The crucial difference between the experiences of the kittens was the amount of control they had over their artificial world. One kitten was free to move the boom whenever it wanted, and where it wanted (within the limits of the rigid boom). It could stop, start, go back or forward. The movement of the other kitten was determined solely by the first kitten's activity. This passive kitten received the same visual information as the active kitten. But its actions had no effect on visual outcomes.
After some time the kittens were removed from the room and carousels and their behaviour observed. The first active kitten did typical kitten things. The behaviour of the second passive kitten was unusual, seeming to have difficulty in making sense of its visual environment. In both situations, the kittens received the same information through their eyes. One had this information under active control, for the other kitten it was a passive process.
2Depressed Dogs: Now consider a second experiment, carried out by Martin Seligman, also in the 1970's. In this experiment two groups of dogs were subjected to electric shocks (one dog at a time!). One group had, in an earlier arrangement, been able to escape from the shock by jumping over a barrier. The previous experience of the second group was that electric shock was inescapable. After a while both groups (again individually) were placed in a different arrangement in which electric shocks could be escaped. What happened? The first group did doggy things, escaping nonchalantly. But the second group, despite being able to now escape the shocks, behaved quite differently. They whimpered and lay down, waiting helplessly for the next shock to come. The dogs were described as behaving in ways similar to human depression.