Direction Selective Silicon Retina 
that uses Null Inhibition 
Ronald G. Benson and Tobi Delbriick 
Computation and Neural Systems Program, 139-74 
California Institute of Technology 
Pasadena CA 91125 
email: benson@cns.caltech.edu and tdelbruck@caltech.edu 
Abstract 
Biological retinas extract spatial and temporal features in an attempt to 
reduce the complexity of performing visual tasks. We have built and tested 
a silicon retina which encodes several useful temporal features found in ver- 
tebrate retinas. The cells in our silicon retina are selective to direction, 
highly sensitive to positive contrast changes around an ambient light level, 
and tuned to a particular velocity. Inhibitory connections in the null di- 
rection perform the direction selectivity we desire. This silicon retina is 
on a 4.6 x 6.8ram die and consists of a 47 x 41 array of photoreceptors. 
1 INTRODUCTION 
The ability to sense motion in the visual world is essential to survival in animals. 
Visual motion processing is indispensable; it tells us about predators and prey, our 
own motion and image stablization on the retina. Many algorithms for performing 
early visual motion processing have been proposed [HK87] [Nak85]. A key salient 
feature of motion is direction selectivity, ie the ability to detect the direction of 
moving features. We have implemented Barlow and Levick's model, [BHL64], which 
hypothesizes inhibition in the null direction to accomplish direction selectivity. 
In contrast to our work, Boahen, [BA91], in these proceedings, describes a silicon 
retina that is specialized to do spatial filtering of the image. Mahowald, [Mah91], 
describes a silicon retina that has surround interactions and adapts over mulitiple 
time scales. Her silicon retina is designed to act as an analog preprocessor and 
756 
Direction Selective Silicon Retina that uses Null Inhibition 757 
Preferred 
ehotoreceptors  ) 
Excitatinhibition 
DS cell 
(a) 
Pixels inhibit to the left 
Preferremrection 
Photoreceptor 
DS cell 
-/Inhibition 
(b) 
Figure 1: Barlow and Levick model of direction selectivity (DS). (a) Shows 
how two cells are connected in an inhibitory fashion and (b) a mosaic of such 
cells. 
so the gain of the output stage is rather low. In addition there is no rectification 
into on- and off-pathways. This and earlier work on silicon early vision systems 
have stressed spatial processing performed by biological retinas at the expense of 
temporal processing. 
The work we describe here and the work described by Delbriick, [DM91], emphasizes 
temporal processing. Temporal differentiation and separation of intensity changes 
into on- and off-pathways are important computations performed by vertebrate 
retinas. Additionally, specialized vertebrate retinas, [BHL64], have cells which are 
sensitive to moving stimuli and respond maximally to a preferred direction; they 
have almost zero response in the opposite or null direction. We have designed and 
tested a silicon retina that models these direction selective velocity tuned cells. 
These receptors excite cells which respond to positive contrast changes only and 
are selective for a particular direction of stimuli. Our silicon retina may be useful 
as a preprocessor for later visual processing and certainly as an enhancement for 
the already existing spatial retinas. It is a striking demonstration of the perceptual 
saliency of contrast changes and directed motion in the visual world. 
2 INHIBITION IN THE NULL DIRECTION 
Barlow and Levick, [BHL64], described a mechanism for direction selectivity found 
in the rabbit retina which postulates inhibitory connections to achieve the desired 
direction selectivity. Their model is shown in Figure l(a) . As a moving edge 
passes over the photoreceptors from left to right, the left photoreceptor is excited 
first, causing its direction selective (DS) cell to fire. The right photoreceptor fires 
when the edge reaches it and since it has an inhibitory connection to the left DS 
cell, the right photoreceptor retards further output from the left DS cell. If an edge 
is moving in the opposite or null direction (right to left), the activity evoked in the 
right photoreceptor completely inhibits the left DS cell from firing, thus creating a 
direction selective cell. 
758 Benson and Delbrfick 
Inhibition to left 
Inhibition from right 
put, Vo 
Q Preferred Direction 
Photoreceptor DS cell 
Figure 2: Photoreceptor and direction selective (DS) cell. The output of the 
high-gain, adaptive photoreceptor is fed capacitively to the input of the DS 
cell. The output of the photoreceptor sends inhibition to the left. Inhibition 
from the right photoreceptors connect to the input of the DS cell. 
In the above explanation with the edge moving in the preferred direction (left to 
right), as the edge moves faster, the inhibition from leading photoreceptors truncates 
the output of the DS cell ever sooner. In fact, it is this inhibitory connection which 
leads to velocity tuning in the preferred direction. 
By tiling these cells as shown in Figure l(b), it is possible to obtain an array of 
directionally tuned cells. This is the architecture we used in our chip. Direction 
selectivity is inherent in the connections of the mosaic, ie the hardwiring of the 
inhibitory connections leads to directionally tuned cells. 
3 PIXEL OPERATION 
A pixel consists of a photoreceptor, a direction selective (DS) cell and inhibition to 
and from other pixels as shown in Figure 2. The photoreceptor has high-gain and 
is adaptive [Mah91, DM91]. The output from this receptor, Vp, is coupled into the 
DS cell which acts as a rectifying gain element, [MS91], that is only sensitive to 
positive-going transitions due to increases in light intensity at the receptor input. 
Additionally, the output from the photoreceptor is capacitively coupled to the in- 
hibitory synapses which send their inhibition to the left and are coupled into the 
DS cell of the neighboring cells. 
A more detailed analysis of the DS cell yields several insights into this cell's func- 
tionality. A step increase of AV at Vp, caused by a step increase in light intensity 
incident upon the phototransistor, results in a charge injection of CeAV at . This 
charge is leaked away by Qr at a rate It, set by voltage Vt. Hence, to first order, 
the output pulse width T is simply 
ccv 
T- 
There is also a threshold minimum step input size that will result in enough change 
Direction Selective Silicon Retina that uses Null Inhibition 759 
1.6 
0.4 
0 40 
Output 
Input intensity 
80 120 160 200 
Figure 3: Pixel response to intensity step. 
is pixel output. 
Time (msec) 
Bottom trace is intensity; top trace 
in lq to pull Vout all the way to ground. This threshold is set by Cc and the gain of 
the photoreceptor. 
When the input to the rectifying gain element is not a step, but instead a steady 
increase in voltage, the current/in flowing into node 1 is 
/in --Ccl)p  
When this current exceeds I there is a net increase in the voltage lq, and the 
output Vout will quickly go low. The condition /in = I defines the threshold 
limit for stimuli detection, i.e. input stimuli resulting in an /in < I are not 
perceptible to the pixel. For a changing intensity I, the adaptive photoreceptor 
stage outputs a voltage Vp proportional to 1/I, where I is the input light intensity. 
This photoreceptor behavior means that the pixel threshold will occur at whatever 
/I causes CcVp to exceed the constant current It. 
The inhibitory synapses (shown as Inhibition from right in Figure 2) provide addi- 
tional leakage from lq resulting in a shortened response width from the DS cell. 
This analysis suggests that a characterization of the pixel should investigate both 
the response amplitude, measured as pulse width versus input intensity step size, 
and the response threshold, measured with temporal intensity contrast. In the next 
section we show such measurements. 
4 CHARACTERIZATION OF THE PIXEL 
We have tested both an isolated pixel and a complete 2-dimensional retina of 47 x 41 
pixels. Both circuits were fabricated in a 2um p-well CMOS double poly process 
available through the MOSIS facility. The retina is scanned out onto a monitor using 
a completely integrated on-chip scanner[MD91]. The only external components are 
a video amplifier and a crystal. 
We show a typical response of the isolated pixel to an input step of intensity in 
Figure 3. In response to the input step increase of intensity, the pixel output goes 
low and saturates for a time set by the bias V in Figure 2. Eventually the pixel 
recovers and the output returns to its quiescent level. In response to the step 
decrease of intensity there is almost no response as seen in Figure 3. 
760 Benson and Delbrick 
-160 ]5 
, 120 
$0 . 
 40. .4 tensity 
Step Contrast 
10 -2 
10 -] 
10  
1- I 
10- 
10 0 
I ! ! 
10 -] 10 0 10 ] 10 2 10 3 
Temporal Frequency (Hz) 
(a) (b) 
Figure 4: (a) Pulse width of response as function of input contrast step size. 
The abscissa is measured in units of ratio-intensity, i.e., a value of 1 means 
no intensity step, a value of 1.1 means a step from a normalized intensity of 1 
to a normalized intensity of 1.1, and so forth. The different curves show the 
response at different absolute light levels; the number in the figure legend is 
the log of the absolute intensity. (b) Receptor threshold measurements. At 
each temporal frequency, we determined the minimum necessary amplitude of 
triangular intensity variations to make the pixel respond. The different curves 
were taken at different background intensity levels, shown to the left of each 
curve. For example, the bottom curve was taken at a background level of 1 
unit of intensity; at 8 Hz, the threshold occurred at a variation of 0.2 units of 
intensity. 
The output from the pixel is essentially quantized in amplitude, but the resulting 
pulse has a finite duration related to the input intensity step. The analysis in 
Section 3 showed that the output pulse width, T, should be linear in the input 
intensity contrast step. In Figure 4(a), we show the measured pulse-width as a 
function of input contrast step. To show the adaptive nature of the receptor, we 
did this same measurement at several different absolute intensity levels. 
Our silicon retina sees some features of a moving image and not others. Detection 
of a moving feature depends on its contrast and velocity. To characterize this 
behavior, we measured a receptor's thresholds for intensity variations, as a function 
of temporal frequency. 
These measurements are shown in Figure 4(b); the curves define "zones of visibil- 
ity"; if stimuli lie below a curve, they are visible, if they fall above a curve they 
are not. (The different curves are for different absolute intensity levels.) For low 
temporal frequencies stimuli are visible only if they are high contrast; at higher 
temporal frequencies, but still below the photoreceptor cutoff frequency, lower con- 
trast stimuli are visible. Simply put, if the input image has low contrast and is 
slowly moving, it is not seen. Only high contrast or quickly moving features are 
salient stimuli. More precisely, for temporal frequencies below the photoreceptor 
cutoff freq. uency, the threshold occurs at a constant value of the temporal intensity 
contrast I/I. 
Direction Selective Silicon Retina that uses Null Inhibition 761 
Preferred 
Preferred  I 
Inhib 
Exclt atlqinhibitio n L 
)DS cell R 
0.1 sec 
(a) (b) 
Figure 5: (a) shows the basic connectivity of the tested cell. (b) top trace is 
the response due to an edge moving in the preferred direction (left to right). 
(b) bottom trace is the response due to an edged moving in the null direction 
(right to left). 
5 NULL DIRECTION INHIBITION PROPERTIES 
We performed a series of tests to characterize the inhibition for various orientations 
and velocities. The data in Figure 5(b) shows the outputs of two photoreceptors, 
the inhibitory signal and the output of a DS cell. The top panel in Figure 5(b) shows 
the outputs in the preferred direction and the bottom panel shows them in the null 
direction. Notice that the output of the left photoreceptor (L in Figure 5(b) top 
panel) precedes the right (R). The output of the DS cell is quite pronounced, but is 
truncated by the inhibition from the right photoreceptor. On the other hand, the 
bottom panel shows that the output of the DS cell is almost completely truncated 
by the inhibitory input. 
A DS cell receives most inhibition when the stimulus is travelling exactly in the null 
direction. As seen in Figure 6(a) as the angle of stimulus is rotated, the maximum 
response from the DS cell is obtained when the stimulus is moving in the preferred 
direction (directly opposite to the null direction). As the bar is rotated toward the 
null direction, the response of the cell is reduced due to the increasing amount of 
inhibition received from the neighboring photoreceptors. 
If a bar is moving in the preferred direction with varying velocity, there is a velocity, 
V,,x, for which the DS cell responds maximally as shown in Figure 6(b). As the 
bar is moved faster than V,,x, inhibition arrives at the cell sooner, thus truncating 
the response. As the cell is moved slower than V,, less input is provided to the 
DS cell as described in Section 3. In the null direction (negative in Figure 6(b)) the 
cell does not respond, as expected, until the bar is travelling fast enough to beat 
the inhibition's onset (recall delay from Figure 5). 
In Figure 7 we show the response of the entire silicon retina to a rotating fan. When 
the fan blades are moving to the left the retina does not respond, but when moving 
to the right, note the large response. Note the largest response when the blades are 
moving exactly in the preferred direction. 
762 Benson and Delbriick 
o = 80 
-0.8 -0.4 00 0.4 0.8 
Velocity (arbitrary units) 
(a) (b) 
Figure 6: (a) polar plot which shows the pixels are directionally tuned. 
(b) shows velocity tuning of the DS cell (positive velocities are in the pre- 
ferred direction). 
(a) (b) 
Figure 7: (a) Rotating fan used as stimulus to the retina. (b) Output of the 
retina. 
Direction Selective Silicon Retina that uses Null Inhibition 763 
6 CONCLUSION 
We have designed and tested a silicon retina that detects temporal changes in an 
image. The salient image features are sufficiently high contrast stimuli, relatively 
fast increase in intensity (measured with respect to the recent past history of the 
intensity), direction and velocity of moving stimuli. These saliency measures result 
in a large compression of information, which will be useful in later processing stages. 
Acknowledgments 
Our thanks to Carver Mead and John Hopfield for their guidance and encourage- 
ment, to the Office of Naval Research for their support under grant NAV N00014- 
89-J-1675, and, of course, to the MOSIS fabrication service. 
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