Christian F. Altmann

Integration of local features into global shapes: monkey and human FMRI studies

The integration of local image features into global shapes was investigated in monkeys and humans using fMRI. An adaptation paradigm was used, in which stimulus selectivity was deduced by changes in the course of adaptation of a pattern of randomly oriented elements. Accordingly, we observed stronger activity when orientation changes in the adapting stimulus resulted in a collinear contour than a different random pattern. This selectivity to collinear contours was observed not only in higher visual areas that are implicated in shape processing, but also in early visual areas where selectivity depended on the receptive field size. These findings suggest that unified shape perception in both monkeys and humans involves multiple visual areas that may integrate local elements to global shapes at different spatial scales.

Shape Saliency Modulates Contextual Processing in the Human Lateral Occipital Complex

Visual context influences our perception of target objects in natural scenes. However, little is known about the analysis of context information and its role in shape perception in the human brain. We investigated whether the human lateral occipital complex (LOC), known to be involved in the visual analysis of shapes, also processes information about the context of shapes within cluttered scenes. We employed an fMRI adaptation paradigm in which fMRI responses are lower for two identical than for two different stimuli presented consecutively. The stimuli consisted of closed target contours defined by aligned Gabor elements embedded in a background of randomly oriented Gabors. We measured fMRI adaptation in the LOC across changes in the context of the target shapes by manipulating the position and orientation of the background elements. No adaptation was observed across context changes when the background elements were presented in the same plane as the target elements. However, adaptation was observed when the grouping of the target elements was enhanced in a bottom-up (i.e., grouping by disparity or motion) or top-down (i.e., shape priming) manner and thus the saliency of the target shape increased. These findings suggest that the LOC processes information not only about shapes, but also about their context. This processing of context information in the LOC is modulated by figureground segmentation and grouping processes. That is, neural populations in the LOC encode context information when relevant to the perception of target shapes, but represent salient targets independent of context changes.

Cognitive Modulation of the Cerebral Processing of Human Oesophageal Sensation using Functional Magnetic Resonance Imaging

Background: While cortical processing of visceral sensation has been described, the role that cognitive factors play in modulating this processing remains unclear. Aim: To investigate how selective and divided attention modulate the cerebral processing of oesophageal sensation. Methods: In seven healthy volunteers (six males, mean age 33 years; ranging from 24 to 41 years old) from the general community, phasic visual and oesophageal (non-painful balloon distension) stimuli were presented simultaneously. During the selective attention task, subjects were instructed to press a button either to a change in frequency of oesophageal or visual stimuli. During a divided attention task, subjects received simultaneous visual and oesophageal stimuli and were instructed to press a button in response to a change in frequency of both stimuli. Results: Selectively focussing attention on oesophageal stimuli activated the visceral sensory and cognitive neural networks (primary and secondary sensory cortices and anterior cingulate cortex respectively) while selective attention to visual stimuli primarily activated the visual cortex. When attention was divided between the two sensory modalities, more brain regions in the sensory and cognitive domains were utilised to process oesophageal stimuli in comparison to those employed to process visual stimuli (p = 0.003). Conclusion: Selective and divided attention to visceral stimuli recruits more neural resources in both the sensory and cognitive domains than attention to visual stimuli. We provide neurobiological evidence that demonstrates the biological importance placed on visceral sensations and demonstrate the influence of cognitive factors such as attention on the cerebral processing of visceral sensation.

Effects of feature-selective attention on auditory pattern and location processing.

Recent neuroimaging studies have suggested that spatial versus nonspatial changes in acoustic stimulation are processed along separate cortical pathways. However, it has remained unclear in how far change-related responses are modulated by selective attention. Thus, we aimed at testing effects of feature-selective attention on the cortical representation of pattern and location of complex natural sounds using human functional magnetic resonance imaging (fMRI) adaptation. We consecutively presented the following pairs of animal vocalizations: (a) two identical animal vocalizations, (b) same animal vocalizations at different locations, (c) different animal vocalizations at the same location, and (d) different animal vocalizations at different locations. Subjects underwent this stimulation under two different task conditions requiring either to match sound identity or location. We observed significant fMRI adaptation effects within the bilateral superior temporal sulcus (STS), planum temporale (PT) and right anterior insula for location changes. For pattern changes, we found adaptation effects within the bilateral superior temporal lobe, in particular along the superior temporal gyrus (STG), PT and posterior STS, the bilateral anterior insula and inferior frontal areas. While the adaptation effects within the pattern-selective temporal lobe areas were robust to task requirements, adaptation within the more posterior location-selective areas was modulated by feature-specific attention. In contrast, inferior frontal cortex and anterior insular exhibited adaptation effects mainly during the location matching task. Given that the location matching task was significantly more difficult than the pattern matching, our data suggest that frontal and insular regions were modulated by task difficulty rather than feature-specific attention.

Distinct gamma-band components reflect the short-term memory maintenance of different sound lateralization angles.

Oscillatory activity in human electro- or magnetoencephalogram has been related to cortical stimulus representations and their modulation by cognitive processes. Whereas previous work has focused on gamma-band activity (GBA) during attention or maintenance of representations, there is little evidence for GBA reflecting individual stimulus representations. The present study aimed at identifying stimulus-specific GBA components during auditory spatial short-term memory. A total of 28 adults were assigned to 1 of 2 groups who were presented with only right- or left-lateralized sounds, respectively. In each group, 2 sample stimuli were used which differed in their lateralization angles (15° or 45°) with respect to the midsagittal plane. Statistical probability mapping served to identify spectral amplitude differences between 15° versus 45° stimuli. Distinct GBA components were found for each sample stimulus in different sensors over parieto-occipital cortex contralateral to the side of stimulation peaking during the middle 200–300 ms of the delay phase. The differentiation between “preferred” and “nonpreferred” stimuli during the final 100 ms of the delay phase correlated with task performance. These findings suggest that the observed GBA components reflect the activity of distinct networks tuned to spatial sound features which contribute to the maintenance of task-relevant information in short-term memory.

Audiovisual Functional Magnetic Resonance Imaging Adaptation Reveals Multisensory Integration Effects in Object-Related Sensory Cortices

Information integration across different sensory modalities contributes to object recognition, the generation of associations and long-term memory representations. Here, we used functional magnetic resonance imaging adaptation to investigate the presence of sensory integrative effects at cortical levels as early as nonprimary auditory and extrastriate visual cortices, which are implicated in intermediate stages of object processing. Stimulation consisted of an adapting audiovisual stimulus S1 and a subsequent stimulus S2 from the same basic-level category (e.g., cat). The stimuli were carefully balanced with respect to stimulus complexity and semantic congruency and presented in four experimental conditions: (1) the same image and vocalization for S1 and S2, (2) the same image and a different vocalization, (3) different images and the same vocalization, or (4) different images and vocalizations. This two-by-two factorial design allowed us to assess the contributions of auditory and visual stimulus repetitions and changes in a statistically orthogonal manner. Responses in visual regions of right fusiform gyrus and right lateral occipital cortex were reduced for repeated visual stimuli (repetition suppression). Surprisingly, left lateral occipital cortex showed stronger responses to repeated auditory stimuli (repetition enhancement). Similarly, auditory regions of interest of the right middle superior temporal gyrus and sulcus exhibited repetition suppression to auditory repetitions and repetition enhancement to visual repetitions. Our findings of crossmodal repetition-related effects in cortices of the respective other sensory modality add to the emerging view that in human subjects sensory integrative mechanisms operate on earlier cortical processing levels than previously assumed.

Alpha synchronization during auditory spatial short-term memory

Recent studies have suggested an active role of cortical &agr; oscillations for cognitive functions including short-term memory. We used magnetoencephalography to assess &agr; activity during an auditory spatial delayed matching-to-sample task compared with a nonmemory control condition. In the memory task, participants had to memorize the lateralization angle of a noise stimulus S1 and compare it with another lateralized sound S2 presented after an 800-ms delay phase. Whereas &agr; desynchronization following S1 was observed over superior temporal areas under both conditions, only the memory task was accompanied by posterior parietal &agr; synchronization during the subsequent delay period. The findings are consistent with the notion of &agr; activity reflecting active inhibition of interfering processes during memory maintenance of spatial sounds.

Processing of Auditory Location Changes after Horizontal Head Rotation

Under natural conditions, our sound localization capabilities enable us to move constantly while keeping a stable representation of our auditory environment. However, since most auditory studies focus on head-restrained conditions, it is still unclear whether neurophysiological markers of auditory spatial processing reflect representation in a head-centered or an allocentric coordinate system. Therefore, we used human electroencephalography to test whether the spatial mismatch negativity (MMN) as a marker of spatial change processing is elicited by changes of sound source position in terms of a head-related or an allocentric coordinate system. Subjects listened to a series of virtually localized band-passed noise tones and were occasionally cued visually to conduct horizontal head movements. After these head movements, we presented deviants either in terms of a head-centered or an allocentric coordinate system. We observed significant MMN responses to the head-related deviants only but a change-related novelty P3-like component for both head-related and allocentric deviants. These results thus suggest that the spatial MMN is associated with a representation of auditory space in a head-related coordinate system and that the integration of motor output and auditory input possibly occurs at later stages of the auditory “where” processing stream.

Similar cortical correlates underlie visual object identification and orientation judgment

Visual object perception has been suggested to follow two different routes in the human brain: a ventral, view-invariant occipital-temporal route processes object identity, whereas a dorsal, view-dependent occipital-parietal route processes spatial properties of an object. Using fMRI, we addressed the question whether these routes are exclusively involved in either object recognition or spatial representation. We presented subjects with images of natural objects and involved them either in object identification or object orientation judgment task. For both tasks, we observed activation in ventro-temporal as well as parietal areas bilaterally, with significantly stronger responses for the orientation judgment in both ventro-temporal as well as parietal areas. Our findings suggest that object identification and orientation judgment do not follow strictly separable cortical pathways, but rather involve both the dorsal and the ventral stream.

Allocentric or Craniocentric Representation of Acoustic Space: An Electrotomography Study Using Mismatch Negativity

The world around us appears stable in spite of our constantly moving head, eyes, and body. How this is achieved by our brain is hardly understood and even less so in the auditory domain. Using electroencephalography and the so-called mismatch negativity, we investigated whether auditory space is encoded in an allocentric (referenced to the environment) or craniocentric representation (referenced to the head). Fourteen subjects were presented with noise bursts from loudspeakers in an anechoic environment. Occasionally, subjects were cued to rotate their heads and a deviant sound burst occurred, that deviated from the preceding standard stimulus either in terms of an allocentric or craniocentric frame of reference. We observed a significant mismatch negativity, i.e., a more negative response to deviants with reference to standard stimuli from about 136 to 188 ms after stimulus onset in the craniocentric deviant condition only. Distributed source modeling with sLORETA revealed an involvement of lateral superior temporal gyrus and inferior parietal lobule in the underlying neural processes. These findings suggested a craniocentric, rather than allocentric, representation of auditory space at the level of the mismatch negativity.