www.elsevier.com/locate/ynimg NeuroImage 29 (2006) 567 – 577 Spatial maps in frontal and prefrontal cortex Donald J. Hagler Jr. and Martin I. Sereno* Department of Cognitive Science, University of California, San Diego, 9500 Gilman Dr. #0515, La Jolla, CA 92093-0515, USA Received 20 April 2005; revised 22 July 2005; accepted 2 August 2005 Available online 11 November 2005 Though the function of prefrontal cortex has been extensively Working memory tasks also activate prefrontal cortex. investigated, little is known about the internal organization of Physiological recordings in monkeys have demonstrated that individual prefrontal areas. Functional magnetic resonance imaging prefrontal areas have spatially selective memory fields that was used to show that some frontal and prefrontal cortical areas primarily represent restricted regions of contralateral space represent visual space in orderly, reproducible, topographic maps. The (Funahashi et al., 1989; Rainer et al., 1998). It seems plausible map-containing areas partly overlap dorsolateral prefrontal areas that maps of visual space might be preserved, rather than engaged by working memory tasks. These maps may be useful for attending to task-relevant objects at various spatial locations, an aspect scrambled, as posterior visual areas project to frontal and of the executive control of attention. prefrontal cortex. Indeed, Sawaguchi and Iba have recently D 2005 Elsevier Inc. All rights reserved. identified one or more spatial maps in monkey dorsolateral prefrontal cortex (DLPFC) related to working memory and attention using delayed saccade paradigms (Sawaguchi and Iba, 2001). These maps may primarily be used to hold locations in Introduction memory for spatial working memory tasks or they might be more generally useful for a variety of tasks that require the allocation The prefrontal cortex has often been implicated in working of working memory and attention. The homologies between memory processes. Several more specific functions have been monkey and human frontal and prefrontal areas are not proposed for this region, including maintenance and manipulation completely clear, and so the relationship between these macaque of information in working memory (Goldman-Rakic, 1987; Owen monkey prefrontal maps and working-memory-related dorso- et al., 1996), attentional control (Corbetta et al., 1998), choosing lateral prefrontal areas in humans remains to be clarified. between alternative responses (Rowe et al., 2000), and representa- In this paper, we ask whether working-memory-related areas in tion and storage of the rules of a task (Miller et al., 2002; Derrfuss human DLPFC contain neurons with preferences for working et al., 2004). The neuronal circuitry required to implement these memory stimuli appearing at particular spatial locations, and if so, functions is not yet well understood. whether these neurons are arrayed in a topological map of visual Mammalian brains represent visual space in multiple retino- space. To address this question, we had subjects perform a working topic visual cortical areas. Though all of the maps in V1, V2, V3, memory task with peripherally displayed images, steadily varying VP, V4v, and V3A represent all or part of the same contralateral the polar angle of presentation. We have found that at least two of visual hemifield, each area provides a unique mixture of the frontal and prefrontal areas active during this task contain such preferences for particular stimulus properties. The topology of maps of visual space. visual space is preserved to some degree in these maps, in that The n-back working memory task is conceptually simple, yet neighboring patches of cortex represent neighboring parts of the it requires a complex set of mental functions to perform. A visual field, forming continuous maps across the cortical surface. A subject monitors for instances in which a stimulus is repeated similar mapping of contralateral visual space across the cortical after n À1 intervening stimuli. This task requires attention to surface was recently revealed in the putative human lateral stimulus features and feature comparison, the encoding and intraparietal area (LIP), located in posterior parietal cortex (Sereno retrieval of stimuli in working memory, the maintenance, et al., 2001). Unlike the maps in many earlier visual areas, temporal ordering, and refreshing of working memory contents, however, this map only became evident when the subject was and finally – when an overt response such as a button press is required to engage attention and working memory. required – the choice, initiation, and sometimes suppression of motor responses. These subtasks require the cooperation of several cortical areas known to be involved in aspects of * Corresponding author. Fax: +1 858 534 1128. attention, working memory, and motor planning, including the E-mail address: [email protected] (M.I. Sereno). frontal eye fields (FEF), posterior parietal cortex (PPC), DLPFC, Available online on ScienceDirect (www.sciencedirect.com). and premotor cortex (Simon et al., 2002). 1053-8119/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2005.08.058 568 D.J. Hagler Jr., M.I. Sereno / NeuroImage 29 (2006) 567–577 Materials and methods or location – for the identity blocks was identical to that for location blocks. The number of randomly chosen locations, eight, Participants was also identical for all three block types. Simultaneous identity and location matches were disallowed to reduce the likelihood that A total of 21 adults participated in this study (7 women). The subjects would focus on the identity of the face during a location mean age was 25 T 7 years (ranging from 19 to 47). All subjects block or vice versa. To prevent spontaneous recognition of were right handed and had normal or corrected to normal vision. matches, no 2-back matches of any kind occurred during the The experimental protocol was approved by the UCSD internal passive blocks. Additional tests outside the scanner with an optical review board, and informed consent was obtained from each eye-tracker confirmed that several subjects included in this study participant. There were two types of experimental designs used in were able to maintain precise, reliable, central fixation during these this study, block design and phase-encoded. Twelve subjects tasks. participated in the block design experiment, and 20 participated in the phase-encoded experiment, with 11 subjects participating in Analysis of block design fMRI experiments both experiments. Before scanning, subjects were allowed, if they desired, to consume caffeinated beverages in order to better Distortion in images was reduced with an image-based, B0-map maintain alertness during the scan session. correction method (Reber et al., 1998). Volume-registered block design experiment images were analyzed using AFNI’s (Analysis fMRI scanning of Functional NeuroImages) (Cox, 1996) 3dDeconvolve (Ward, B.D., Medical College of Wisconsin, 1998). A quadratic poly- A Varian 4 T or a GE 3 T scanner was used with a head coil, a nomial was used to fit the baseline. Motion estimates from single small surface coil, or a whole-head array of four or eight 3dvolreg were added to the baseline model to further reduce the small surface coils (Nova Medical, Wakefield MA; General contribution of motion to activation patterns. Correlation coef- Electric). See Supplementary Table 1 for individual scan informa- ficients and F statistics were generated for the area under the tion for each subject. An echo planar T2*-weighted gradient echo hemodynamic response function, which freely varied over 15 s. pulse sequence (8V 32VV scan time, TR = 2000 or 4000 ms for For each subject in the group analysis, their cortical white matter phase-encoded, 8V 15VV scan time, TR = 3000 ms for block design, surface was inflated to a sphere and warped into a best-fit sulcal TE = 26.3 ms, flip angle = 90-, bandwidth = 125 kHz, 64  64 alignment with an average of 40 spherical surfaces using Free- matrix, 21–36 axial slices, 3  3  3 mm or 3.75  3.75  3.8 Surfer (Fischl et al., 1999). After resampling the deconvolution mm voxels) was used for functional scans. A T1-weighted coefficients onto the average spherical surface, 10 steps of surface- MPRAGE scan (TR = 10.5 ms, TE = 4.8 ms, flip angle = 11-, based smoothing were performed, equivalent to a 4 mm full-width, bandwidth = 50 kHz, 256Â256 matrix, 84 axial slices, 1  1Â1.5 half-maximum (FWHM) Gaussian filter (Hagler et al., in prepa- mm voxels) was also acquired during each session to align the ration). AFNI’s 3dttest was used to generate t statistics and means functional images to a previously obtained (1.5, 3, or 4 T) high for each condition and the identity vs. location contrast. To resolution (1Â1Â1 mm) T1-weighted MPRAGE scan. Subjects’ facilitate viewing, results were sampled back onto the surface of a heads were immobilized with subject-specific dental impression single subject. Cluster size exclusion was used for multiple bite-bars supported by a 4-ball-joint yoke. A custom image display comparison correction (Forman et al., 1995), with t statistics program, run on a Silicon Graphics O2, was used to present images thresholded at P < 0.05 and surface clusters smaller than 145 (¨5- wide¨6- high) centered at ¨8- eccentricity. Images were contiguous vertices excluded. A cortical-surface-based program projected onto a screen over the subject’s ribcage using a standard adapted from AFNI’s AlphaSim (Ward, 2000) was used to estimate video projector with a 7.38–12.3VV focal length Xtra Bright Zoom that these thresholds result in a corrected P value of 0.05 (Hagler et lens (Buhl Optical, USA). Subjects viewed the screen indirectly via al., in preparation). For comparison, group statistics were also a mirror above the eyes. generated from Talairach-transformed, 4 mm FWHM Gaussian smoothed, 3D volume data ( P < 0.05, uncorrected). Stimuli Analysis of phase-encoded fMRI experiments Gray-scale images of 68 different faces were selected from the FERET database (Phillips et al., 1998), excluding those with facial A Fourier transform of a time series generates a vector with real hair or glasses.