Short answer: Because their brains aren’t as complex as human brains. Unfortunately that’s the price we people with prefrontal cortexes pay. In bipolar disorder, as in schizophrenia, people with these illnesses can become out of touch with reality. This is called psychosis, or being psychotic. Auditory hallucinations happen to 90% of people with schizophrenia, i.e. they hear voices, this also happens up to 80% of people with bipolar d/o. There are also visual hallucinations (seeing things), even olfactory hallucinations, where you may smell something that isn’t there! (Luckily for me, I have never had auditory hallucinations, I am forever grateful for this! Interestingly enough, I have had olfactory hallucinations, I smelled the scent of Camay soap once when it was nowhere to be found.)

Let’s get back to the point of this article from Scientific American. It basically says that schizophrenia 9and I assume bipolar d/o in psychosis) are the price we pay for a much more complex brain. It is a defect of the gamma amino butyric acid (GABA) system. This is an inhibitory neurotransmitter, meaning it inhibits neurons from firing, in part by suppressing dopamine in certain parts of the brain. So when there is a problem with this system, then neurons that wouldn’t normally be firing are firing, and dopamine is also not suppressed, and this is happening in the prefrontal cortex (PFC). This leads to hallucinations. See quote below.

Yes the psychotic brain, whether in schizophrenia or bipolar d/o runs amok. And it can run so crazily amok because it is so complicated. So complicated that when things go wrong, they go wrong in a big way. Hence hallucinations.

http://www.scientificamerican.com/article/why-don-t-animals-get-schizophrenia-and-how-come-we-do/

“They also found that these culprit genes are involved in various essential human neurological functions within the PFC, including the synaptic transmission of the neurotransmitter GABA. GABA serves as an inhibitor or regulator of neuronal activity, in part by suppressing dopamine in certain parts of the brain, and it’s impaired transmission is thought to be involved in schizophrenia. If GABA malfunctions, dopamine runs wild, contributing to the hallucinations, delusions and disorganized thinking common to psychosis. In other words, the schizophrenic brain lacks restraint.”