As per Scheggia et al (2022)

Experimental Environment:

• The entire setup was placed inside a sound-attenuating cubicle to minimize external environmental influences.
• The cubicle was homogeneously and dimly lit to maintain consistent lighting conditions (6 ± 1 lux) and prevent gradients in light, temperature, and sound.
• The controlled environment aimed to prevent any biases that could result from differences in sensory conditions.
• The actors and recipients were mildly food-restricted and housed together before the experiment. Unfamiliar recipients were used for specific conditions.

Task design

The task design included three conditions:

• With recipient (cagemate)
• No recipient (empty compartment)
• With a toy (inanimate object).

The actor could make selfish choices (receive reward alone) or altruistic choices (share reward with the recipient) by poking into left or right nose poke holes. Both choices were reinforced with a food reward delivery, and the order of choices was balanced. After each nose poke, a 5-second interval occurred.

Testing

Actors were tested for five days in 40-minute sessions, either with a cagemate, no recipient, or a toy. In some conditions, an opaque or transparent partition was used between the actor and recipient to control visual and social cues. The setup was monitored by a digital camera connected to a behavioral tracking system.

Fixed ratio schedules

This protocol involved testing mice’s voluntary choices to benefit others under increasing fixed ratio (FR) schedules for altruistic decisions. The FR schedule for altruistic decisions ranged from FR2 to FR8, meaning that the number of responses required to give food to the recipient increased each day (from 2 to 8 responses). Selfish responses were consistently reinforced on an FR1 schedule. In the ‘no recipient’ condition, each actor’s preferred nose poke was reinforced using the increasing FR schedule, while the other nose poke remained on an FR1 schedule. This setup aimed to investigate whether subject would continue to make altruistic choices even when the effort required was higher, and it included controls without a recipient to isolate the effects of the schedule.

SDM task without concurrent reward

In this phase of the study, mice were initially trained in the social decision-making (SDM) task. The task was then altered, so that one nose poke led to food rewards exclusively for the mice themselves (selfish choice), while the other nose poke resulted in food rewards solely for the recipient (altruistic choice). During testing, the mice were engaged in longer sessions lasting 120 minutes, allowing researchers to observe whether the mice’s choices were influenced by feelings of satiety over an extended period of time.

Satiety-induced reward devaluation

In this phase of the study, mice were subjected to satiety-induced reward devaluation tests. After undergoing five days of the social decision-making (SDM) task, the mice were individually housed for two hours and allowed to consume reward pellets to the point of satiety in their cages. Following this, they were moved to the operant chamber and tested in a session without reinforcement.

Two experiments were conducted with sated actor mice:

In Experiment 1, two groups were tested: one where neither the actor nor the recipient received rewards (‘no reward’), and the other where actor mice did not receive reinforcement, but could still allocate food rewards to their recipients (‘reward to recipient only’).
In Experiment 2, mice were tested in the ‘SDM without concurrent reward’ condition after making altruistic choices in the initial SDM task.

SDM task with sated recipients

In this part of the study, the social decision-making (SDM) task was modified to include sated recipients. Both actor and recipient mice were subjected to food restriction. Prior to each session of the SDM task, recipient mice were separated from their cage mates (actors) and were fed to satiety, providing them with free access to reward pellets in their own cages. Subsequently, both actor and recipient mice were moved to the operant chamber and were tested in a session with reinforcement.

In a separate group of mice, satiety-induced reward devaluation was also tested in recipient mice after they had undergone standard training in the SDM task for five days. In this scenario, one group of actor mice was tested with food-restricted recipients, and another group with sated recipients following the reward devaluation induced by satiety.