Coordinate and categorical judgements in spatial imagery. An fMRI study

Dissociating Verbal and Nonverbal Conceptual Processing in the Human Brain

Functional neuroimaging has highlighted a left-hemisphere conceptual system shared by verbal and nonverbal processing despite neuropsychological evidence that the ability to recognize verbal and nonverbal stimuli can doubly dissociate in patients with left- and right-hemisphere lesions, respectively. Previous attempts to control for perceptual differences between verbal and nonverbal stimuli in functional neuroimaging studies may have hidden differences arising at the conceptual level. Here we used a different approach and controlled for perceptual confounds by looking for amodal verbal and nonverbal conceptual activations that are common to both the visual and auditory modalities. In addition to the left-hemisphere conceptual system activated by all meaningful stimuli, we observed the left/right double dissociation in verbal and nonverbal conceptual processing, predicted by neuropsychological studies. Left middle and superior temporal regions were selectively more involved in comprehending words—heard or read—and the right midfusiform and right posterior middle temporal cortex were selectively more involved in making sense of environmental sounds and images. Thus, the neuroanatomical basis of a verbal/nonverbal conceptual processing dissociation is established.

Sex differences in visuo-spatial processing: an fMRI study of mental rotation

Following the theoretical framework of coordinate and categorical principals for visuo-spatial processing, originally formulated by [Kosslyn, S. M. (1987). Seeing and imagining in the cerebral hemispheres: AQ computational approach. Psychological Review, 94, 148-175], we present data from an fMRI study on mental rotation, using the classic [Shepard, R. N., & Metzler, J. (1971). Mental rotation of three-dimensional objects. Science, 171, 701-703] task, comparing males and females. Subjects were presented with black-and-white drawings of 3-D shapes taken from the set of 3-D perspective drawings developed by [Shepard, R. N., & Metzler, J. (1971). Mental rotation of three-dimensional objects. Science, 171, 701-703], alternated with 2-D white bars as control stimuli. The drawings were presented pairwise, as black and white drawings against a black circular background. On half of the trials, the two 3-D shapes were congruent but portrayed with different orientation, in the other half the two shapes were incongruent. Analysis of response accuracy and reaction times did not reveal any significant differences between the sexes. However, clusters of significant neuronal activation were found in the superior parietal lobule (BA 7), more intensely over the right hemisphere, and bilaterally in the inferior frontal gyrus (BA 44/45). Males showed predominantly parietal activation, while the females, in addition, showed inferior frontal activation. We suggest that males may be biased towards a coordinate processing approach, and females biased towards a serial, categorical processing approach.

Recovering Space in Unilateral Neglect: A Neurological Dissociation Revealed by Virtual Reality

Neglect patients often show deficits in responding to targets in the contralesional side of space. Past studies were able to ameliorate these deficits through manipulation of visual input. Here, the neural bases of the recovery of space following virtual reality (VR) training in neglect patients were investigated. Neglect patients were trained to respond to targets in the left side of space that appeared in the central or in the right side of space in a VR system. It was found that only patients with lesions that spared the inferior parietal/superior temporal regions were able to benefit from the VR training. It was concluded that these regions play a crucial role in the recovery of space that underlies the improvement of neglect patients when trained with VR. The implications of these results for determining the neural bases of a higher order attentional and/or spatial representation and for treating patients with unilateral neglect are discussed.

Prefrontal cortex hemispheric specialization for categorical and coordinate visual spatial memory

During visual spatial perception of multiple items, the left hemisphere has been shown to preferentially process categorical spatial relationships while the right hemisphere has been shown to preferentially process coordinate spatial relationships. We hypothesized that this hemispheric processing distinction would be reflected in the prefrontal cortex during categorical and coordinate visual spatial memory, and tested this hypothesis using functional magnetic resonance imaging (fMRI). During encoding, abstract shapes were presented in the left or right hemifield in addition to a dot at a variable distance from the shape (with some dots on the shape); participants were instructed to remember the position of each dot relative to the shape. During categorical memory retrieval, each shape was presented centrally and participants responded whether the previously corresponding dot was 'on' or 'off' of the shape. During coordinate memory retrieval, each shape was presented centrally and participants responded whether the previously corresponding dot was 'near' or 'far' from the shape (relative to a reference distance). Consistent with our hypothesis, a region in the left prefrontal cortex (BA10) was preferentially associated with categorical visual spatial memory and a region in the right prefrontal cortex (BA9/10) was preferentially associated with coordinate visual spatial memory. These results have direct implications for interpreting previous findings that the left prefrontal cortex is associated with source memory, as this cognitive process is categorical in nature, and the right prefrontal cortex is associated with item memory, as this process depends on the precise spatial relations among item features or components.

Differential Effects of Transcranial Magnetic Stimulation of Left and Right Posterior Parietal Cortex on Mental Rotation Tasks

A recently published study used the interference strategy of transcranial magnetic stimulation (TMS) to demonstrate the role of the right posterior parietal cortex (PPC) in the mental rotation of alphanumeric stimuli. We used similar stimulation parameters over the same left and right PPC regions, and examined the ability to rotate more complex 3D Shepard and Metzler (1971) images. There was reduced accuracy of performance with both right and left PPC stimulation for different angles of rotation of the visual stimuli. Right PPC stimulation led to reduced accuracy to rotate stimuli by 1200, whereas left PPC stimulation affected 180 degrees C rotation. We hypothesise that the two hemispheres make different contributions to the processing underlying visuospatial mental imagery: the right PPC is important for spatial rotations through smaller angles; the left hemisphere has a unique role when the stimuli to be compared are rotated through 180 degrees C, a task that engages verbal strategies due to the well-documented special nature of enantiomorphs.

Neural mechanisms underlying spatial judgements on seen and imagined visual stimuli in the left and right hemifields in men

We investigated the neural networks involved in spatial judgements on visual and imagined stimuli in the left and right hemifields. Twenty healthy male right-handers were scanned using fMRI. In an adaptation of the Mental Clock Test, subjects judged whether the angle between visually presented or to be imagined clock hands at a given time (e.g. "12:20") was >90 degrees or <90 degrees . The angle formed by the clock hands was always located either on the left or right hemifield of the clock face, constituting a 2x2 factorial design with factors stimulus type (visual "V", imagined "I") and attention to spatial hemifield (left "L", right "R"). The contrast I>V revealed a bilateral neural network which included the superior parietal, prefrontal and antero-dorsal cingulate cortex. While right superior parietal cortex was activated during both RV and LV, left superior parietal cortex was only active during RV. The contrast RI versus LI (and vice versa) did not reveal significant differences with respect to activation height. However, a masking procedure and the calculation of a laterality index showed that RI recruited larger areas in the left than in the right hemisphere, while LI led to symmetrical activations in both hemispheres. Our data thus confirm that there are hemifield-specific asymmetries in brain activity during spatial processing of both visual and imagined stimuli. Our data indicate, however, that these asymmetries are less clear and of a different nature in the latter. Overall, our findings converge with neuropsychological data showing that representational and visuospatial neglect do not always co-occur.

Divergence of categorical and coordinate spatial processing assessed with ERPs

The spatial relation between two objects may be described either precisely or more coarsely in abstract terms, denoted as coordinate and categorical descriptions, respectively. These descriptions may reflect the outcomes of two spatial coding processes, which are realized in the left- and right-hemisphere. Support for this account comes from visual field effects in categorical and coordinate judgment tasks and from patient studies. In the current study, this hypothesis was tested by using event-related potentials (ERPs) and source localization. ERPs yield information about the processing stage at which the hypothesized categorical and coordinate processing diverge due to different task demands, especially in our S1-S2 version of the Bar Dot task. A centrally presented Bar Dot (S1) was followed after 2.5s by a second one (S2) in the left or right visual field; participants had to judge whether S2 matched S1 at the categorical, or, in a second task, at the coordinate level. Behavioral measures revealed a left-field advantage in the coordinate task that was absent in the categorical task. S1s elicited stronger early and late bilateral posterior responses in the coordinate than in the categorical task, possibly related to a compensatory strategy at the level of encoding and spatial memory. S2s elicited only stronger early contralateral responses, and stronger late right-hemisphere responses in the categorical task. It is proposed that the left-field advantage in the coordinate task may be due to differences in spatial resolution in perceptual encoding of the left- and right-hemispheres that are largely unaffected by the task at hand.

Categorical and coordinate spatial processing in the imagery domain investigated by rTMS

Using repetitive transcranical magnetic stimulation (rTMS), we investigated the functional relevance of posterior parietal cortex for categorical and coordinate judgements in the spatial imagery domain. In the coordinate task, subjects were asked to imagine two analogue clock faces based on acoustically presented pairs of times, and to judge at which of the two times the clock hands form the greater angle (mental clock task); in the categorical task subjects were again asked to imagine an analogue clock face showing the time verbally presented by the examiner, but in this case they had to judge whether both hands lay in the half of the clock face cued by an auditorily presented label. We matched the performance of three groups of subjects, two of which received rTMS stimulation over left and right posterior parietal cortex, respectively, while the third group received a sham stimulation. The results showed that right parietal stimulation interfered with the execution of the coordinate task, while left parietal stimulation mainly affected the categorical task, but also reduced the learning effect on the coordinate task. The present findings support the hemispheric specialization of the posterior parietal cortex in different spatial information processing in the imagery domain.

Mapping multiple visual areas in the human brain with a short fMRI sequence

It is a fundamental insight of neuroscience that the cerebral cortex is divided into spatially separated and functionally distinct areas. In this study, we tried to map a large number of visual areas in individual subjects passively viewing a simple stimulus sequence during functional magnetic resonance imaging (fMRI) at 1.5 T. The blocked stimulus sequence contrasted static object photographs with video takes of movement through natural indoor and outdoor scenes, alternated with a control fixation task. Two runs of the 5-min sequence sufficed to invoke 29 distinguishable activations, 16 (13 bilateral) of which were observed in all 10 participants. At the ventral side, object responsive activations were organized along the lateral occipital-temporal surface and near the collateral and occipital-temporal sulci. The latter activations, corresponding to the lateral occipital complex, showed a different activation profile from those near the collateral sulcus, most likely corresponding to the color constancy areas V4/V8-V4alpha. A potentially new fusiform object area was seen in 6 subjects, even more anterior than the parahippocampal place area. At the dorsal side, consistent activations were mainly related to motion stimuli and included the well-known areas V3a, VIPS, POIPS, hV5+, STS and the cingulate sulcus. There was consistent activation in the parietal-occipital sulcus, containing the areas V6a and V6. In addition, all subjects showed activation in the superior-anterior precuneus. Thus, the short stimulus sequence robustly invoked multiple visual areas and can be used to map the organization of the visual system in normal and brain-damaged individuals.

Neural impact of the semantic content of visual mental images and visual percepts

The existence of hemispheric lateralization of visual mental imagery remains controversial. In light of the literature, we used fMRI to test whether processing of mental images of object drawings preferentially engages the left hemisphere to compared non-object drawings. An equivalent comparison was also made while participants actually perceived object and non-object drawings. Although these two conditions engaged both hemispheres, activation was significantly stronger in the left occipito-temporo-frontal network during mental inspection of object than of non-object drawings. This network was also activated when perception of object drawings was compared to that of non-object drawings. An interaction was nonetheless observed: this effect was stronger during imagery than during perception in the left inferior frontal and the left inferior temporal gyrus. Although the tasks subjects performed did not explicitly require semantic analysis, activation of this network probably reflected, at least in part, a semantic and possibly a verbal retrieval component when object drawings were processed. Mental imagery tasks elicited activation of early visual cortex at a lower level than perception tasks. In the context of the imagery debate, these findings indicate that, as previously suggested, figurative imagery could involve primary visual cortex and adjacent areas.

Age-dependent brain activation during forward and backward digit recall revealed by fMRI

In this study, brain activation associated with forward and backward digit recall was examined in healthy old and young adults using functional MRI. A number of areas were activated during the recall. In young adults, greater activation was found in the left prefrontal cortex (BA9) and the left occipital visual cortex during backward digit recall than forward digit recall. In contrast, the activation in the right inferior frontal gyrus (BA 44/45) was more extensive in forward digit recall than in backward digit recall. In older adults, backward recall generated stronger activation than forward recall in most areas, including the frontal, the parietal, the occipital, and the temporal cortices. In the backward recall condition, the right inferior frontal gyrus (BA44/45) showed more activation in the old group than in the young group. These results suggest that different neural mechanisms may be involved in forward and backward digit recall and brain functions associated with these two types of recall are differentially affected by aging.

Combining transcranial magnetic stimulation and functional imaging in cognitive brain research: possibilities and limitations

Transcranial magnetic stimulation (TMS) is a widely used tool for the non-invasive study of basic neurophysiological processes and the relationship between brain and behavior. We review the physical and physiological background of TMS and discuss the large body of perceptual and cognitive studies, mainly in the visual domain, that have been performed with TMS in the past 15 years. We compare TMS with other neurophysiological and neuropsychological research tools and propose that TMS, compared with the classical neuropsychological lesion studies, can make its own unique contribution. As the main focus of this review, we describe the different approaches of combining TMS with functional neuroimaging techniques. We also discuss important shortcomings of TMS, especially the limited knowledge concerning its physiological effects, which often make the interpretation of TMS results ambiguous. We conclude with a critical analysis of the resulting conceptual and methodological limitations that the investigation of functional brain-behavior relationships still has to face. We argue that while some of the methodological limitations of TMS applied alone can be overcome by combination with functional neuroimaging, others will persist until its physical and physiological effects can be controlled.

Brain regions involved in simple and complex grammatical transformations

Grammatical transformation is a verbal reasoning task requiring judging the veracity of statements describing the spatial order of letter sets. We studied 18 adults with FMRI while they performed grammatical transformations of varying complexity levels (2-letter, 3-letter, and 4-letter sentences). Brain regions activated by 2-letter sentences included the visuospatial processing regions of the bilateral parietal lobes and the frontal operculum. A linear increase in sentence complexity engaged dorsolateral and ventrolateral prefrontal cortex as well as significantly increased activation within 2LTR areas. These data provide evidence that grammatical transformation reasoning relies primarily on the posterior visuospatial working memory system and need not necessarily engage the prefrontal cortex. Increasing the complexity of grammatical transformation, though, activates prefrontal cortex.

Spatial cognition: evidence from visual neglect

Recent work on human attention and representational systems has benefited from a growing interplay between research on normal attention and neuropsychological disorders such as visual neglect. Research over the past 30 years has convincingly shown that, far from being a unitary condition, neglect is a protean disorder whose symptoms can selectively affect different sensory modalities, cognitive processes, spatial domains and coordinate systems. These clinical findings, together with those of functional neuroimaging, have increased knowledge about the anatomical and functional architecture of normal subsystems involved in spatial cognition. We provide a selective overview of how recent investigations of visual neglect are beginning to elucidate the underlying structure of spatial processes and mental representations.