Embodiments of the presently-disclosed subject matter include methods and systems for measuring a level of a neurotransmitter in a subject. Embodiments of the present methods comprise displaying a fixation point, a reward target, and a non-reward target, and measuring one or more saccade movement parameters for reward saccades and non-reward saccades. The saccade movement parameters can include velocity, amplitude, reaction time, or a combination thereof. The present methods can further include determining a reward modulation of the subject, the reward modulation being equal to a difference between the reward and the non-reward values for a respective saccade movement parameter. Some embodiments further include identifying the subject as including a deficiency of the neurotransmitter if there is a statistically measurable difference between the reward modulation of the subject and a reference reward modulation and/or if the non-reward and the reward saccade movement parameters are statistically equivalent.
