Download J Comp Physiol (1982) 149: 179 193

Journal of
J Comp Physiol (1982) 149: 179 193
Comparative
Physiology • A
Springer-Verlag 1982
Common Visual Response Properties
of Giant Vertical Cells in the Lobula Plate of
the Blowfly Calliphora
R. Hengstenberg
Max-Planck-Institut für Biologische Kybernetik, Spemannstrasse 38, D-7400Tübingen 1, Federal Republic of Germany Accepted
June 21, 1982
Summary. 1. The common response properties
to simple visual stimuli (light impulses, light
steps, and movement of simple patterns at
different speeds) has been investigated by
intracellular recording from Giant Vertical Cells
(VS) in the lobula plate of the blowfly
Calliphora erythtroccephala.
2. The impulse response begins < 10 ms after
onset of the photoreceptor signal (Fig. 6), and
shows several phases which gradually subside
within about 0.5 s. Very late events, which
would hint at recurrent or far-reaching
sidepaths, were not observed.
3. The step response is highly non-linear in that
both, the increase and decrease of brightness
elicit transient depolarization. The excitatory
transients are followed by inhibitory waves
(Figs. 7, 8), similar to those observed in impulse
responses. The possible significance of this
succession of excitation and inhibition is
discussed.
4. Vertical movement of arbitrary
patterns (dot, edges, bar, and gratings) elicit,
invariably and irrespective of contrast polarity,
depolarizing
responses
with
downward
movement, and hyperpolarizing responses with
upward movement (Fig. 10). Both responses
increase nonlinearly with contour length (Fig.
11 ). Possible mechanisms, and the functional
significance of such nonlinear summation are
discussed.
5. The velocity dependence of movement responses to periodic gratings was investigated at
both high and low pattern luminance and
contrast. Under these conditions VS-cells
respond best at a contrast frequency of ≈2 Hz,
which corresponds with that of velocity
dependent optomotor reactions.
6. These results confirm earlier findings that
Abbreviations: HS horizontal system; VS
vertical system
giant vertical cells have many response
properties in common. They are best suited to
perceive widefield motion, which occurs when a
fly performs rotatory and translatory movements
in a resting environment. VS-cells are therefore
most likely involved in the visual control of
such movements.
7. The present results are not sufficient to
indicate which of the VS-cells contribute to which
of the optomotor reactions. A subsequent
publication will be addressed to these questions.
Introduction
The lobula plate of the blowfly Calliphora
contains amongst others two classes of
remarkably large tangential neurons:
3 ‘Horizontal Cells’ (HS), and 11 ‘Vertical
Cells' (VS) (Fig. 1). They were originally
termed this way for anatomical reasons (Pierantoni 1976). By subsequent intracellular recording, and dye injection, HS-, and VS-cells were
found to respond specifically to movements in the
visual field. Their preferred directions correspond
roughly with their class names. Extensive studies
of the response properties of HS-cells indicate
that they probably contribute to the visual
control of yaw movements (for review see
Hausen 1981).
Much less is known about VS-cells. Despite
the general similarity of its members this class is
quite heterogeneous. A study on the structure of
VS-cells (Hengstenberg et al. 1982) has shown
that each of the 11 VS-cells is distinguished from
the others by its position in the neuropil and its
particular dendritic structure. It was further
shown that major structural aspects of the
different VS-cells are surprisingly constant in
different individuals of Calliphora. Similarly,
signs of physiological hetero0340-7594:82:014910179 803.00
180
R. Hengstenberg: Common Visual Responses in Giant Vertical Cells of Calliphora
geneity of VS-cells have been found (Hausen 1976a;
Eckert and Bishop 1978; Hengstenberg 1981 b).
According to this dualism of gross similarity, which
defines the class of VS-cells, and the distinct
individuality of its members, the present work deals
with response properties that are common to all
VS-cells, and which define physiologically the class
of VS-cells. A subsequent article will aim at the
discrete response characteristics of its individual
members.
Materials and Methods
Animals
All experiments were performed with female Calliphora erythrocephala Meig. (Insecta, Diptera) from laboratory cultures, 4-6
days post emergence.
Preparation
Flies were anaesthetized ≤ 1 min in nitrogen, wings and legs
amputated, wounds sealed with wax, and the body fixed to a metal
holder. After at least 4 h of recovery from anaesthesia, the head was
tilted ca. 30° ventrally and secured to the thorax with wax. Access
to the lobula complex was gained through a window in the occipital
cuticle. The overlying fatbody, and M. retractor haustelli were
removed. Neck muscles and the pulsatile head organ were
transsected. After amputation of the proboscis near its base, the
esophagus was pulled caudad out of the brain, closed by wax, and
fixed to the thorax. The tracheal system was usually left intact: for
some experiments, how
ever, it was necessary to remove one branch, which overlies the
axons of VS-cells.
Control of Viabilty
In view of the extensive dissection, necessary for stable intracellular recording, the viability of the preparation was checked by
extracellularly recording from a prominent movement sensitive
interneuron (H1) in the lobula plate. It responded for > 24 h in the
same manner, as in much less extensively dissected specimens.
Experiments were usually completed within <6 h after dissection.
Penetration and Cell Identification
Recording electrodes (3%, aqueous Procion Yellow M4-RAN 200500 MΩ), and reference electrodes were placed under microscopic
control. The recording electrode was carried by a hydraulic
microdrive (D. Kopf), and cell penetration was aided by a
piezoelectric jolter (Hengstenberg 1981 a). All potential
differences stated in text and figures refer to the reference electrode placed in the hemocoel. Mostly the axons of VS-cells were
impaled at the proximal margin of the lobula plate. Dye was
injected by dc-iontophoresis ( ≤ 5 nA , ≥ 1 µC) through the
recording electrode by a constant current source, which allowed
observation of the electrical activity of the neuron during iontophoresis. Detailed procedures for cell penetration, dye injection,
histology, fluorescence microscopy. and cell reconstruction are
published elsewhere (Hengstenberg and Hengstenberg 1980:
Hengstenberg 1981 a; Hengstenberg et al., in press).
Recording and Staining Strategy
The lifetime of intracellular penetrations varies from a few seconds in most cases up to 2 h in very few cases. About 10 min
R. Hengstenberg: Common Visual Responses in Giant Vertical Cells of Calliphora
181
Data Processing
To evaluate the recorded data off-line, the signals were either
filmed or processed with a signal analyzer (Intertechnique,
Histomat S). Statistical analyses were performed if necessary
by electronic integration of single responses during a period of
stimulation. Further evaluation involved a data printer (Kienzle,
D44c), and a programmable calculator (Hewlett Packard, 97).
Results
Giant Tangential Neurons
are required for dye injection if satisfactory stainings are to be
obtained. At least the same time is required if even simple
measurements are to be made under stationary recording conditions. Since the end of a penetration is never predictible, a
complementary strategy has been applied in successive penetrations: (a) One or only a few stimulus response cycles were
recorded, at the risk of statistical uncertainty of the results, and
emphasis was placed upon dye injection and cell identification.
(b) Most of the time was dedicated to recording, in order to
ensure stationarity and statistical significance of the results, at
the risk of injecting too little or even no dye for cell identification.
(c) In a few cases, dye was injected by low current ( < 1 nA) dciontophoresis while responses were recorded. Data from the latter
experiments were only used for control purposes.
Stimulus Arrrangement
The flies faced two circular backprojection screens of 60° or 90°
diameter, whose axes were in the equatorial plane of the fly's head,
and displaced by ±45 laterally (Fig. 2). A dull black diaphragm
between the screens prevented optical crosstalk between the two
stimulus fields. Since the visual field of Calliphora covers almost
the entire solid angle around the animal, only 13%-30% of the total
visual field could be stimulated this way.
Figure 1 shows 12 of the 14 giant tangential cells of
the lobula plate, in a frontal view of the right optic lobe
from behind, as they are displayed by retrograde
transsynaptic cobalt diffusion, and silver
intensification. Three Horizontal Cells (HS) and 11
Vertical Cells (VS) are present per lobula plate in
different depths of the neuropil. Figure 1 shows HSand VS-cells separately, because each class covers
completely the area of the lobula plate, i.e. the
retinotopic input array of this neuropil. Only VS-cells
will be considered here; their individual structure has
been previously described in detail (Hengstenberg et
al. 1982).
Electrical Activity in the Neuropil
The vast majority of neurons in the lobula plate and
other visual neuropils are unipolar (Strausfeld 1976)
and their initial process, connecting the perikaryon
with the rest of the cell, is very thin (≈ 2 µm) and
comparatively long (20-50 µm). When somata are
impaled, a steady, membrane potential of - 75 mV
< U < -40 mV is recorded, but neither spontaneous
nor stimulus-induced potential changes could be
observed. This indicates that cell somata are
electrically uncoupled from the bulk of VS-cells, and
all recordings have to be made by blind penetration of
fibres in the neuropil. Conventional types of electrical
signals are encountered there, depending upon which
type or part of neuron has been impaled.
The giant tangential cells (HS and VS) have in
most penetrations a comparatively low average
membrane potential of -40 to -50 mV (Fig. 3). lt
appears ‘noisy’ by continuous synaptic activity
182
R. Hengstenberg: Common Visual Responses in Giant Vertical Cells of Calliphora
tic potentials. A study of the regenerative processes,
associated with the output side of these cells, will be
presented later.
Impulse Responses
(e.g.
Fig. 9). Both, the average potential, and the
degree of fluctuation depend upon the stationary level
of brightness. Figure 3 shows amplitude histograms
of a VS-cell, when dark-adapted (I ≤_ 0.1 cd/m2and
when bright-adapted (I=100 cd/ m2). lt is on average
depolarized by 2 mV in brightness, and the
fluctuation around the mean value is about twice of
that in darkness. Responses to visual stimulation
consist usually of such membrane potential shifts,
and concomitant changes of fluctuation (e.g. Fig. 9).
So far, overshooting action potentials could not
be demonstrated to occur anywhere in these
neurons under the prevailing recording conditions.
Indications of electrical excitability have however
been found in the axons and terminal arborizations of
HS- and VS-cells (Hengstenberg 1977; Eckert 1979;
Eckert and Bishop 1978; Hausen 1981 , Eckert and
Hamdorf 1981). For the present work, the graded
potential changes in response to visual stimulation are
used to characterize the input organization of VScells, because this activity is probably caused by
summation of many excitatory and inhibitory
postsynap-
Very bright, brief flashes, when diffusely delivered
to large areas of the compound eye, present a strong,
instantaneous perturbation of the visual system.
Several thousand photoreceptors are simultaneously
excited. Their signals travel along an equally large
number of parallel fibres centripetally, and along
particular pathways even through the whole central
nervous system (Hengstenberg, unpublished).
Several kinds of results can be expected when the
response of central interneurons to such an unspecific
perturbation is measured: (a) A neuron may not
respond at all, and may therefore lack any
connection with the visual system. (b) Neurons,
with very specific visual stimulus requirements
may be ‘surprised’ this way, and induced to reveal
their visual input, even if their specific stimulus is
not available or not yet known. (c) For apparently
visual interneurons, the response delay indicates
their position in a chain of neurons, relative to
photoreceptors on the input side, and to other
neurons in the vicinity. (d) Different pathways
between photoreceptors, and an interneuron under
investigation may be revealed by impulse responses
consisting of several excitatory and/or inhibitory
waves.
Impulse responses of VS-cells were recorded in
29 penetrations. The responses are very similar in
different VS-cells, and will therefore be presented in
general terms.
Figure 4 shows impulse responses of an unidentified VS-cell at two different flash frequencies.
Both were recorded after ≥30 s adaptation to the
particular flash frequency. Under both conditions..
the first sign of a response is a small negative deflection, which is due to a field potential that can also
be recorded extracellularly (Fig. 6, inset), or in any
of the cell somata. At 0.2 Hz, the next event is a
singular action potential, whose amplitude depends
upon the average resting potential in darkness, upon
the degree of dark adaptation.. and possibly other
factors. In penetrations of low average membrane
potential (U > -45 mV), or in the bright adapted
state only a small, positive peak follows the field
potential at the time where the action potential is
elicited in dark adapted penetrations. The variable
amplitude of the action potential indicates that the
regenerative mechanism is partially inactivated in
these neurons (Hengstenberg 1977). At 0.2 Hz flash
frequency, the action
R. Hengstenberg: Common Visual Responses in Giant Vertical Cells of Calliphora
potential is followed by a series of humps and
notches, superimposed upon a transient depolarization
of < 10 mV. At 2 Hz flash frequency, the depolarizing
transient is smaller, and the potential fluctuations are
reduced (Fig. 4).
Figure 5 shows averaged impulse responses of the
same VS-cell as in Fig. 4 in comparison to
impulse responses of a photoreceptor axon and a
lamina monopolar cell, which were recorded under
the same conditions, but in a different animal. In all
records, the transient increase of activity decays with
approximately the same time course. That is to say
that an instantaneous perturbation does not seem to
elicit longlasting reverberations, which might occur if
the wave of excitation would travel along very
complicated and/or recurrent pathways. The high
frequency fluctuations, seen in single responses (Fig.
4) are largely suppressed in the averages,
indicating that they are not specifically related to
the synchronizing flash. There are, however, two
negative going waves (Fig. 5, asterisks). the first at
about 60 ms after the flash, and the second after
about 120 ms. The first is present in practically all
impulse responses recorded so far in VS-cells, the
second is often less clearly recognizable. It is currently
not known whether these negative going waves,
superimposed upon a transient depolarization arise
from a temporary inactivity of the depolarizing
inputs or from distinct hyperpolarizing inputs. Such
delayed inhibition could in principle be an essential
part of the elementary process of movement detection
(Thorson 1966; Torre and Poggio 1978). The results
presented
183
here, are, however, not sufficient to prove or disprove
this possibility (see Discussion).
Figure 6 shows impulse responses of RA, LM and
VS at higher time resolution to reveal how the
impulse perturbation travels through the optic lobe.
The inset shows records at 2 Hz flash frequency inside
and just outside the same VS-cell. The initial
negative deflection in intracellular records of
VS-cells at 0.2 and 2 Hz flash frequency is equally
present in the extracellular recording.
184
R. Hengstenberg: Common Visual Responses in Giant Vertical Cclls of Calliphora
lobula complex. This is a very short time compared
to the further 5 ms which are required to elicit the
action potential in the VS-cell. It is therefore probable that the field potential is not locally generated
by the input elements of VS-cells- it may even originate in the lobula or medulla.
Step Response
The impulse responses (Figs. 4- 6) give no indication
whether the transient excitation of VS-cells is caused
by the increase or decrease of brightness which occur
almost simultaneously in flashes of 20 µs duration.
Step responses are appropriate to decide this
question, and have been recorded in 21 VSpenetrations. Since impulse responses decline within
about 500 ms, square pulses of ≥0.5 s may be
considered as a succession of ‘ON’- and ‘OFF’-steps.
Figure 7 shows the step response of a VS-cell: (a)
in the dark-adapted state, and (b) in a state of bright
adaptation, which corresponds roughly to that of
movement stimuli, which are used later in this work.
The dark adapted response is quite complex: it starts
again with a negative going field potential, followed
by an abortive action potential, a transient,
depolarizing peak and, a considerable increase of
fluctuation of the membrane potential about its
average dark value (Fig. 7a). Light off is followed
by even larger fluctuations, superimposed upon a
somewhat slower depolarizing transient.
When flashes of 0.5 s duration are repeated at 1 s
intervals, the step response changes gradually from
the form shown in Fig. 7a to that in Fig. 7b.
presumably due to an average bright adaptation. As
before, the response shows depolarizing transients
both at the onset and offset of light, but the potential
fluctuations during brightness are much smaller
than in the dark adapted state.
The detailed time course of step responses thus
varies with adaptation, but also from flash to flash,
when a steady state of adaptation is reached. Similar variations are seen in different penetrations from
presumably different VS-cells, and have not been
studied in detail.
lt is therefore rather a fieldpotential of unknown
origin than a constituent part of the transmembrane
signal of VS-cells. For comparison, the latency of the
responses was defined as the intersection of the
tangent through the inflexion point of the rising
phase with the resting potential level. Figure 6
shows that a large proportion of the total latency of
the VS-cell response is caused by the photoreceptor
latency (6.5 ms), that the perturbation needs less than
5 ms to travel from the lamina through the external
chiasma, through the medulla, and through the
internal chiasma into the
The general features of step responses of VScells are revealed, when a large set of measurements are pooled and compared with averaged step
responses of a photoreceptor and of a lamina
monopolar cell. The photoreceptor responds in a
tonic manner to the input step, and the monopolar
cell gives a phasic signal (Fig. 8). In both cases, the
sign of the potential change reverses when the
direction of brightness change is reversed. VS-cells,
however, respond to increasing as well as to de-
R. Hengstenberg: Common Visual Responses in Giant Vertical Cells of Calliphora
185
lowed by inhibitory waves, similar to those observed
in impulse responses. Figure 8 shows that only one
such wave occurs after each of the respective
depolarizing transients in step responses. The
inhibitory wave after 'o n ' is comparatively
shallow, and attains its minimum about 120 ms
after the onset of the depolarizing transient. The
inhibitory wave after light 'off' is shorter, and
attains its minimum at about 45 ms after the beginning of the depolarizing ‘off’-transient. Both characteristics of the inhibitory waves in step responses
(the delay after the depolarizing transient, and the
different duration) bear striking similarity with the
two inhibitory waves, observed in the impulse response (Fig. 5): the first, short-lived inhibitory
wave occurs about 46 ms after the onset of the
VS-response, and the second, slower inhibitory
wave occurs at about 115 ms. Comparison with the
inhibitory waves of step responses suggests that the
first inhibitory wave of impulse responses is caused
by the 'off-aspect' of the light impulse, whereas the
later, and slower inhibitory wave of the impulse
response is caused by the 'on-aspect' of the light
impulse. The close correspondence of delayed
inhibitory waves in step responses, and in impulse
responses, is quite surprising in view of the different
stimulus conditions (step: Ipeak=100 cd/ m2, dI/d t≈
2 x 104 cd .s/ m2; impulse: Ipeak=250,000 cd
/m2,dI/dt≈2.5 x 1010 cd.s/m2).
Similar nonlinear responses of VS-cells, and
especially the succession of excitation and inhibition, have also been recorded in VS-cells of Phaenicia (Soohoo and Bishop 1980).
creasing brightness with a transient depolarization.
Such nonlinear responses are expected in movementsensitive interneurons like VS-cells, because the
essential process in directionally specific movement
detection is a nonlinear interaction between adjacent
input channels (Hassenstein and Reichardt 1951).
The nonlinearity seen here, is, however, not
necessarily due to this mechanisms. It could equally
be caused by an unspecific 'self-interaction' (Poggio
and Reichardt 1976).
In step responses, the initial depolarizing transients after the onset and offset of light are fol-
The succession of transient excitation and inhibition, as demonstrated in the present investigation
may be related to the elementary process of directionally specific movement detection (Torre and
Poggio 1978). A delayed inhibition could however
equally arise from other mechanisms, for example
from recurrent inhibition of input channels (Poggio
et al. 1981). Specific experiments will he necessary
to reveal the significance of the successive excitatory
and inhibitory waves in impulse-and step responses
of VS-cells.
Movement Responses and Directionality
VS-neurons have been termed 'Vertical cells' because their dendritic arborizations span the whole
dorso-ventral extent of the retinotopic input array of
the lobula plate (Pierantoni 1976). Some of them
have later been found to respond predominantly to
vertical movement in the ipsilateral visual field
(Dvorak et al. 1975; Hausen 1976a, b: Hengstenberg
1977. 1981 b; Eckert and Bishop 1978:
186
R. Hengstenberg: Common Visual Responses in Giant Vertical Cells of Calliphora
companied by an increase of fluctuation frequency, and
a decrease of fluctuation amplitude relative to
periods where the pattern does not move. This can be
expected if the response is generated by
superposition of many asynchronous ipsp's and' or
exclusion of epsp's.
The movement responses of VS-cells have been
tested in 47 penetrations, and all respond in a very
similar manner to vertical pattern movement.
Some VS-cells (VS1, VS5-VS10) respond
additionally to horizontal movements. The functional
implications of such responses will be reported separately. Physiological data about VS11 are presently
not available.
The similarity of responses to vertical movements
in the visual field defines the most important common
response property of these neurons. Together with
their structural similarities (Hengstenberg et al.
1982), the notion of a distinct class of neurons
('Vertical System', Pierantoni 1976) appears
justified. VS-cells should however not be regarded
as ‘general purpose neurons’ for any vertical
movements, because several vertically sensitive
neurons with different structural and functional
characteristics are known in the lobula plate (cf.
Hausen 1981).
Responses to Movements of Basic Patterns
Soohoo and Bishop 1980). This response behaviour
is illustrated in Fig. 9 for VS2. The meaning of the
arrow symbols, which specify movement stimuli is
illustrated in Fig. 2. This cell does not respond to any
kind of horizontal movement. It is, however,
depolarized as long as the pattern is moving
downwards, and at the same time the frequency and
amplitude of membrane potential fluctuation
increase. probably because of an increased rate of
epsp's impinging upon the cell. Upward movement
causes a steady hyperpolarization, ac-
In previous investigations, the movement responses of VS-cells were elicited by extended
striped patterns, moving at constant angular velocity. In order to characterize the functional organization of the receptive field of VS-cells, a series of 12
experiments was performed, where four local, and
one extended pattern were used: (a) A 10° x
10° black square on white background; (b, c) single
edges of either positive or negative brightness
change, when moved across the receptive field; (d) a
black bar of 10° width on white background; and (e)
a periodic grating of 20° spatial wavelength. The
patterns were moved 60° up and 60° down at 24°:s
by a position-controlled servo system. The stimulus
fields of 70° x 70° were arranged as in Fig. 2, i.e.,
their center axes coinciding with the equatorial plane
of the eye. Because of the fixed arrangement of the
stimulus fields in the anterior part of the visual field,
only distal VS-cells (VS2-VS4) could be studied
this way. In order to ensure stationarity of the
responses, the phases of pattern movement were
separated by 3 s rest, where the pattern was present
and illuminated, but not moving. Furthermore, the
sequence of measurements shown in Fig. 10 was
repeated several times, usually until the cell was lost.
R. Hengst enberg: Common Visual Responses in Giant Vertical Cells of Calliphora
Figure 10 illustrates the essential results of these
experiments, which will he discussed sequentially:
VS-cells respond to arbitrary patterns in a
qualitatively similar manner; all patterns elicit depolarizing responses when moving downwards, and
all elicit hyperpolarizing responses when moving
upwards. There is neither a unique preference for
small discrete objects (Fig. 10a e), nor a
recognizable threshold which would indicate a 'least
area requirement'. The moving square stimulates at
any time no more than 25 out of about 6,000 visual
elements of the compound eye, i.e. less than 0.5%.
The sign of the responses to moving edges (Fig.
10b, c) is determined by the direction of
187
movement, and not by the sign of the local brightness
change. This invariance of the responses against
contrast reversal immediately proves the directional
selectivity of movement responses in VS-cells.
Responses to discrete patterns (Fig. 10a-d) attain
maximal amplitudes shortly after the beginning of
the downward movement and towards the end of the
upward movement. This indicates the presence of a
sensitivity maximum for preferred (downward), and
reverse (upward) movement about 10° dorsal of
the equatorial plane of the head. The limited extent of
the stimulus field prohibits, however, definite
statements about the spatial sensitivity distribution
of VS-cells.
The amplitudes of both depolarizing and hyperpolarizing responses increase with contour
length of the pattern. This is to say that VS-cells
respond most vigorously when sufficiently textured, extended patterns are moved in the appropriate
direction and speed across the receptive field.
The electrical activity of VS-cells suggests that
their graded responses are due to superposition of a
multitude of asynchronous synaptic potentials. The
vigorous responses to extended, well-structured
patterns is therefore likely to arise from widefield
summation of local signals as provided by
elementary movement detectors with small receptive
fields (Buchner 1976: Buchner et al. 1978). In the
simplest case, the response amplitude may increase
linearly with increasing contour length of the pattern,
because an increasing number of movement
detectors is activated.
To determine the dependence of VS-cell responses
from the contour length of the pattern, live cycles
of up/down movement were averaged for each of the
patterns of Fig. 10. Data on edge movements were
pooled to restitute the dorsoventral symmetry as in
the other patterns. The peak amplitudes are plotted
against the contour length of the respective
patterns. Hyperpolarizing responses are plotted
with inverse sign to facilitate comparison. Figure
11 shows the result of this procedure: The
amplitudes of de- and hyperpolarizing responses
increase monotonically with contour length, but
by no means linearly. If the slope of the curves is
taken to express the response gain (mV/degree of
contour length), it is seen that this gain decreases from
> 30 mV/deg for small patterns to < 0.01 mV /deg for
extended patterns. This kind of saturation
nonlinearity is to be expected when the driving
potential of synaptic channels is reduced by large
changes of the postsynaptic membrane potential (see
Discussion).
Figure 11 shows further that the relative ampli-
188
R. Hengstenberg: Common Visual Responses in Giant Vertical Cells of Calliphora
tudes of de- and hyperpolarizing responses to patterns of low contour length ( < 150°) arc about equal.
At high contour length, however, the depolarizing
response is significantly smaller than the
hyperpolarizing one (Fig. 11, 490° ; 8 penetrations. 4 4
tests). When the luminance of the moving grating is
reduced (I ≈ 0.1cd/m2; 4 penetrations, 23 tests), this
asymmetry vanishes, as in case of patterns with
low contour length. The additional suppression of
depolarizing responses at high stimulus strength
cannot be explained by synaptic saturation (see
Discussion).
These results show that VS-cells respond to
arbitrary patterns, moved anywhere in their receptive
field, if the direction and velocity of movement are
appropriate. The sign of the responses depends only
upon the direction of movement, and their size
increases with brightness, contrast and contour length
as long as the pattern texture is transmitted with full
contrast by the optics of the eye (Götz 1964, 1965).
Apparently, VS-cells collect movement signals
which originate at different locations in their
receptive field, and integrate these over large parts of
the visual field. The positional information associated
with local signals is lost by this process.
Contrast Frequency Dependence
The observation that VS-cells respond best, when
stimulated by large field movements, suggests that
they contribute to compensatory optomotor reflexes
which stabilize a fly's position and orientation in
space. When studied with periodic gratings of
spatial wavelength λ (deg), moved at different
angular velocities w (deg/s), such reflexes depend
upon the contrast frequency (CF=w/λ,), rather than
upon w. Maximum reactions were observed between
1 < CF < 5 Hz (Hassenstein and Reichardt 1956;
Fermi and Reichardt 1963; Götz 1964, 1968, Götz and
Wenking 1973; Srinivasan 1977, Wehrhahn 1978a;
Blondeau and Heisenberg 1982). Similar values
hold for various visual interneurons (cf. Hausen
1981). However, this range is not universal in flies,
since other movement dependent behaviour like the
landing reaction is best elicited between 6 <CF< 10 Hz
(Eckert and Hamdorf 1981; Wehrhahn et al. 1981),
and a few interneurons are known which respond
best at high contrast frequency (Hengstenberg 1973;
cf. Kirschfeld 1979).
In order to corroborate the suggestion that VScells contribute to velocity-dependent optomotor
reactions, the contrast frequency dependence of
R. Hengstenberg: Common Visual Responses in Giant Vertical Cells of Calliphora
their movement responses was measured in 34 penetrations. In some of these experiments, response
saturation was minimized either by reduction of
pattern luminance (I= 10 cd,/m2, n=7) or by reduction of pattern contrast (m=0.06; n=15). Figure
12 shows one of the response curves, obtained with
low contrast. Under all conditions and in all
penetrations, responses were confined to 0.01 < CF <
10 Hz. Maximal responses were obtained with CF≈2
Hz for both, depolarizing and hyperpolarizing
stimuli. By dye injection this has been shown to
hold true for the vertical cells VS1-VS4, and VS6VS9. VS5 has not been stained in these
experiments, but responds vigorously at CF=
2 Hz. No data are presently available for VS 10
and VS 11.
These results show that the majority, and most
likely all VS-cells respond best at contrast frequencies which coincide with those of different velocity
dependent behaviour. Their responses are not compatible with the higher range of contrast frequencies
which optimally elicit the landing response.
Discussion
Common Properties of Vertical Cells
Studies of the structure of Vertical Cells in Calliphora
have shown that each of these neurons has a
characteristic structure, which is largely invariant in
different individuals (Hengstenberg et al. 1982) and
even in different species (Eckert and Bishop 1978).
On the other hand, VS-cells are quite similar to one
another. This similarity of structure was found to
correlate with a similarity of basic response
properties. The results presented here agree, where
comparable, with those of previous investigations
(Hausen 1976a, b; Hengstenberg 1977, Eckert and
Bishop 1978; Soohoo and Bishop 1980). Apparently,
VS-cells respond best, when appropriately textured,
extended patterns are moved at appropriate speed,
and in appropriate direction across their receptive
fields. Their optimal contrast frequency (2 Hz)
coincides with that of velocitydependent optomotor
reactions, and there is little doubt, that VS-cells are
involved in their control.
Electrical Activity
The electrical activity of the movement sensitive
giant tangential neurons (HS, VS, CH) in the lobula
plate has been controversial ever since it has become
possible to record from these neurons. Under the
present recording conditions, VS-cells respond
consistently with graded potential chan-
189
ges, occasionally with small spikes superimposed.
They are, however, capable to generate overshooting action potentials, at least when hyperpolarized.
The regenerative mechanism of the axon membrane
seems therefore to be largely inactivated under the
prevailing conditions. The current state of the
discussion about coding in these neurons is given in
Hausen (1981).
For the present investigation, the graded potential changes in response to visual stimulation, were
used to characterize the synaptic input organization
of VS-cells. These signals depend quantitatively
upon the average membrane potential level, because
the dynamic range of synaptic channels depends
upon their driving voltage (V = E - U), and
therefore upon U. Under the prevailing recording
conditions the average resting potential of VS-cells
ranges from -40 mV to -50 mV. Consequently, a
variable degree of asymmetry of the graded potential
changes in response to visual stimulation was
observed: in records with small resting potentials
( U ≈ -40 mV), depolarizing, and hyperpolarizing
responses were about symmetrical. In records with
larger resting potentials (U≈ -50 mV), depolarizing
responses were enhanced, and hyperpolarizing ones
reduced. This is to be expected if the reversal
potential of ipsp's is, as usual, more negative than
that of epsp's.
Impulse- and Step Responses
At first sight, it may not seem sensible, to investigate
quite sophisticated interneurons like VS-cells by
simple stimuli like diffuse impulses, and steps of
light. It is in fact true that very little about the specific
response properties of any interneuron can be
revealed this way. The instantaneous, and
synchronous perturbation, set up by a light
impulse, does however yield information which is
not accessible by specific movement stimuli, where
the input elements of VS-cells are excited sequentially. Especially the impulse response has proved to
be helpful to reveal the time history of an instantaneous perturbation, travelling through the optic
lobe. The transition time of 9.7 ms from the
beginning of the photoreceptor response to the first
event (action potential) in VS-cells leaves plenty of
time for several synaptic interactions, and for signal
conduction even along very thin fibres. If a minimum
of three synaptic stages is assumed (one in the
lamina, one in the medulla, and one in the lobula
plate), each requiring maximally 0.7 ms at 22° C
(Hengstenberg 1971), then there are still 7.5 ms left
for signal conduction, and further synaptic
interactions. In order to localize more
190
R. Hengstenberg: Common Visual Responses in Giant Vertical Cells o1 Calliphora
accurately the time consuming processes during the
passage of an impulse perturbation through the optic
lobe, comparable measurements at different stages
will be necessary. Since the photoreceptor latency
contributes a large proportion of the total delay, and
since this depends strongly upon different
parameters like dark adaptation, flash intensity, and
temperature (Payne and Howard 1981), it will be
advisable to record the photoreceptor response in
such experiments as an internal standard.
The nature of the later phases of impulse- and step
responses is at present not clear, especially that of the
delayed negative going waves. They may either- he
due to a temporary interruption of excitatory input
or due to delayed inhibitory input. The fact that the
peak of the negative wave following the on-transient
in step responses is more negative than the resting
potential in darkness seems to favour the second
possibility. The same is seen in step responses of VScells of Phaenicia (Eckert and Bishop 1978, Fig. 10;
Soohoo and Bishop 1980, Fig. 6). The nature, and
functional significance of these delayed inhibitory
waves is at present obscure. They are, however, of
considerable interest because the nature and location
of `elementary movement detectors' (cf. Poggio and
Reichardt 1976; Götz and Buchner 1978; Buchner et
al. 1978; Torre and Poggio 1978; Srinivasan and
Dvorak 1980) is still unknown. As proposed previously (Hausen 1976a, 1981) two radically different organizations are possible: (a) local movement
detection takes place in the medulla, directionally
specific movement signals are conducted along
small field fibres into the lobula plate, and are integrated by widefield tangential neurons. (b) the input
signals to VS-cells are not movement-specific
but provide appropriately filtered flicker signals.
Elementary movement detection may then take place
by nonlinear interaction between adjacent synapses
on small patches (possibly dendritic `spines') of
tangential cells (cf. Torre and Poggio 1978).
Current knowledge about the response
properties, and identity of medullary neurons neither
suffices to discriminate these possibilities, nor to
suggest further ones.
The Torre-Poggio model of directionally specific
movement detection requires a fast excitatory, and a
delayed inhibitory channel. whose signals interact in
close vicinity on a patch of postsynaptic membrane.
The optimal contrast frequency of such a movement
detector can be estimated from the time delay
between excitatory and inhibitory signals by
CF ≈ 1/4·∆t (Poggio and Reichardt 1973). The
sequence of excitation, and inhibition,
as observed in impulse- and step responses is very
suggestive in this respect. An optimal contrast frequency of about 2 Hz could be predicted from the 120
ms delay of the slow inhibitory wave, which follows
the excitatory on-transient (Fig. 8). This would agree
very well with the observed optimal contrast
frequency of VS-cells (Fig. 13). Similarly, an
optimal contrast frequency of about 5.6 Hz could be
predicted from the 45 ms-delay of the fast inhibitory
wave, which seems to be superimposed upon the
excitatory off-transient (Fig. 8). The present results
on the contrast frequency dependence of VS-cells,
does not suggest the existence of such movement
detectors. However, in previous studies of various
other tangential cells of the lobula plate (Hausen
1981) and of velocity-dependent optomotor
behaviour (Wehrhahn 1978a), the contrast frequency
optimum of responses was found close to 5 Hz.
The delayed inhibitory waves could equally arise
from entirely different processes. The vertical system
of movement detection could, for example, include a
gain control mechanism, based upon feedback
inhibition of the input channels, as proposed for the
horizontal yaw torque system (Poggio et al. 1981).
When such a mechanism is synchronously
activated by light impulses or -steps, a
succession of excitatory, and inhibitory, events can
be expected. In this case, the time delay of the
inhibitory wave would reflect the time of transmission
along the inhibitory feedback loop.
The observation of delayed inhibitory waves in
impulse- and step responses of VS-cells is certainly
not sufficient to prove, or to discard any of these
possibilities. Further evidence will be required to
reveal their functional significance. The synchronous
activation of a multitude of input channels by light
flashes reveals, however. functional properties of VScells. which remain obscured, when input channels
are asynchronously activated by pattern movement.
Movement Responses
The movement responses of VS-cells, agree, where
comparable, very well with previous findings
(Dvorak ei al. 1975, 1976a, b; Hengstenberg 1977,
1981, Eckert and Bishop 1978; Soohoo and
Bishop 1980). In particular it was found that the responses of all VS-cells (except VS11) to vertical
pattern movement are very similar. From the responses
to diffuse brightness changes one should expect that
even VS-cells like VS2 show a flicker response when
stimulated with horizontal movement. The local
depolarizing transients after light-
R. Hengstenbcrg: Common Visual Responses in Giant Vertical Cells of Calliphora
191
to explain the asymmetry of de- and hyperpolarizing
responses seen in Fig. 11 at high contour length,
because the ratio of these responses should remain
constant if only the number of input elements is varied.
The voltage-dependent potassium conductance of the
axon membrane of VS-cells (Hengstenberg 1977) is
likely to produce a variable shunt conductance
which must be expected to suppress depolarizing
membrane potential changes selectively.
Although the membrane mechanisms of' nonlinear
summation are trivial, their functional significance is
not. If flies move through a variegated environment,
they are confronted with rapid changes of' overall
‘movement stimulus strength’ and may want to
correct their flight maneuvres independent of this
quantity. Nonlinear summation, as observed in VScells, is the first, simplest und fastest mechanism
which can contribute to such adaptation because it is
an inherent property of the summation process itself.
This mechanism is also economic when only the
energy expenditure of VS-cells is considered because
synaptic saturation reduces the transmembrane flow
of ions. It is, however, uneconomic with respect to the
energy expenditure of input elements. It may therefore
well be that the optomotor control network for vertical
movements contains also means of feedback inhibition
Nonlinear Summation
of the input elements as in case of the horizontal
The process of widefield integration of a multitude system (Hausen 1981).
of local movement signals in VS-cells could in principle be almost linear, if the activation of single input
channel would only produce a minute voltage change
( < 1 mV) at the output site of the dendrite. If
however single input channels produce a sizeable Contrast Frequency Dependence
voltage change ( > 1 nmV), summation of many such It was shown for 8 of the 11 VS-cells in Calliphora
signals yields a large change of membrane potential that these neurons respond best to a moving grating at
towards the equilibrium potential of the involved about 2 Hz contrast frequency. A similar value was
synaptic channels. This in turn reduces the driving previously stated for a vertical cell in Phaenicia
voltage acting upon the synaptic conductance, and (Soohoo and Bishop 1980). Responses of opposite
the current per synaptic site is smaller than in case polarity which are elicited by movement in
of single activation. Nonlinear summation may opposite directions, have the same optimum and
therefore be conceived as a means to maintain a high range of contrast frequencies (Fig. 11). This suggests
gain for singly activated input elements and to reduce that the underlying processes of movement detection
the risk of overloading the output of a neuron when have very similar properties in either direction
many input elements are active.
despite of the different synaptic mechanisms which
Varying the number of simultaneously must underly de- and hyperpolarization. The range
activated input elements of VS-cells by choice of patof contrast frequencies to which VS-cells respond
terns with different contour length (Fig. 11) has
best, coincides very well with that of velocity
revealed that widefield integration in VS-cells
dependent optomotor behaviour, and not with the
probably involves this mechanism because moved
higher range of contrast frequencies (CFmax≈ 8Hz)
gratings elicit large average potential changes. This
which optimally elicits the landing response, and
effect must be even more dramatic when instead of' the which was so far only found in one tangential neuron
of the lobula plate and in descending fibres of the
limited stimulus field (∆θ=60° ) the whole
cervical connectives.
receptive field of VS-cells (∆θ=180° ) is stimulated.
The common response properties of giant vertiSynaptic saturation is however not sufficient
on and -off should add to a net depolarizing response
even if they arise asynchronously at different sites of
the dendritic arborization. The absence of such an
average flicker response with horizontal pattern
movement (Fig. 9) is most likely due to the
transient nature of the intensity responses (Figs.
5- 8) and the much lower rate of local brightness
change with the moving pattern (dI/dt <103 cd·s/m2)
than with step stimuli (dI/dt ≈ 2 x 104 cd·s/m2). The
movement responses to different patterns have
proved the previous assumption that VS-cells
respond maximally, when extended, well textured
patterns are moved at appropriate speed, and in
appropriate direction across the receptive field of
VS-cells. This behaviour is in sharp contrast to that
of the orthopteran LGMD (lobula giant movement
detector; Rowell et al. 1977), which responds best,
when small objects are moved in arbitrary direction
anywhere in the receptive field, and which is
inhibited by large field movement of extended
patterns. Therefore, there is little doubt that VS-cells
are involved in the optomotor control of locomotion,
and that VS-cells are used by the fly to perceive its
movement in space.