The human and animal brain is made of neural networks. Is it possible to train an animal so that it becomes as intelligent as a human?
There are a lot of examples of animals that have been trained by humans (to perform some specific task). For example, dogs, tigers or chimpanzees. Nonetheless, none of them have exhibited a general intelligence comparable to that of humans. Why is that?
It is believed that the intelligence of mammals is (at least partially) determined by the size of the neocortex. Humans have one of the largest neocortices (among all mammals). Intelligence is also determined by the connection between the neurones.
It is likely the case that, possibly among other factors, the size of the neocortex, the connection between the neurones (the synapses) and the number of the neurones limits the intelligence of a mammal. Consequently, by nature, animals are limited in terms of (general) intelligence.
A serious attempt to understand and map the animal brains was done within the Brain/Minds project in Japan. The idea is to scan the brain of a marmoset monkey and build a computer model of it. In this model, certain questions can be answered, especially if it's possible to train the brain to become intelligent.
A biological neural network is similar to an artificial neural network able to adapt to external stimulus. If a convolution neural network in software is able to recognize words in a stream of audio signals, the probability is high, that a biological brain can do the same task. This would help to cure Alzheimer disease for monkeys and other animals.
 Wada, Akihiko, et al. "Differentiating Alzheimer’s Disease from Dementia with Lewy Bodies Using a Deep Learning Technique Based on Structural Brain Connectivity." Magnetic Resonance in Medical Sciences (2018): mp-2018.