Material and Methods

9.4.1 Subjects

All surgical and experimental procedures followed the Regulation for the Welfare of Experimental Animals issued by the Federal Government of Germany and were approved by the local authorities. Training procedures were carried out with 3 male macaque monkeys (Macaca mulatta) with body weight of ~11 (M1, M2) and 13 kg (M3), implanted with a headpost. Surgical procedures followed previously published protocols [Schledde et al. 2017;

Wegener et al. 2004]. All animals had been used in other projects before. They were familiar with the laboratory conditions and with a dimming task at fixation (see below) but were naive to the specific stimulus and task conditions reported here. Out of training sessions, animals were kept in a species-appropriate, environmentally enriched husbandry. M1 and M2 were each pair-housed and had daily access to an outdoor compartment; M3 lived in an indoor compartment with visual and auditory contact with other monkeys. The animals received free fruits and water on Friday afternoon and during the weekend as well as during nontraining periods. Health and well-being were checked by daily monitoring of behavior and body weight and regular checks by veterinarians.

9.4 Material and Methods

9.4.2 Visual stimulation, reward schedule, and behavioral paradigms

Training sessions were performed in a lightly dimmed room. Animals sat in a primate chair 80 cm in front of a 22-in. cathode ray tube monitor (1152 × 864 pixels, 100-Hz refresh rate).

Eye movements were measured at a spatial resolution of 0.2° visual angle, using a custom-made remote video-oculography system. Reward consisted of water or diluted red grape juice, applied by a simple, valve-controlled gravity liquid dispenser. A “high reward”

consisted of ~25-30 ml/100 correct responses, a “medium reward” of ~15-20 ml/100 corrects, a “small reward” of ~10-15 ml/100 corrects, and a “very small reward” of ~5-10 ml/100 corrects. “Zero reward” refers to trials that were not rewarded but neither terminated. Details about the reward schedule in different training situations are given in Results. Training sessions were not terminated before the monkey showed no further interest in performing the task. During training days, monkeys usually did not receive additional liquid in their home compartment.

Visual stimulation was carried out with custom-made software, run on a Pentium computer.

The computer software chose the amount of reward depending on the monkeys’ behavior (e.g., precision of fixation or reaction time) and saved all stimulation and behavioral data in a trial-description file. We applied NB-PRT during four different early and late periods of training attention tasks, as well as investigating its effectiveness regarding fixation accuracy and reaction time (RT) acceleration in simple fixation tasks. For better traceability, the general task requirements are explained in the corresponding Results.

Trial timing in the different experiments and conditions is summarized in Table 1. Common to all examples reported in this article, monkeys were required to initiate a trial by gazing at a fixation point (FP; 0.11°- 0.18° side length) at the center of the screen and pressing a lever.

With the exception of example 1, they had to keep fixation within a maximally 1.5° radius throughout the trial. For the experiments described in example 1, the fixation window was divided into two zones of different radius. Trials during which the eyes did not leave the inner fixation zone received a higher reward than trials during which eyes entered the outer zone.

Leaving the outer zone resulted in termination of the trial without reward. In all experiments, monkeys were required to respond to a change in a stimulus (described below for each example separately) by releasing the lever within a time window of 150-750 ms after this

9.4 Material and Methods

event. If not stated differently, providing the response in the appropriate time window had been trained before and was considered a well-established behavior. As such, releasing the lever too soon (false alarm) or too late (miss) resulted in immediate trial termination without reward. Intertrial intervals in all examples had a length of maximally 1,000 ms, followed by an interval of maximally 3,000 ms for trial initiation. Not initiating the trial during this time resulted in a reset of the trial clock. In any case, monkeys performed as many trials as they wanted. When they showed no further interest in performing the task, they were carried back to their home compartment.

Experiments were performed using either a simple fixation task or two different tasks requiring covert attention to either a moving Gabor stimulus (speed change task) or a static object (object dimming task). Simple fixation tasks (example 1, M2; and example 2, M2 and M3) were carried out with no other objects on the screen. The FP was shown on a dark background at the center of the display. Dimming occurred at a pseudorandom time after FP onset (cf. Table 1 for details). For all experiments, dimming was clearly above threshold. The speed change task (examples 1-3, M1) required detection of an instantaneous speed increment of a Gabor element (2 cycles/° spatial frequency, enveloped by a Gaussian with 0.75° at half height, 10 cd/m² mean luminance) placed 8° away from the FP. The Gabor inherently drifted with 2.17°/s and increased speed by 100% (examples 1 and 2) or 80% (example 3) at a pseudorandom point in time 1,360-3,520 ms after trial initiation. If cued, Gabor onset was preceded by displaying a 3° × 3° rectangular frame at the upcoming target position for 500 ms. When the Gabor was shown together with another Gabor, both objects were displayed at opposite positions across the FP (Fig. 25E). In the object dimming task (example 4, M3 and M1), visual stimulation consisted of 26 isoluminant objects, each of which was unique in shape and color. Objects were arranged on three imaginary, circular rings around the FP, centered at 3, 5, and 7° eccentricity. Object size on the inner ring was ~0.9° width and ~0.9°

height, and increased by a factor of ~1.2 and ~1.5 on the middle and outer ring, respectively, to account for larger receptive fields in the periphery. Objects had a luminance of 10 cd/m² and were presented on a dark background. Thirteen (M3) and 15 (M1) of all objects were possible targets, distributed over the inner and middle rings (M3) or over all rings (M1). The monkeys’ task was to covertly attend to one of these targets and to indicate a dimming of this object (M3) or of a small spot (0.15°, 0.22°, or 0.23° side length on inner, middle, or outer ring, respectively) on the object (M1). The target in each upcoming trial was cued by a

9.4 Material and Methods

smaller (factor 0.75), slightly blurred version of the respective object, located at the center of the screen, behind the FP (see Fig. 29A). Object locations were kept constant during all training sessions. On 0-20% of the trials, the cued change was preceded by an uncued change in another, randomly chosen distractor object (catch trial). Unless stated differently, monkeys were allowed to respond to both the cued and uncued change within 150-750 ms after the change. Dimming of cued and uncued objects was separated by at least 750 ms. For the first five sessions of the training to introduce catch trials, however, the minimal temporal separation between changes at uncued and cued objects was allowed to be as short as 250 ms.

In such cases, when the temporal separation between two changes was too short to allow a definite assignment of the response, we considered responses below 750 ms after the first change as a response to the first, uncued change and responses occurring later as a response to the second, cued change.

Tab 1: Trial timing and cue validity. Values for cue onset, stimulus onset, and first and second change event refer to time of trial start (lever press). SCD, speed change detection task; FPD, fixation point dimming task;

OBD, object dimming task; NT, normal trials; CT, catch trials.

Monkey Task Session Cue Validity,

%

Cue Onset, ms

Stimulus Onset, ms

First Change, ms

Second Change, ms Example 1

M1 SCD 1-5 200 860-3020

6-13 100 200 500 1,360-3,520

M2 FPD All 1,500-4,000

Example 2

M1 SCD All 100 200 500 1,360-3,520

M2, M3 FPD All 1,500-4,000

Example 3

M1 SCD All 90-75 200 500 1,360-3,520

Example 4

M3 OBD 1-5 100 1,000 250 1,700-4,700

6-45, NT 90 1,000 250 1,700-4,700

6-45, CT 90 1,000 250 1,700-2,700 1,950-5,150

M1 OBD All, NT 80 1,000 250 2,500-4,500

All, CT 80 1,000 250 2,500-3,500 4,000-5,300

9.4 Material and Methods

9.4.3 Data analysis

Data were analyzed using MATLAB (The MathWorks, Natick, MA). Data analysis was based on the amount of correct and error responses (false alarms, misses, eye errors) and RT distributions. RT distributions were fit by an ex-Gaussian probability density function (https://github.com/bramzandbelt/exgauss), which is a convolution of a Gaussian and an exponential distribution. The ex-Gaussian fit provides the parameters µ and σ to describe the mean and the variability, respectively, of the normally distributed component of the distribution, and parameter τ to describe the exponential part, which accounts for the skew of the distribution [Heathcote et al. 1991; Tarantino et al. 2013].

For some analyses, training sessions with a low number of trials were disregarded to allow conclusive statistics. Such sessions occurred on Mondays only, as a result of ad libitum supply of water and fruits during the weekend. Exclusion of sessions is reported in the corresponding Results. Statistical testing was done using nonparametric tests throughout. P levels are reported for two-tailed testing. Significance is reported at the α = 5% level.

Statistical details are reported separately for each comparison. If only Z and P values are reported, they were obtained using the same statistical test and the same sample size as in the directly preceding comparison. The effect size R for Mann-Whitney’s U-test to test whether two independent, unpaired samples come from the same distribution was calculated as

R= Z

√

where Z is drawn from the Mann-Whitney test statistics and N1 and N2 correspond to the number of elements in each of the samples. The estimated probability EP to randomly draw a value from one data group that is larger (smaller) than a randomly drawn value from a second data group was calculated as

EP= U N1×N2

where U is the smaller (larger) of the two U values derived from the Mann-Whitney test statistics.