Annotating Blinks
A blink is noted as a single point in time. This is the moment at which the eyeball is first substantially occluded. It lasts until the first frame at which the eyeball is no longer substantially occluded. In the cartoon below, the blink lasts for 4 frames.
Blink types
The most fundamental criteria to be considered in categorizing a blink is the direction of gaze before and after the blink. A second criterion to be considered is the length of the blink. If the blink is judged to last for 5 frames or less (at 25 frames/sec), then it is a short blink; a blink of 6 to 10 frames is a long blink, and any longer event is an eye closure, for which we notate the start and end, using the labels "gd" and "gu".
| Gaze before | Gaze after | Length | Label | Comments |
|---|---|---|---|---|
| Towards | Towards | Short | bstt | Most common blink type |
| Away | Away | Short | bsaa | Gazing away. No obvious change of direction. |
| Towards | Away | Short | bsta | At the point of gaze switch |
| Away | Towards | Short | bsat | At the point of gaze switch |
| Away | Away (changed) | Short | bsab | Change of direction, but gaze is not towards partner at either end |
| Towards | Towards | Long | bltt | Often together with a head nod or back channel |
| Away | Away | Long | blaa | Rare |
| Towards | Away | Long | blta | Rare |
| Away | Towards | Long | blat | Rare |
| Away | Away (changed) | Long | blab | Very rare |
Examples
Several illustrated examples of each blink type are provided, with notes. These should be studied before the ambiguous or borderline cases presented below are addressed. For clarity, many of these examples make use of subjects wearing eye makeup. This makes blinks considerably easier to observe. Each blink is provided in real time video and in a slowed down format for detailled study.
| bstt/bltt | bsaa/blaa | bsta/blta | bsat/blat | bsab/blab |
Potential Sources of Ambiguity
Incomplete blinks
Some people frequently display blinks in which the upper eyelid fails to meet the lower. If there is only an occasional blink of this nature, the annotater should be guided by the simple principle that if the visible surface of the eyeball is substantially occluded (>50%), a blink should be recorded.
If a subject displays a lot of incomplete blinks, the annotater will need to form a judgement as to what degree of occlusion ought to count as a blink for that subject. Here is a good example.
A special case of incomplete blink that occurs frequently is one associated with gaze change. The degree to which the visible part of the eyeball is occluded is very dependent upon the vertical direction of gaze, and the degree of change in the vertical sense. Here, the simple rule of thumb shoudl be applied that if you judge that the eyeball is substantially occluded, a blink should be recorded.
Blink clusters
Sometimes blinks occur in clusters. That can be two blinks in rapid succession, or it can be many more. It is often difficult to tell the direction of gaze in the middle of such a cluster. I suggest that the most important thing is to ensure that the direction of gaze is accurately recorded at the point before and after the cluster, and a "best guess" is made for gaze direction at intermediate points. This is not a very common problem. Here is an example.