1. Error Signals in Motor Cortices Drive Adaptation in Reaching
Masato Inoue, Motoaki Uchimura, Shigeru Kitazawa 
Neuron 
Volume 90, Issue 5, Pages (June 2016)
DOI: /j.neuron
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2. Figure 1 Design of the Experiments
(A) The reaching task. The monkey made a rapid reaching movement toward a target that appeared at a random location on the screen. The vision was blocked by a liquid crystal shutter during the movement. The shutter was opened again for 300 ms to demonstrate the final error in reaching.
(B) Left: a PC-controlled wedge prism was used to introduce 9 × 9 visual displacements in a random manner that covered a 40 mm × 40 mm square (8.2° × 8.2°, prism displacement). Middle and right: a target was presented randomly in a 40 mm × 40 mm square (real target zone) that was displaced from the center of the screen so that the square was placed (perceived) in the straight-ahead direction after visual displacement. As a result, the animals were kept unaware of the visual displacement until the touch. The figure illustrates an example of the real target zone when a chosen displacement was 20 mm to the right (20, 0).
(C) A diagram illustrating the visual error and the motor error. We defined visual errors as the vector between the perceived target (virtual target position) and the perceived touch position (virtual touch position), which were displaced from the real positions by the prism. The motor error was defined as the vector between the virtual target position and the real touch position.
(D) Recording sites of visual error coding neurons superimposed on MRI pictures. The black circle indicates the primary motor cortex (M1) and the white circle indicates the premotor cortex (PM). CS, central sulcus; IPS, intraparietal sulcus; AS, arcuate sulcus; PS, principal sulcus.
Neuron  , DOI: ( /j.neuron )
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3. Figure 2
Behaviors
(A) A distribution of end-point errors when there was no visual displacement (n = 2,017 trials, Monkey A). An error ellipse that corresponds to one SD is shown in yellow.
(B and C) Distributions of visual (B) and motor (C) errors when a random visual displacement was introduced. Note that the visual error covered the 40 mm × 40 mm square area as designed, whereas the motor error was distributed in a Gaussian manner with an error ellipse larger than that of the non-visual displacement trials (A).
(D) Visual and motor errors over a 127-trial block with a random visual displacement. Red lines indicate visual errors, and blue dots indicate motor errors in the horizontal (H error) and vertical (V error) directions. Thick blue lines indicate predictions yielded from a trial-by-trial adaptation model in which the motor error was assumed to be shifted by a small amount in response to the visual error of the previous trial. Note a clear decrease in the motor error over a few trials when large visual errors occurred in succession (shaded area). The text provides the coefficients of determination (d.c.) and the learning rate (B) of the model.
(E) Distribution of the coefficient of determination when the trial sequence shown in (D) was randomly shuffled 100 times. Note that the coefficients of determination yielded using the original data sequence (arrows) were twice as large as the largest value yielded from the 100 sequences prepared by random permutations.
(F) Estimated retention factors (A) and learning rates (B) for the two monkeys (n = 59 for Monkey A; n = 5 for Monkey S). The edges of the box plots show the 25th, 50th, and 75th percentiles. Each whisker extends from the box to the most extreme data value within 1.5 times the interquartile range.
(G) Saccade onset probability plotted against time aligned to the touch (100 trials, monkey A). Note that no saccade occurred during the 300-ms post-movement period when the shutter was opened again (shaded).
(H) Target positions relative to the gaze position at the time of target presentation (Target ON) and touch (Touch). Note that the target was nearly foveated by the time of touching regardless of the visual displacement used.
Neuron  , DOI: ( /j.neuron )
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4. Figure 3 Cluster Analysis of Neuronal Activities
Activation functions from 237 neurons (157 M1 and 80 PM neurons) were classified into four clusters.
(A) Activities of typical neurons for each cluster. Data for a single cell with the largest Silhouette values in each cluster are shown on the left (typical cells), and mean data for typical neurons with Silhouette values exceeding 0.5 are shown on the right (mean). Data were aligned at the release of the button (release). Error bars and thin lines show the SD.
(B) Distribution of activation patterns in a three-dimensional space defined by the first, second, and third principal component scores (PC 1–3). Each symbol represents a single neuron with a color that represented each of the four clusters (#1, red; #2, green; #3, blue; and #4, magenta). The size of each symbol represents the silhouette value.
(C) Percentage of each of the four clusters in M1 (n = 157) and PM (n = 80) neurons.
Neuron  , DOI: ( /j.neuron )
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5. Figure 4 Visual Error Information Encoded by M1 and PM Neurons
(A and F) Firing patterns of typical M1 (A, cluster #2) and PM (F, cluster #3) neurons. Raster plots (top) and the peri-event spike timing histograms (bottom) are aligned at the touch (A, n = 163; F, n = 107).
(B, C, G, and H) Distribution of visual errors (black dots) with spike counts (red circles) for the M1 (B and C) and PM (G and H) neurons. Spike counts are shown for two time windows, one before (B and G) and another after (C and H) the touch. The two time windows are shown in (A) and (F) with shadings in cyan (−200 to −100 ms) and magenta (100 to 200 ms). The diameter of each red circle is proportional to the spike counts. Insets show the sum of the spike counts in each quadrant. Note contrasts in the distribution of spike counts before (evenly distributed) and after the touch (skewed).
(D and I) Time course of the information on the end-point error for the M1 (D) and PM (I) neurons. Arrows show the information calculated from distributions of spike counts shown in (B), (C), (G), and (H). Filled regions show that the information was significant in terms of the chi-square test. Error bars show the SD of the timings of target presentation and the button release.
(E and J) Time course of the sum of significant information across 157 M1 (E) and 80 (J) PM neurons. Only significant information was summed up, after applying the chi-square test at each time bin for each cell. Thus, different numbers of cells were included in different time bins (see Figure S1 for additional information). Intervals between thin lines show contributions from single neurons. Peak latencies from the touch (opening of the shutter) are shown with arrows.
(K and L) Comparison of the information profiles with zero delay (red) and a 100-ms delay (green) in opening the shutter. (K) Data from a single M1 neuron. (L) Sum of 10 neurons (five M1 and five PM neurons).
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6. Figure 5
Comparison of Information across Cell Clusters, #1–#4, and M1/PM
(A) Proportion of visual error-coding neurons in each cluster for M1 (filled bars) and PM (open bars).
(B) Distribution of the peak error information in each cluster. Two-way ANOVA revealed that none of the main effects (cluster: F(3,75) = 0.13; M1/PM, F(1,75) = 0.04) or their interaction (F(3,75) = 2.7) were significant. It is worth noting that peak error information was acquired after touch for all of the visual error-coding neurons.
Neuron  , DOI: ( /j.neuron )
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7. Figure 6
Trial-by-Trial Increases in the End-Point Error in Response to Intracortical Microstimulation to PM
(A) Activation profiles of a PM neuron to the position at which intracortical microstimulation (ICMS) was delivered. The PM neuron belonged to Cluster #3, with its major activity in the post-movement period.
(B) Information profiles of the PM neuron. The visual error information was significant from 80 to 220 ms after the touch (filled). The inset shows spike counts during a post-movement period between 40 and 140 ms after the touch. The arrow indicates its preferred direction (weighted sum of the end-point errors according to the spike counts).
(C) End-point errors before (black dots), during (red dots), and after (open circles) ICMS. ICMS was delivered after each movement during the post-movement period from 0 to 200 ms.
(D) End-point errors plotted against trial numbers. Each block consisted of 30 trials. Errors were measured in the x and y directions shown in (C), where the y direction shows the preferred direction of the PM neuron. Note a gradual increase of error in the anti-preferred direction during the stimulation block (trials 31–60), and an exponential wash-out process during the post-test block (trials 61–90). Thick lines show the results of linear regressions (pre-test and stimulation blocks) and the fitting of a discrete model (post-test block).
(E and F) Effects of a 100-ms delay in delivering ICMS. Note that the ICMS was no longer effective when the ICMS to the same PM region was delayed by only 100 ms (100 to 300 ms after the touch).
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8. Figure 7 Effects of ICMS to 31 Locations: 18 M1 and 13 PM
(A) Distribution of the slopes of regression lines in the preferred and orthogonal directions. Note a skewed distribution in the left quadrants. The slope in the preferred direction was significant at the 15 locations shown by large symbols (M1, filled; PM, open circles).
(B) The mean changes in errors averaged across 31 locations. Dashed lines show the standard deviation. Note a clear increase in error in the anti-preferred direction during the stimulation block and an exponential wash-out thereafter.
(C and D) Distribution of correction rates (C) and after-effects (D) during the post-test period. The after-effect was measured in the anti-preferred direction.
Neuron  , DOI: ( /j.neuron )
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9. Figure 8 Muscle Activity
(A–D) EMG activity of wrist extensors (A), wrist flexors (B), elbow extensors (C), and elbow flexors (D). Top: the digitized EMG activity aligned to the touch. Bottom: the information transmission rate (Ir) of the virtual target position (green) and visual error position (blue) plotted against the time from touch. Filled areas show significant information. Note that the succession criterion (significance for five consecutive time bins) was not applied to the data drawn from elbow extensors. Shaded areas (cyan) show 100-ms time bins that yielded each peak information.
(E–H) Activity tuning maps for neurons and muscles on the visual error plane. Spikes of neurons, supra-threshold stimulation to which evoked wrist extension (E, n = 9), wrist flexion (F, n = 2), elbow extension (G, n = 5), and elbow flexion (H, n = 30), were averaged across neurons after normalizing the maximum activity level to one for each neuron. As for muscle activity levels, each map was prepared during each 100-ms period as shown in (A)–(D). The maximum activity level was normalized to one for each muscle group. Arrows show the “preferred” direction that connected the origin and maximum point for each map. Note that two vectors, one for neurons and another for EMG signals, pointed in a similar direction within each group.
Neuron  , DOI: ( /j.neuron )
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