PS1061: Sensation and Perception
Term 2,      THURSDAY 11am -1pm      (Boiler House)

Lecture 5:   Eye Movements and Perception

Lecturer: Szonya Durant, szonya.durant@rhul.ac.uk, (Room W 245)

Lecture topics

• Background – why study eye movements and anatomy of the eye   
• Types of eye movements
• Measuring eye movements
• Why do we have eye movements?
• When eye movements go wrong
• Active vision - eye movements in active searching of the visual scene
• Reading - gaze patterns
• Attention - where we look and where we pay attention

Useful links

http://viperlib.york.ac.uk/categories/121-eye-movements (requires you to register)

http://www.cudos.ac.uk/roger/oculo.html

Why study eye movements in Psychology?

• If we aim to study visual perception we need to know where participants are looking   
• Eye movements can be used as indicators of visual attention - with theoretical and applied uses
• Eye movements can inform us about complex cognitive processes such as reading
• Eye movements help us see humans as active viewers
• Eye movements provide an example of interaction between sensory and action systems
• Eye movements are a tool in studying clinical populations

Background - the eye and the retina

Images taken from https://www.stanford.edu/group/vista/cgi-bin/FOV/chapter-3-the-photoreceptor-mosaic

The most sensitive part of the retina is the fovea. Further away from the fovea neurons have larger receptive fields a poorer resolution.

Types of eye movements

Gaze shifting (orienting) mechanisms - Voluntary Saccades - fast, ballistic, movements Smooth pursuit - for tracking a moving object Vergence - to adjust for the distance of the object to be viewed	Saccades - jerky movements for scanning the scene Smooth pursuit - following movements Vergence - adjust to near/far objects
Stabilising mechanisms - Involuntary Vestibulo-ocular reflex (VOR)- for correcting for head motion Optokinetic reflex (OKR) - made in correction to the visual scene moving	Spinning people round on a chair causes them to make eye movements to compensate- VOR OKR can be seen when people look out of train windows - it manifests itself as nystagmus movie Some people experience nystagmus spontaneously, in this case it is considered a visual dysfunction http://www.nystagmus.co.uk/facts/the-basics
Further miniature eye movements Microsaccades Drifts and tremor	These small eye movements happen when we are fixing our eyes (fixating) on an object. You can read all about them here.

Measuring eye movements

There have been various ways developed, but most commonly now an eye-tracker is used. This involves a small camera focused on the eye and software that tracks where the pupil moves. By calibrating the pupil position against fixed points on the screen one can work out where the participant is looking.

Demo movie of eye tracking here As well as the examples shown above, remote eye tracking (with no restriction on the head is now possible using two remote image sensors to capture images of the eyes and the reflection patterns. Using this kind of technology we can build a picture over time of where a participant was looking. We can measure: trajectory frequency of fixation dwell time frequency of fixation pre-define regions of interest

How to represent and analyse eye movement data

Scanpaths and fixations

Can simply trace the eye movements, or mark each fixation either with the time spent at each point or with a circle that represents dwell time by its size.

Top image taken from Yarbus (1969), Chapter 7 here Bottom image taken from eye tracker company Tobii website here: http://www.tobii.com

Heatmaps and regions of interest

In this heatmap taken from Mosimann et al. (2004), where the warmer colours represent more fixations - in this study they were looking at differences in attention in Alzheimer's sufferers.

In this study by Norbury et al. (2009) they chose regions of interest within movie clips of social  interactions and compared the amount of fixations between different developmental groups within these regions e.g. eye, mouth, body.

Why do we need eye movements?

So we don't have to walk around like pigeons!!! They don't have eye movements and bob their heads to compensate for their own motion and keep the image stable on their eye. We use eye VOR and OKR to ensure stabilisation of the visual scene on the back of the eye (the retina).

To avoid fading of the visual scene, if a retinal image doesn't change neurons stop responding - eye movements act as a 'refresh'. This fading is called Troxler fading.

See a demo here: http://www.michaelbach.de/ot/col_lilacChaser/index.html for a great illusion using retinal after-images and Troxler fading.

In order to focus the most important part of the scene on the fovea, the part of the retina with highest resolution. The image on the left illustrates equivalent size of letters according to our ability to see them if we are looking in the middle of the image

Why don't we notice eye movements?

• In order to build up a detailed representation of the scene we are constantly moving our eyes, yet we do not notice this motion.
• “Saccadic suppression” (Zuber & Stark, 1966) means that stimuli are more difficult to detect during the movement of the eyes.
• In particular it appears that the motion of the whole visual scene is ignored.
• If a change is introduced in the scene whilst we make an eye movement it is difficult to notice.
• This is still a much investigated area – the question of perceptual stability across consecutive saccades

The brain needs to know which part of the motion on the retina is caused by eye movements and which part is caused by something moving in the scene. This can be done by cancelling out the signal using an “efference copy” from eye muscles Also, cues from the scene, such as whole image motion can be cancelled out. Some illusions are caused by our inability to cancel out the motion of the eyes. In these cases we can see our own eye movements (see image on right). Image on right is taken from here: http://www.ritsumei.ac.jp/~akitaoka/index-e.html, where you can see similar beautiful illusions.

The importance of the active vision approach

• A lot of research on vision perception ignores the role of eye movement.
• Often the eye as just receiving information from the outside world that happens to be there.
• The active approach emphasis the dynamic process of the sampling of the visual scenes with reference to the role of the observer in guiding eye movements to relevant parts of the scenes.

Active vision emphasises the role of eye movements linked with attention in real-life tasks. Eye tracking studies such as Land et al. (1999) have measured how eye movements are made on to the next task, before the current one is completed.

Eye movements during reading

Reading is complex cognitive process. Many models exists to try and describe the ways in which we extract meaning from written words. Eye movements are part of the evidence that can be used to test these models. The brain needs to know which part of the motion on the retina is caused by eye movements and which part is caused by something moving in the scene. They can also be used to manipulate the type of information present at fixation at any given time - called a gaze contingent paradigm

Saccades during test reading Rightward movements followed by return sweep Large leftward movement to start of next line . Fixations 200-250 ms, some regressions (10%) depending on text difficulty.

Aoccdrnig to a reresearchs at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoatnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe. It has been shown that we do not fixate on each letter. Typically we move about 8 characters with each saccade. Many short words we do not even fixate on at all. The example above illustrates that reading does not rely on fixating each letter in order as we are still bale to make sense of the paragraph,

Gaze contingent paradigm This involves displaying the text according to where the eye is looking e.g.: Normal … move smoothly across the text… Shown . …xxxxxxmoothly axxxxxxxxxx….. Eye position ............*.............................. This can be used to see how many letters around fixation we use for reading.

The link between eye movements and attention

Summary

• There are eye stabilising and orienting mechanisms.
• Complex processes mean we are constantly able to compensate for our own motion as well as motion in the visual scene and the movement of our own eyes.
• Eye movements are closely linked with attentional mechanisms and are the key to seeing vision as an active process.
• This involves closely coupled interaction of visual and motor processes at all levels.

Reading:

•    Zanker, JM (2010)  Sensation Perception and Action , Palgrave (Beginning of Chapter 12)
•    Goldstein, EB (2007)  Sensation and Perception (7th ed.), Wadsworth (152.1 GOL) (Beginning of Chapter 6)
•    Findlay, J. M. & Gilchrist, I. D. (2003) Active vision :the psychology of looking and seeing  Oxford ; New York : Oxford University Press; click here to download Chapter 2 from Moodle

Specific References:

• Anstis S. (1974) Chart demonstrating variations in acuity with retinal position. Vision Research 14(7):589-592
• Land, M., Mennie, P., Rusted J. (1999) The roles of vision and eye movements in the control of activities of daily living. Perception 28(11):1311-1328 ; click here 
• Martinez-Conde, S., Macknik, S. (2007) Windows on the mind. Scientific American 297(2): 56-63 
• Mosimannn, U. P., Felblinger, J., Ballinari, P., Hess, C. W., Muri, R. M. (2004) Visual exploration behaviour during clock reading in Alzheimer’s disease. Brain 127:431-438
• Norbury, C. F., Brock, J., Cragg, L., Einav, S, Griffiths, H., Nation, K. (2009) Eye-movement patterns are associated with communicative competence in autistic spectrum disorders. J. Child Psychology and Psychiatry 50(7):834-842
• Rawlinson, G. (1976) The Significance of Letter Position in Word Recognition. PhD Thesis, Nottingham University
• Yarbus, A. L. (1967) Eye Movements and Vision. New York: Plenum Press; click here (download Chapter 7 from Moodle)
• Zuber, B. L., Stark, L. (1996) Saccadic suppression – elevation of visual threshold associated with saccadic eye movements. 16(1) 65-79

to download pdf-file of e-handout click here

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last update 11-02-2014
Szonya Durant