Abstract
Although the functional role of the optic tectum has yet to be well characterized in teleosts, it
is thought to be responsible for initiating and coordinating visuomotor behaviors associated with
the looming response. In this study, behavioral and electrophysiological responses to the looming
stimulus were measured by differential suction electrodes at the optic tectum and retinal ganglion
cells of adult male zebrafish (Brachydanio rerio). The looming stimulus was simulated by a
light-emitting diode and by increasing irradiance durations. Responses to off-light events were
found at both the retina and optic tectum, suggesting that light-induced startle responses begin as
early in the visual synaptic pathway as the retina. However, several differences in signal shape
and amplitude between recordings at these two regions suggest that higher-order visual
processing of on- and off-responses occurs at the optic tectum. Latency times were measured
between the presentation of the stimulus and the onset of the response, but data are currently
inconclusive. Habituation was observed through decreases in startle response as a function of
flash repetition. Future studies are necessary to correlate electrode position with signal strength
to resolve neural organization at the optic tectum. Öther parameters such as flash duration,
interstimulus interval, and age of the zebrafish can also be varied to obtain a more complete
understanding of the startle response.
Introduction
In natural habitats, predator-prey interactions often provide the selection pressure for the
evolution of behavioral escape responses, which range from immobility, changes in color, active
flight, and the seeking of refuge (Roberts 1992). In this study, we focused on one specific escape
response, the startle reaction, which occurs following a sudden, unexpected stimulus. The adult
zebrafish (Brachydanio rerio) was used as our model organism because its startle reaction has
been well characterized: a distinct tail flip, more pronounced than that in normal swimming.
which both accelerates and turns the fish (Kimmel 1974). The net result is rapid movement at an
angle to the original axis of swimming (Fig. 1). In general, these motions enable the fish to evade
oncoming predators quickly and efficiently.
In many teleosts, including the zebrafish, an abrupt decrease in light intensity triggers a
particular startle reaction, now identified as the looming response (Diamond 1970). Shadows
created from above can indicate an impending attack, especially by avian predators. Therefore,
these fish tend to dive into deeper waters for safety when another creature "looms" above it and
blocks the ambient sunlight. The pathway that mediates light received by the eye and ultimately
generates muscle fiber contraction has been studied extensively. The looming response is
understood to be transmitted via Mauthner’s neuron, or the M-cell, which initiates large
muscular contractions to produce a full startle reaction following certain visual cues (Bartelmez
1915). Nevertheless, by what method certain structures function, such as the exact processes
occurring within the optic tectum, is not yet well understood.
The retina contains two distinct types of ganglion cells: on-center and off-center cells. On-
center cells fire action potentials when their center is exposed to light and do not fire when the
region around its center, the surround, is subjected to light. Off-center cells react in just the
opposite manner. The axonal projections of retinal ganglion cells aggregate to form the optic
nerve. In non-mammalian vertebrates, these axons synapse onto the optic tectum (Fig. 2). In fish,
the optic tectum is the largest center for visual processing and appears to play a significant role
in predator/prey discrimination by initiating either escape or hunting, respectively. The
mammalian analog of the optic tectum is known as the superior colliculus. Another group of
neurons connects the superior colliculus to the cerebral cortex, the primary visual center in
mammalian vertebrates that is absent in lower vertebrates. Interestingly, the number of retinal
fibers in the visual afferent pathway to the optic tectum decreases considerably from lower
vertebrates to mammals (Cajal 1995). From the optic tectum, signals are relayed to the pair of
Mauthner neurons that surround the brainstem (Zottoli 1987). Each Mauthner cell sends a
crossed projection to a group of motoneurons and simultaneously inhibits the M-cell located on
the opposite side of the fish. These motoneurons are responsible for the contraction of muscle
fibers along the entire length of the zebrafish flank, which ultimately creates a visible startle
response.
In this study, we recorded both behavioral and electrophysiological responses of zebrafish to
the looming stimulus. We stimulated on-center and off-center cells in the retina to create action
potentials that traveled along the optic nerve to the optic tectum, which were then measured
using electrophysiological techniques. Our objective was to resolve differences in the processing
of on- and off-responses in the optic nerve and at the optic tectum. By observing discrepancies in
shape, relative amplitude, and duration, we hoped to elucidate whether higher-order processing
of visual input occurs in the optic tectum of adult zebrafish.
Materials & Methods
Subjects
Adult zebrafish were bred in the lab or purchased from a commercial supplier and kept in
aerated tanks with flowing water at 28°C. Only males approximately 3-4 cm in body length were
used. Animal care and handling was conducted in accordance with guidelines established by
Stanford University
Behavioral Observations
Five zebrafish were placed in a tank measuring 24 cm 12 cm 16 cm with aerated water at
28°C and allowed to acclimate for at least 1 hour. The tank was surrounded on three sides by
black poster board and illuminated by ambient room light, as well as by a flashlight positioned
approximately 1 m above the water’s surface.
Shadows were created by swiftly interrupting the flashlight beam with a circular piece of
cardboard 10 cm in diameter. The shadow was presented every 10 seconds until a noticeable
decrease in animal response occurred. Behavioral responses were recorded by a high-speed
digital video recorder (Sony DSC-T1).
Retinal and Tectal Preparations
Prior to surgery, the zebrafish was isolated from the supply tank and immobilized
temporarily by hypothermic shock in an ice water bath. Skeletal muscle paralysis was induced by
an intramuscular injection of 0.1-0.3 mL of curare into the mid-lateral tail line.
Recordings from the retina and the tectum were performed in different fish. For retinal
experiments, the eye was isolated from the fish by gently pulling and severing the optic nerve
and surrounding muscle tissue (Fig. 3). The eye was submerged in aerated 75% Hanks in a glass¬
holding dish during experimentation.
For tectal experiments, the skull was removed dorsally with forceps to expose the optic
tectum (Fig. 4). Äfter surgery, the fish was positioned upright in a glass-holding dish, and the
gills were perfused with aerated 75% Hanks solution. The solution level was maintained below
the gills. Adequate oxygen delivery was monitored throughout the experiment by observing the
blood flow in the gills and in the vessels on the optic tectum.
The glass-holding dish was placed in a light-isolated Faraday cage for both retinal and tectal
experiments.
Electrode Placement
Differential glass suction electrodes were used for recording from the retina and the tectum.
The electrodes consisted of an Ag-AgCl wire encased in a glass pipette filled with 75% Hanks
solution. The tectum electrode was positioned on the surface of either the right or the left tectal
hemisphere. The retinal electrode was placed on the stump of the detached optic nerve. Suction
and a mineral oil coating were used to establish a tight seal. A reference electrode was placed
approximately 2 cm from the fish. Once the electrodes were positioned, 30 minutes were given
for darkness adaptation.
Light Stimuli and Recordings
On- and off-light stimuli were produced by a white light-emitting diode (LED) positioned
approximately 3 cm to the left of the subject. The intensity of the LED was constant during all
periods of irradiance, and ambient light was minimal during dark periods. The flash duration and
interstimulus interval were controlled remotely by a computer (National Instruments Lab VIEW).
Experiments consisted of 5 repetitions of an irradiance interval followed by a darkness
interval (interstimulus interval). Light stimulus duration started at 100 ms and was incremented
to 2 minutes. Irradiance was preceded by a 100-ms delay and was followed by an interstimulus
interval totaling the irradiance and delay time in duration. Approximations for these parameters
were obtained and modified from Bullock (1990).
Electrical signals were sent to an AC differential amplifier with low-pass filtering of 1 Hz
and high-pass filtering of 1 kHz. The amplified signal (10,000—) was sent to an oscilloscope, a
DAT recording deck, and a computer. Measurements were recorded as single sweeps and as
averaged responses of five consecutive repetitions. Light frequencies were simultaneously
recorded by a DAT.
Results
In each figure, the lower schematic depicts when the flashes occurred: the LED is on
whenever the voltage is at a value significantly higher than the baseline. The upper diagram, on
the other hand, is the recording from the electrode placed either in the optic nerve or at the optic
tectum.
Optic Nerve
Recording from the retina was done from the distal stump of the optic nerve. A baseline flash
duration of 100 ms and interstimulus interval of 1900 ms was established (Fig. 5). From the
retina, only a single dip in response to each of the five light flashes was observed. Each of these
potentials lasted longer than the light flash itself and was relatively sharp in its characteristic
shape. The end of each signal was marked by hyperpolarization before the resting potential was
reached.
As the flash duration was increased gradually, two distinct dips began to appear: an on-
response when the light appeared and an off-response when the light disappeared (Fig. 6). In
order to see the two separate troughs, however, it was necessary to increase the flash duration
past a certain threshold value. This ensured that the on- and off-responses did not overlap, which
occurred in the initial trial. Although both of these responses may have been present
physiologically at a flash duration of 100 ms, they were indistinguishable according to the
voltage recordings. In addition, the on- and off-responses were similar in magnitude over five
repetitions.
As the flash duration and interstimulus interval were increased further, it was noticed that
even with an increasing flash duration, the relative durations of both on- and off-reactions remain
constant, within an approximate range of 200 and 300 ms (Fig. 7).
Right Tectum
Due to the anatomical connections that link the retina to the brain in lower vertebrates, when
the LED was positioned more proximal to the left eye, the contralateral tectum received all of the
neuronal signals output by the left retina (Fig. 8). As a result, it was expected that signals
detected at the right tectum to correspond better to the stimulus being presented to the zebrafish
when compared to those measured at the left tectum.
When the same initial trial from the retinal experiment was repeated, two peaks became
visible in the right tectum, one that represented the on-response and another that represented the
off-response (Fig. 9). Moreover, the on- and off-responses appeared to be comparable in
magnitude given these baseline parameters. Even though the recordings from the right tectum
show slightly higher levels of electrical noise compared to those made in the optic nerve, the
overall shapes present in a series of flashes remain consistent over repeated trails.
As both the flash duration and interstimulus interval were lengthened, complex relationships
between flash duration, on-response, and off-response magnitude were observed. For example,
when the flash duration was increased from 100 ms to 5 seconds, the ratio between
corresponding off- and on-responses was almost 50 percent greater in magnitude (Fig. 10). As
flash duration was increased further, the off-response continued to grow relative to the on-
response, but the increases were not as apparent, and the off-response reached its maximum
amplitude at a flash duration of 10 seconds. The on-response also grew with longer flashes, but
only very slightly.
Once comparisons were made with regard to flash duration, patterns within a single trial of
five repetitions were examined. The observation that the off-response decays markedly as a
function of repetition number was made (Fig. 11). The greater the number of times our stimulus
is presented to the zebrafish, the smaller the magnitude of the resulting off-response. As it turns
out, within a set of five repetitions, there was an approximate 25 percent decrease in the
magnitude of the off-response as repetition number increases. The on-response, on the other
hand, was much more variable and showed both decreases and increases with repetition.
Both the on- and off-responses were characterized by a short delay period during which only
background electrical activity was measured, and a sharp increase in activity that immediately
followed. The observed delay, or the time between the onset of a stimulus and the beginning of a
signal, was referred to as the latency period. For both the on- and off-response, the latency period
ranged between 20 and 100 ms. The signals themselves, however, had durations around 20 ms
for the on-response and 130 ms for the off-response. Thus, although the on- and off-responses
were generated at similar times after a light stimulus, they persisted to different extents and at
varying magnitudes.
In addition, changes in latency times were analyzed to determine a relationship as a function
of flash duration. At a flash duration of 100 ms, the average latency period for the ÖFF response
was 100 ms in the right tectum. When flash duration was extended to 5 s, however, the
corresponding latency value was only around 20 ms. For the on-response, latency times
remained relatively constant between 15 and 20 ms.
Left Tectum
Although the left optic tectum represented an ipsilateral configuration to the retina receiving
most of the light input, recordings made from the left tectum displayed strong activity that
corresponded to stimulus times. From our baseline trial with a flash duration of 100 ms, two
peaks with highly variable amplitudes over five repetitions were the result (Fig. 12), one for the
on-response and another for the off-response. The first peak was characterized by a high-
amplitude sharp wave, whereas the second peak was slower and less pronounced.
When flash duration was increased enough to distinguish between on- and off-responses, the
off-response was consistently smaller in magnitude than its associated on-response (Fig. 13). As
flash duration was increased further, off-responses increased disproportionately relative to their
corresponding on-responses until a flash duration of 10s was reached. However, the ratio of off-
responses to the first on-response of five successive repetitions showed no consistent trend over
repeated trials. Although differences in the relative amplitudes of on- and off-responses
increased with longer flash durations, they were not as significant as those observed in the right
optic tectum.
The components of both the on- and off-responses in the left tectum included a delay period
that lasted between 15 and 30 ms. However, the on-response contained two distinct peaks,
whereas the off-response involved only one such wave. Within the on-response, the first wave
was very sharp in its shape and persisted from 40 to 70 ms. This wave was followed by a slower,
shallower wave with a duration of around 130 ms. On the other hand, the off-response consisted
of a single, long wave of activity that ranged between 200 and 300 ms in length.
Latency periods were again compared over varying flash durations. At a flash duration of
100 ms and an interstimulus interval of 1900 ms, the average latency time for the ÖFF response
was approximately 15 ms. When flash duration was increased to 5 s, the corresponding latency
value was almost 200 ms. Similar to what occurred in the right tectum, the ON response latency
times ranged very little, between 15 and 20 ms.
Qualitative Comparisons
In comparing recordings made at the optic nerve and two optic tecta, on- and off-responses
were observed in all three locations. However, their relative magnitudes were found to be more
similar in the retina. Signals observed in the tecta consisted of double peaks, whereas those
measured in the retina consisted only of single peaks. The most distinct difference that was
noticed was the reversal of the relative magnitudes of the on- and off- responses between the
right and left tecta. In the right tectum, the off-response was considerably and consistently larger
in magnitude than the on-response that preceded it. In contrast, in the left tectum, the on¬
response was larger than the off-response, although not quite as consistently in comparison with
the right tectum.
Moreover, the shapes of on- and off- responses were more defined in the right tectum than in
the left tectum. Äfter a response was evoked, an extended series of several oscillations occurred
at the left optic tectum before baseline potentials were established. These fluctuations were
entirely absent in the right tectum. In addition, latency times for the OFF response decrease as a
function of flash duration in the right tectum, but increase in the left tectum. The presence of
such polar differences between the two tecta may indicate one method of determining the
direction of a light stimulus.
Discussion
In this study, we investigated the behavioral and electrophysiological responses of adult male
zebrafish to a looming stimulus. Electrical activity at both the optic nerve and tectum in response
to sudden introduction and elimination of light showed that the on- and off-responses originate
from the retina. However, several significant differences in the waveforms elicited at the post¬
synaptic layers of the tectum were observed, which might suggest the occurrence of higher-order
processing of on- and off- responses in the optic tectum.
Evidence of Higher-Order Tectal Processing
Visible signals recorded from the optic nerve in response to on- and off-events at all tested
durations of irradiation and interstimulus intervals show that neuron potentials elicited by sudden
illumination or darkening around the field of vision begin as early in the visual synaptic pathway
as the retina. This follows current consensus on the functional organization of retinal ganglion,
on-, and off- bipolar cells in mammals (Masland 2001) and teleosts (Daw 1968, Yang et al.
1988).
However, marked differences in waveforms and relative amplitudes were observed between
the signals recorded from the retina and optic tectum, which seems to indicate higher tectal
processing of the on- and off-stimulated signals from the ganglion cells. Signals may be
processed" at the tectum for any number of quantitative and qualitative characteristics of the
visual stimulus, such as light intensity, diffusivity, duration, and orientation, as well as
integration of these various aspects. Additionally, differences between signals recorded from the
11
right and left hemispheres of the optic tectum may indicate independent processing at individual
tectal lobes.
The waves recorded from the ganglion nerve in response to the on- and off-light stimulus
exhibited approximately equal amplitudes at all tested durations of irradiance. Differences in
amplitude between the on- and off-stimulus were immediately apparent at the optic tectum. The
on-response was visibly weaker than the off-response at the right tectum, whereas the reverse
was observed at the left tectum. It is too early to postulate the functional significance of the
amplitude differences observed in the waves recorded from the retina and optic tectum.
Nevertheless, it is possible that such differences are the result of visual orientation and
directionality processing, since the ganglion cells from the left eye synapse primarily onto the
right optic tectum (the LED was positioned on the left side of the animal in this study). Any
amplitude differences between the right and left tectum are thus likely to be influenced strongly
by the position of the light source relative to the visual field of the zebrafish.
While at least two depolarization events occurred at the optic tectum at all tested durations of
irradiance (one in response to the on-stimulus and one in response to the off-stimulus), only one
depolarization event was recorded from the optic nerve at the flash duration at 100 ms. However,
durations of light longer than 100 ms produced distinct on- and off-responses at the optic nerve.
These observations suggest the presence of at least three different functional cell populations in
the ganglion, including on-center, off-center, and on-off response cells, in which on-off response
cell potentials are evoked by a brief flash of light (Monasterio et al. 1975). Accordingly, it
appears that the visual synaptic input to the optic tectum during a looming response derives
primarily from on- and off-response cells in the ganglion.
Comparisons of relative amplitudes of on- and off-responses as flash durations were
increased indicated that the size of the off-response relative to the on-response at both the left
and right optic tectum increased. In contrast, little to no increase was observed at the retina.
Thus, the off-evoked potential at the optic tectum appeared more pronounced as the fish was
exposed to longer periods of light. This suggests that the tectal processing of information
regarding dark-induced visual input from the retina is influenced by the time of light exposure
prior to the off-stimulus. Additional measurements of on- and off-responses to a short flash
duration (100 ms) were recorded immediately after measurements of responses to a series of
increasingly longer flash durations to confirm that the increases in the ratio of the off- and on¬
response amplitudes were not the result of temporal habituation during repeated testing.
Off-Response Attenuation
The magnitude of consecutive off-responses decreased relative to the first off-response
within each experiment (5 repetitions per experiment), indicating the occurrence of habituation
to the looming stimulus. Attenuation of responses to shadows was also observed in the
behavioral experiments. While the occurrence of habituation in these studies suggests that
zebrafish may become less responsive to repeated appearances of shadows of predators from
overhead or may judge that such shadows do not represent imminent danger, this has yet to be
studied in natural habitats.
Limitations and Directions for Future Research
This study represents a preliminary investigation of the electrophysiological responses of
zebrafish to light-induced startle responses and is limited in several respects. Electrode
placement was determined largely by the position yielding the most visible signal and did not
13
account for possible input biases resulting from the synaptic organization of the retina and optic
tectum. Comparisons of signals from different locations on the tectum may help provide a more
detailed understanding of the functional organization of the retina and optic tectum.
Furthermore, a number of parameters have yet to be studied, including responses to flash
durations of increased resolution, decreased interstimulus intervals, and from fish in different
stages of development. Also, measurements of latency may reveal more retinal and tectal
differences. While preliminary recordings seem to indicate a decrease in latency of the off-
response at the right optic tectum and an increase at the left optic tectum with increasing
durations of light, they are currently inconclusive and require a more accurate and standardized
measurement method. Additionally, the looming stimulus was produced only as square-wave
pulses of uniform light intensity in this study. It remains for future studies to employ variations
in the simulation of the looming stimulus.
Acknowledgements
We are deeply indebted to Professor Stuart Thompson for his expertise and guidance, without
which, this project would not have been possible. We also express our sincerest gratitude to
Christian O’Reilly and Chris Patton for their encouragement and assistance. Additionally, we
would like to thank all the members of the Thompson laboratory for their support and
collaboration.
14
Literature Cited
Bartelmez, G. W. 1915. Mauthner’s Cell and the Nucleus Motorius Tegmeni. J. Comp. Neurol.
25: 87-129.
Bullock, T. H., M. Hofmann, F. Nahm, J. New, and J. Prechtl. 1990. Event-Related Potentials ir
the Retina and Optic Tectum of Fish. J. Neurophysiol. 64: 903-914.
Cajal, S. R. 1995. The Optic Lobe of Lower Vertebrates. pp. 161-185 in N. Swanson and L. W.
Swanson, ed. Histology of the Nervous System of Man and Vertebrates, Vol. II., Oxford
University Press, New York, NY.
Cajal, S. R. 1995. The Optic Nerve, Chiasm, and Tract. pp. 303-313 in N. Swanson and L. W.
Swanson, ed. Histology of the Nervous System of Man and Vertebrates, Vol. II., Oxford
University Press, New York, NY.
Daw, N.W. 1968. Colour-coded ganglion cells in the goldfish retina. J. Physiol. 197: 567-592.
Diamond, J. 1970. The Mauthner Cell. pp. 265-346 in W. S. Hoar and D. J. Randall, ed. Fish
Physiology, Vol V., Academic Press, New York, NY.
Eaton, R. C., R. A. Bombardieri, and D. L. Meyer. 1977. The Mauthner-Initiated Startle
Response in Teleost Fish. J. Exp. Biol. 66: 65-81.
Kimmel, C. B., J. Patterson, and R. Kimmel. 1974. The Development and Behavioral
Characteristics if the Startle Response in the Zebra Fish. Dev. Psychobiol. 7: 47-60.
Masland, R. H. 2001. The fundamental plan of the retina. Nature Neurosci. 4: 877-886.
Monasterio, F. M. and P. Gouras. 1975. Functional properties of ganglion cells of the rhesus
monkey retina. J. Physiol. 251: 167-195.
Roberts, B. L. 1992. Neural mechanisms underlying escape behaviour in fishes. Rev. Fish Biol.
Fish. 2: 243-266.
Yang, X. L., K. Tornqvist, and J. E. Dowling. 1988. Modulation of cone horizontal cell activity
in the teleost fish retina. J. Neurosci. 8: 2259-2268.
Zottoli, S. J., A. R. Hordes, and D. S. Faber. 1987. Localization of Optic Tectal Input to the
Ventral Dendrite of the Goldfish Mauthner Cell. Brain Res. 401: 113-121.
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Fig. 1 Dorsal view of typical startle response in zebrafish. Silhouettes are drawn at 5 ms
intervals beginning approximately 5 ms before onset of vibrational stimulus (reproduced
from Eaton et al. 1977)
Fig. 2 Frontal section through a teleost (Barbus fluviatilis) optic tectum (after P. Ramón)
stained using the Golgi method. A, deep regions of layer 5; B, middle region and central
plexus of layer 5; C, superficial region of layer 5 consisting primarily of fusiform cell
dendritic branches; a, layer 5 ganglion cell; b, pyramidal cells with a central axon; c,
shepherd’s crook cell with some of its dendritic branches extending into the central
plexus; d, layer 5 transverse cell; e, small stellate cell (reproduced from Cajal 1995).
Dorsal view of zebrafish eye and surrounding tissue after extraction from socket. The
Fig. 3.
thin, light tissue is the optic nerve, which contains the axonal projections of ganglion
cells. Encircling the optic nerve are fat globules that supply nutrients to the region.
Top-down view of zebrafish tectal region after surgical removal of protective head plate.
Fig. 4.
Clearly visible are the right and left lobes of the optic tectum, along with the cerebellum,
which lies posterior to and between the tecta.
Fig. 5 Diagram of an optic chiasm and the compensatory decussation of motor and
somatosensory pathways in lower vertebrates. C, second-order visual centers; G, spinal
ganglia and sensory roots; M, motor pathway; O, crossed optic nerves; R, spinal motor
roots; S, crossed second-order somatosensory pathway (reproduced from Cajal 1995).
Voltage data recorded from the retina at five repetitions given baseline parameters (flash
Fig. 6.
duration of 100 ms and an interstimulus interval of 1900 ms). Single dip responses occur
following each flash. Each response lasts longer than the corresponding flash itself.
Voltage data recorded from the retina given a flash duration and an interstimulus
Fig. 7
interval of 10 s. Both an on- and an off-response are visible and are similar in peak
amplitude.
Voltage data recorded from the retina given a flash duration and an interstimulus
Fig. 8
interval of 20 s. On- and off-responses remain constant in the duration, between 200 and
300 ms, as flash duration increases.
Voltage data recorded from the right optic tectum at five repetitions given baseline
Fig. 9
parameters. Both on- and off-responses result from repeated light stimuli. High levels of
electrical noise are present in this run.
Fig. 10 Voltage data recorded from the right optic tectum at five repetitions given a flash
duration and an interstimulus interval of 5 s. The ratio of off-response magnitude to on-
response magnitude increases as a function of repetition number.
Fig. 11 Voltage data recorded from the right optic tectum at five repetitions given a flash
duration and an interstimulus interval of 10 s. Off-response magnitude displays a
noticeable decrease of approximately 25 percent over the complete series of light
flashes.
Fig. 12 Voltage data recorded from the left optic tectum at five repetitions given baseline
parameters. Both on- and off-responses result from repeated light stimuli, and no clear
trend governs their magnitudes. High levels of electrical noise are present in this run.
Fig. 13. Voltage data recorded from the left optic tectum at five repetitions given a flash duration
and an interstimulus interval of 5 s. The ratio of off-response magnitude to on-response
magnitude decreases as a function of repetition number.
29