Functional Neuroimaging techniques The current status of

Functional Neuroimaging techniques § The current status of non-invasive techniques applied for human and animal brain mapping could be reviewed by integrating hemodynamic and electrophysiological principles. § There are several functional neuroimaging techniques based on hemodynamic principle which reflect the neuronal activation indirectly: - functional magnetic resonance imaging (f. MRI) ! - positron emission tomography (PET), - single-photon emission computed tomography (SPECT). § More frequently used electrophysiological techniques include - electroencephalography (EEG), - sensory evoked potentials (EP), - magnetoencephalography (MEG).

Electroencephalogram (EEG) The EEG - an oscillating voltage recorded on scalp surface A typical adult human EEG signal is about 10µV to 100 µV in amplitude when measured from the scalp and is about 10– 20 m. V when measured from subdural electrodes. • • • Bands of activity Delta 0. 5 -4 Hz Theta 4 -8 Hz Alpha 8 -13 Hz Beta 13 -30 Hz Gamma 30 -50 (100) Hz

Recording EEG The "10 -20" system or "International 10 -20" system. The "10" and "20" refer to the fact that the actual distances between adjacent electrodes are either 10% or 20% of the total front-back or right-left distance of the skull. The letters F, T, C, P and O stand for Frontal, Temporal, Central, Parietal and Occipital respectively.

Recording References • Measure voltage potential differences – Difference between what and what else? • “Monopolar” versus Bipolar – No truly inactive site, so monopolar is a relative term – Relatively monopolar options • Body – BAD IDEA • Head – Linked Ears or Mastoids – Tip of Nose – Hypothetical advantages of Monopolar – seldom realized

Recording References • Bipolar recording – Multiple active sites • Sensitive to differences between electrodes • With proper array, sensitive to local fluctuations (e. g. spike localization) • Off-line derivations – Averaged Mastoids – Average Reference (of EEG Leads) • With sufficient # electrodes and surface coverage, approximates inactive site (signals cancel out) • Artifacts “average in”

Artifacts • Three sources – 50 Hz noise – Muscle artifact – Eye Movements

Movement in reference lead

Chewing

Vertical eye roll

Excessive muscle – notice saturation of T 5

Talking and moving head

Yawn

Eye Closure and reopening

Blink and Triple blink

Dealing with artifacts • 50 -cycle noise – Ground subject – 50 Hz Notch filter • Muscle artifact – – – No gum! Use headrest Measure EMG and reject/correct for influence Statistically control for EMG Hand score • Eye movements – Eyes are dipoles – Reject ocular deflections including blinks – Computer algorithms for EOG correction

High and low pass filtering • Do not eliminate frequencies of interest • Polygraphs have broad rolloff characteristics • Digitization rate (Nyquist) • For example, 0. 01 - 100 Hz bandpass, sampled at 500 Hz

Time Domain vs. Frequency Domain Analysis • Time Domain Analysis involves viewing the signal as a series of voltages as a function of time, [x(0), x(t 1), x(t 2), . . . , x(tn-1)] – e. g. , skin conductance response, event-related potential – Relevant dependent variables • latency of a particular response • amplitude of that response within the time window

Time Domain Vs Frequency Domain Analysis • Frequency Domain Analysis involves characterizing the signal in terms of its component frequencies – Assumes periodic signals • Periodic signals (definition): – Repetitive – Repetition occurs at uniformly spaced intervals of time • Periodic signal is assumed to persist from infinite past to infinite future

Speaking non-technically, a time domain graph shows how a signal changes over time, whereas a frequency domain graph shows how much of the signal lies within each given frequency band over a range of frequencies.

Digitization rate (Nyquist)

Fourier Series Representation • If a signal is periodic, the signal can be expressed as the sum of sine and cosine waves of different amplitudes and frequencies • This is known as the Fourier Series Representation of a signal

Fourier Series Representation • Pragmatic Details – Lowest Fundamental Frequency is 1/T • T=period sampled by the N samples – Resolution is 1/T • Phase and Power – There exist a phase component and an amplitude component to the Fourier series representation • Using both, it is possible to completely reconstruct the waveform. • Psychophysiologist usually only interested in amplitude component

Sine wave 9. 75 Hz wave with noise added Noise Only < 20 Hz Time Domain Mixture of Frequency Domain 3 waveforms

Synaptic activity in the pyramidal cells is the principe source of the EEG acivity The pattern of electrical current flow for an excitatory postsynapric potential (EPSP) on the apical dendrite of a pyramidal neuron in the cerebral cortex.

The polarity of the surface EEG depends on the location of the synaptic activity within the cortex. EPSPs in superficial layers and IPSPs in the deeper layers appear as upward negative potential. EPSPs in deeper layers & IPSPs in superficial layers have downward (positive) potential.

Comparison of EEG bands Type Frequency (Hz) Delta Theta up to 3 Hz 4 – 7 Hz Alpha 8 - 12 Hz Beta 12 - 30 Hz Gamma 26– 100 Hz Location Normally frontally in adults, posteriorly in children • adults slow wave sleep • in babies • young children • drowsiness or arousal in older children and adults Pathologically • subcortical lesions • diffuse lesions • metabolic encephalopathy hydrocephalus • deep midline lesions. • focal subcortical lesions • metabolic encephalopathy • deep midline disorders • some instances of hydrocephalus posterior regions of head, both sides, higher in • closing the eyes and by amplitude on dominant • coma relaxation. side. Central sites (c 3 -c 4) at rest. both sides, symmetrical • active, busy or anxious • benzodiazepines distribution, most evident thinking, active frontally concentration • certain cognitive or motor functions The normal EEG varies by age. The neonatal EEG is quite different from the adult EEG. The EEG in childhood is generally comprised of slower frequency oscillations than the adult EEG.

Sleep Stages Stage I sleep (drowsiness in some systems) appears on the EEG as drop-out of the posterior basic rhythm. There can be an increase in theta frequencies. Stage II sleep is characterized by sleep spindles-transient runs of rhythmic activity in the 12 -14 Hz range (sometimes referred to as the "sigma" band) that have a frontal-central maximum. Most of the activity in Stage II is in the 3 -6 Hz range. Stage III and IV sleep are defined by the presence of delta frequencies and are often referred to collectively as "slow-wave sleep. " Stages I-IV are comprise non-REM (or "NREM") sleep. The EEG in REM (rapid eye movement) sleep appears somewhat similar to the awake EEG.

EEG activity in a patient with epilepsy shows focal sharp waves in the EEG electrode located over the right temporal area (enclosed in boxes) Focal epileptiform discharges represent fast, synchronous potentials in a large number of neurons in a somewhat discrete area of the brain. These can occur as inter-ictal activity, between seizures, and represent an area of cortical irritability that may be predisposed to producing epileptic seizures.

Epileptic spike and wave discharges monitored with EEG Generalized epileptiform discharges often have an anterior maximum, but the are seen synchronously throughout the entire brain. They are strongly suggestive of a generalized epilepsy.

The pathways for seizure propagation in partial seizures and primary generalized seizures 1. Partial seizure. Seizure activity can spread from a focus in neocortex via intrahemispheric commissural fibers (1) to the homotopic contralarteral cortex (2) and subcortical centers (3). 2. Secondary generation. The second generation of partial seizure activity spread to subcortical centers via projections to the thalamus (4). Wide spreading thalamacortical interconnections then cause rapid activation of both hemispheres. 3. In primery generalized seizure, such as typical absence seizure, diffuse interconnections between the thalamus and cortex are the primary route of seizure propagation.

Non-epileptic abnormal activity As with the epileptic activity, Focal non-epileptiform abnormal activity may occur over areas of the brain where there is focal damage of the cortex or white matter. It often consists of an increase in slow frequency rhythms and/or a loss of normal higher frequency rhythms. It may also appear as focal or unilateral decrease in amplitude of the EEG signal. Diffuse non-epileptiform abnormal activity manifest as diffuse abnormally slow rhythms or bilateral slowing of normal rhythms.

Overview q Event-related potentials are patterned voltage changes embedded in the ongoing EEG that reflect a process in response to a particular event (e. g. , visual or auditory stimuli) q ERPs are measured from the same “raw data” (i. e. , scalp electrical activity over time and space) as EEG

Three class of the sensory evoked potentials. q Visual evoked potential (VEP). Commonly used visual stimuli are flashing lights, or checkerboards on a video screen that flicker between black on white to white on black (invert contrast). VEP are useful in detecting visual perception, blindness + optic neuritis, multiple sclerosis(delayed signal conductance). The term "visual evoked potential“ usually refers to responses recorded from the occipital cortex. q Auditory evoked potential (AEPs) can be used to trace the signal generated by a sound, from the cochlear nerve, through the lateral lemniscus, to the medial geniculate nucleus, and to the cortex. Commonly used auditory stimuli are different tones and clicks, speech sounds. q Somatosensory evoked potential Somatosensory Evoked Potentials (SSEPs) are used in neuromonitoring to asses the function of a patient's spinal cord during surgery. They are recorded by stimulating peripheral nerves, for ex. , median nerve, typically with an electrical stimulus. The stimulus is then recorded from the patient's scalp.

Acoustic tumors : (a) in VIII th cranial nerve (peak I): e. g. absence of peaks I – V (b) in brainstem (begins with peak II): e. g. absence of peaks II – V

Overall Stimulus Probability

The amplitude modulated EEG A 4 -h amplitude-integrated electroencephalography (a. EEG) recording, with 34 s of corresponding EEG below, from a 6 -week-old preterm infant born at 24 gestational weeks (i. e. 30 postmenstrual weeks). The a. EEG background is discontinuous, the low-amplitude EEG corresponds with the lower border of the a. EEG, while the high-voltage bursts of activity in the EEG corresponds with the upper border of the a. EEG. The sinusoidal variability in the lower border of the a. EEG tracing represents sleep ewake cycling. The broader bandwidth represents discontinuous background activity during quiet sleep (EEG in term infants), and the narrower bandwidth corresponds to more continuous activity during wakefulness and active sleep. L. Hellström-Westas, I. Rosen (2006)