The humble saccade. A very, quick, short eye movement, driven by a movement or shift of attention outside of central vision. This quick movement allows us to assess our environment quickly and find potential dangers, as well as developing a map of our surroundings. They are also vital to reading accuracy.
The Accurate Saccade
As one enters a room, the the eyes quickly survey the room. Our attention is drawn from one thing to another…picture of a dog, the TV playing a novel commercial. We smell something delicious and eyes quickly make saccades around the table until we find the source. These exploratory saccades, help to develop an accurate spatial map of this new, lively environment. If one closed their eyes, they could still point to items in the room and even touch specific items, because of the accuracy of this spatial map.(2)
The Dorsal Stream
This spatial mapping is a part of the function of the dorsal stream (magnocelluar tract). It receives its information from the peripheral areas of the retina. The dorsal stream guides both the eyes in exploration of the environment and guides the hand as it reaches for something(1) within the environment. The accuracy of a saccade is dependent upon this spatial map for accuracy. A well functioning dorsal stream helps in accurate saccades as well as accurate reach.
The Inaccurate saccade
When the information gathered about the environment via the dorsal stream is not accurate , then a saccade cannot be accurately targeted and we observe saccade over shoots and undershoots. A poorly developed spatial map could cause a person to be clumsy as they could be prone to knocking thiings over or tripping over things as their awareness of the location of items is impaired. Functionally our patient/student could have problems with reading as well.
Is it the movement or the fixation?
As we attempt to objectively measure saccade accuracy, technology such as RightEye has certainly been helpful but the cost of these tools makes it prohibitive in many contexts. Number reading tasks such as the King-Devick and the Developmental Eye Movement test are frequently used as they are lower cost and administered and scored quickly. While the DEM specifically has been shown to correlate well to reading fluency, it does not generally identify those with saccade inaccuracy when compared to data gather from eye tracking devices.(3) While the inaccurate movement is measured by these devices, the DEM, King Devick and (reading in general) may be equally dependent on the accuracy of the fixation between saccades making acuity another factor in reading accuracy. This of course makes sense. Good eye teaming during saccades makes for that accurate fixation allowing for more fluent reading.
Its not “just a saccade”
While its easy to the role saccades play in reading, the humble saccade does so much more!
1.Galletti C, Fattori P. The dorsal visual stream revisited: Stable circuits or dynamic pathways? Cortex. 2018 Jan;98:203-217. doi: 10.1016/j.cortex.2017.01.009. Epub 2017 Jan 23. PMID: 28196647.
2. Zimmermann E, Morrone MC, Burr DC. Buildup of spatial information over time and across eye-movements. Behav Brain Res. 2014 Dec 15;275:281-7. doi: 10.1016/j.bbr.2014.09.013. Epub 2014 Sep 16. PMID: 25224817; PMCID: PMC4378607.
3. Ayton LN, Abel LA, Fricke TR, McBrien NA. Developmental eye movement test: what is it really measuring? Optom Vis Sci. 2009 Jun;86(6):722-30. doi: 10.1097/OPX.0b013e3181a6a4b3. PMID: 19417709.
