VLSI Phase Locking Architectures for 
Feature Linking in Multiple Target 
Tracking Systems 
Andreas G. Andreou 
andreou@j hunix.hcf.jhu.edu 
Department of Electrical and 
Computer Engineering 
The Johns Hopkins University 
Baltimore, MD 21218 
Thomas G. Edwards 
tedwards@src.umd.edu 
Department of Electrical Engineering 
The University of Maryland 
College Park, MD 20722 
Abstract 
Recent physiological research has shown that synchronization of 
oscillatory responses in striate cortex may code for relationships 
between visual features of objects. A VLSI circuit has been de- 
signed to provide rapid phase-locking synchronization of multiple 
oscillators to allow for further exploration of this neural mechanism. 
By exploiting the intrinsic random transistor mismatch of devices 
operated in subthreshold, large groups of phase-locked oscillators 
can be readily partitioned into smaller phase-locked groups. A 
multiple target tracker for binary images is described utilizing this 
phase-locking architecture. A VLSI chip has been fabricated and 
tested to verify the architecture. The chip employs Pulse Ampli- 
tude Modulation (PAM) to encode the output at the periphery of 
the system. 
I Introduction 
In striate cortex, visual information coming from the retina (via the lateral genic- 
ulate nuclei) is processed to extract retinotopic maps of visual features. Some cells 
in cortex are receptive to lines of particular orientation, length, and/or movement 
direction (Hubel, 1988). A fundamental problem of visual processing is how to 
866 
VLSI Phase Locking Architectures for Feature Linking in Multiple Target Tracking Systems 867 
associate certain groups of features together to form coherent representations of 
objects. Since there is an almost infinite number of possible feature combinations, 
it seems unlikely that there are dedicated "grandmother" cells which code for ev- 
ery possible feature combination. There probably exists a type of adaptive and 
transitory method to "bind" these features together. The Binding Problem (Crick, 
1990) is the problem of making neural elements which are receptive to these visual 
features temporarily become active as a group that codes for a particular object, 
yet maintaining the group's specificity towards that object, even when there are 
several different interleaved objects in the visual field. 
Temporal correlation of neural response is one solution to the binding problem 
(von der Malsburg, 1986). Response from neurons (or neural oscillating circuits) 
which are receptive to a particular visual feature are required to have high temporal 
correlation with responses to other visual features that correspond to the same 
object. This would require that there is stimulus-driven oscillation in visual cortex, 
and that there is also a degree of oscillation synchronization between neural circuits 
receptive to the same object. Both of these requirements have been found in visual 
cortex (Gray, 1987; Gray, 1989). Furthermore, there have been several computer 
simulations of the synchronization phenomena and related visual processing tasks 
(Baldi, 1990; Eckhorn, 1990). 
This paper describes a phase-locking architecture for a circuit which performs a 
multiple-target tracking problem. It will accomplish this task by establishing a zero 
valued phase difference between oscillators that are receptive to those features to be 
"bound" together to form an object. Each object will then be recognized as a group 
of synchronous oscillators, and oscillators that correspond to different objects will 
be identified due to their lack of synchronization. We assume these oscillators have 
low duty-cycle pulsed outputs, and the oscillators which correspond to the same 
object will all pulse high at the same time. Target location will be communicated 
to the periphery by Pulse Amplitude Modulation (PAM). 
2 The Neural Oscillator 
The oscillator for the target tracker must have two qualities. It needs to be capable 
of producing a fairly smooth phase representation so that it is easy to compare 
the difference between oscillator phases to allow for robust phase-locking. It is also 
useful to have a pulsed output present so that one group of oscillators can be easily 
discerned from another group of oscillators when their outputs are examined over 
time. The self-resetting neuron circuit (Mead, 1989) provides both of these outputs 
(Figure 1). Current Ii, provided by FET Q1 charges capacitor C1 until positive 
feedback though the non-inverting CMOS amplifier and capacitor C2 brings Vphas, 
all the way to Vaa. This causes the output voltage to go high, which turns on Q2 
thus draining charge from C1 by I,.,s,t through Q3 and lowering Vpha,. When 
the Vh,,,, is brought low enough, positive feedback brings both Vpn,, and the 
output voltage down to V,,. This turns transistor Q2 off, and the cycle repeats. 
The duration of output pulses is inversely proportional to I,.,, - Ii,, and the time 
between output pulses is inversely proportional to Ii,. Figure 2 is a plot of the 
pulse output voltage and Vph,, vs. time. 
868 Andreou and Edwards 
Vfreq 
Vphase .' 
Q1 
-- C1 C2 
Q2 
Q3 
.T,. 
Vreset 
Pulse 
Output 
Figure 1' Self-Resetting Neural Oscillator 
Oscillator Output 
-1 
/ 
f 
f 
1 
/ 
t 
1 
t 
/ 
/ 
/ 
/ 
/ 
/ 
f 
I/ 
t t 
t / 
/ / / 
/ / / 
/ / / 
/ / / 
/ / t 
I I I I I I I I I I 
0 1 2 3 4 5 6 7 8 9 
(10 -6sec) 
Figure 2: Plot of Pulse Output (line) and Phase Voltage (dashed) vs. 
Neural Oscillator 
Time for 
VLSI Phase Locking Architectures for Feature Linking in Multiple Target Tracking Systems 869 
3 Phase Locking 
To achieve stable and reliable performance, the Comparator Model (Kammen, 1990) 
of phase-locking was used. Oscillator phase is adjusted according to 
Where O(x,t) is the phase of oscillator x at time t, w(x) is the intrinsic phase 
advance of oscillator x, n is the total number of oscillators, and f is a sigmoid 
squashing-function. 
Each object in the visual field requires one averaging circuit to achieve phase-locking 
of its receptive oscillators. But at any time we do not know the number of objects 
which will be in the visual field. Therefore, instead of having a pool of monolithic 
averaging circuits, it is preferable to distribute the averaging function over all the 
oscillator cells in a way which allows partitioning of the visual field into multiple 
phase-locked groups of oscillators. The follower-aggregator circuit (Mead, 1989) can 
be used to develop the average phase information using current-mode computation. 
It consists of transconductance amplifiers connected as voltage-followers with all 
outputs tied together to form the average of all input voltages. 
The phase averaging circuitry can be distributed among the oscillators by placing 
one transconductance amplifier in each oscillator cell, and linking those oscillators 
to be phase-locked by a common line. The visual field can be partitioned into 
multiple phase-locked groups with separate average phases by using FETs to gate 
whether or not the averaging information can pass through an oscillator cell to its 
neighbors. 
To lock an oscillator in phase with the rest of the oscillators which are attached to 
the averaging line, extra current is provided to the oscillator by a transconductance 
amplifier to slightly speed up or slow down the oscillator to match its phase to 
the average phase of the oscillators in the group. Figure 3 shows the circuit for a 
complete phase-locking oscillator cell. 
Computer simulations of this phase-locking system were carried out using the Ana- 
log circuit simulator. Figure 4 shows the result of a simulation of two oscillators. 
Vgate is the voltage controlling the NFET of the transmission gate which links the 
phase averaging lines of the two oscillators together (the PFETs are controlled 
complementary). As soon as the Vgat, is brought high, the oscillators rapidly phase 
lock. 
4 Target Location 
We will assume that the input to a visual tracking chip is a binary image projected 
onto the die. Phototransistors detect the brightness of each pixel, and if it is above 
a threshold level, the pixel control circuitry will turn the pixel's oscillator on. If a 
pixel oscillator is turned on, gating circuitry will allow the propagation of the phase 
averaging line through the pixel's oscillator cell to its nearest-neighbors. Illuminated 
870 Andreou and Edwards 
To top neighbor 
vgt  
To left 
neighbor 
VgateP 
x  VgateP 
VgateN 
To right 
neighbor 
Vol 
VO2 
To bottom neighbor 
Figure 3: Phase-Locking Oscillator Cell 
Vr 
Pulse 
Output 
VLSI Phase Locking Architectures for Feature Linking in Multiple Target Tracking Systems 871 
-5 
10 sec 
Figure 4: Phase-Locking Simulation 
5V 
nearest-neighbor connected pixels will thus have their oscillators turned on and will 
become phase-locked. 
The follower-aggregator circuit can be modified to determine linear position (Maher, 
1989) by using voltage taps off of a resistive line as inputs to the transconductance 
amplifiers, and biasing the amplifiers by currents that correspond to the pulsing 
outputs of the oscillators (see Figure 5). 
During the time that a group of oscillators are spiking, the output of the tracking 
circuitry will yield a location corresponding to the average position of the distribu- 
tion of those oscillators. There can be many different nearest-neighbor connected 
objects projected onto the die, and the position of the center of each object is com- 
municated to the periphery via PAM. Thus, we can use multiplexing in time to 
simplify connectivity of communication with the periphery of the chip. 
5 Test Chip 
A chip to test the Comparator Model phase-locking method and multiple-target 
tracking system was fabricated by the MOSIS service in 2.0 pm feature size CMOS. 
To keep this test chip simple, the oscillators were arranged in a one-dimensional 
chain, and voltage inputs to the chip were used to control whether or not a pixel 
was considered "illuminated." A polysilicon resistive line was used to provide linear 
position information to the tracking system. All transistors used were minimum 
size (6 pm wide and 4 pm long). 
The test chip was able to rapidly and robustly phase-lock groups of nearest-neighbor 
872 Andreou and Edwards 
vpu.e(n-O  ' 
Postional Voltage Line 
vp,ase(n) .  
Vpulse(a+l)  
Average Position Line 
Figure 5: Circuit to determine object location 
connected oscillators. This phase-locking could occur with oscillator frequencies set 
from 10 Hz to 4 KHz. A phase-locked group of oscillators would almost instantly 
split into two separate phase-locked groups with little temporal correlation between 
them when a connected chain of on oscillators was severed by turning off an oscillator 
in the middle of the original group. Mismatch in the transconductances of the 
oscillator transistors provided easy desynchronization. 
Position tracking was measured by examining the resistive-line aggregator output 
during the time a certain phase-locked group of oscillators was pulsing. When 
multiple phase-locked groups of oscillators were active, it was still quite easy to 
make out the positional PAM voltage associated with each group by triggering an 
oscilloscope off of the pulsing output of an oscillator in that group. While there are 
occasional instances of two or more groups pulsing at the same time, if the duty 
cycle of the spiking oscillator is kept relatively small, there is little interference on 
average. 
6 Discussion 
It is becoming obvious that oscillation and synchronization phenomena in cortex 
may play an important role in neural information processing. In addition to striate 
cortex, the olfactory bulb also has oscillatory neural circuits which may be impor- 
tant in neural information processing (Freeman, 1988). It has been suggested that 
temporal correlation may be used for pattern segmentation in associative memories 
(Wang, 1990), and correlations between multiple oscillators may be used for storing 
time intervals (Miall, 1989). 
We have described a circuit which performs Comparator Model phase-locking. The 
distributed and partitionable qualities of this circuit make it attractive as a possible 
physiological model. The PAM representation of object position shows one way that 
connectivity requirements can be minimized for communication in a neuromorphic 
VLSI Phase Locking Architectures for Feature Linking in Multiple Target Tracking Systems 873 
system. The chip has been fabricated using subthreshold CMOS technology, and 
thus uses little power. 
Acknowledgements 
The authors are pleased to acknowledge helpful discussion with C. Koch and J. 
Lazzaro. Chip fabrication was provided by the MOSIS service. 
P. Baldi & R. Meir. (1990) Computing with arrays of coupled oscillators: an appli- 
cation to preattentive texture discrimination. Neural Computation 2,458-471. 
F. Crick &: C. Koch. (1990) Towards a neurobiological theory of consciousness. 
Seminars in the Neurosciences 2,263-275. 
R. Eckhorn, H. J. Reitboek, M. Arndt &: P. Dicke. (1990) Feature linking via 
synchronization among distributed assemblies: simulations of results from cat visual 
cortex. Neural Computation 2,293-307. 
W. J. Freeman, Y. Yao, &: B. Burke. (1988) Central pattern generating and recog- 
nizing in olfactory bulb: a correlation learning rule. Neural Networks 1, 277-288. 
Gray &: W. Singer. (1987) Stimulus-specific neuronal oscillations in the cat 
cortex: A cortical functional unit. Soc. Neurosci. Abstr. 13(404.3) 
Gray, P. KSnig, A. K. Engel &: W. Singer. (1989) Oscillatory responses in cat 
cortex exhibit inter-columnar synchronization which reflects global stimulus 
C. M. 
visual 
C.M. 
visual 
properties. Nature (London) 338, 334-337. 
D. H. Hubel. (1988) Eye, Brain, and Vision. 
Library. 
D. M. Kammen, C. Koch &: P. J. Holmes. 
New York, NY: Scientific American 
(1990) Collective oscillations in the 
visual cortex. In D. S. Touretzky (ed.) Advances in Neural Information Processing 
Systems 2. San Mateo, CA: Morgan Kaufman Publishers. 
C. A. Mead. (1989)Analog VLSI and Neural Systems. Reading, MA: Addison- 
Wesley. 
M. A. Maher, S. P. Deweerth, M. A. Mahowald & C. A. Mead. (1989) Implementing 
neural architectures using analog VLSI circuits. IEEE Trans. Cite. Sys. 36,643- 
652. 
C. Miall. (1989) The storage of time intervals using oscillating neurons. Neural 
Computation 1,359-371. 
C. von der Malsburg. (1986) A neural cocktail-party processor. Biological Cyber- 
netics. 54,29-40. 
D. Wang, J. Buhmann &: C. von der Malsburg. (1990) Pattern segmentation in 
associative memory. Neural Computation 2, 95-106. 
