Neuron, Vol. 48, 901–911, December 22, 2005, Copyright ª2005 by Elsevier Inc. DOI 10.1016/j.neuron.2005.11.034 Neurotechnique Mapping Cortical Activity Elicited with Electrical Microstimulation Using fMRI in the Macaque Andreas S. Tolias, 1,4, * Fahad Sultan, 2,4 Mark Augath, 1 Axel Oeltermann, 1 Edward J. Tehovnik, 3 Peter H. Schiller, 3 and Nikos K. Logothetis 1 1 Max Planck Institute for Biological Cybernetics Tuebingen, 72076 Germany 2 Hertie-Institute for Clinical Brain Research Department of Cognitive Neurology University of Tuebingen Auf der Morgenstelle 15 72076 Tuebingen Germany 3 Department of Brain and Cognitive Sciences Massachusetts Institute of Technology Building 46-6041 Cambridge, Massachusetts 02139 Summary Over the last two centuries, electrical microstimulation has been used to demonstrate causal links between neural activity and specific behaviors and cognitive functions. However, to establish these links it is imper- ative to characterize the cortical activity patterns that are elicited by stimulation locally around the electrode and in other functionally connected areas. We have de- veloped a technique to record brain activity using the blood oxygen level dependent (BOLD) signal while ap- plying electrical microstimulation to the primate brain. We find that the spread of activity around the electrode tip in macaque area V1 was larger than expected from calculations based on passive spread of current and therefore may reflect functional spread by way of hor- izontal connections. Consistent with this functional transynaptic spread we also obtained activation in ex- pected projection sites in extrastriate visual areas, demonstrating the utility of our technique in uncover- ing in vivo functional connectivity maps. Introduction Electrical stimulation has been used extensively to study the functional organization of the brain by establishing relationships between specific regions of the brain and behavior. This method has been central in the discovery of topographic maps involved in the generation of ocular and skeletomotor behaviors (e.g., Fritsch and Hitzig, 1870; Scha¨ fer, 1888; Penfield and Boldrey, 1937; Wool- sey et al., 1952; Robinson and Fuchs, 1969; Robinson, 1972; Tehovnik and Lee, 1993; Graziano et al., 2002; Seidemann et al., 2002). Furthermore, it has played a prominent role in the elucidation of brain areas involved in sensory and cognitive processes (Doty, 1969; Schiller and Tehovnik, 2001; Moore et al., 2003; Cohen and Newsome, 2004; Tehovnik et al., 2005; Bartlett et al., 2005). There is a vast literature on the neural elements activated by electrical microstimulation (e.g., Ranck, 1975; Doty and Bartlett, 1981; Yeomans, 1990; Tehovnik, 1996; Rattay, 1999). Despite this, we still know very little about the local and distal brain circuits recruited during the delivery of currents to brain tissue. Most methods available for studying functional connectivity are largely inferential and depend on a reliable stimulation-elicited behavioral response (e.g., Yeomans and Tehovnik, 1988; Yeomans, 1995) or are very labor intensive, involving the isolation and behavioral testing of a plethora of single cells that respond to the electrical excitation of a distant site (e.g., Sommer and Wurtz, 2002; Moore and Arm- strong, 2003). Here we have developed a technique of simultaneous electrical microstimulation and functional magnetic res- onance imaging (fMRI) in the primary visual cortex (area V1) of the macaque monkey to characterize the activity patterns that are generated locally at the site of stimula- tion and distally at regions innervated by the site of stim- ulation. To establish that the microstimulation was in- deed activating neurons and not the smooth muscle of the vasculature, we determined the excitability (i.e., chronaxie) of the stimulated tissue by using the BOLD signal. We also measured the functional spread of activ- ity around the electrode tip for a range of current ampli- tudes. Finally, the functional connectivity between V1 and extrastriate cortex, i.e., areas V2, V3, V3A, V4, and middle temporal cortex (area MT/V5), was examined by stimulating V1 while simultaneously imaging extras- triate cortex. Results BOLD Elicited by Electrical Microstimulation The microstimulation experiments were conducted with the use of six rhesus monkeys while they were under an anesthesia protocol specifically developed for func- tional imaging (Logothetis et al., 1999, 2001). A 4.7 Tesla scanner was used with protocols described in detail else- where (Figure 1A; Logothetis et al., 1999, 2001). During an experimental session, a single microelectrode was advanced into the gray matter of the operculum of V1. A fiber optic system was used to present different visual stimuli that were moved manually to map out the multi- unit response field. The receptive-field centers of the units at the electrode tip were between 1 to 6 degrees of eccentricity, usually situated within the lower contra- lateral visual field (i.e., contralateral with respect to the cortical hemisphere under study). The location of the mi- croelectrode within V1 was confirmed with the use of an anatomical MRI sequence (see Experimental Proce- dures). Typically, a microelectrode was positioned within the granular or infragranular layers of V1, not ex- tending more than 1.8 mm below the cortical surface, which is the approximate thickness of striate cortex in macaque monkeys (Peters and Sethares, 1991). Brain activity in V1 and extrastriate visual areas was measured with BOLD during microstimulation of V1. Four-second-long trains of pulses were delivered *Correspondence: andreas.tolias@tuebingen.mpg.de 4 These authors contributed equally to this work.