Week 5 - Movement Disorders

What many of us might regard as ‘simple’ actions or movements (such as reaching out for an object or taking a step) actually involve intricate and complex interactions of the central nervous system and skeletal muscle.    

Different brain regions are responsible for a variety of roles in movement.   For example, situated behind the brainstem at the back of the skull, the cerebellum is understood to interact with the spinal cord and frontal lobes to guide balance, posture and motor skill learning.  Containing the caudate nucleus, putamen and globus pallidus, the basal ganglia are subcortical structures which also interact with the frontal lobes and are involved in the control of voluntary movement.   It is damage to this area in particular that most commonly gives rise to a variety of motor dysfunctions described as extrapyramidal disorders, characterised by either excessive or restricted movement.

Excessive and rapid involuntary movement (or hyerkinesia) is a prominent feature of Huntington’s disease. A rare and inherited degenerative motor disorder, patients with HD exhibit jerky, dance-like (choreiform) movements which they are unable to control.  A significant loss of neurons is found in the globus pallidus whilst reduced glucose metabolism is evident in the caudate nucleus.

In contrast to HD, Parkinson’s disease is characterised by akinesia, a generalised reduction in -  or lack of - movement.  Also characteristic of PD is rigidity and tremor at rest.  Patients with PD are found to have depleted levels of dopamine in the caudate nucleus, putamen, substantia nigra and globus pallidus.

Week 4 - Visual Perception II

Blakemore, Wolpert and Frith (2002) propose a framework of motor (or action) control and describe several disorders which appear to support this framework, including optic ataxia, utilization behaviour and phantom limbs.

Central to this framework are two internal models of the central nervous system.  The ‘inverse model’ works on the basis of goal perception and the initiation of any action or sequence of events that are necessary in order to achieve that goal. Blakemore et al.(2002) offer the example of picking up a cup – affordances in the form of the visual features of the cup (the shape and angle of the handle for example) will guide how you position your hand in order to grasp it and pick it up.  However, whilst perception and awareness of the cup are conscious, the details of each step in the process of reaching out and picking up the cup are outside of awareness.

Whilst the ‘inverse model’ appears to employ goal perception to shape a plan of action, the ‘forward model’ operates on the basis of prediction of the consequences of action by using efference copy.  Whenever a movement is made, the brain simultaneously generates a mental ‘copy’ of that movement or motor command from which a prediction can then be made about the effect of that action.  Importantly, comparisons between the predicted and desired outcome of an action as well as between the predicted and actual sensory feedback are made.  Although the results of these comparisons appear to take place outside of conscious awareness (if the intended action is achieved), it is suggested that the ‘forward model’ has the effect of determining not only our subjective experience, but also our awareness of action control.

Reference: Blakemore, S., Wolpert, D. M. & Frith, C. D. (2002). Abnormalities in the awareness of action. Trends in Cognitive Sciences, 6, 237-242

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.

Hello!

Welcome to my Cognitive Neuropsychology blog!  I am a 3rd year undergraduate psychology student at Kingston University, Surrey.  A course requirement for my Cognitive Neuropsychology module is the creation of this blog will essentially act as a personal revision tool where I pick up some aspects of weekly topics to discuss. Should you happen across my blog, please bear in mind that it is not intended as definitive guide to what is obviously an incredibly complex and dynamic field of study!