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COPYRIGHT 2001 Massachusetts Institute of Technology
V. W. K. Ng [1]
E. T. Bullmore [4]
G. I. de Zubicaray [3]
A. Cooper [1]
J. Suckling [2]
S. C. R. Williams [1]
Abstract
* With the advent of functional neuroimaging techniques, in particular functional magnetic resonance imaging (fMRI), we have gained greater insight into the neural correlates of visuospatial function. However, it may not always be easy to identify the cerebral regions most specifically associated with performance on a given task. One approach is to examine the quantitative relationships between regional activation and behavioral performance measures. In the present study, we investigated the functional neuroanatomy of two different visuospatial processing tasks, judgement of line orientation and mental rotation. Twenty-four normal participants were scanned with fMRI using blocked periodic designs for experimental task presentation. Accuracy and reaction time (RT) to each trial of both activation and baseline conditions in each experiment was recorded. Both experiments activated dorsal and ventral visual cortical areas as well as dorsolateral prefrontal cortex. More regionally specific associations with task performance were identified by estimating the association between (sinusoidal) power of functional response and mean RT to the activation condition; a permutation test based on spatial statistics was used for inference. There was significant behavioral--physiological association in right ventral extrastriate cortex for the line orientation task and in bilateral (predominantly right) superior parietal lobule for the mental rotation task. Comparable associations were not found between power of response and RT to the baseline conditions of the tasks. These data suggest that one region in a neurocognitive network may be most strongly associated with behavioral performance and this may be regarded as the computationally least efficient or rate-limiting node of the network.
INTRODUCTION
It is now generally accepted that complex mental functions are often subserved by large-scale, spatially distributed neurocognitive networks (Lin, Nein, & Lin, 1999; Damasio, Grabowski, Tranel, Hichwa, & Damasio, 1996; Haxby et al., 1991). Many of these networks have recently been investigated using functional neuroimaging techniques such as positron emission tomography (PET) (Fox & Raichle, 1984) and functional magnetic resonance imaging (fMRI) (Bullmore, Rabe-Hesketh, et al., 1996; Ogawa, Lee, Kay, & Tank, 1990). Inferences about the roles of cerebral structures are commonly based on regional differences in the activation maps obtained from different tasks. Although this "subtraction" approach yields important information regarding the anatomical substrates of task performance, it does not always elucidate the relative contribution of each activated area to different aspects of a task. One complementary way to accomplish this is to examine the quantitative relationships between regional activation and behavioral performance measures (e.g., Honey, Bullmore, & Sharma, 2000; Tagaris et al., 1996).
We have chosen to study visuospatial processing systems because although there is a large body of published literature on visuospatial tasks, the specific contributions of the (usually) multiple areas involved in overall performance of these tasks are not always clear. For example, posterior parietal and extrastriate visual cortices are engaged in the mental rotation of objects as demonstrated by PET (Alivisatos & Petrides, 1997), fMRI (Cohen et al., 1996; Tagaris et al., 1996), and event-related potential (ERP) (Perronet & Farrah, 1989) techniques. However, they are also involved in performance on other visuospatial processing tasks such as those involving judgements of axis/line orientation (Taira, Kawashima, Inoue, & Fukuda, 1998; O'Donnell, Swearer, Smith, Hokama, & McCarly, 1997; Hannay et al., 1987; Benton, Varney, & Hamsher, 1978). Which of these two regions is specifically critical to performance on each of these rather different tasks remains an open question.
In their computational model of mental rotation, Just and Carpenter (1992) proposed that the degree of mental computation and extent of information maintenance required would scale with a concomitant consumption of neural resources. More recently, Carpenter, Just, Keller, Eddy, & Thulborn (1999) proposed that the amount of cortical activation required for visuospatial processing is related to the amount, as well as to the type, of computational demand. This proposal was based in part on an observed linear relationship between fMRI (i.e., blood oxygen level dependent [BOLD]) activation in intraparietal, fusiform, and inferotemporal regions and increasing angular disparity on the Shepard-Metzler mental-rotation task. Carpenter et al. equated increasing angular disparity with greater task demand and larger resource utilization as it results in a monotonic increase in the average decision time (Shepard & Cooper, 1982; Shepard & Metzler, 1971). A control condition that had a lower computational demand for object recognition and rotation processes and a similar demand for visual scanning produced less activation in the parietal and inferotemporal regions, although considerable activation in the precentral and middle frontal gyri.
The present study was designed to further examine Carpenter et al.'s (1999) proposal with respect to visuospatial processing. In addition to a mental rotation task, we...
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