An inexpensive, multi-channel, electrophysiological recording system

AN INEXPENSIVE, MULTI-CHANNEL, ELECTROPHYSIOLOGICAL RECORDING SYSTEM' DONALD W.

DEMOTT

University of Rochester, Rochester, N.V. (U.S.A.) (Received for publication: O c t o b e r 28, 1960)

T h e a p p a r a t u s to be described was designed to attack the p r o b l e m o f recording the electrical activity o f the cerebral cortex over the entire surface o f one hemisphere. With slight modification, however, t h e s a m e basic design

a l u m i n u m b o o t h (Fig. 1), which carries the g r o u n d e d side o f the supply voltage a n d provides electrostatic shielding. T h e b o o t h is six by seven feet, a n d eight feet high. A three foot s q u a r e sheet metal cage in the left rear corner houses

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Fig. 1 Layout o f a p p a r a t u s . T h e b o o t h is m a d e o f g r o u n d e d sheet metal, to provide electrostatic shielding. Amplifier a n d power supply racks are additionally shielded. Broken lines indicate the m a i n cable runs. T h e experimental animal is placed in t h e cage at left rear, facing the s t i m u l u s panel, with the probe attached to his electrodes. the preparation, a n d provides additional shielding. T h e amplifiers are plugged into shielded racks built against the right wall o f the booth, while o u t p u t stages and power supplies plug into similar racks built against the outside o f t h e walls. A l o n g the left wall is the experimenter's desk a n d control panel, as well as i n s t r u m e n t s for m o n i t o r i n g various aspects o f t h e apparatus. Separated f r o m the booth, b u t c o n n e c t e d by cables, are the o u t p u t display panel a n d t h e high speed movie camera, which serve to record the data.

could be applied in m a n y situations where discrete samples o f t h e o u t p u t (up to 1000 samples/sec) would give adequate information, a n d where t h e cost o f the m a n y channels o f amplification needed is a m a j o r obstacle to research. T h e entire system is built into a n d a r o u n d a g r o u n d e d t This research was s u p p o r t e d by U S P H S G r a n t No. M-2413 f r o m t h e N a t i o n a l Institute o f Neurological Diseases a n d Blindness, U.S. Public H e a l t h Service. 467

468

D.W.

DEMOTT if the n u m b e r o f channels was small e n o u g h to warrant a t t e m p t i n g to adjust the gain. T h e bias a d j u s t m e n t , R I 0 , is c o m m o n to all channels, a n d controls the zero-signal brightness o f the NE-2 n e o n display tubes. As in the m a i n amp, sixteen o u t p u t stages are operated in parallel from a single B + supply. T h e NE-2 n e o n tube which is used to display the o u t p u t draws little power. Since the circuit parameters do

T h e a p p a r a t u s is designed to a c c o m m o d a t e up to 2000 c h a n n e l s o f amplification; 160 c h a n n e l s are n o w in use. CIRCUITRY T h e basic amplifier circuit is s h o w n in schematic f o r m in Fig. 2. T h e pre-amplifier (VI) receives a direct coupled i n p u t f r o m the preparation. T h e grid leak resistor (R4), (one for each channel), a n d the variable g r o u n d i n g

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Fig. 2 Schematic o f amplifiers. V1 (pre-amp): 1/2 12AX7; V2 (main a m p stage 1): 1/2 12AX7; V 3 : 1 / 2 12BZ7; V4 (output): 1/2 12AT7. N o t e t h a t V1 a n d V2 are n o t one tube. T h e two triodes in each tube are used by adjacent channels, n o t by two stages within the s a m e channel. R9 adjusts in-phase noise rejection, R I 0 adjusts zero-signal brightness o f the NE-2 display tubes. Both p o t e n t i o m e t e r s are c o m m o n to all channels. resistor (R9), ( c o m m o n to all channels), form a voltage divider which serves two purposes. First, t h e cathodes are all referred to the i n s t a n t a n e o u s m e a n o f all channels. This type o f reference has several advantages, particularly where m a n y channels are being used. Secondly, the voltage divider allows e n o u g h o f a n y in-phase noise to appear between grid a n d cathode so that, w h e n amplified a n d inverted by t h e pre-amp, it cancels t h e r e m a i n d e r o f the in-phase noise appearing between cathode and ground. By careful a d j u s t m e n t o f R9, therefore, in-phase signals can be a l m o s t completely eliminated. T h e pre-amplifier 90 volt B + battery, o f c o u r s e ; . n ~ s t be c o m m o n to all channels. T h e m a i n amplifier is in no way u n u s u a l . Sixteen c h a n n e l s are operated in parallel f r o m a single power supply, b u t otherwise each c h a n n e l is electrically independent. T h e o u t p u t stage is a simple p o w e r amplifier. T h e voltage divider in the grid circuit serves to fix t h e gain of the amplifier. This could be replaced by a potentiometer,

not permit the tube to de-ionize, light intensity is a linear function o f current. T h e amplifiers have a moderately high input impedence, a total voltage gain of 106, noise equivalent to 3-5 #V, g o o d linearity, and a frequency response range f r o m l0 to at least 2000 cycles (higher frequencies were n o t tested). T h e filaments o f the pre a n d m a i n a m p tubes are operated f r o m storage batteries, eight tubes in series. T h e o u t p u t stage filaments are r u n o n rectified, unfiltered D.C. T h e pre a n d m a i n a m p s are m a d e up in printed circuits, sixteen channels of pre a n d m a i n amplifiers on a single, 1 2 " x 1 2 " printed circuit board. Each board plugs into a pigeon-hole in the shielded rack. T h e o u t p u t stages are also printed, with sixteen channels to a 1 2 " × 3" board, which also plugs into a shielded rack. ELECTRODE ARRANGEMENT O n e o f t h e m a j o r p r o b l e m s in designing this system was to provide a m e t h o d by which an animal could be

INEXPENSIVE MULTI-CHANNEL RECORDING plugged into as m a n y as 2000 channels, w i t h o u t c r u s h i n g h i m u n d e r a b u r d e n o f plugs. T h e difficulty was avoided by a technique referred to as the " s t a i n l e s s steel cortex." A s t a n d a r d c r a n i o t o m y is carried o u t a n d a n impression m a d e o f the exposed brain a n d s u r r o u n d i n g skull with a fast-setting silicone rubber. W o r k i n g f r o m this impression, a plastic plate is constructed, which precisely fits the exposure. This plate carries a close-packed array o f stainless steel wires, F o r m v a r insulated, whose inner ends rest o n d u r a or pie, depending o n t h e original exposure. T h e o u t e r ends are t h e n cut, g r o u n d a n d polished to f o r m a flat, h a r d surface (Fig. 3). T h e plate is held to the skull with bone screws, a n d t h e scalp cut to fit a r o u n d the a r r a y o f electrodes. A pair o f holes is drilled and tapped into the plastic just outside the elec-

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SKUll Fig. 3 Design o f the "stainless steel cortex.'" T h e underside o f the electrode array is shaped by placing the electrodes o n a mold o f the exposed brain before p o u r i n g the plastic a m o n g a n d a r o u n d the electrodes. T h e u p p e r surface is g r o u n d a n d polished after the plastic is cured. The electrodes are F o r m v a r insulated.

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trode array, so that a probe m a y be screwed onto the plate. T h e probe n o w in use (Fig. 4) carries 160 stainless steel wires, spaced 1.5 m m a p a r t in a h e x a g o n a l array. T h e wires are e m b e d d e d in a silicone rubber, with a b o u t 1 m m o f each wire projecting f r o m the r u b b e r at an acute angle. T h e spring of the projecting wire, plus the elasticity of the r u b b e r matrix, a s s u r e s that each wire will m a k e g o o d p r e s s u r e contact with the plate when the probe is m o u n t e d to it. A l t h o u g h the a r r a n g e m e n t o f electrodes in the skull plate is r a n d o m , a n d that in the probe regular, each probe wire necessarily m a k e s contact with one or two electrodes in t h e plate, a n d thus m a k e s contact with a circumscribed area o f cortex directly b e n e a t h it. In effect, therefore, the active electrode sites on the brain form a pattern identical to that o f the probe wires. T h e plastic plate is insensitive, and resistant to accidental damage. T h e a n i m a l s do not object to having t h e probe attached, a l t h o u g h s o m e training is necessary to persuade t h e m to accept the resultant restriction o f movement. In order to minimize t h e cable sway artifact resulting f r o m m o v e m e n t s o f the preparation, the cable was s a t u r a t e d with a self-vulcanizing latex rubber, mixed with three v o l u m e s o f powdered a l u m i n u m . A braided shield covers the latex. T h e powdered a l u m i n u m reduces cable sway artifact, p r e s u m a b l y by creating a distributed capacitance between each c o n d u c t o r and ground.

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::::::::::::::::::::::::: mounting i si itt/// i ~i Fig. 4 Detail of t h e 160 wire probe n o w in use (actual size). T h e wires, properly spaced, are e m b e d d e d in silicone r u b b e r to p e r m i t each wire to m a k e good pressure contact with the electrode array. T h e shielding a n d latex are s h o w n cut away to s h o w h o w the cable is s a t u r a t e d a n d coated with t h e l a t e x - a l u m i n u m m i x t u r e before shielding is applied.

Fig. 5 O u t p u t display panel. T h e h e x a g o n a l p a t t e r n near the center represents the array o f n e o n display tubes, g r o u p e d in blocks o f sixteen, held by metal strips in a pattern duplicating the p a t t e r n of the probe (see Fig. 4). T h e vertical row o f lights to the left are the s t i m u l u s indicator lights. T h e lighted panel at the left o f the board is used to record vital statistics. T h e entire panel is p h o t o g r a p h e d by the high speed camera.

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D . W . DEMOTT DISPLAY AND RECORDING

The neon tubes which display the output current of the amplifiers are arranged in groups of sixteen, corresponding to the grouping of amplifiers and power supplies. The sixteen tubes are arranged in an irregular hexagon, as shown in Fig. 5, which permits the groups to be arranged in a continuous pattern, duplicating the arrangement of wires in the probe. Thus, spatial relationships are preserved from the preparation to the output display. The display panel is photographed with a high speed movie camera. Good records can be obtained with speeds up to, and probably beyond, 1000 frames/sec. In addition to recording the primary data from the output display, stimulus indicator lights are recorded by the high speed camera, so that the onset and offset of any of eight visual and auditory stimuli are known. A n electrically driven 8 m m movie camera, switched in parallel with the high speed camera, records movements of the animal. COST ANALYSIS An approximate analysis of the cost of the present

installation is as follows: amplifiers, including pre-amp, output, brightness and noise rejection controls: $4.25 per channel. Power supplies: 81.75 per channel. Connecting cable and plugs: 81.50 per channel. The booth described cost about $300.--: the high speed camera was built for $20.--. Obviously, the cost will vary with general prices, and with the size of the installation. Large installations will have a lower cost per channel. In use, the cost of film for recording will soon exceed the initial cost of the apparatus. At a camera speed of 300 frames/sec, the cost of recording amounts to about 8660.--/hour. One does not push the button casually. The author will be happy to supply additional details to anyone wishing to duplicate or modify this apparatus. SUMMARY A system is described for recording from neural tissue with as many as 2000 channels in simultaneous operation. The output is recorded as a pattern of lights, photographed at high speed. The unique feature of the system is its cost of less t h a n ten dollars per channel. In addition, a technique is described for the chronic implantation of a large number of cortical electrodes.

Reference: DEMoTT, D. W. A n inexpensive, multi-channel, electrophysiological recording system. Electroenceph. clin. Neurophysiol., 1961, 13: 467--470.