Biomedical subjects

Uri Hasson

Publications and source records attributed to Uri Hasson.

17 recordsLinked to original sources

PubMed2006

Extrinsic and intrinsic systems in the posterior cortex of the human brain revealed during natural sensory stimulation.

When exposing subjects to a continuous segment of an audiovisual movie, a large expanse of human cortex, especially in the posterior half of the cerebral cortex, shows stimulus-driven activity. However, embedded within this widespread activity, there are cortical regions whose activity is dissociated from the external stimulation. These regions are intercorrelated among themselves, forming a functional network, which largely overlaps with cortical areas previously shown to be deactivated by task-oriented paradigms. Moreover, the network of areas whose neuronal dynamics are associated with external inputs and the network of areas that appears to be intrinsically driven complement each other, providing coverage of most of the posterior cortex. Thus, we propose that naturalistic stimuli reveal a fundamental neuroanatomical partition of the human posterior cortex into 2 global networks: an "extrinsic" system, comprising areas associated with the processing of external inputs, and an "intrinsic" system, largely overlapping with the task-negative, default-mode network, comprising areas associated with--as yet not fully understood--intrinsically oriented functions.

Acoustic Stimulation↗

PubMed2006

Widespread functional connectivity and fMRI fluctuations in human visual cortex in the absence of visual stimulation.

To what extent does the visual system's activity fluctuate when no sensory stimulation is present? Here, we studied this issue by examining spontaneous fluctuations in BOLD signal in the human visual system, while subjects were placed in complete darkness. Our results reveal widespread slow fluctuations during such rest periods. In contrast to stimulus-driven activity, during darkness, functionally distinct object areas were fluctuating in unison. These fMRI fluctuations became rapidly spatially de-correlated (39% drop in correlation level, P < 0.008) during visual stimulation. Functional connectivity analysis revealed that the slow spontaneous fluctuations during rest had consistent and specific neuro-anatomical distribution which argued against purely hemodynamic noise sources. Control experiments ruled out eye closure, low luminance and mental imagery as the underlying sources of the spontaneous fluctuations. These results demonstrate that, when no stimulus is present, sensory systems manifest a robust level of slow organized fluctuation patterns.

Adult↗

PubMed2006

Repetition suppression for spoken sentences and the effect of task demands.

We examined whether the repeated processing of spoken sentences is accompanied by reduced bold oxygenation level-dependent response (repetition suppression) in regions implicated in sentence comprehension and whether the magnitude of such suppression depends on the task under which the sentences are comprehended or on the complexity of the sentences. We found that sentence repetition was associated with repetition suppression in temporal regions, independent of whether participants judged the sensibility of the statements or listened to the statements passively. In contrast, repetition suppression in inferior frontal regions was found only in the context of the task demanding active judgment. These results suggest that repetition suppression in temporal regions reflects facilitation of sentence comprehension processing per se, whereas in frontal regions it reflects, at least in part, easier execution of specific psycholinguistic judgments.

Adult↗

PubMed2006

Human brain activation during viewing of dynamic natural scenes.

To what extent do brains of different human individuals operate in a similar manner? Here we explored the organization and function of different brain regions under progressively more natural conditions. Applying an unbiased analysis, in which spatiotemporal activity patterns in one brain were used to 'model' activity in another brain, we found a striking level of voxel by voxel synchronization between individuals during free viewing of an audio-visual movie. This intersubject correlation was evident not only in primary and secondary visual and auditory areas, but also in association cortices. The results reveal a surprising tendency of individual brains to 'tick collectively' during natural vision. Moreover, our results demonstrate that the unitary nature of conscious experience in fact consists of temporally interleaved and highly selective activations in an ensemble of specialized regions, each of which 'picks-up' and analyses its own unique subset of stimuli according to its functional specialization. Applying reverse correlation to the movie stimuli provides a powerful methodology for revealing both known and unexpected functional specializations in those cortical areas activated by the movie.

Brain↗

PubMed2005

Coupling between neuronal firing, field potentials, and FMRI in human auditory cortex.

Functional magnetic resonance imaging (fMRI) is an important tool for investigating human brain function, but the relationship between the hemodynamically based fMRI signals in the human brain and the underlying neuronal activity is unclear. We recorded single unit activity and local field potentials in auditory cortex of two neurosurgical patients and compared them with the fMRI signals of 11 healthy subjects during presentation of an identical movie segment. The predicted fMRI signals derived from single units and the measured fMRI signals from auditory cortex showed a highly significant correlation (r = 0.75, P < 10(-47)). Thus, fMRI signals can provide a reliable measure of the firing rate of human cortical neurons.

Adult↗

PubMed2005

Believe it or not: on the possibility of suspending belief.

We present two experiments that cast doubt on existing evidence suggesting that it is impossible to suspend belief in a comprehended proposition. In Experiment 1, we found that interrupting the encoding of a statement's veracity decreased memory for the statement's falsity when the false version of the statement was uninformative, but not when the false version was informative. This suggests that statements that are informative when false are not represented as if they were true. In Experiment 2, participants made faster lexical decisions to words implied by preceding statements when they were told that the statements were true than when the veracity of the statements was unknown or when the statements were false. The findings suggest that comprehending a statement may not require believing it, and that it may be possible to suspend belief in comprehended propositions.

Adult↗

PubMed2005

Detailed exploration of face-related processing in congenital prosopagnosia: 2. Functional neuroimaging findings.

Specific regions of the human occipito-temporal cortex are consistently activated in functional imaging studies of face processing. To understand the contribution of these regions to face processing, we examined the pattern of fMRI activation in four congenital prosopagnosic (CP) individuals who are markedly impaired at face processing despite normal vision and intelligence, and with no evidence of brain damage. These individuals evinced a normal pattern of fMRI activation in the fusiform gyrus (FFA) and in other ventral occipito-temporal areas, in response to faces, buildings, and other objects, shown both as line drawings in detection and discrimination tasks and under more naturalistic testing conditions when no task was required. CP individuals also showed normal adaptation levels in a block-design adaptation experiment and, like control subjects, exhibited evidence of global face representation in the FFA. The absence of a BOLD-behavioral correlation (profound behavioral deficit, normal face-related activation in the ventral occipito-temporal cortex) challenges existing accounts of face representation, and suggests that activation in these cortical regions per se is not sufficient to ensure intact face processing.

Adult↗

PubMed2004

One picture is worth at least a million neurons.

How many neurons participate in the representation of a single visual image? Answering this question is critical for constraining biologically inspired models of object recognition, which vary greatly in their assumptions from few "grandmother cells" to numerous neurons in widely distributed networks. Functional imaging techniques, such as fMRI, provide an opportunity to explore this issue, since they allow the simultaneous detection of the entire neuronal population responding to each stimulus. Several studies have shown that fMRI BOLD signal is approximately proportional to neuronal activity. However, since it provides an indirect measure of this activity, obtaining a realistic estimate of the number of activated neurons requires several intervening steps. Here, we used the extensive knowledge of primate V1 to yield a conservative estimate of the ratio between hemodynamic response and neuronal firing. This ratio was then used, in addition to several cautious assumptions, to assess the number of neurons responding to a single-object image in the entire visual cortex and particularly in object-related areas. Our results show that at least a million neurons in object-related cortex and about two hundred million neurons in the entire visual cortex are involved in the representation of a single-object image.

Adult↗

PubMed2004

Intersubject synchronization of cortical activity during natural vision.

To what extent do all brains work alike during natural conditions? We explored this question by letting five subjects freely view half an hour of a popular movie while undergoing functional brain imaging. Applying an unbiased analysis in which spatiotemporal activity patterns in one brain were used to "model" activity in another brain, we found a striking level of voxel-by-voxel synchronization between individuals, not only in primary and secondary visual and auditory areas but also in association cortices. The results reveal a surprising tendency of individual brains to "tick collectively" during natural vision. The intersubject synchronization consisted of a widespread cortical activation pattern correlated with emotionally arousing scenes and regionally selective components. The characteristics of these activations were revealed with the use of an open-ended "reverse-correlation" approach, which inverts the conventional analysis by letting the brain signals themselves "pick up" the optimal stimuli for each specialized cortical area.

Adult↗

PubMed2004

Functional analysis of the periphery effect in human building related areas.

Several studies have shown that a region in the anterior collateral sulcus (CoS) and a region in the vicinity of the transverse occipital sulcus (TOS) are preferentially activated by images of buildings and scenes. We have found recently that these regions show a strong activation bias to stimuli located in the peripheral visual field. We explore in detail the source of this "periphery" effect. Our results show that the periphery effect can be generated by a large single object occupying the peripheral visual field as well as by multiple small peripheral objects. We also investigated whether the periphery effect was related to the annular shape used in conventional mapping of the visual field periphery and found that the mere presence of a stimulus in the visual field periphery, regardless of object shape, is sufficient to enhance activation. We also found that a small bias toward the peripheral visual field was shown even when the stimulated areas in the central and peripheral parts of the visual field are equated. Finally, our results demonstrate that the periphery effect shows object selectivity that can be obtained even with face images, which are the non-optimal stimulus for this region. In summary, our study shows that the building-related CoS and TOS manifest a true but graded retinotopic bias toward the peripheral visual field.

Adult↗

PubMed2004

The importance of being nonalignable: a critical test of the structural alignment theory of similarity.

The structural alignment theory of similarity distinguishes 2 types of difference that may occur between stimuli: Alignable differences are those related to a commonality, whereas nonalignable differences are not related to a commonality. Alignment theory predicts that alignable differences should be more heavily weighted than nonalignable differences in similarity judgment. Experiments 1 and 2 demonstrate that, contrary to this prediction, nonalignable differences exerted a greater impact than alignable differences in similarity and difference judgments of geometric stimuli. Experiment 3 revealed that the relative weight accorded a given difference was also affected by contextual constraints. Thus, although the experiments supported the validity of the distinction between alignable and nonalignable differences, results were discordant with the specific prediction of structural alignment theory.

Form Perception↗

PubMed2003

Face-selective activation in a congenital prosopagnosic subject.

Congenital prosopagnosia is a severe impairment in face identification manifested from early childhood in the absence of any evident brain lesion. In this study, we used fMRI to compare the brain activity elicited by faces in a congenital prosopagnosic subject (YT) relative to a control group of 12 subjects in an attempt to shed more light on the nature of the brain mechanisms subserving face identification. The face-related activation pattern of YT in the ventral occipito-temporal cortex was similar to that observed in the control group on several parameters: anatomical location, activation profiles, and hemispheric laterality. In addition, using a modified vase-face illusion, we found that YT's brain activity in the face-related regions manifested global grouping processes. However, subtle differences in the degree of selectivity between objects and faces were observed in the lateral occipital cortex. These data suggest that face-related activation in the ventral occipito-temporal cortex, although necessary, might not be sufficient by itself for normal face identification.

Adult↗

PubMed2003

Large-scale mirror-symmetry organization of human occipito-temporal object areas.

We have combined functional maps of retinotopy (eccentricity and meridian mapping), object category, and motion in a group of subjects to explore the large-scale topography of higher-order object areas. Our results reveal seven consistent category-related entities situated in the occipito-temporal cortex adjoining early visual areas. These include two face-related regions, three object-related regions, and two building-related regions. Interestingly, this complex category-related pattern is organized in a large-scale dorso-ventral mirror symmetry of object category. Furthermore, correlating this pattern to the map of visual field eccentricity, we found that the entire network of areas could be related to a single and unified eccentricity map. We hypothesize that this large-scale organization points to a possible development of high-order object areas through extension and specialization of a single proto-representation.

Brain Mapping↗

PubMed2003

Counterexamples in sentential reasoning.

How do logically naive individuals determine that an inference is invalid? In logic, there are two ways to proceed: (1) make an exhaustive search but fail to find a proof of the conclusion and (2) use the interpretation of the relevant sentences to construct a counterexample--that is, a possibility consistent with the premises but inconsistent with the conclusion. We report three experiments in which the strategies that individuals use to refute invalid inferences based on sentential connectives were examined. In Experiment 1, the participants' task was to justify their evaluations, and it showed that they used counterexamples more often than any other strategy. Experiment 2 showed that they were more likely to use counterexamples to refute invalid conclusions consistent with the premises than to refute invalid conclusions inconsistent with the premises. In Experiment 3, no reliable difference was detected in the results between participants who wrote justifications and participants who did not.

Concept Formation↗

PubMed2002

Analysis of the neuronal selectivity underlying low fMRI signals.

BACKGROUND: A prevailing assumption in neuroimaging studies is that relatively low fMRI signals are due to weak neuronal activation, and, therefore, they are commonly ignored. However, lower fMRI signals may also result from intense activation by highly selective, albeit small, subsets of neurons in the imaged voxel. We report on an approach that could form a basis for resolving this ambiguity imposed by the low (mm range) spatial resolution of fMRI. Our approach employs fMR-adaptation as an indicator for highly active neuronal populations even when the measured fMRI signal is low. RESULTS: In this study, we first showed that fMRI-adaptation is diminished when overall neuronal activity is lowered substantially by reducing image contrast. We then applied the same adaptation paradigm, but this time we lowered the fMRI signal by changing object shape. While the overall fMRI signal in category-related regions such as the face-related pFs was drastically reduced for non-face stimuli, the adaptation level obtained for these stimuli remained high. We hypothesize that the relatively greater adaptation level following exposure to "nonoptimal" object shapes is indicative of small subsets of neurons responding vigorously to these "nonoptimal" objects even when the overall fMRI activity is low. CONCLUSIONS: Our results show that fMR-adaptation can be used to differentiate between neuronal activation patterns that appear similar in the overall fMRI signal. The results suggest that it may be possible to employ fMR-adaptation to reveal functionally heterogeneous islands of activity, which are too small to image using conventional imaging methods.

Adult↗

PubMed2002

Eccentricity bias as an organizing principle for human high-order object areas.

We have recently proposed a center-periphery organization based on resolution needs, in which objects engaging in recognition processes requiring central-vision (e.g., face-related) are associated with center-biased representations, while objects requiring large-scale feature integration (e.g., buildings) are associated with periphery-biased representations. Here we tested this hypothesis by comparing the center-periphery organization with activations to five object categories: faces, buildings, tools, letter strings, and words. We found that faces, letter strings, and words were mapped preferentially within the center-biased representation. Faces showed a hemispheric lateralization opposite to that of letter strings and words. In contrast, buildings were mapped mainly to the periphery-biased representation, while tools activated both central and peripheral representations. The results are compatible with the notion that center-periphery organization allows the optimal allocation of cortical magnification to the specific requirements of various recognition processes.

Adult↗

PubMed2002

The topography of high-order human object areas.

Cortical topography is one of the most fundamental organizing principles of cortical areas. One such topography - eccentricity mapping - is present even in high-order, ventral stream visual areas. Within these areas, different object categories have specific eccentricity biases. In particular, faces, letters and words appear to be associated with central visual-field bias, whereas buildings are associated with a peripheral one. We propose that resolution needs are an important factor in organizing object representations: objects whose recognition depends on analysis of fine detail will be associated with central-biased representations, whereas objects whose recognition entails large-scale integration will be more peripherally biased.

Journal Article↗