Is it true that a person's emotional state (such as arousal, fear, etc) can be determined by looking solely at the persons eyes?
Here I am assuming that this may be the case only in limited circumstances (specific scenarios or specific basic emotions such as fear) but if emotional state can be determined from the eyes, is there any research on the extent to which this is possible?
This question has mostly been asked in reverse in the research literature--not whether the eyes can show emotion (which is often called affect by psych researchers), but whether humans can accurately judge affective state based only on the eyes. That ability varies somewhat depending on demographic parameters (age, gender, and socioeconomic class, as well as autism spectrum disorder status).
The "Reading the Mind in the Eyes" test, developed by Simon Baron-Cohen, tests exactly that, using stimuli like these.
Participants are asked to choose the most applicable emotion word for a picture restricted to the eye area. Neurotypical controls are more successful at this task than autism spectrum participants; since most of the research has focused on showing differences between populations on this task and not on quantifying normal performance, the reported data on controls alone is difficult to interpret by itself. The test has fairly good test-retest reliability, but it has been criticized for gender bias since there are considerably more male eyes in the stimuli set than female eyes. This is particularly problematic in light of Baron-Cohen's "extreme male brain" theory of autism.
Humans do commonly use the eyes, area around the eyes, and other eye-related information such as movement and orientation to deduce the emotional states of others. What isn't as clear from research examining this capability however, putting aside subjects' confidence in their own skills, is how accurate / valid such assessments actually are.
But it turns out that the eyes alone can be used objectively to determine a wide variety of information about a person's mental state.
A fairly large research sub-field in this area is Pupillometry, the measurement of pupil diameter. Just copying from a Scientific American article:
Scientists have ... used pupillometry to assess everything from sleepiness, introversion and sexual interest to race bias, schizophrenia, moral judgment, autism and depression.
Other research also shows a connection between mental states and eye-related information, such as eye contact and cognitive load, blinking and certain mental disorders related to dopamine, eye movement and memory recall, eye movement and beliefs, and of course, there's crying.
Though humans may not usually use those particular cues, we presumably evolved certain skills to discern emotional states from eyes as a survival mechanism, suggesting that the cues we use are generally valid. For example, fear causes widening of the eyes / increase in exposure of the sclera (white of the eye), that has the evolutionary advantage of increasing peripheral vision, and the side-effect of alerting others around us to danger, and even to where the danger is. Even infants are found to be quite skilled at discerning emotional state from the sclera, suggesting a genetic origin.
However in general, physical or physiological cues are only part of the picture, as other context information is also crucial for correct attribution of emotional states. So in practice, eyes do not generally provide sufficient information for determining emotional state - humans use a lot of additional context information for that.
I'd like to add some nuance here. While not isolated to reading emotion in the eyes, consider these images from Hillel Aviezer's work (1, 2):
In all four images above, the face is the same. What changes is the body language and objects in the scene. What also changes is the emotions you perceive in each image (in alphabetical order: disgust, anger, sadness, fear).
Can you tell above which tennis players have just won or just lost? It's hard, isn't it? They all look like they're in pain, but 1,4,6 have just lost and 2,3,5 have just won. If you stick the faces back onto the bodies, you'll have better luck at telling who just won or lost.
The point is that we don't interpret the features of the face independently of its context. When we lose context (like in the images Krysta linked), we will rely on concept knowledge about emotions (e.g., the prototypical Ekmanian fear emotion involves wide eyes, as Arnon pointed out). But if you situate those wide eyes in several different contexts, then how you judge the emotion will change.
First we need to define emotional state. If we define it as basic emotions then eyeballs are connected to those and we can see effects of PNS (Adrenal gland, fight or flight (or freeze)
More interesting is if we can find effects of higher order emotions (care, love, guilt) There are not much research about it. According to ie. Ledoux theory of emotion we could even guess about effect of higher emotions on state but it will not be precise.
http://dericbownds.net/bom99/Ch10/Ch10-4.gif
Third very important thing is eye movement, eyemovement can tell us a lot about cognition especialy perception and memory formantion (and rehearsal) and it could be one more puzze for educated gues of higher order emotions: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0064937
