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Electrophysiology -I Chapter 1: Electric Fields of Synchronous Neural Activity Electric Fields of Synchronous Neural Activity Voltage changes in neocortical nerve cells can be recorded with electrodes Combined electrical activity recording is called the electroencephalogram (EEG) EEG changes as a function of brain state and depends on electrode location Frequency Domain Analysis Most common strategy for analyzing EEG is signal averaging Assumes time-locked sequences Signal-to-noise ratio is improved by square root of epochs averaged Overview During stimulation with sound, EEG undergoes changes in a time-locked fashion in response to sound changes These changes are called auditory evoked potentials (AEPs) These potentials are averaged: the AEPs add up and become larger than the background waves (EEG) Overview Time between onset of acoustic change and occurrence of peak latency ranges from 1ms to over 500 ms (.5 sec) AEPs can be used to obtain thresholds at various audiometric frequencies Objective testing Short-latency is used more for threshold while long-latency is used primarily for cognitive processing AEPs recorded from the scalp are generated in the cochlea (CM,SP,CAP), in the brainstem (ABR), and in the auditory cortex (MLR, long latency AEPs) AEPs can be classified as either compound action potentials or compound postsynaptic potentials Response to Pure Tones Cochlear nerve fibers respond best to specific range of tone freq. Response to tone is excitatory (increase in discharge rate above the spontaneous) Initial response is greater if stimulus is abrupt Shorter onset time means less freq. specificity Rate of discharge is less important than total number of active neurons Responses to Pure Tones, cont. At minimum threshold, a cochlear neuron usually responds to only one freq. It’s important to remember that AEPs are the activity of hundreds of neurons together The most commonly used stimulus is the acoustic click good synchrony but poor freq. resolution Resting Activity Most afferent cochlear neurons are spontaneously active There is a range of discharge rate from low (<0.5 spikes/sec), medium(0.5-18spikes/sec), to high(>18 spikes/sec) The most sensitive neurons have high rate of spontaneous discharge Spontaneous discharge is most likely from random release of neurotransmitter at the hair cell synapse History Hallowell Davis (1896-1992) is called the father of evoked- response audiometry, as he was first to use long-latency AEPs to estimate hearing thresholds – objective audiograms. Pauline Davis (Hallowell’s wife) was the first to spot repetitive changes in the ongoing EEG (1939). But, until signal averaging was introduced they could record only ALRs. Electrocochleography (ECochG) was established by using the works of Portmann, Le Bert, and Aran (1967) in combination with the work done by Yoshie and coworkers in Tokyo History (cont.) Jewett and colleagues (1970) are credited with being the first to develop auditory brainstem response (ABR) recordings, although several researchers from all over the world were working at those recordings at that time, including Ernest Moore. Selters and Brackmann (1977) published their landmark findings on prolonged inter-peak latencies in tumor cases (greater than 1 cm) History (cont.) Geisler et al., (1958) recorded short-latency cortical evoked potentials (now called middle-latency responses, MLR). The latency range of MLRs (10-50 ms) overlaps with the Postauricular muscle (PAM) potential. This lead to two decades of fighting about the true nature of the MLRs. Finally ending when Maurice Mendel volunteered to be paralyzed with atropine but, still showed MLRs. Classification of AEPs Exogenus, Mesogenous and Endogenous responses- Harkrider et al., 2001 Exogenous is often used interchangeably with stimulus-related potentials implying that conditions external to the listener (e.g. stimulus intensity or duration) affect exogenous potentials. Exogenous AEPs are associated with automatic processing within the CNS. Mesogenous AEPs fall somewhere in between exogenous and endogenous as both external and internal conditions can affect the response. The division between automatic and controlled processing is not as well defined. Functional use of the signal affects response amplitude but is not necessary to produce the response. The MLR and 40-Hz response are most often classified as mesogenous AEPs. Endogenous is often used interchangeably with event-related potential meaning that conditions internal to the listener (e.g. attention, vigilance) determine endogenous potentials. Endogenous AEPs are related to controlled processing within the CNS potentials (for review, see Goldstein and Aldrich, 1999; Picton, 1980). Classification of AEPs by latency Auditory brainstem response (ABR) peaks denoted by Roman numerals; I, II, III, IV, V, VI, VII Middle latency response (MLR) are indicated by Po, Na, Pa, Nb, and Pb Auditory late response (ALR) are indicated by P1, N1, P2, and N2. Short Latency AEPs Latencies of less than 10 ms and the shortest responses are the cochlear potentials CM and SP Generated by hair cells CM largely generated by outer hair cells, while the SP is generated by inner hair cells These help determine if hearing loss occurs at the level of the hair cells or higher up CAP and ABR Dominant neural representatives of the short-latency AEPs originate from neural structures peripheral to the auditory midbrain CAP and ABR little affected by sleep or sedation Best results come from patients in either state Main use is to obtain threshold information and in differential diagnosis of vestibular schwannomas and Meniere’s disease MLRs and ALRs Latencies range from 10-50 ms (but recording is from 0-50 ms) Longer-latency MLR responses are affected by sleep and are difficult to measure under sedation Long-latency AEPs comprise every component with latency above 50 ms Further subdivisions to determine what caused the response to occur (i.e. stimulus vs. task) Time boundary: MLRs and long-latency AEPs border at 50 ms Also functions to separate components affected by attention and those that are not Mismatch Negativity (MMN) Formed by responses following an unexpected sound, such as an infrequent tone of 1000 Hz among a series of more frequent tones of 110 Hz MMN is also called the deviant or oddball sound MMN reflects neural information in the brain that allows behavioral detection of a difference in two sounds If the subject is required to press a button or count the deviant tones, an additional positive peak will occur at a latency of 300 ms The N400 can be elicited later when a word at the end of a sentence is perceived as semantically wrong Classification by Source Can be made on the basis of what type of electrical activity generated by nerve cells contributes to the AEPs When neuron not activated, the potential is -70 mV (the resting potential) Transient signals can depolarize neurons, called the excitatory postsynaptic potential Changes are measured by electrodes placed close to a cell Depolarizations are small, so many have to occur synchronously for the electrodes to measure the changes Nerve cells produce slow localized membrane voltage changes (postsynaptic potentials) and fast voltage changes (action potentials or spikes) Synchronous Activity in Spatially Aligned Structures If large number of neurons are activated at the same time, then changes in membrane and action potential firing rates occur together across neurons. The corresponding currents add up in phase and become so large they can be detected at the scalp Only structures with spatial alignment of neurons with the same orientation of their current-producing parts produce far-field potentials Action potentials are produced in axons. Spatially aligned axons can produce far-field potentials If all axons from a group of cells are oriented in a parallel fashion (as in a nerve),they form a nerve that can produce far-field potentials Auditory nerve is made up of about 30,000 parallel nerve fibers When activated by a transient sound a compound action potential (CAP) is produced ABRs A typical ABR has a sequence of up to 7 vertex positive waves with negative valleys between Wave peaks are typically labeled with Roman numerals Wave I & II are the compound action potentials of the auditory nerve (N1 and N2) Peaks with numbers from wave III up to and including wave V likely are generated sequentially in the auditory brainstem ABR Recording Typically, the recording of ABR is done in a way that slow activity (below 100Hz) is filtered out. This is the activity that is composed of PSPs (but also of muscle potentials) Synchronous activity and a spatial alignment determine the amplitude of the various AEP components: The more fibers that are simultaneously active, the larger the amplitude Higher stimulus levels typically activate more fibers and produce larger AEP amplitudes Another factor is the change in surrounding tissue resistance The largest response amplitudes are found for the most abrupt and largest changes in resistance AEPs of Cortical Origin There is a tonotopic map, where the CF is systematically mapped along the cortical surface The sites of depolarization are called sinks, because of the inward current of positive ions The sites where the current leaves the dendrites is called a source Amplitude of Cortical AEPs If voltage is spread out over larger surface then amplitude is reduced Cancellations from opposing polarity contributions can also reduce amplitude Peak overlap can also affect amplitude Latencies of Cortical Periods The cochlear traveling wave delay amounts to approx 2 periods of the tone and becomes important for LF components The delay is .5 ms for 4 kHz but reaches 4 ms for 500 Hz The time it takes for neural activity to go from cochlea to cortex Includes synaptic delays (6 synapses with a delay of 1-2 ms each) and neural conduction delays Adds up to 17 ms for the Na component of the MLR, considered the first sign of cortical activity Latencies of Cortical Periods The latencies of the MLRs go up in steps of 10-15 ms These latencies become understandable if one assumes buildup times for the PSPs (about 4 ms) and slow intracortical neural conduction (1 m/s) After Pb and P1, the latency differences between subsequent peaks are too long to be the result of large conduction delays and a single synaptic delay A solution is by neural activity looping around in reverberant circuits in cortex and thalamus and synchronizing about every 50-100 ms Latency of Cortical Potentials Cochlear traveling wave delay amounts to 2 periods of the tone. (Exp: delay for 4KHz is 1/4000 X 2 = 0.5ms) Time it takes for neural activity to go from cochlea to cortex (includes six synaptic delays of 1-2 ms each and neural conduction delays) Buildup times for postsynaptic potentials(about 4ms) and slow intracortical neural conduction (1m/s) The 10-20 international electrode system
"Auditory Evoked Potentials"