What it means to be a person is an issue that has been made substantially more complicated over the past century by the rise of artificial intelligence. One way to start thinking about it is via the "Turing Test". Imagine that you are communicating with an avatar in a computer chat room or on a social media site. You cannot see whether the avatar is being controlled by a human or a computer. How could you tell whether you were chatting with person or machine? Would a machine that could pass as human to most interlocutors in this situation be considered human? What the Turing test and recent studies on animal intelligence make us reconsider is whether we should consider "humanity" a matter of our physical or biological nature or something having to do with our abilities to think and feel.
When we think about decision making, we general consider factors grounded in three mental characteristics, emotions, reason, and memory. All three of these are parts of how we define humanity. What makes us somewhat uncomfortable with considering a machine that passes a Turing test human is precisely our doubts concerning the ability of machines to experience emotion. Memory is a more complex challenge. The borderline cases might be a baby (or fetus) with few or no memories or an elderly person with dementia. In both cases, we have obviously biological humanity. In the case of the very young, we have a future memory-forming potential and in the case of someone with advanced dementia, a past capability.
There are several brain-based factors that influence the way we make decisions and think about things. Scientists have begun to isolate brain areas involved in different types of decisions using patients who have suffered injuries to one section of their brains. For example, scientists have found that the front part of the frontal lobe is involved in decisions that involve abstract reasoning, as this region of the brain controls planning, organization, and the so-called "executive functions" (the ability to decide on and execute a task). Decisions that involve concrete reasoning involve the back of the frontal lobe. Other decisions that involve integrating visual information are processed through the parietal lobe, which processes sensory information. When part of the brain is involved in a decision, it shows increased activity in the neurons in that region. Memory is processed in an area of the brain called the limbic system, which includes the amygdala, hippocampus, and other structures. Damage to the hippocampus, for example, can result in anterograde amnesia, or the inability of people to form new memories.
This means that cognition and many of the qualities we associate with being human are, in reality, based on the firing of neurons in different parts of the brain. The sense we make of these cognitive functions is in part what makes us human and adds to our individuality.