This relatively simple electronic device was developed to test and debug our experimental setup. It emulates simple cells which can be found in layer 4 of cat's area V1 (primary visual cortex).
The emulator has 3 subfields: two "ON" subfields on the sides and one "OFF" subfield in the middle. It is insensitive to full-field stimulation, demonstrates orientation tuning and has no direction tuning. The outputs of the emulator are: "membrane potential" (MP), TTL pulses and spike-like pulses.
The subfield blocks are made of photo sensors, amplifiers and filters. ON subfields produce excitation on dark-light transitions and inhibition on light-dark transition. OFF subfield is the opposite: excitation on light-dark and inhibition on dark-light.
The summator produces weighted sum of all three subfield signals. Weights for the ON subfields are set so that each of the ON subfields produces approximately equal response to full-field stimulus. The weight of the OFF subfield is set to produce zero sum response to full-field stimulus. The output of the summator can be considered as a "membrane potential" (MP) of the emulator neuron.
To produce random spiking, the comparator compares noise signal to the output of the summator. MP acts as a threshold modulated by the stimulus. The lower the threshold, the higher the probability of spiking.
The refractory period one-shot generates 4-ms pulses and emulates the refractory period. The spike pulse one-shot generates 0.4 ms pulses which are used as a TTL output. The spike-shape filter produces spike-alike output from the TTL pulses.
The schematics are shown in 3 pages: page 1 covers the subfields and the summator, page 2 details the rest of the block diagram, and page 3 shows power supply filters.
All three subfields have the same design except for the output amplifier. They all use Panasonic's PIN photodiode PN334 (Digi-Key Part# PN334PA-ND) in the photovoltaic mode. The photodiode signal is amplified by a factor of 5 by the Op-Amp. The amplified signal is processed by the peak detector followed by the band-pass filter with the time constants of 15ms (for charging) and 47ms (for discharging). The output of the filter is boosted by the Op-Amp with the gain of 1 (for ON-subfields) or gain of -1 (for OFF-subfield). The signals form the three subfields are summed up with the weights set by the trim resistors. Total gain of the summator is set by the trim resistor. At the next stage a DC offset is added. The DC offset potentiometer is mounted at the front panel and labeled as "Threshold". The output of the summator is available through a BNC connector (labeled as "MP") at the front panel.
The noise is generated by n-p junction in reverse polarity: in this mode it acts as a noisy Zener diode. The Op-Amp amplifies the noise 680 times. The amplified noise signal is compared against the MP. When the MP becomes larger than noise, the comparator output changes from -Vp to +Vp (Vp is the power supply voltage - about 9V). Note that the positive feedback resistor provides 18mV of hysteresis (4.7k/4.7M *2Vp) . A down-up transition of the comparator triggers the one-shot that generates a 4-ms pulse which is used to emulate the refractory period. The refractory period is "soft" since the one-shot can be retriggered in the late stage of the pulse generation, when it is close to the flip-back threshold. Note that the output voltage of the comparator is divided by 2 to match the comparator's output voltage range of 2Vp to the digital input range of 1Vp. The spike one-shot is triggered by the falling edge of the refractory period pulse. It generates a 0.4ms pulse which is is available through a BNC connector (labeled as "TTL") at the front panel. At the input of the spike-shape amplifier the amplitude of the pulse is divided by 10 (to avoid saturation). The signal is high-pass filtered by the input RC-circuit with the time constant of 0.33ms and then it is low-pass filtered by the Op-Amp with the time-constant of 0.22ms. The resulting "spike" is output to the front-panel BNC.
The device is powered by two 9V batteries. Digital power is filtered with the LC circuit to suppress interference..
Click on the icon to see the picture.
Photodiodes were mounted in empty shells of 9V batteries. The shells were squashed to give them ellipse-like shape. The photodiodes were inserted into a piece of black foam and then plugged into the shell. The shells were taped together with an electric tape. The electronic circuit was assembled on a prototype board and mounted in a box along with the controls.
Here are some demos of the emulator.
An example of the sinusoidal grating presentation. From left to right:
You can also see the whole process in a movie. Two formats are available: mpeg-2 (2.2MB) and asf (0.55MB).
An orientation tuning curve of the emulator. 72 directions were presented with the step of 5 degrees, the response is shown in spikes per second. Note that there is no direction selectivity.
"Receptive field" (RF) measured with flashing gratings. Frame update rate was 24Hz and refresh rate was 120Hz. Three RFs correspond to delays of 1, 2 and 3 frames (0-41.7ms, 41.7-83.3ms, 83.3-125ms) correspondingly.
Hand-mapping of the receptive field is demonstrated in the movie
available in two
formats: mpeg-2 (5.5MB) and
asf (1.2MB).
The emulator was designed and implemented by Sergei Rebrik <rebrik@phy.ucsf.edu>.
The author would like to thank Ken Miller for support and Andrey Kurgansky for help with measuring the characteristics of the emulator.
Last Modified: February 3, 2004.
