P urpose To better understand the eye movement control by the cerebral cortex using recent techniques such as transcranial magnetic stimulation and functional magnetic resonance imaging.
Why study eye movement? “It gives new meaning to the distinction between 'quantitative' and 'qualitative' techniques for evaluating brain-behavior relationships.” William B. Barr, Chief of Neuropsychology, Comprehensive Epilepsy Center, New York University School of Medicine Quantify relatively complex neuropsychological processes 1. attention 2. spatial memory 3. motivation 4. decisional processes
I ntroduction Classic Methods –Lesion –Electrical Stimulation Two recent methods to study eye movement –Transcranial magnetic stimulation –Functional magnetic resonance imaging
New Methods vs. Old Methods
Transcranial Magnetic Stimulation Temporarily disrupts the functioning of a specific region of the brain. –Magnetic field is applied to a subject’s head which crosses the scalp and skull –Electric current from Magnetic field disrupts the neural activity –Inactivation lasts <1 sec Advantage: increased temporal resolution intra.ninds.nih.gov/Lab.asp?Org_ID=104
Functional Magnetic Resonance Imaging Non-invasive method MRI scanner exploits the natural magnetic properties present in our bodies to obtain image of blood flow in the brain. Advantage: increased spatial resolution flickr.com/photos/macronin47/ /
Control of Human Eye Movement in Cerebral Cortex Frontal Lobe Parietal Lobe Cingulate Cortex
Frontal Lobe Three main areas involved in eye movement control –Frontal Eye Field (FEF) [BA 8] –Supplementary Eye Field (SEF) –Dorsolateral prefrontal cortex (DLPFC)[BA9 and BA46]
F rontal Eye Field Controls Pursuit Eye Movements (PEMs) –Smooth tracking of an object Preparation and triggering of Intentional saccades –Visually guided saccades –Predictive saccades –Memory guided saccades –Antisaccades
FEF and PEMs Control two type of PEMs Ipsilateral PEMs (main control) contralateral PEMs (superficial) Controls optokinetic nystagmus Involuntary eye movement to foveate a moving target to maintain the perception of self-motion –In the monkey, FEF neurons controlling PEMs are also involved in vergence. However, no literature shows vergence FEF activation in humans.
FEF and Saccades Antisaccades: intentional saccades to a target located in the opposite direction. Two different Mechanisms –1. Inhibition of an unwanted reflexive misdirected saccade PEF triggers towards the target. DLPFC inhibits and the error between the PEF and DLPFC reflect the inhibition function –2.Simultaneous triggering of an intentional correct antisaccade, made in the direction opposite to the target by FEF
Supplementary Eye Field (SMA) SEF: connected with FEF, the DLPFC, the anterior cigulate cortex and posterior parietal cortex Location: Medial surface of the superior frontal gyrus, in the upper part of the paracentral sulcus. Function: involved in motor programmes comprising of saccade with a body movement or successive saccades.
Supplementary Eye Field Cont. TMS and fMRI studies show that pre-SEF is involved in motor learning SEF is involved in the execution of motor sequence.
Dorsolateral Prefrontal Cortex Involved in –saccade inhibition –short-term spatial memory TMS support that DLPFC is exerted during delay period when spatial memory is involved. fMRI show activity up to 24 second. –decisional processes lesion studies show that percentage of predictive saccades decreased significantly.
Parietal Lobe Posterior Parietal Cortex Involved in the control of saccades and attention Parietal Eye Field –Corresponds to lateral intraparietal area –Involved in control of saccades and attentional processes. –Projects to both FEF and the superior colliculus Parieto-FEF projection: visual fixation Parieto-superior colliculus projection: saccadic involvement
Cingulate Cortex Divided into the anterior cingulate cortex (ACC) [BA 24] and the posterior cingulate cortex (PCC) [BA 23].
Cingulate Cortex ACC: involved in intentional saccade control Cingulate Eye Field: located between BA 23 and 24, prepare all the frontal ocular motor areas involved in intentional saccade control to act in the forthcoming motor behaviour. PCC: reflexive saccade control (?) –fMRI study shows that the PCC is active during reflexive saccades. –Activation during PEM
Conclusion Recent data have summarized some of the cortical pathways and mechanisms involved in saccade control. TMS and fMRI are currently used to understand new information and interpret cortical control of eye movements in humans.
Reference Approach-to-Neuropsychological-Assessment Intra.ninds.nih.gov/Lab.asp?Org_ID=104 I