Mary-Ellen Large

Independent Processing of Form, Colour, and Texture in Object Perception

Most investigations of object recognition have focused on the form rather than the material properties of objects. Nevertheless, knowledge of the material properties of an object (via its surface cues) can provide important information about that objects identity. In this study, we used Garners speeded-classification task to explore whether or not the processing of form and the processing of surface properties are independent. In experiment 1, participants made length and width classifications in an initial form task. Participants were unable to ignore length while making width classifications, and were unable to ignore width while making length classifications. This suggests that the perception of length and the perception of width share common processing resources. In a subsequent task, we examined possible interactions between the processing of form and the processing of surface properties. In contrast to the findings with the form task, participants were able to ignore form while making surface-property classifications, and to ignore surface properties while making form classifications. This suggests that the form of objects and their surface properties are processed independently. In experiment 2, we went on to show that the two prominent surface-property dimensions of colour and texture can also be processed independently. In other words, participants were able to ignore colour while making texture classifications, and vice versa. Finally, in experiment 3, we examined the possibility that the stimuli and required responses that we used in experiment 2 were too categorical and thus not optimal for assessing whether or not colour and texture share common processing resources. Using a different stimulus set, participants were again able to ignore colour while making texture classifications, and vice versa. Taken together, these results provided convincing evidence that the separate ventral-stream brain regions identified for form, texture, and colour in a recent neuroimaging study (Cant and Goodale, 2007 Cerebral Cortex 17 713–731) can indeed function independently.

Dissociable neural mechanisms for determining the perceived heaviness of objects and the predicted weight of objects during lifting: An fMRI investigation of the size-weight illusion

In size-weight (SW) illusions, people learn to scale their fingertip forces for lifting small and big objects of equal weight even though they fail to learn perceptually that both objects have the same weight. The question then arises as to what the separate neural mechanisms are for determining the perceived heaviness of objects and the predicted weight of these objects during lifting. To answer this question, we used fMRI to first identify areas that code for the size, weight, and density of objects using an adaptation paradigm. We then contrasted BOLD in the SW illusion condition in which subjects falsely perceived the smaller of two equally weighted objects as heavier versus a condition in which size and weight did not differ between objects. Sensory areas in the parietal and temporal cortex adapted to the size of objects and the primary motor area (M1) contralateral to the lifting hand adapted to the weight of objects. The ventral premotor area (PMv), which did not adapt to either the size or the weight of objects, adapted instead to the density of objects, and responded more when subjects falsely perceived differences in weight between objects in the SW illusion condition. Taken together, we conclude that the real-world properties of objects, such as size and weight, are computed by sensory areas and by M1 respectively, whereas the perceived heaviness of objects, presumably based on their apparent density, is computed by PMv, a higher-order area well placed to integrate sensory information about the size of objects and the weight of objects.

Task-related laterality effects in the lateral occipital complex

Using functional imaging, we investigated the effects of two different tasks on activation in the lateral occipital complex (LOC). Alternating blocks of intact and scrambled objects were presented. In one task, subjects responded when an object repeated (matching task). In a second task subjects silently named objects (naming task). Identical objects (tools, animals and letters) were presented for both tasks. A relative measure of the number of voxels activated in LOC in left and right hemispheres was calculated for each task across a range of thresholds. Also the effects of task demands on category specific areas in LOC were examined. The object matching task resulted in proportionally more activity in the right hemisphere. The object naming task resulted in proportionally more activity in the left hemisphere, most prominently in the anterior portion of LOC. Effectively, changing the task changed the lateralization of activation to intact objects in LOC. In contrast, changing the task did not change the lateralization of category-specific activations. The results suggest that there are task-related top-down influences on the activation of neural populations in LOC as a whole, but the lateralization of category-specific regions in LOC is independent of task demands and may reflect bottom-up processing.

fMRI reveals greater within‐ than between‐hemifield integration in the human lateral occipital cortex

Early visual areas within each hemisphere (V1, V2, V3/VP, V4v) contain distinct representations of the upper and lower quadrants of the contralateral hemifield. As receptive field size increases, the retinotopy in higher‐tier visual areas becomes progressively less distinct. Using functional magnetic resonance imaging (fMRI) to map the visual fields, we found that an intermediate level visual area, the lateral occipital region (LO), contains retinotopic maps with a contralateral bias, but with a combined representation of the upper and lower visual field. Moreover, we used the technique of fMRI adaptation to determine whether neurons in LO code for both the upper and lower contralateral quadrants. We found that even when visual stimulus locations are equivalent across comparisons, the LO was more sensitive to location changes that crossed hemifields than location changes within a hemifield. These results suggested that within high‐tier visual areas the increasing integration of visual field information is a two‐stage process. The upper and lower visual representations are combined first, in LO, then the left and right representations. Furthermore, these results provided evidence for a neural mechanism to explain behavioral findings of greater integration within than between hemifields.

The Role of Temporal Synchrony as a Binding Cue for Visual Persistence in Early Visual Areas: An fMRI Study

We examined the role of temporal synchrony-the simultaneous appearance of visual features-in the perceptual and neural processes underlying object persistence. When a binding cue (such as color or motion) momentarily exposes an object from a background of similar elements, viewers remain aware of the object for several seconds before it perceptually fades into the background, a phenomenon known as object persistence. We showed that persistence from temporal stimulus synchrony, like that arising from motion and color, is associated with activation in the lateral occipital (LO) area, as measured by functional magnetic resonance imaging. We also compared the distribution of occipital cortex activity related to persistence to that of iconic visual memory. Although activation related to iconic memory was largely confined to LO, activation related to object persistence was present across V1 to LO, peaking in V3 and V4, regardless of the binding cue (temporal synchrony, motion, or color). Although persistence from motion cues was not associated with higher activation in the MT+ motion complex, persistence from color cues was associated with increased activation in V4. Taken together, these results demonstrate that although persistence is a form of visual memory, it relies on neural mechanisms different from those of iconic memory. That is, persistence not only activates LO in a cue-independent manner, it also recruits visual areas that may be necessary to maintain binding between object elements.