using functional magnetic resonance imaging and a motor sequence learning paradigm in humans, we found evidence of an experience-dependent shift of activation from the cerebellar cortex to the dentate nucleus during early learning, and from a cerebellar-cortical to a striatal-cortical network with extended practice. The results indicate that intrinsic modulation within the cerebellum, in concert with activation of motor-related cortical regions, serves to set up a procedurally acquired sequence of movements that is then maintained elsewhere in the brain.

Adult↗
PubMed2002

Task-independent functional brain activity correlation with skin conductance changes: an fMRI study.

Lesions of the ventromedial prefrontal cortex cause a loss of skin conductance response (SCR) to stimuli with affective content and an inability to integrate information with social consequences into decisions. Previous behavioral studies using a gambling task were able to differentiate patients with lesions in this region from normal subjects. In the present imaging study, this region, among others, was shown to be "spontaneously" activated during three different cognitive states: a gambling task, a working memory task, and resting state. SCR data were simultaneously collected during the scanning process. Six subjects were scanned at 1.5 T during all three states, and one subject was scanned at 3 T during the resting state only. SCR data were used as a reference function for correlation analysis with the fMRI time series during each of the three tasks. SCR changes were evident during the gambling and two-back tasks as well as during rest. SCR activity was not observed to be specifically related to reward-based decisions in the gambling task. Correlation of the fMRI time series directly with the SCR data revealed a consistent set of activated regions. The activity of these regions showing correlation with the SCR appeared independent of the cognitive state. Further, the subject scanned only at rest (without the possible confound of task-related carryover activity) replicated the findings in the original six subjects. From these data, SCR appears to be a marker of a network that is active during, but independent of, the task being studied.

Adult↗
PubMed2002

Visual imagery of famous faces: effects of memory and attention revealed by fMRI.

Complex pictorial information can be represented and retrieved from memory as mental visual images. Functional brain imaging studies have shown that visual perception and visual imagery share common neural substrates. The type of memory (short- or long-term) that mediates the generation of mental images, however, has not been addressed previously. The purpose of this study was to investigate the neural correlates underlying imagery generated from short- and long-term memory (STM and LTM). We used famous faces to localize the visual response during perception and to compare the responses during visual imagery generated from STM (subjects memorized specific pictures of celebrities before the imagery task) and imagery from LTM (subjects imagined famous faces without seeing specific pictures during the experimental session). We found that visual perception of famous faces activated the inferior occipital gyri, lateral fusiform gyri, the superior temporal sulcus, and the amygdala. Small subsets of these face-selective regions were activated during imagery. Additionally, visual imagery of famous faces activated a network of regions composed of bilateral calcarine, hippocampus, precuneus, intraparietal sulcus (IPS), and the inferior frontal gyrus (IFG). In all these regions, imagery generated from STM evoked more activation than imagery from LTM. Regardless of memory type, focusing attention on features of the imagined faces (e.g., eyes, lips, or nose) resulted in increased activation in the right IPS and right IFG. Our results suggest differential effects of memory and attention during the generation and maintenance of mental images of faces.

Adult↗