Sensory Processes written assignment

Single-Neuron Responses

Current Biology 20, 750–756, April 27, 2010 ª2010 Elsevier Ltd All rights reserved DOI 10.1016/j.cub.2010.02.045

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in Humans during Execution and Observation of Actions

Roy Mukamel,1,2,3,* Arne D. Ekstrom,1,5 Jonas Kaplan,2,3,6

Marco Iacoboni,2,3,4 and Itzhak Fried1,3,4,7 1Department of Neurosurgery 2Ahmanson-Lovelace Brain Mapping Center 3Semel Institute for Neuroscience and Human Behavior 4Brain Research Institute David Geffen School of Medicine, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA 5Center for Neuroscience, 1544 Newton Court, University of California, Davis, Davis, CA 95618, USA 6Brain and Creativity Institute and Department of Psychology, University of Southern California, Los Angeles, CA, 90098, USA 7Functional Neurosurgery Unit, Tel Aviv Medical Center and Sackler School of Medicine, Tel Aviv University, Tel Aviv 69978, Israel

Summary

Direct recordings in monkeys have demonstrated that

neurons in frontal and parietal areas discharge during execution and perception of actions [1–8]. Because these

discharges ‘‘reflect’’ the perceptual aspects of actions of others onto the motor repertoire of the perceiver, these

cells have been called mirror neurons. Their overlapping sensory-motor representations have been implicated in

observational learning and imitation, two important forms of learning [9]. In humans, indirect measures of neural

activity support the existence of sensory-motor mirroring mechanisms in homolog frontal and parietal areas [10, 11],

other motor regions [12–15], and also the existence of multisensory mirroring mechanisms in nonmotor regions

[16–19]. We recorded extracellular activity from 1177 cells in human medial frontal and temporal cortices while patients

executed or observed hand grasping actions and facial emotional expressions. A significant proportion of neurons

in supplementary motor area, and hippocampus and envi- rons, responded to both observation and execution of these

actions. A subset of these neurons demonstrated excitation during action-execution and inhibition during action-obser-

vation. These findings suggest that multiple systems in humans may be endowed with neural mechanisms of mirror-

ing for both the integration and differentiation of perceptual and motor aspects of actions performed by self and others.

Results

We recorded extracellular activity from a total of 1177 neurons in 21 patients while they observed and executed grasping actions and facial gestures. In the observation conditions, subjects observed various actions presented on a laptop screen. In the execution conditions, the subjects were cued to perform an action by a visually presented word. In a control task, the same words were presented and the patients were

*Correspondence: rmukamel@ucla.edu

instructed not to execute the action (see Experimental Proce- dures and Figure S1A available online). In the medial frontal cortex, we recorded from 652 neurons (369 single units, and 283 multiunits) in the supplementary motor area (SMA; both SMA proper and pre-SMA), and anterior cingulate cortex (ACC; both the dorsal and rostral aspects [20]). In the medial temporal lobe we recorded from 525 neurons (296 single units, and 229 multiunits) in the amygdala, hippocampus, parahippo- campal gyrus (PHG), and entorhinal cortex (EC) (see Fig- ure S1B for anatomical location of electrodes). The number of cells recorded in each region is provided in Table 1A.

Significant changes in firing rate were tested with a two- tailed paired t test between the firing rate during baseline (21000 ms to 0 ms relative to trial onset) and a window of +200 to +1200 ms after stimulus onset (see Experimental Procedures). For each action (smile, frown, precision grip, or wholehand grip) we examined the neural response during action-observation and action-execution. A response to action-execution was considered only if there was no signifi- cant response to the corresponding control task.

After examination of the cell’s response to each action sepa- rately, the cell was classified as follows:

Action-observation neuron: a cell responding only during one or more action-observation conditions and not during any of the action-execution conditions (e.g., a cell respond- ing to smile observation and frown observation). Action-execution neuron: a cell responding only during one or more action-execution conditions and not during any of the action-observation conditions (e.g., a cell responding to precision-grip execution). Action observation/execution nonmatching neuron: a cell responding during action-observation in one condition and action-execution in a different condition (e.g., a cell responding to smile observation and frown execution). Action observation/execution matching neuron: a cell responding during both the execution and the observation of the same action (e.g., a cell responding to smile observa- tion and smile execution).

Table 1B provides the number of cells in each category described above, according to anatomical region. The majority of cells responded to one dimension of the stimuli (observation or execution). In the SMA [c2(1) = 14.5, p = 1023] and pre-SMA [c2(1) = 4.2, p = 0.03], the proportion of responses to action- execution relative to action-observation was significantly higher. In the other regions examined (ACC and medial temporal lobe) there was no significant difference between the two conditions. Six cells responded to observation, execu- tion, and also the control task of one action and were therefore not considered as action observation/execution matching cells (three cells in PHG, two in EC, and one in SMA). Within the population of action-observation cells, there were more responses to hand grasps (precision grip or wholehand prehension) in PHG relative to facial gestures (smile or frown; c2(1) = 3.9, p = 0.04), and more responses to observations of facial gestures relative to hand grasps in ACCd [c2(1) = 4.8, p = 0.02]. The distribution of responses within the population

Table 1. Location and Response Types of Recorded Cells

A. Location of Recorded Cells

Region A H EC PHG SMA Pre-SMA ACCd ACCr Total

SU/MU 11/22 92/71 102/73 91/63 82/43 79/65 66/59 142/116 665/512

Right 13 77 81 48 23 68 80 168 558

Left 20 86 94 106 102 76 45 90 619

Total 33 163 175 154 125 144 125 258 1177

B. Response Types

Region A H EC PHG SMA Pre-SMA ACCd ACCr %

Action-execution 11 (4, 7)

(33%)

36 (19, 17)

(22%)

37 (18, 19)

(21%)

39 (22, 17)

(25%)

41 (27, 14)

(33%)

34 (23, 11)

(24%)

28 (12, 16)

(22%)

50 (28, 22)

(19%)

23%

Action-observation 4 (1, 3)

(12%)

29 (18, 11)

(18%)

32 (21, 11)

(18%)

35 (19, 16)

(23%)

13 (9, 4)

(10%)

19 (11, 8)

(13%)

26 (15, 11)

(21%)

45 (25, 20)

(17%)

17%

Observation/

Execution matching

2 (1, 1)

(6%)

18 (8, 10)

(11%)

14 (9, 5)

(8%)

19 (13, 6)

(12%)

17 (10, 7)

(14%)

6 (4, 2)

(4%)

2 (2, 0)

(2%)

12 (8, 4)

(5%)

8%

Observation/Execution

nonmatching

1 (0, 1)

(3%)

16 (8, 8)

(10%)

11 (5, 6)

(6%)

11 (8, 3)

(7%)

13 (7, 6)

(10%)

10 (5, 5)

(7%)

1 (0, 1)

(1%)

14 (8, 6)

(5%)

7%

(A) Number of single units (SU) and multiunits (MU) recorded in the left and right hemispheres in various anatomical regions.

(B) Response types of cells across all recorded regions. Absolute number (single unit, multiunit) and percentages of cells (calculated from total number of

recorded cells in each region; see A). The last column represents the percentage of responses across all regions. For definitions of response types, see text.

The following abbreviations are used: A, amygdala; H, hippocampus; EC, entorhinal cortex; PHG, parahippocampal gyrus; SMA, supplementary motor area;

ACCd, dorsal aspect of anterior cingulate; and ACCr, rostral aspect of anterior cingulate. See also Table S2.

Single-Cell Mirroring Responses in Humans 751

of action-observation cells and action-execution cells is pro- vided in Table S1.

We subsequently focused our analyses on the action obser- vation/execution matching cells responding during both observation and execution of particular actions. Figure 1A displays one such cell in the SMA responding to the observa- tion and execution of two grip types (precision and whole- hand). This cell did not respond to the control tasks or any of the facial gesture conditions. Figure 1B displays another cell in entorhinal cortex responding to observation and execution of facial gestures (smile and frown). Again, this cell did not respond to the control tasks or to observation and execution of the various grips.

Next, we tested whether the proportion of action observa- tion/execution matching neurons in each anatomical region is significantly higher than that expected by chance (chance level set at 5%). We performed a chi-square test on the propor- tion of such cells in each region (except for the amygdala where we performed Fischer’s exact test due to small number of cells). The proportion of cells in the hippocampus [c2(1) = 12.5, p = 2 3 1024], parahippocampal gyrus [c2(1) = 17.4, p < 1024], entorhinal cortex [c2(1) = 3.3, p < 0.05], and SMA [c2(1) = 19.4, p < 1024] was significantly higher than expected by chance. In amygdala, pre-SMA, ACCd, and ACCr the proportions were not significantly higher than chance. In addi- tion to the chi-square test, we performed a bootstrap analysis to test whether or not the number of action observation/execu- tion matching neurons is higher than the null distribution (see Experimental Procedures). Figure S2A displays the null distribution (blue) together with the actual number of cells in our data set (red arrow). In agreement with the chi-square test described above, the number of cells in SMA (p = 0.003), entorhinal cortex (p = 0.001), hippocampus (p < 1024), and par- ahippocampal gyrus (p < 1024) were significantly higher than expected by chance. In addition, we performed the same anal- ysis, this time taking into account only cells defined as single units and obtained similar results (SMA (p = 0.02), EC (p = 0.004), H (p = 0.02), and PHG (p = 0.007); see Figure S2B). Furthermore, the proportion of action observation/execution matching neurons in these regions was significantly higher

compared with Poisson generated spike trains with similar firing rates (Figure S2C). The distribution of joint p values for these action observation/execution matching neurons is pro- vided in Figure S2D for the different regions.

Next, we focused on the action observation/execution matching neurons in the anatomical regions where the propor- tion of such cells was significant (SMA, parahippocampal gyrus, hippocampus, and entorhinal cortex). Figure 2 displays the responses of six additional neurons from these various regions. The complete response details of all action observa- tion/execution matching cells are provided in Table S2. The majority of these cells (40 out of 68) were classified as single units (see Experimental Procedures). Among the 68 action observation/execution matching cells, 33 increased their firing rate during both observation and execution of a particular action (e.g., Figures 2A–2D). In contrast, 21 other neurons decreased their firing rate during both conditions (Figure 2E). These types of responses have been previously reported in monkeys (e.g., [21]) and birds [22]. Furthermore, 14 neurons increased their firing rate during one condition and decreased it during the other. The majority of these cells (n = 11) increased their firing rate during action-execution and decreased their firing rate during action-observation (Figure 2F), whereas the remaining neurons did the opposite [c2(1) = 6.2, p = 0.01]. For anatomical distribution of response types see Table S3A. Obviously, the breaking down of responses by type and anatomical region makes it difficult to test for regional differ- ences and therefore to draw any firm conclusion on these distributions.

We subsequently examined the temporal profiles of neural activity by computing the average response profile of all action observation/execution matching neurons. This was con- ducted separately for cells exhibiting excitation to both condi- tions (Figure 3A), inhibition to both conditions (Figure 3B), and cells exhibiting excitation during action-execution and inhibi- tion during action-observation (Figure 3C). In order to accom- modate for differences in firing rates across different cells before averaging, similar to [21] we normalized each excitatory response to range between 0 and +1, and each inhibitory responses to range between 0 and 21 (see Experimental

Figure 1. Neural Responses of Two Cells during All Experimental Conditions and Tasks

Rasters (top) are aligned to stimulus onset (red vertical line at time = 0). Bin size for peristimulus time histogram (bottom) is 200 ms. Red box highlights

responses passing statistical criteria.

(A) An action observation/execution matching multiunit in left SMA for the two grips (precision and wholehand).

(B) An action observation/execution matching single unit in right entorhinal cortex for two facial gestures (smile and frown). See also Figure S1.

Current Biology Vol 20 No 8 752

Procedures). Excitatory cells reached peak firing rate faster during action-observation compared with action-execution and inhibitory cells returned to baseline faster during action- observation. It is interesting to note that excitatory observa- tion/execution matching cells had firing rates significantly lower than baseline during the control task (Figure 3A). Average baseline firing rates for cells exhibiting excitation during both action-execution and action-observation was 4.8 6 3.7 Hz, whereas the average baseline firing rates for cells exhibiting inhibition during both conditions was 9.4 6 6.0 Hz (mean and standard deviation). Average baseline firing rate for cells exhibiting excitation to action-execution and inhibi- tion to action-observation was 6.5 6 3.2. For relative and abso- lute response amplitudes see Figure S3. In terms of response latencies, no significant difference between observation and execution was found (see Table S3B).

The majority of action observation/execution matching neurons in our data set matched only one action (54 cells), and 14 cells matched the execution and observation of two actions. No significant difference between the proportion of cells matching facial gestures or hand grasps was found [c2(1) = 0.6, p = 0.4; see Table S1C].

Discussion

We recorded extracellular neural activity in 21 patients while they executed and observed facial emotional expressions and hand-grasping actions. In agreement with the known motor properties of SMA and pre-SMA, our results show a significantly higher proportion of cells responding during action-execution compared with action-observation in these regions. Although the majority of responding cells across all

Figure 2. Raster Plots and Peristimulus Time Histograms of Six Different Observation/Execution Matching Neurons during Execution, Observation, and the

Control Task

(A) Single unit in left entorhinal cortex increasing its firing rate during both frown execution and frown observation.

(B) Single unit in right parahippocampal gyrus increasing its firing rate during whole hand grasp execution and whole hand grasp observation.

(C) Single unit in left entorhinal cortex increasing its firing rate during smile execution and smile observation.

(D) Single unit in right parahippocampal gyrus increasing its firing rate during precision grip execution and precision grip observation.

(E) Single unit in left SMA decreasing its firing rate during smile execution and smile observation.

(F) Single unit in left parahippocampal gyrus increasing its firing rate during frown execution and decreasing it during frown observation. See also Figure S2.

Single-Cell Mirroring Responses in Humans 753

regions responded only to one aspect of a particular action (either perception or execution), we also found cells respond- ing to both. Significant proportions of such cells were found both in medial frontal lobe (SMA) and in medial temporal lobe—namely, hippocampus, parahippocampal gyrus, and entorhinal cortex. In the amygdala, ACC (both rostral and dorsal aspects), and pre-SMA, the number of such cells did not reach significance levels. Finally, within the population of cells responding to both observation and execution of action, our results indicate a subpopulation of cells respond- ing with excitation during action-execution and inhibition during action-observation.

What is the relationship between the cells recorded in SMA—on the medial wall of the frontal lobe—that responded during both execution and observation of actions and the ‘‘mirror neurons’’ reported previously in monkeys? The critical feature of mirror neurons is the functional matching between a motor response and a perceptual one [23]. The population of cells we found exhibited this critical functional feature for grasping actions and facial expressions. In this regard, there is obviously similarity between the human and the monkey cells. In monkeys, however, neurons with mirroring properties have been reported in a variety of areas on the lateral wall of the primate brain [3, 7, 21, 24, 25]. In the current study we did not record from these regions because the placement of

electrodes was determined only by clinical considerations. Neurophysiological data suggest that whereas areas on the lateral wall such as F5 seem to contain a vocabulary of actions, from grasping to facial expressions, areas on the medial wall such as SMA seem relevant to movement initiation and move- ment sequences [26]. Thus, it is possible that the action obser- vation/execution matching neurons we recorded from SMA represent cellular mirror mechanisms for these particular aspects of hand and facial actions.

One of the striking features of our findings is the presence of action observation/execution matching neurons in the medial temporal lobe (MTL). Connections such as the uncinate fascic- ulus and other cortico-cortical white matter tracts between the MTL and motor regions in the frontal lobe exist [27–31]. Although there is some evidence for responses in the hippo- campus during voluntary actions [32], unlike SMA, lesions in the medial temporal lobe do not result in obvious motor defi- cits, and electrical stimulation in these areas does not result in overt movement. It might be argued that the visual input (rather than the motor output) during action-execution is what elicited the responses in these medial temporal lobe neu- rons. In our study, however, the visual inputs during action- observation and action-execution were widely different (only a word is visually presented to cue action-execution compared with a video/picture presented during action-observation).

Figure 3. Average Normalized Response Profile of all Action Observation/

Execution Matching Neurons

(A) Average of 41 excitatory responses (from 33 different neurons) during

action execution and action observation.

(B) Average of 26 inhibitory responses (from 21 different neurons).

(C) Average of 11 response profiles (from 11 different neurons) exhibiting

excitation during action-execution and inhibition during action-observation.

Bins size = 200 ms.

For the normalization procedure, see Experimental Procedures. Error bars

represent standard error of the mean across all neurons. Asterisks on the

observation/execution plots denote time bins at which the difference

between the temporal profile of action-execution and action-observation

were significant (see Experimental Procedures). Asterisks on the control

task plot denote time bins at which the control condition is significantly

different than zero. See also Figure S3.

Current Biology Vol 20 No 8 754

Furthermore, the visual input during the control and action- execution of the face experiment is identical although these cells did not respond to the control condition (see Figures 1 and 2). Additionally, in some patients we used auditory tones to cue action-execution (and as appropriate control) and we obtained similar results for these patients (i.e., responses to the tone during action-execution and not during the control condition). It follows that the purely visual explanation for action observation/execution matching cells cannot hold, at least for the execution of facial expressions where no addi- tional visual input is available. In principle, the visual input of the patient’s grasping hand may explain the discharge of the cells during grasping execution and grasping observation. However, this argument would require two separate mecha- nisms to explain the mirroring responses for facial expressions and for grasping: a ‘‘true’’ mirroring mechanism for facial expression and a ‘‘purely visual’’ mechanism for grasping action. Although this possibility cannot be excluded, it is less parsimonious than invoking a unitary mirroring mechanism for both facial expressions and grasping actions.

It may also be argued that the neurons with mirroring properties respond in an invariant manner to different visual stimuli sharing the same concept, e.g., a picture of a smiling face and the execution cue word ‘‘smile’’ [33]. Indeed we found six neurons that responded to observation, to execu- tion, and also to the control condition of a specific action. However, the argument that the observation/execution match- ing neurons in the medial temporal lobe represent the concept of the action is untenable because we only considered cells that did not respond during the control conditions where the word stimuli were presented again but did not cue the patient to perform an action. An alternative account for the responses in medial temporal lobe during action-execution is that they represent proprioceptive processing. At this stage we cannot rule out this alternative account.

We have recently demonstrated that neurons in medial temporal lobe are reactivated during spontaneous recall of episodic memory [34]. The action observation/execution matching neurons in the medial temporal lobe may match the sight of actions of others with the memory of those same actions performed by the observer. Thus during action-execu- tion, a memory of the executed action is formed, and during action-observation this memory trace is reactivated. This interpretation is in line with the hypothesis of multiple mirroring mechanisms in the primate brain, a hypothesis that can easily account for the presence of mirroring cells in many cortical areas [1, 3–5, 7, 8, 24, 25].

The functional significance of the mirror mechanism most likely varies according to the location of mirror neurons in different brain areas [35]. For example, the mirror mechanism in the insula might underlie the capacity to understand a specific emotion (disgust) in others [16, 19], whereas the mirror mechanism in the parietofrontal circuit may help under- standing the goal of observed motor acts and the intentions behind them [21]. Here we show cellular mirroring mechanisms in areas relevant to movement initiation and sequencing (SMA) and to memory (medial temporal lobe). Whereas these hypoth- eses have yet to be tested more carefully, these results demonstrate the presence of mirror mechanisms in humans at the single neuron level and in areas functionally different from the ones previously described in the literature.

Mirroring activity, by definition, generalizes across agency and matches executed actions performed by self with per- ceived action performed by others. Although this may facilitate

Single-Cell Mirroring Responses in Humans 755

imitative learning, it may also induce unwanted imitation. Thus, it seems necessary to implement neuronal mechanisms of control. The subset of mirror neurons responding with oppo- site patterns of excitation and inhibition during action-execu- tion and action-observation seem ideally suited for this control function. Indeed, extensive brain lesions are associated with compulsory imitative behavior in neurological patients [36, 37]. Recently, it has been reported that the majority of pyra- midal tract neurons in monkey F5 that display mirror-like activity suppress their firing rate during action observation [6], in accord with our own data. Interestingly, some fMRI studies have also reported decreased BOLD signal in primary motor cortex during action-observation [12]. A recent model proposes a direct mirror pathway for automatic, reflexive imitation and an indirect mirror pathway for parsing, storing, and organizing motor representations [38]. The observation/ execution matching cells with opposite response patterns are compatible with the direct pathway. Finally, mirroring may generate the problem of differentiating between actions of the self and of other people. The opposing pattern of activity for actions of self and others may also form a simple neuronal mechanism for maintaining self-other differentiation.

In conclusion, these data demonstrate mirroring spiking activity during action-execution and action-observation in human medial frontal cortex and human medial temporal cortex—two neural systems where mirroring responses at single-cell level have not been previously recorded. A subset of these mirroring cells exhibited opposing pattern of excita- tion and inhibition during action-execution and action-obser- vation, a neural feature that may help preserving the sense of being the owner of an action during execution, and exert control on unwanted imitation during observation. Taken together, these findings suggest the existence of multiple systems in the human brain endowed with neural mirroring mechanisms for flexible integration and differentiation of the perceptual and motor aspects of actions performed by self and others.

Experimental Procedures

For detailed description of methods see Supplemental Experimental

Procedures.

Patients

We recorded extracellular single and multiunit activity from 21 patients with

pharmacologically intractable epilepsy. Patients were implanted with intra-

cranial depth electrodes to identify seizure foci for potential surgical treat-

ment. Electrode location was based solely on clinical criteria and the

patients provided written informed consent to participate in the experi-

ments. The study conformed to the guidelines and was approved by the

Medical Institutional Review Board at UCLA.

Experiment Design

The entire experiment was composed of three parts: facial expressions,

grasping, and a control experiment. Stimuli were presented on a standard

laptop at the patient’s bed. In the grasping experiment there were two

conditions: action-observation and action-execution. In the action-obser-

vation conditions, the subjects observed a 3 s video clip depicting a hand

grasping a mug with either precision grip or whole-hand prehension. In

the action-execution condition, the word ‘‘finger’’ appearing on the screen

cued the subject to perform a precision grip on a mug placed next to the

laptop. Similarly, the word ‘‘hand’’ cued the subject to perform a whole-

hand prehension. Observation and execution trials were randomly mixed.

The facial expressions experiment was also composed of execution and

observation trials. In the execution trials, the subjects smiled or frowned

whenever the word ‘‘smile’’ or ‘‘frown,’’ respectively, appeared on the

screen. In the observation conditions they simply observed an image of

a smiling or frowning face. Observation and execution trials were randomly

mixed. In the control experiment, the subjects were presented with the

same cue words used in the execution conditions of the facial expressions

and grasping experiments (i.e., the words ‘‘finger,’’ ‘‘hand,’’ ‘‘smile,’’ or

‘‘frown’’). This time, the subjects had to covertly read the word and refrain

from making facial gestures or hand movements.

Recording and Analysis

Data were recorded at 28 kHz with a 64-channel acquisition system (Neura-

lynx, Tucson, AZ) and the signals were band-pass filtered between 1 Hz and

9 kHz. During off-line analysis, the raw signal was band-pass filtered

between 300 and 3000 Hz and action potentials were clustered and manu-

ally sorted with an algorithm based on superparamagnetic clustering.

For each neuron, and each condition, we assessed responsiveness by

comparing the firing rate during baseline (21000 ms to 0 ms relative to stim-

ulus onset) and firing rate during the experimental condition (+200 ms

to +1200 ms relative to stimulus onset) on a trial-by-trial basis with a two-

tailed paired t test. The statistical significance threshold for the paired

t test across trials was set at 0.05. For calculation of the average response

profile of cells during execution and observation (Figure 3), excitatory

responses were normalized by subtracting the average response during

baseline (21000 to 0 ms relative to trial onset), and dividing by the maximum

firing rate of the response (bin size = 200 ms). Inhibitory responses were

normalized by removing the average response during baseline and dividing

by the absolute value of the minimum of the response (see Supplemental

Experimental Procedures for further details).

Supplemental Information

Supplemental Information includes three figures, three tables, and Supple-

mental Experimental Procedures, and can be found with this article online at

doi:10.1016/j.cub.2010.02.045.

Acknowledgments

The authors thank the patients for participating in the study. We also thank

E. Behnke, R. Kadivar, T. Fields, E. Ho, K. Laird, and A. Postolov for technical

assistance; B. Salaz and I. Wainwright for administrative help; and G. Rizzo-

latti for fruitful comments on the manuscript. This work was supported by

a National Institute of Neurological Disorders and Stroke grant (to I. Fried).

R. Mukamel was supported by European Molecular Biology Organization

and Human Frontier Science Program Organization. For generous support

the authors also wish to thank the Brain Mapping Medical Research

Organization, Brain Mapping Support Foundation, Pierson-Lovelace Foun-

dation, The Ahmanson Foundation, William M. and Linda R. Dietel Philan-

thropic Fund at the Northern Piedmont Community Foundation, Tamkin

Foundation, Jennifer Jones-Simon Foundation, Capital Group Companies

Charitable Foundation, Robson Family, and Northstar Fund. The project

described was supported by grant numbers RR12169, RR13642, and

RR00865 from the National Center for Research Resources (NCRR),

a component of the National Institutes of Health (NIH); its contents are solely

the responsibility of the authors and do not necessarily represent the official

views of NCRR or NIH.

Received: October 30, 2009

Revised: February 16, 2010

Accepted: February 17, 2010

Published online: April 8, 2010

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