Cortical fields participating in spatial frequency and orientation discrimination: Functinal anatomy by positron emission tomography

Age-related changes in the neural correlates of degraded and nondegraded face processing

In order to explore the neural correlates of age-related changes in visual perception of faces, positron emission tomographic scans were obtained on young and old adults while they were engaged in tasks of nondegraded and degraded face matching. Old adults were less accurate than were young adults across all face matching conditions, although the age difference was greatly reduced when degraded performance was adjusted for nondegraded performance. The interaction of age and degree of degradation on performance measures was not significant. Brain activity patterns during nondegraded face matching were similar in the two groups with some differences in parietal and prestriate cortices (greater activity in young adults) and in prefrontal cortex, thalamus, and hippocampus (greater activity in old adults). Increases in activity related to increasing degradation of the faces were seen mainly in prefrontal cortices in both age groups. Despite this similarity in the brain response to face degradation, there were striking differences between groups in the correlations between brain activity and degraded task performance. Different regions of extrastriate cortex were positively correlated with behavioural measures in the two groups (fusiform gyrus in the young adults and posterior occipital regions in old adults). In addition two areas where older adults showed greater activity during nondegraded face matching, thalamus and hippocampus, also showed positive correlations with behaviour during the degraded tasks in this group, but not in the young group. Thus, although the elderly are not more vulnerable to the effects of increasing face degradation, the brain systems involved in carrying out these visual discriminations in young and old adults are not the same. These results are consistent with the idea of functional plasticity in face processing over the life span.

Spatial information is processed even when it is task-irrelevant: implications for neuroimaging task design

Many neuroimaging studies have been designed to differentiate domain-specific processes in the brain. A common design constraint is to use identical stimuli for different domain-specific tasks. For example, an experiment investigating spatial versus identity processing would present compound spatial-identity stimuli in both spatial and identity tasks, and participants would be instructed to attend to, encode, maintain, or retrieve spatial information in the spatial task, and identity information in the identity task. An assumption in such studies is that spatial information will not be processed in the identity task, as it is irrelevant for that task. We report three experiments demonstrating violations of this assumption. Our results suggest that comparisons of spatial and identity tasks in existing neuroimaging studies have underestimated the amount of brain activation that is spatial-specific. For future neuroimaging studies, we recommend unique stimulus displays for each domain-specific task, and event-related measurement of post-stimulus processing.

Feature uncertainty: a novel test to probe prefrontal dysfunction in unaffected siblings of schizophrenia patients

Previous studies indicated that neuropsychological impairments are potential endophenotypes of schizophrenia. However, the sensitivity of these procedures is not sufficient and their brain substrates are poorly defined. The aim of this study was to measure the behavioral performance of siblings of schizophrenia patients and controls on a novel feature uncertainty (FU) task that selectively activates dorsal anterior cingulate cortex relative to orientation (OR) and spatial frequency (SF) discrimination. During the FU task, two subsequent sinusoidal gratings are presented and participants are asked to remember both the OR and SF of the gratings. After the disappearance of the gratings, a color cue signifies the perceptual dimension (OR or SF) to discriminate. Results revealed that the siblings of schizophrenia patients (n = 25) showed a selective deficit on the FU task as compared with controls (n = 20). The FU deficit was more severe than that found on neuropsychological tests of executive functions, psychomotor speed, and verbal memory. These results suggest that anterior cingulate dysfunction is a potential endophenotype of schizophrenia.

Spatial frequency of visual image modulates neural responses in the temporo-occipital lobe. An investigation with event-related fMRI

Our visual environment consists of information ranging from coarse to fine patterns with respect to spatial frequency (SF). Neurophysiological studies using experimental animals have shown that there exist specific SF channels in striate and extrastriate visual cortices. In the present study, we used event-related functional magnetic resonance imaging (fMRI) and healthy subjects to investigate whether manipulation of the SF of visual images modulates neural responses in the temporo-occipital lobes. Subjects were scanned while performing "one-back task" with high-pass or low-pass filtered images of a face and house. We demonstrated that visual attention to the stimuli with high SF more specifically involves cortical activation in the left fusiform gyrus and inferior occipital gyrus as compared to that with low SF. High-SF specificity in the left fusiform gyrus was confirmed by voxel-by-voxel comparison of original images with left-right flipped images. There was no low-SF region in the right hemisphere; however, processing of low-SF images may be category-specific in face- and house-related regions. These results may shed light on the neural correlates of behavioral evidence that high-SF stimuli are handled faster and more accurately when presented to the right visual hemifield than to the left counterpart. The present results were also discussed in a viewpoint of local/global processing and functional asymmetry of cerebral hemispheres.

Feature uncertainty activates anterior cingulate cortex

In visual discrimination tasks, the relevant feature to discriminate is defined before stimulus presentation. In feature uncertainty tasks, a cue about the relevant feature is provided after stimulus offset. We used (15)O-butanol positron emission tomography (PET) in order to investigate brain activation during a feature uncertainty task. There was greater activity during the feature uncertainty task, compared with stimulus detection and discrimination of orientation and spatial frequency, in the lateral and medial prefrontal cortex, the cuneus, superior temporal and inferior parietal cortex, cortical motor areas, and the cerebellum. The most robust and consistent activation was observed in the dorsal anterior cingulate cortex (Brodmann area 32; x = 0 y = 16, z = 40). The insula, located near the claustrum (x = -38, y = 8, z = 4), was activated during the discrimination tasks compared with the feature uncertainty condition. These results suggest that the dorsal anterior cingulate cortex is important in feature uncertainty conditions, which include divided attention, expectancy under uncertainty, and cognitive monitoring.

A pet study of human skill learning: changes in brain activity related to learning an orientation discrimination task

Using 15O-water 3D positron emission tomography we investigated the effect of training in orientation discrimination upon cerebral activity in healthy human adults. When subjects are trained in this discrimination task, they learn the visuo-motor stimulus-response association required by the task and they increase their perceptual abilities in orientation discrimination. The present study was designed to investigate the rCBF modifications related to both these learning processes induced by training in orientation discrimination. PET data were acquired on two separate days (before and after training). Comparing the activation pattern related to orientation discrimination before and after the training period we observed activity decreases located in the left cerebellar cortex, in the right precentral gyrus and bilaterally in the fusiform gyri. The only region showing an activity increase was located in the body of the right caudate nucleus. These findings confirm the role of the neostriatum in skill learning and highlight the importance of mechanisms resulting in cortical and cerebellar neuronal activity decreases in this type of learning.

Orientation discrimination of objects and gratings compared: an fMRI study

We used functional magnetic resonance imaging to compare the human brain regions involved in orientation discrimination of two-dimensional (2D) objects and gratings. The orientation discrimination tasks, identification and successive discrimination, were contrasted to a dimming detection control condition with identical retinal input. Regions involved in orientation discrimination were very similar for the two types of tasks and for the two types of stimuli and both belonged to the dorsal and ventral visual pathways. They included posterior occipital, lingual, posterior fusiform, inferior temporal, dorsal intraparietal and medial parietal regions. The main difference between the two types of stimuli was a larger activation of precuneus when 2D objects were used compared to gratings. The main difference between discrimination tasks was an enhanced activity, at the group level, in superior frontal sulcus in identification compared to successive discrimination, and at least at the single subject level, a larger activity in right fusiform cortex in successive discriminations compared to identification. Thus, in contradiction to generally accepted views, orientation discrimination of gratings and objects involve largely similar networks including both ventral and dorsal visual regions.

Neuronal mechanisms of perceptual learning: changes in human brain activity with training in orientation discrimination

Using 15O-water 3D positron emission tomography, regional cerebral blood flow was measured twice in six human subjects: before and after extensive training in orientation discrimination. In each session subjects performed two orientation discrimination tasks, during which they discriminated the orientation of a grating at either the trained or untrained reference orientation, and a control task, during which they detected a randomly textured pattern. By comparing the discrimination to the detection tasks, we observed a main effect of task bilaterally in the posterior occipital cortex, extending into the left posterior fusiform gyrus and the right inferior occipital gyrus, bilaterally in the intraparietal sulcus, as well as in the cerebellum, thalamus, and brainstem. When we compared the activation pattern before and after the training period, all the changes observed were activity decreases. The nonspecific changes, which were not related to the orientation used during the training, were situated in the cerebellum and bilaterally in the extrastriate visual cortex. The orientation-specific changes, on the other hand, were restricted to the striate and extrastriate visual cortex, more precisely the right calcarine sulcus, the left lingual gyrus, the left middle occipital, and the right inferior occipital gyrus. These findings confirm our hypothesis concerning the existence of learning related changes at early levels of visual processing in human adults and suggest that mechanisms resulting in neuronal activity decreases might be involved in the present kind of learning.

Regional differences in the effects of task difficulty and motor output on blood flow response in the human anterior cingulate cortex: a review of 107 PET activation studies

We reviewed 107 blood flow activation studies carried out with positron emission tomography and published between January 1993 and November 1996. These studies had reported their findings as peaks of significant difference in cerebral blood-flow (CBF) between two scans/tasks and had located the peaks in standardized stereotaxic space. We coded each task along several dimensions, including the type and rate of input and output, the types of cognitive processes, and the relative difficulty of tasks within a study. Based on this coding, a difference score (A-B) was calculated for each subtraction. Subsequently, the frequency distributions of the difference scores for subtractions yielding a peak in the anterior cingulate region (cingulate peak) were compared with those distributions obtained from subtractions without a cingulate peak (no cingulate-peak). The cingulate peak subtractions (n = 158) differed from the no cingulate peak subtractions (n = 229) in terms of difficulty level (p = 0.001) and the presence of a remote memory component (p = 0.01). Regional differences in the frequency distribution of certain task parameters, such as difficulty level, recent memory and the use of the hand for responding, were also observed when peaks found in the anterior cingulate cortex (ACC) were further classified as located in the rostral vs caudal ACC, supracallosal vs subcallosal ACC, and limbic vs paralimbic parts of the supracallosal ACC. We conclude that task difficulty plays a major role in modulating blood-flow response in the ACC, possibly interacting with other parameters such as the nature of the response and memory demands.

Structural divisions and functional fields in the human cerebral cortex

The question of what is a cortical area needs a thorough definition of borders both in the microstructural and the functional domains. Microstructural parcellation of the human cerebral cortex should be made on multiple criteria based on quantitative measurements of microstructural variables, such as neuron densities, neurotransmitter receptor densities, enzyme densities, etc. Because of the inter-individual variations of extent and topography of microstructurally defined areas, the final microstructurally defined areas appear as population maps. In the functional domain, columns, patches and blobs signifying synaptically active parts of the cortex appear as cortical functional fields. These fields are the largest functional entities of the cerebral cortex according to the cortical field hypothesis. In its strong version, the cortical field hypothesis postulates that all neurons and synapses within the fields perform a co-operative computation. A number of such fields together provide the functional contribution of the cerebral cortex. The functional parcellation of the human cerebral cortex must be based on field population maps, which after intersection analysis appear as functional domains. The major structural-functional hypothesis to be examined is whether these functional domains are equi-territorial to the microstructurally defined meta-maps. The cortical hypothesis predicts that, if two brain tasks make use of one or several identical or largely overlapping fields, they cannot be performed simultaneously without errors or increases in latency. Evidence for such interference is presented. This evidence represents a restriction in the parallel processing of the human brain. In the posterior part of the brain not only visual cortical areas may qualify for parallel processing, but also the somatosensory cortices appear to have separate functional streams for the detection of microgeometry and macrogeometry.

A PET study of axis orientation discrimination

We examined the cortical areas showing increased activation related to axis orientation discrimination in hand movement and perception by positron emission tomography (PET) in right-handed normal human subjects while they performed a hand orientation (HO) task and an orientation discrimination (OD) task. In the OD task, fields in the parietal cortex and a field in the inferior frontal gyrus showed increased activation relative to the control task. In the HO task, fields in the intraparietal sulcus, the inferior and middle frontal gyri, the fusiform gyrus and the middle temporal gyrus showed increased activation relative to the control task. The results indicate that areas involved in axis orientation discrimination seemed to be segregated in the parietal cortex, with one field in the left intraparietal sulcus related to hand movement and another field in the right intraparietal sulcus related to perception.

Visual form discrimination from texture cues: a PET study

With the purpose of localising the cerebral cortical areas participating in the discrimination of visual form generated exclusively by texture cues, we measured changes in regional cerebral blood flow (rCBF) with positron emissions tomography (PET) and 15O-butanol as the tracer. The subjects performed two odd-one-out discrimination tasks: a form-from-texture discrimination task (in which a visual form was defined by differences in texture) and its reference task, the discrimination of texture. During task performance, activated fields were present bilaterally in the primary visual cortex and its immediate extrastriate cortex, the right lateral occipital gyrus, bilaterally in the fusiform and superior temporal gyri and posterior parts of the superior parietal lobules, along the medial bank of the right intraparietal sulcus, and in the right supramarginal gyrus. Other fields were found in the cingulate and prefrontal cortex. The findings demonstrate that the discrimination of visual form as defined by texture engages cortical fields that are widely distributed ion the human brain. In the visual cortex, the activated fields are present in both the occipito-temporal and occipito-parietal visual areas. These results suggest that the perception and discrimination of forms in the visual system requires the joint-activation of neuronal populations in the visual cortex.

Human brain activity related to orientation discrimination tasks

In order to relate regional activity in the human brain to the different components of discrimination tasks, we compared regional cerebral blood flow, measured with positron emission tomography, under four conditions: successive orientation discrimination, orientation identification, detection and passive viewing. By adding successive discrimination and passive viewing at a second, lower rate we were able to investigate the main effects and interaction between task and presentation rate. Four occipital regions--the posterior calcarine region bilaterally, the right lingual gyrus and the right interior occipital cortex--displayed a main effect of presentation rate. Two regions--a right posterolateral occipital region and a right posterior fusiform region--displayed a significant main effect of task. The involvement of this posterior fusiform region in successive discrimination was also revealed by the subtraction of detection from successive discrimination, as was that of the right middle fusiform gyrus. Finally, a more anterior right middle fusiform region was differentially active in successive discrimination compared to identification, suggesting that activity in this region is related to the temporal comparison of orientation.