Behavioral paradigm

The monkey was sitting in a dark room in a custom-made primate chair with the head restrained 30 cm away from a 27” LED display (60 Hz refresh rate, model HN274H, Acer Inc. USA). The gaze position of the right eye was monitored at 220 Hz using an MCU02 ViewPoint infrared eyetracker (Arrington Research Inc. USA). The monkey’s face and body

1 session 12 sessions

1 session 4 sessions

More anterior site:

More posterior site:

y -13.5

y -14.75 y -14.5 y -14.25 y -14.0

y -13.75 y -13.25 y -13.0

PM PL PI bsc

R L

Fig. 2.2 Probability map of electrode positions across sessions projected onto a high-resolution MR image of the animal’s brain in AC-PC space. Pulvinar nucleus outlines (PM, PL, PI) were adapted from the NeuroMaps atlas (Rohlfing et al., 2012). Red/yellow map: overlap of the estimated electrode positions between 12 sessions with stimulation at a more anterior dPul stimulation site. Blue map: overlap of the estimated electrode positions between 4 sessions with stimulation at a more posterior dPul stimulation site. Y: distance from AC-PC origin in the anterior/posterior plane in millimeters, L: left, R: right, PM: medial pulvinar, PL: lateral pulvinar, PI: inferior pulvinar, bsc: brachium of the superior colliculus.

2.2 Methods 17 were monitored with infrared cameras to ensure that microstimulation did not elicit abrupt movements or signs of discomfort. A MATLAB-based task controller (version R2012b, The MathWorks, Inc., USA) and the Psychophysics Toolbox (Brainard, 1997) were used to control stimulus presentation.

Fixation task

At the beginning of each stimulation session the monkey performed an eye fixation task in order to determine the optimal current strength and electrode position for subthreshold microstimulation that did not evoke saccades. To this end, we first placed the electrode at the estimated location in the center of the dPul. Then, in several blocks consisting of 20 trials each the animal was presented with a dark grey central fixation spot (0.5° diameter) which turned light grey as soon as the animal acquired eye fixation. The monkey was required to maintain eye fixation for 2000 ms within a radial window of 5° around the fixation spot. The inter-trial interval (ITI) was 2000 ms and 1000 ms long for successful and aborted trials, respectively. In half of the trials, 500 ms after eye fixation was acquired one current-pulse train (see section 3.2.3 for stimulation parameters) was delivered and the online, MATLAB-based representation of the eye position and the infrared camera images were carefully monitored to detect eye movements including eye blinks. The current strength started at 50 µA and was increased in steps of 50 µA after each block without obvious stimulation-induced eye movements until the final current strength of 200 or 250µA was reached. As soon as obvious eye movements time-locked to the delivery of the pulse train were observed the electrode was retrieved by approximately 0.25 mm and the respective block was repeated. This procedure was repeated until an electrode position was found that allowed stimulation with a current strength of at least 200 µA without evoking eye movements.

Distractor task

Fig. 2.3 shows a schematic of the distractor task. Each trial started with the presentation of a dark grey fixation spot. The animal initiated each trial by acquiring eye fixation within 500 ms after the onset of the fixation spot. As soon as the animal’s eye position entered the 5°

radial window around the fixation spot, the fixation spot turned bright grey. After maintaining fixation for 500-900 ms the fixation spot turned dark again and the animal was presented with one or two peripheral dots representing potential options for an upcoming saccade. This luminance change of the fixation spot with the simultaneous onset of the peripheral visual stimuli represented the time the animal was allowed to give a response (go signal). Red

dots represented target stimuli whereas yellow and orange dots represented distractors. In conditions with a single peripheral stimulus either one target or one distractor was presented in the left or the right hemifield and the monkey was required to make saccades to targets while ignoring distractors by maintaining central fixation. In conditions with two peripheral stimuli the monkey was shown two dots in opposite hemifields. In double-target trials two equally-rewarded targets were presented and the animal could freely choose either one of them as a saccade target. In double-distractor trials two distractors were presented which had to be ignored by maintaining central fixation. In target-distractor trials a target was presented with a distractor and the animal was required to make a saccade towards the target while ignoring the distractor. Targets and distractors could be presented contralateral to the side of stimulation (left, contraversive side) or ipsilateral to the side of stimulation (right, ipsiversive side). The animal had to make his choice within 500 ms (target acquisition epoch). As soon as the eye position entered the 5° radial window around one of the peripheral stimuli the stimulus was considered to be selected and the monkey was not allowed to reverse his decision. Note that in each condition the animal had to choose between making a saccade to one of the peripheral dots and ignoring the peripheral stimuli by maintaining eye fixation resulting in two response options for conditions with one peripheral stimulus (saccade, fixation) and three response options for conditions with two peripheral stimuli (contraversive saccade, ipsiversive saccade, fixation). The selected stimulus, either the selected peripheral dot for saccade responses or the fixation spot for maintaining eye fixation, turned bright to confirm the animal’s selection. After fixating the selected stimulus for another 500 ms (target hold epoch) correct responses were followed by a feedback sound, a fluid reward, and an ITI of 2000 ms whereas after incorrect responses the next trial started after an ITI of 1000 ms.

All stimuli were matched in luminance (dim stimuli: 11 cd/m², bright stimuli: 35 cd/m²) and size (1° diameter). Targets and distractors were displayed at one of three locations per hemifield (six locations in total) with an eccentricity of 20° of visual angle. Stimulus locations were arranged concentrically around the fixation spot at 0° (mid left), 20° (up left), 160° (up right), 180° (mid right), 200° (down right), and 340° (down left). In conditions with two peripheral dots the two stimuli were presented either on a horizontal or a diagonal axis.

All experimental conditions were pseudorandomly interleaved in such a way that trials that were aborted before the monkey selected a stimulus returned to the pool of trials from which the next trial was chosen randomly. Distractor colors were determined in six initial sessions of psychophysical assessment (see section 2.2.7). In a total number of 16 stimulation sessions in half of the trials electrical microstimulation of the right dPul started either 80 ms before the go signal (early stimulation condition), simultaneously with the go signal (stimulation at

2.2 Methods 19

+80 ms

Fixation GO

acquisition Fixation Reward /

ITI

up to 500 ms 500 - 900 ms up to 500 ms 500 ms

Target-distractor Double distractor Double target

Stimulation periods

200 ms -80 ms

0 ms (GO)

up to 2500 ms

Trial time Target

acquisition Target

hold

Single distractor Single target

60 pulses, 300 Hz

Fig. 2.3 Time course of one trial of the distractor task paradigm. After an initial period of central eye fixation the animal was presented with one or two peripheral stimuli. Stimulus onset together with the dimming of the fixation spot (grey dot) was the go signal for the animal to give the response. Dashed circles represent the animal’s eye position in a successful trial for each condition with the radial tolerance window around the stimulus that is currently being looked at. Red dots represent targets, yellow dots represent distractors. Electrical microstimulation was applied either 80 ms before the go signal (-80 ms), simultaneously with the go signal (0 ms, onset of peripheral stimuli), or 80 ms after the go signal (+80 ms).

go), or 80 ms after the go signal (late stimulation condition) (see section 3.2.3 for stimulation parameters).