Biomedical subjects

Timothy J Andrews

Publications and source records attributed to Timothy J Andrews.

8 recordsLinked to original sources

PubMed2005

fMR-adaptation reveals a distributed representation of inanimate objects and places in human visual cortex.

The way information about objects is represented in visual cortex remains controversial. It is unclear, for example, whether information is processed in modules, specialized for different categories of objects or whether information is represented in a distributed fashion across a large network of overlapping visual areas. In this study, we used fMR-adaptation to investigate the extent to which 'specialized' regions of visual cortex are involved in representing information about inanimate objects and places. We found adaptation in the object-selective lateral occipital complex (LOC) following repeated presentations of the same inanimate object. However, we also found fMR-adaptation to inanimate objects in fusiform face area (FFA) and the parahippocampal place area (PPA). Furthermore, this adaptation was not affected by changes in the size of the stimulus. In the second part of the experiment, we found adaptation to repeated images of places in the place-selective PPA, which was both size- and viewpoint-invariant. fMR-adaptation to repeated images of places was also observed in the LOC, but not in the FFA. These results suggest that the representation of inanimate objects and places is not restricted to those regions showing maximal responses to these particular categories of objects, but is distributed across human visual cortex and can include 'face-selective' regions such as the FFA.

Adaptation, Psychological↗

PubMed2005

Independent binocular integration for form and colour.

Although different features of an object are processed in anatomically distinct regions of the cerebral cortex, they often appear bound together in perception. Here, using binocular rivalry, we reveal that the awareness of form can occur independently from the awareness of colour. First, we report that, if both eyes briefly view a grating stimulus prior to the presentation of the same grating in one eye and an orthogonal grating in the other, subjects tend to report perceptual dominance of the non-primed grating. The primer was most effective when it was similar in orientation, spatial frequency and spatial phase to one of the rival images. Next, we showed that the process underlying the binocular integration of chromatic information was selectively influenced by the colour of a previously presented stimulus. We then combined these paradigms by using a primer that had the same colour as one rival stimulus, but the same form as the other stimulus. In this situation, we found that rival stimuli differing in form and colour can sometimes achieve states of dominance in which the chromatic information from one eye's image combines with the form of the other eye's image temporarily creating a binocular impression that corresponds with neither monocular component. Finally, we demonstrated that during continuous viewing of rival stimuli differing in form and colour, chromatic integration could occur independently of form rivalry. Paradoxically, however, we found that changes to the form of the stimulus had more of an influence on chromatic integration than on form rivalry. Together these phenomena show that the neural processes involved in integrating information from the two eyes can operate selectively on different stimulus features.

Color Perception↗

PubMed2005

Visual cortex: how are faces and objects represented?

The way in which information about complex objects and faces is represented in visual cortex is controversial. One model posits that information is processed in modules, highly specialized for different categories of objects; an opposing model appeals to a distributed representation across a large network of visual areas. A recent paper uses a novel imaging technique to address this controversy.

Animals↗

PubMed2004

Fusion and rivalry are dependent on the perceptual meaning of visual stimuli.

We view the world with two eyes and yet are typically only aware of a single, coherent image. Arguably the simplest explanation for this is that the visual system unites the two monocular stimuli into a common stream that eventually leads to a single coherent sensation. However, this notion is inconsistent with the well-known phenomenon of rivalry; when physically different stimuli project to the same retinal location, the ensuing perception alternates between the two monocular views in space and time. Although fundamental for understanding the principles of binocular vision and visual awareness, the mechanisms under-lying binocular rivalry remain controversial. Specifically, there is uncertainty about what determines whether monocular images undergo fusion or rivalry. By taking advantage of the perceptual phenomenon of color contrast, we show that physically identical monocular stimuli tend to rival-not fuse-when they signify different objects at the same location in visual space. Conversely, when physically different monocular stimuli are likely to represent the same object at the same location in space, fusion is more likely to result. The data suggest that what competes for visual awareness in the two eyes is not the physical similarity between images but the similarity in their perceptual/empirical meaning.

Color Perception↗

PubMed2004

Neural responses to Mooney images reveal a modular representation of faces in human visual cortex.

The way in which information about objects is represented in visual cortex remains controversial. One model of human object recognition poses that information is processed in modules, highly specialised for different categories of objects; an opposing model appeals to a distributed representation across a large network of visual areas. We addressed this debate by monitoring activity in face- and object-selective areas while human subjects viewed ambiguous face stimuli (Mooney faces). The measured neural response in the face-selective region of the fusiform gyrus was greater when subjects reported seeing a face than when they perceived the image as a collection of blobs. In contrast, there was no difference in magnetic resonance response between face and no-face perceived events in either the face-selective voxels of the superior temporal sulcus or the object-selective voxels of the parahippocampal gyrus and lateral occipital complex. These results challenge the concept that neural representation of faces is distributed and overlapping and suggest that the fusiform gyrus is tightly linked to the awareness of faces.

Adult↗

PubMed2004

Distinct representations for facial identity and changeable aspects of faces in the human temporal lobe.

The neural system underlying face perception must represent the unchanging features of a face that specify identity, as well as the changeable aspects of a face that facilitate social communication. However, the way information about faces is represented in the brain remains controversial. In this study, we used fMR adaptation (the reduction in fMRI activity that follows the repeated presentation of identical images) to ask how different face- and object-selective regions of visual cortex contribute to specific aspects of face perception. We report that activity in the face-selective region of the fusiform gyrus (FG) was reduced following repeated presentations of the same face. Adaptation in this area was not sensitive to changes in image size, but was sensitive to changes in viewpoint. In contrast, face-selective regions in the superior temporal lobe failed to adapt to identical presentations of the same face, but showed an increased response when the same face was shown from different viewpoints and with different expressions. These results reveal a largely size-invariant neural representation in the inferior temporal lobe that could be involved in the recognition of facial identity, and a separate face-selective region in the superior temporal lobe that could be used to detect changeable aspects of faces. The absence of fMR-adaptation in object-selective regions of visual cortex challenges the idea that a more distributed network of areas is used to represent information about faces.

Adaptation, Psychological↗

PubMed2002

Integration of motion information during binocular rivalry.

When two moving gratings are superimposed in normal viewing they often combine to form a pattern that moves with a single direction of motion. Here, we investigated whether the same mechanism underlies pattern motion when drifting gratings are presented independently to the two eyes. We report that, with relatively large circular grating patches (4 deg), there are periods of monocular dominance in which one eye's orientation alone is perceived, usually moving orthogonal to the contours (component motion). But, during the transitions from one monocular view to the other, a fluid mosaic is perceived, consisting of contiguous patches, each containing contours of only one of the gratings. This entire mosaic often appears to move in a single direction (pattern motion), just as when two gratings are literally superimposed. Although this implies that motion signals from the perceptually suppressed grating continue to influence the perception of motion, an alternative possibility is that it reflects a strategy that involves integrating directional information from the contiguous single-grating patches. To test between these possibilities, we performed a second experiment with very small grating stimuli that were about the same size as the contiguous single-grating patches in the mosaic (1-deg diameter). Despite the fact that the form of only one grating was perceived, we report that pattern motion was still perceived on about one third of trials. Moreover, a decrease in the occurrence of pattern motion was apparent when the contrast and spatial frequency of the gratings were made more different from each other. This phenomenon clearly demonstrates an independent binocular interaction for form and motion.

Humans↗

PubMed2002

Activity in the fusiform gyrus predicts conscious perception of Rubin's vase-face illusion.

We localized regions in the fusiform gyrus and superior temporal sulcus that were more active when subjects viewed photographs of real faces than when they viewed complex inanimate objects and other areas in the parahippocampal gyrus and the lateral occipital lobe that showed more activity during the presentation of nonface objects. Event-related functional magnetic resonance imaging was then used to monitor activity in these extrastriate visual areas while subjects viewed Rubin's vase-face stimulus and indicated switches in perception. Since the spontaneous shifts in interpretation were too rapid for direct correlation with hemodynamic responses, each reported percept (faces or vase) was prolonged by suddenly adding subtle local contrast gradients (embossing) to one side or the other of the figure-ground boundary, stabilizing the percept. Under these conditions, only face-selective areas in the fusiform gyrus responded more strongly during the perception of faces. To control for effects of the physical change to Rubin's stimulus (i.e., addition of embossing), we compared activity when the face contours were embossed after the subject had just reported the onset of perception of either faces or vase. Activity in the fusiform face area responded more strongly under the first condition, despite the fact that the physical stimulus sequences were identical. Moreover, on a trial-to-trial basis, the activity was statistically predictive of the subjects' responses, suggesting that the conscious perception of faces could be made explicit in this extrastriate visual area.

Adult↗