During recording, a headstage is mechanically and electrically connected to the microdrive. The headstage includes two epoxy encapsulated quad surface mount JFET-input operational amplifiers (TL074 Texas Instruments), a small DC lamp, a metal alligator clip and approximately 3 m of twisted pair hearing-aid wire. The operational amplifier has an RC high pass filter at the front end (AC coupling), and acts as a voltage follower. The amplifier circuit drives the cable, to the recording equipment, with the voltage difference between the electrode and a ground level taken from a skull screw. The operational amplifier has been selected for its very high input impedance and its low leakage current (<10 nanoamps).
The headstage cable also includes the power supply voltages for the amplifiers (± 12 V) and a separate power and ground for the DC lamp. The alligator clip provides mechanical contact between the headstage and the microdrive. The mechanical contact is intentionally separated from the electrical connector in order to avoid recording electrical artefacts, which can occur due to intermittent contact between the parts of the connector if it is mechanically stressed. The lamp is used for tracking the animal's location (discussed below).
The eight potentials (four from each of the two tetrodes) are led through the hearing-aid wires to purpose-built differential amplifiers and filters (passband 400 Hz - 5.4 kHz). Each of the four channels of one tetrode is fed into one input o f a differential amplifier. None of the signals are multiplexed, each electrode has an independent wire from the headstage to the amplifiers. The signals for the other input are derived from one of the channels o f the other tetrode to provide a common rejection signal to minimise noise, movement and chewing artefacts.
The programmable amplifiers have three stages. The first stage is a differential amplifier with a fixed gain (400 x). Following this, the signal is filtered with a fourth order high pass Bessel filter with a cut-off at 600 Hz and a second order low pass Bessel filter. In the final stage the gain is independently programmable for each channel (range 1 to 100,000 x). The reference for each electrode signal can be selected independently as the headstage cable is divided into miniature BNC cable for each electrode. The basic system includes four differential amplifiers, each of
which contains a filter module. The amplifiers and filters are battery powered to minimise the power line (50 Hz) pickup.
Each channel is continuously sampled at 20 or 40 KHz., with 12 bit resolution. When a spike event is detected (as a voltage above a set level on any one o f the channels) the sampled points around the event are stored to yield 25 or 50 points for each of the four channels for each spike event. Each spike event was stamped with the time since the beginning o f data collection and the animal’s location at the time (see below). Data were usually collected in blocks o f one, two or four minutes and stored on a local hard disk.
V M E system Analog S y stem ; Disk ; Parallel
controlled | 1/0 ;conirollt7 Amplifiers and fillets________ : Digital converter Parallel port / Disk drive Tracking system Hearing aid wire Sun workstation .amera Disk drive
I Head stage and DC lair^
F ig u re 4.4 Schem atic diagram o f the recording apparatus. The signals are am plified in a headstage am plifier that is connected to the im planted m icrodrive and pass along the hearing aid cable to a bank o f battery operated differential am plifiers and filters. The EEG is also filtered by Neurolog amplifiers. The am plified signals are fed into the ADC and recorded by the Force CPU onto the local disk storage. T he APAL T T L -I/0 board is used to read the anim al's location from a cam era based tracking system.
During the recording of hippocampal unit activity, the animal's location (or more precisely, the location of the DC lamp attached to the headstage) was monitored with a television tracking system ( S P l l l , HVS Image Analysing, Kingston, UK). The location was monitored at video rate (50 Hz) and with a resolution of 256 x 256 pixels. The effective resolution of the y-axis is reduced to 128 pixels by the video interleave of alternate scan lines. These data enabled off line analysis o f the influence of the animal's spatial position and velocity on hippocampal unit activity. Figure 4.4 contains a schematic diagramof the full recording apparatus.
The EEG data were taken either from an individual channel o f the tetrode or by adding the signals from all four channels of a tetrode. Only single ended recording was used. For some recordings the data were filtered using Neurolog variable filters (Neurolog System, Digitimer Inc., Welwyn Garden City, UK) but for most of the cells purpose-built EEG amplifiers and filters have been used.
In the days following recovery from surgery, the electrodes were moved down into the C A l or CA3 pyramidal cell layers while the animal sat on a small 40 cm^ holding platform. Once a suitable recording location was achieved, the electrodes were usually left in place for at least one day before data collection began in order to ensure maximum recording stability. The pyramidal layer was located by using the presence of 100-200 Hz ripple activity (O'Keefe 1976).