Experimental setup.

<p>Darwin XIII is positioned in front of the DMS apparatus. The apparatus can rotate the stimulus object (light-colored J-shape) to one of four selected orientations or flip sides to expose a different object that is the mirror image of the first. During the delay period and whenever the stimulus is changed, a mechanical blinder is lowered in front of the camera in the robot’s head. To indicate a “match” response, Darwin XIII reaches toward the object with its left arm to touch the clear Plexiglas panel, which activates a sensor. The time of activation is recorded for behavioral response analysis.</p

Comparison of the percentages of neurons active during mental rotation task and during stimulus presentation prior to experience with rotating objects.

<p>Comparison of the percentages of neurons active during mental rotation task and during stimulus presentation prior to experience with rotating objects.</p

Parameters for composition of neural areas, connectivity, and learning in Darwin XIII.

<p>The neural areas area assumed to be 2mm by 2mm toroidal sheets for the purposes of the connectivity distributions described here. See [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0162155#pone.0162155.ref014" target="_blank">14</a>] for detailed descriptions of these parameters.</p

Parameters for the five types of neurons in the simulations and the neural areas in which they are used.

<p>The nine parameters, applied in the Izhikevich spiking neuron equations [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0162155#pone.0162155.ref016" target="_blank">16</a>], determine the spiking behavior of the simulated neurons (see [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0162155#pone.0162155.ref014" target="_blank">14</a>]).</p

The time course of activity patterns in the Decision area during the Response period of match trials causes the linear relationship between response time and angular difference.

<p>(A) Match trials. The yellow line reflects the time course of the match score, averaged across subjects for all trials in which the stimulus B angle matched the stimulus A angle. The Decision area became active when the Sequence area pattern (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0162155#pone.0162155.g007" target="_blank">Fig 7</a>) progressed in the cycle to the one matching that in Working Memory. The response times were progressively longer as the angle between Stimulus A and B increased (green, red, and cyan lines). (B) Non-match trials. Low match scores for all patterns in the Decision area during non-match trials indicating low overall activity, as expected, resulted in Darwin XIII correctly withholding a motor response on most non-match trials.</p

Images of the objects and rotational orientations used.

<p>The two three-dimensional wooden block constructions are related by mirror symmetry. Successive rotations are by 90 degrees in the counter-clockwise direction.</p

The Sequence area shows increased activity during mental rotation.

<p>Mean firing rates were calculated per cell type over the 8 trials for each subject where Stimulus A and B were identical in both object and orientation. The mean firing rates during the mental rotation task were subtracted from the corresponding rates prior to training with rotating objects. The primary difference was in the Sequence area E2/3 neurons, which were cycling through the activity patterns corresponding to the object orientations only during the mental rotation task. Statistically significant differences (P<0.01, Wilcoxon Rank Sum, N = 5) are indicated by double asterisks.</p

Population activity in the Sequence area during the Response period occurs in the same order as when viewing a rotating object.

<p>The match score indicates the similarity of the activity pattern at each time point to the reference activity patterns for a given object orientation. Reported match values are averaged over all subjects and all trials. Each colored line shows the match in relation to one of the four orientations, and the progression of peaks (starting at angle theta, the orientation of Stimulus B) in the colored lines indicates that the activity patterns recapitulate the sequence learned when viewing a counter-clockwise rotating object.</p

Delayed match-to-sample version of the mental rotation task.

<p>The timeline for a single trial shows the four task phases. Images of the visual input at each phase are shown above the timeline. First, Stimulus A is presented for one second. Then a blinder is lowered for a one-second delay during which the object may be switched or rotated. Stimulus B is then presented for one second, and the blinder is again lowered. Darwin XIII is given up to two seconds to respond with a movement if the objects match, as they do in this trial. The elapsed time from the offset of Stimulus B until Darwin XIII touches the Plexiglas panel (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0162155#pone.0162155.g001" target="_blank">Fig 1</a>) is recorded as the response time.</p

Behavioral evidence for mental rotation in Darwin XIII.

<p>Response time of Darwin XIII during match trials from a mental rotation experiment. The mean and standard deviations of response times from five different subjects is plotted as a function of the angular difference between the stimuli. The time to respond to a pair of images of the same object was proportional to the degree of angular rotation of the object between the two views, which matches the key finding from Shepard and Metzler’s study of human subjects [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0162155#pone.0162155.ref003" target="_blank">3</a>].</p