Week 3 - Visual Perception (Disorders - part 1)

In Cognitive Physiology: Moving the Mind's Eye Before the Head's Eye, Treue and Martinez-Trujillo (2003) review an investigation by Moore and Armstrong (2003) suggesting that orientation of attention and eye movement not only stem from the same area of the cortex but are integrated systems. 

Using electrodes to stimulate the FEF, Moore and Armstrong appeared to demonstrate a marked improvement in a monkey’s performance in a visual task.  Specifically, stimulation of the FEF resulted in increased activity in Area V4 – an area of the visual cortex associated with the perception of colour and form. Together, stimulation of the FEF and the subsequent rise in neuronal activity in Area V4 appeared to have the effect of enhancing spatial attention.

 

It appears then that not only does the Frontal Eye Field generate the motor commands necessary to direct our gaze, it also performs an analysis to determine the saliency or relevance of a target. In other words, the FEF appears to be involved in assessing whether something is actually worth closer inspection (or attention). Depending on the outcome of this analysis, an eye movement may or may not be executed towards any given target. Thus, the Frontal Eye Field (FEF) - alongside other areas of the brain - has an apparent role in modulating attention.  

Reference: Treue, S. and Martinez-Trujillo, J.C. (2003). Cognitive physiology: moving the mind’s eye before the head’s eye. Current Biology, 13, R442–R444

Why do we need to move our eyes across a scene?

Located in the centre of the retina, the fovea is a small depression measuring half a millimetre in diameter, densely packed with colour-sensitive cones (photoreceptors).  The high concentration of cones in this small area means that visual acuity (sharpness of vision) is greatest when images fall directly onto the fovea.

Moving outwards from the fovea across the retina, the numbers of cones reduces whilst rods (which function best in dim light) increase in number.  This has the effect of ‘blurring’ peripheral vision, so that whilst we may be consciously aware of objects away from our centre of vision, we cannot see them in fine detail.  Thus, in order to fully analyse a scene we must move our eyes, or more specifically direct our fovea, to small areas at a time.

Would it not be easier if we could see the whole scene in front of us at once?

The size of the human brain would have to increase enormously in order to accommodate the many more neurons we would need in order to analyse whole scenes entirely at once.  Thus, evolution has determined that we only foveate those targets high in saliency.  In other words, the small size of the fovea means that we are equipped to ‘ignore’ the vast majority of inconsequential visual information that we are met with in order to concentrate on that which is relevant at any given time.  
 

What does FEF mean? And what is its role in vision?

FEF refers to the Frontal Eye Field (located in the fontal cortex) responsible for generating motor commands that direct the eyes towards a target.   Recent research, as reviewed by Treue and Martinze-Trujillo (2003) suggest that further to this, the FEF is implicated in orientating attention.