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Journal of Physiology (1991), 440, pp. 697-722 With 13 figures Printed in Great Britain

MECHANISMS OF INHIBITION IN CAT VISUAL CORTEX

BY NEIL J. BERMAN*, RODNEY J. DOUGLAS*t, KEVAN A. C. MARTIN* AND DAVID WHITTERIDGE* From the *MRC Anatomical Neuropharmacology Unit, Department of Pharmacology, South Parks Road, Oxford OXI 3QT and the tDepartment of Physiology, University of Cape Town Medical School, Observatory 7925, Cape Town, South Africa

(Received 6 March 1990) SUMMARY

1. Neurones from layers 2-6 of the cat primary visual cortex were studied using extracellular and intracellular recordings made in vivo. The aim was to identify inhibitory events and determine whether they were associated with small or large (shunting) changes in the input conductance of the neurones. 2. Visual stimulation of subfields of simple receptive fields produced depolarizing or hyperpolarizing potentials that were associated with increased or decreased firing rates respectively. Hyperpolarizing potentials were small, 5 mV or less. In the same neurones, brief electrical stimulation of cortical afferents produced a characteristic sequence of a brief depolarization followed by a long-lasting (200-400 ms) hyperpolarization. 3. During the response to a stationary flashed bar, the synaptic activation increased the input conductance of the neurone by about 5-20 %. Conductance changes of similar magnitude were obtained by electrically stimulating the neurone. Neurones stimulated with non-optimal orientations or directions of motion showed little change in input conductance. 4. These data indicate that while visually or electrically induced inhibition can be readily demonstrated in visual cortex, the inhibition is not associated with large sustained conductance changes. Thus a shunting or multiplicative inhibitory mechanism is not the principal mechanism of inhibition. INTRODUCTION

Inhibition is an integral facet of the organization of receptive fields in the visual cortex (Hubel & Wiesel, 1959, 1962; Bishop, Coombs & Henry, 1973; Goodwin, Henry & Bishop, 1975; Sillito, 1975; Heggelund, 1981; Palmer & Davis, 1981 a, b; Ganz & Felder, 1984). It is evoked in almost all neurones with optimal stimuli, but also appears with stimuli presented at non-optimal orientations or directions of motion. Physiological experiments using electrical stimulation show that inhibitory postsynaptic potentials (IPSPs) are evoked in virtually every neurone by electrical stimulation of cortical afferents (Li, Ortiz-Galvin, Chou & Howard, 1960; Watanabe, Konishi & Creutzfeldt, 1966; see Douglas & Martin,1990). Structural studies show that all neurones in visual cortex receive the type 2 synapses that are GABA (yMS 8326

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aminobutyric acid)-ergic and are thought to be inhibitory in function (see Martin, 1984; Somogyi, 1989; White, 1989). Thus the cortical circuits contain an intimate integration of inhibitory and excitatory processes. The mechanisms underlying neocortical inhibition remain relatively unexplored. In the absence of any structural evidence to the contrary, we assume that all cortical inhibitory mechanisms involve postsynaptic inhibition. In this paper we consider two postsynaptic mechanisms, which lie at opposite ends of the same spectrum. In the first, the principal action of the inhibitory synapses is to hyperpolarize the membrane and so prevent the excitatory potentials from reaching threshold (Coombs, Eccles & Fatt, 1955; Eccles, 1961). In the second mechanism the inhibitory synapses act to 'shunt' the excitatory current through low-resistance channels in the membrane (Coombs et al. 1955; Eccles, 1961). The two inhibitory mechanisms are nominally distinguishable through the changes they produce in the membrane potential and the input resistance of the neurone (see Douglas & Martin, 1990). For the first mechanism the shifts in membrane potential can be produced by small changes in synaptic conductance, which would have little effect on the input resistance of the neurone. In the second case the synaptic conductances are large and produce a large change in the input resistance of the neurone, without necessarily producing much alteration in the membrane potential. The so-called 'silent' inhibition (Koch & Poggio, 1985) is a special case of shunting inhibition and occurs when the reversal potential of the inhibitory synapses is the same as the membrane potential. The case of silent inhibition has received rigorous theoretical analysis (Rall, 1962, 1964, 1967; Jack, Noble & Tsien, 1975) because it is the simplest case. Shunting inhibition has also received close attention because of its 'computational' potential (see Blomfield, 1974; Koch & Poggio, 1985; Martin, 1988; Mead, 1989; Koch, Douglas & Wehmeier, 1990), which makes possible the non-linear logical operations like And-Not. Shunting can have a much more selective and local effect than polarizing inhibition, because it need not involve membrane potential changes, which would inevitably be propagated through the dendritic tree (Jack et al. 1975). Theoretical models of orientation and direction selectivity have used the nonlinearities available through the shunting synaptic mechanism (Blomfield, 1974; Koch & Poggio, 1985; Koch et al. 1990). Experimental support for a shunting mechanism, however, has only come from indirect studies where the changes in the tuning curves of cortical cells during inhibition have been interpreted as evidence for non-linear inhibitory mechanisms (Rose, 1977; Dean, Hess & Tolhurst, 1980; Morrone, Burr & Maffei, 1982; Bonds, 1989). In our intracellular recordings (Douglas, Martin & Whitteridge, 1991) we found instances where there was little, if any, change in the membrane potential during non-optimal stimulation. Such evidence could be interpreted as the consequence of 'silent' inhibition. In this paper we examine the mechanisms of inhibition in cat visual cortex by direct measurement of the input resistance of neurones during visual stimulation. In particular, we examine whether those periods of the response during which inhibition is thought to occur are associated with large or small synaptic conductances. A recent study (Koch et al. 1990) calculated the size of the conductance changes that would be visible during intracellular recording from the soma during different regimes of inhibition. Actual anatomical data were used in the simulations, which showed that

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a 100-200 % change in the conductance measured at the soma should be recorded if the inhibition were produced by a shunting mechanism. The experiments presented here thus complement this theoretical work. Our results show that large sustained changes of input conductance do not occur during periods of inhibition, whether the inhibition is evoked electrically or visually. A brief account of part of this work has appeared (Douglas, Martin & Whitteridge, 1988; Berman, Douglas & Martin, 1989). METHODS

Preparation and maintenance of animals Twenty-nine cats (2-3-3-0 kg body weight) were prepared for intracellular recording in visual cortex according to the same anaesthetic, recording and stimulation protocols described in the preceding paper (Douglas et al. 1991). Recordings The micropipettes were filled with a 4 % solution of horseradish peroxidase (HRP) in a 0-2 M-KCl solution buffered with 0-05 M-Tris, pH 7-9. This was ejected with positive current pulses ranging from 2 to 4 nA. The pipettes were bevelled to 100 MQ. In some experiments unbevelled electrodes filled with 2 M-potassium citrate were used. The visual cortex was exposed and recorded from as described by Douglas et al. (1991). Visual stimulation Visual stimuli were displayed on a high-resolution monochrome cathode ray tube display as described by Douglas et al. (1991). Two seconds of control recording were followed by 2 s during which the moving bar stimulus occurred. Any combination of stimulus size, direction, speed and contrast could be selected. For flash stimuli, the contrast chosen was the minimum necessary to produce a robust inhibitory or excitatory response. The background was set at a neutral contrast. The bar (length 8 deg) was of optimum orientation and placed inside a subfield, as determined by hand plotting of the receptive field prior to impalement. Receptive fields were classified as simple (S-type) or complex (C-type). Following Gilbert's (1977) classification of the complex receptive fields into 'special' (showing little length summation) and 'standard' (showing considerable length summation) we have divided the C-type receptive fields into Cs. and CST respectively.

Data capture and analysis Since the data presented here were collated from an evolutionary series of experiments, the data capture protocols differed in later experiments. In general, the electronic and computer methods used in this study were similar to those described by Douglas et al. (1991). In later experiments the Kemo VBF/3 anti-alias filter was replaced by a Kemo VBF/33 135 dB octave-' elliptic filter. In addition, the filter cut-off frequency and preconditioning gains and offsets were computer controlled. The protocols used to investigate responses to moving bar stimuli have been described in Douglas et al. (1991). Briefly, intracellular signals were digitized (12 bit, 2 kHz) after appropriate anti-alias filtering. Each trial consisted of a 2 s control period followed by a 2 s test period. The intertrial interval was approximately 8 s. The stimuli were presented in random order. In later experiments, intracellular responses to stationary flash stimuli and electrical stimulation of the optic radiation were recorded. The stimuli were presented in multiple trials. Each trial consisted of a control period that was one-quarter the trial duration, followed by a test period. The intertrial interval was at least 1 s longer than the trial duration. The digitizing frequency was 10 kHz for 200 ms, 5 kHz for 400 ms, and 2-5 kHz for 800 ms trials. Responses to electrical or visual stimulation were saved as averaged (number of sweeps indicated in legends) or single sweeps. Measurements of input resistance Previous attempts to discover whether there are hidden or 'silent' inhibitory processes operating have used the device of depolarizing the neurone well away from the reversal potential of the

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synapses involved in the inhibition (Innocenti & Fiore, 1974; Ferster, 1986, 1987, 1988). The interpretation of the potential changes seen in neurones depolarized to values above the spike threshold depends on the reversal potential of the synapses involved and potential to which the neurone is depolarized by intracellular injection of current. These values were not specified in the studies mentioned above, making interpretation of the potential shifts problematical. Even if the values were known, current injections that depolarize the neurone to near the excitatory postsynaptic potential (EPSP) reversal potential have several disadvantages. First, blocking the action potentials makes the interpretation of potential shifts impossible to validate by reference to changes in action potential discharge since the direction of the shift depends critically on the relation between the reversal potential of the postsynaptic potential (PSP) and the depolarized membrane potential. Second, by preventing the neurone from producing an output, the neurone has effectively been disabled. Thus the circuit in which it participates may not be operating normally. This may be significant if the output of the neurone contributes to a feedback circuit. Third, polarizing the neurone far from its resting potential seems certain to change the activity of the voltage-sensitive membrane channels (Crill & Schwindt, 1983). In particular the input resistance may change significantly (Connors, Gutnick & Prince, 1982), so that a neurone depolarized to near the EPSP reversal potential would respond quite differently to synaptic input than the same neurone at 'resting' potential. Here we used a standard technique that avoids the above difficulties and provides nearsimultaneous sampling of both input resistance and membrane potential. The technique is to inject brief constant-current pulses through the recording pipette. The amplitude of the voltage deflection gives an estimate of the input impedance and the membrane potential immediately preceding and following the pulse serves as the control potential. This is the preferred method routinely used in in vitro preparations (e.g. Connors, Malenka & Silva, 1988), but has been applied rarely in vivo (Pollen & Lux, 1966; Dreifuss, Kelly & Krnjevic, 1969). We spent between 15 and 60 min in the extracellular space outside the cell before penetration. During this time the receptive field was hand-plotted, latencies to optic radiation stimulation were measured, and the receptive field properties were quantified using peristimulus time histograms of action potential discharge. In addition, the DC offset, capacitance compensation, and bridge balance were trimmed. Once inside the cell, the balance and capacitance compensation were checked using a biphasic rectangular current pulse. If possible, further trimming of the amplifier was avoided until the pipette was withdrawn from the cell. Where necessary the bridge was rebalanced inside the neurone so that the charging of the membrane capacitance at the onset of the current appeared to begin smoothly from the resting potential (Purves, 1981). Thereafter extracellular controls were performed to detect any error in the bridge balance. The error on withdrawal was at the most 10 % of the input resistance. Sometimes premature impalement of the cell began to occur during extracellular recording. Attempts to trim the amplifier again risked injuring the neurone. Under these circumstances we ignored capacitance compensation unless the error was severe, and used biphasic rectangular injection current to balance the 'bridge'. On withdrawal extracellular controls were performed as usual. In these experiments measurement of the input resistance in vivo proved to be a great deal more difficult than the same operation performed in vitro because of the fluctuations in membrane potential. The input resistance was measured by injecting a regular train of constant-current pulse of 15-50 ms duration into the neurone and recording the resultant voltage deflections. In the case of the flashed responses the constant-current pulses occurred at the same time on separate trials, and so could be averaged. In the case of moving stimuli individual responses were much more variable. To ensure that both voltage and input resistance were adequately sampled for the limited number of trials that were possible, the train of pulses was staggered from trial to trial and so could not be averaged. In early recordings we used a very low frequency (about 6 Hz) because of concern that the visual response might be affected by the pulses. In later trials this frequency was increased without the pulses having any detrimental effect on the visual response. In all eleven cells (eight simple cells, three complex cells) in layers 2-6 were tested with this method, involving 195 trials in total. The input resistance of the recorded neurone was monitored by intracellular injection of a train of known amplitude (0-1 or 0-2 nA hyperpolarizing) constant-current pulses. Changes in the amplitude of the voltage deflection caused by each current pulse indicate changes in the neurone's input resistance. The pulses were distorted by action potentials so few measurements were possible during the regions of spontaneous or stimulus-driven discharge.

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For the purposes of graphical representation and for comparison with theoretical accounts, we have expressed the changes in input resistance as the conductance (i.e. the inverse of resistance). The conductance changes were normalized against the mean of the conductance measures obtained during the control period. Where single sweeps are available the standard error of the measurements are plotted. Where unavailable, measurements are taken from the averaged record. Other procedures were as described by Douglas et al. (1991).

RESULTS

Biophysical measurements In some neurones we derived a baseline input impedance by making complete current Vs. voltage plots. Figure IA shows current-voltage plots from a layer 6 pyramidal neurone measured at different stages through the 1 h 15 min of intracellular recording and after withdrawal from the cell. The control plot (A) indicated that the amplifier 'bridge' was balanced. This recording was obtained using an HRP-filled pipette. Similar current-voltage plots were obtained using the potassium citrate-filled pipettes (Fig. 1B-D). During the 2 h 50 min of intracellular recording the input resistance dropped from the initial value of 98 to 79 MQ and then remained constant. The control trace taken after the electrode was removed indicates that the 'bridge' was balanced (Fig. 1 C and D). The traces in Fig. I D show the charging curve and the control curve obtained after withdrawing the electrode into the extracellular space. The hyperpolarizing portions of the current-voltage plots were relatively linear, but in the depolarizing range the slope could increase, decrease, or stay the same, as has been observed both in vitro and in vivo (e.g. Dreifuss et al. 1969; Connors et al. 1982; Deisz & Prince, 1989). If recording conditions deteriorated, for example, a decrease in membrane potential, then the input impedance usually also decreased. The small polarizing currents injected into the neurone at various times did not seem to affect the input resistance. However, ionophoresis of HRP, which required 200 ms current pulses of several nanoamperes, generally had a deleterious effect on the membrane potential and input resistance of most neurones. Response to flashed and electrical stimuli Subfield excitation and inhibition were two of the cardinal properties of simple receptive fields studied originally by Hubel & Wiesel (1962) and subsequently by many investigators (see Douglas et al. 1991). Here we repeated their tests while injecting a train of constant-current pulses to measure the input resistance of the neurone. Figure 2 shows the excitatory response of a layer 5 pyramidal neurone with a single On (S1) receptive field. A light bar came on at time 0 and stayed on for the duration of the sampling period. The stimulus onset elicited a strong depolarization leading to a train' f action potentials, shown in the single trace (Fig. 2, top left). The mean latency of on t of the EPSP was 29 ms (Fig. 2, middle left), with the action potential discharge commencing at 44 ms after stimulus onset. The discharge rose to a peak frequency at about 250 ms and then declined rapidly after 300 ms (Fig. 2, bottom left). The averaged membrane potential showed the same trend (Fig. 2, middle left).

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Hyperpolarizing constant-current pulses (-01 nA) were then injected during the trial with the averaged result for six trials shown in Fig. 2 (middle right). The input resistance was 110 MQ measured 2 min after impaling the neurone and 98 MQ 22 min later, so the 0.1 nA current pulses produced a deflection of about 10 mV. The trace A

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Fig. 1. A, current-voltage plots taken from a single neurone (details in Figs 7 and 12) recorded intracellularly for 75 min. Three series of intracellular measurements were taken at different times. *, 102 MCI, 5 min after impalement; 0, 102 MCI, approximately 50 min after impalement; aL, 104 MQ, 70 min after impalement. Control (A, -1 MQ2) taken immediately after withdrawing pipette after intracellular recording. HRP-filled pipette. Neurone no. P1C4.E38. B, current-voltage plots taken from a single neurone (details Figs 10 and 13) recorded intracellularly for 2 h 50 min. Three intracellular measurements were taken at different times. *, 98 MQ, 20 min after impalement; 0, 79 MQ2, 110 min after impalement; L, 79 MQ, 130 min after impalement. Control plot (A, 2 MO) taken immediately after withdrawing pipette after intracellular recording. Potassium citratefilled pipette. Neurone no. P3C1.E32. C, voltage response of neurone shown in B to a 0-25 nA hyperpolarizing current step (45 ms duration). The trace shows the full charging curve. The displaced trace is control taken after withdrawing the pipette from the neurone. D, same voltage response as that shown in C, at expanded time base to show initial portion of charging curve.

during the onset of the stimulus obviously contains action potentials that distort the shape of the pulses. The mean change in membrane conductance is shown in the normalized plot in Fig. 2 (bottom right) and shows that the membrane conductance increased as the neurone became excited. The conductance change was between 20 and 40 %.

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The test was repeated for a bar of opposite contrast (Fig. 3). As Hubel & Wiesel (1959) observed, decreasing the luminance on an On-excitatory subfield evokes inhibition. At the onset of the dark bar the membrane hyperpolarized (Fig. 3, top left). The onset of the hyperpolarization occurred about 40 ms after stimulus onset

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Time (s) Time (s) Fig. 2. Response to optimum orientation stationary light bar flash placed over On subfield of S1-type receptive field (represented at right of window) of layer 6 pyramidal cell (onset at vertical dashed line, t = 0; duration 600 ms). Left column, response to flashed bar. Top left, single trial showing intracellular response. Middle left, averaged intracellular response, action potentials removed (eighteen trials; average membrane potential 55 mV during control, horizontal dashed line). Bottom left, spike histogram derived from above trials showing mean action potential discharge rate. Right column, conductance changes during response to light bar flash. Top right, single trial with constant current pulses applied (-0-1 nA, 20 Hz, 25 ms; single sweep). Middle right, averaged response of eight trials similar to top right, continuous trace. Action potential amplitudes are truncated as a result of averaging. Control without current injection (same as middle left), dotted trace. Bottom right, mean percentage change in conductance normalized against control values (S.E.M. bars, n = 8 trials). Rin, 120 MQ. In this and all subsequent figures: voltage window amplitudes are indicated on their left; the electrode capacitance artifact at the onset and the offset of each current pulse has been removed for clarity. For this and Figs 3-6 the timing of the flash or electrical stimulation is indicated by the continuous lines labelled stimulus. Neurone no. P5C1.C2/89.

and reached a maximum value after about 300 ms, about the time of the rapid decline in discharge seen for the excitatory response (compare with Fig. 2, bottom left). The constant-current pulses showed that the input resistance of the neurone

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decreased during the hyperpolarization (Fig. 3, middle and bottom left). The normalized conductance showed a mean increase of 10-20 %, i.e. about half as large as that seen during excitation (Fig. 2). The inhibitory response to the visual stimulus was also compared with the neurone's response to electrical activation of the thalamic afferents (Fig. 3, right-

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Fig. 3. Responses of neurone with Si-type receptive field, previously shown in Fig. 2, to dark bar flash (left column) and electrical stimulation (bolt) of afferents from OR2 (right column). Top left, averaged response (six trials) to stationary dark bar flashed over On subfield. Middle left, averaged response (six trials), dotted trace; averaged response with hyperpolarizing constant current pulses (seven trials), continuous trace. Bottom left, mean percentage change in input conductance derived from the average response above, normalized against control period mean (n = 7). Top right, response to electrical stimulation showing EPSP and action potential followed by a long-duration hyperpolarizing IPSP (single sweep). Middle right, averaged response to electrical stimulation (five trials, action potentials averaged down), dotted trace; averaged response with hyperpolarizing current pulses (-0-1 nA; five trials), continuous trace. Bottom right, mean percentage change in input conductance derived from traces above, normalized against control period (±S.E.M., n = 5). Neurone no. P5C1.C2/89.

hand traces). The typical response for the neurones recorded in this study was a brief EPSP, often leading to action potential discharge, followed by a long duration IPSP during which spontaneous activity was inhibited (Fig. 3, top right; see also Douglas & Martin, 1991). This was similar to the form originally reported by Phillips (1959) for Betz cells in motor cortex. The duration of the IPSP was comparable in time with the visually evoked hyperpolarization in this instance, although the amplitude of the

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visual response is less than half that of the electrically evoked hyperpolarization. The constant-current pulses (Fig. 3, middle right) indicate that the conductance increased about 10% during the electrically evoked IPSP (Fig. 2, bottom right), which is comparable to the visually evoked conductance changes (Fig. 2, bottom left).

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Fig. 4. Intracellular responses of a layer 5 pyramidal neurone, with an St-type receptive field. Dark bar of optimum orientation flashed over On field. Onset, duration and direction of contrast step indicated by continuous line below. Firing rate of neurone was increased by injection of depolarizing current step (+ 0-2 nA, onset at t = -0-2 s, duration 0-8 ms). Top window, intracellular membrane potential recorded during the response to injected current and flashed bar. Injected current depolarized the cell to threshold causing it to discharge action potentials. Onset of dark bar hyperpolarized the neurone and inhibited discharge. Middle window, averaged spike rate (seven trials). Discharge rate decreased (latency 40 ms) rapidly after onset of the flash. Bottom window, membrane polarizations associated with the inhibition of firing (action potentials removed). Average membrane potential during the seven sweeps above, continuous trace; average (seven trials) without injected current, dotted trace. Averages have their control mean membrane potentials superimposed (horizontal dashed line) to facilitate comparison of their hyperpolarizations. The magnitude of the flash evoked hyperpolarization was increased in the presence of depolarizing current. Neurone no. P5C1.C2/89.

It could be demonstrated that the hyperpolarization was not simply the result of withdrawal of excitation due to inhibition at a prior stage in the visual pathway. By depolarizing the neurone with a constant current of 0-2 nA we elicited a train of action potentials that was inhibited by a dark bar flashed on the On field 200 ms after the onset of the constant depolarizing current (Fig. 4: top trace, single trial; middle window, average spike rate; bottom trace, averaged membrane potential). About 23

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40 ms after the flash the membrane potential began to hyperpolarize, reaching its peak value after 140 ms, and remaining relatively hyperpolarized for the duration of the flash (600 ms). This mirrors the time course of the action potential discharge. The peak potential was 8 mV hyperpolarized relative to the baseline. When no current

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Fig. 5. Response of a layer 2 pyramidal cell to dark bar flashed over Off subfield of S1type receptive field. Top window, biphasic depolarizing/hyperpolarizing response (single sweep). Middle window, averaged biphasic response (eleven trials, action potentials removed, mean potential of control period, -49 mV). Bottom window, mean firing rate (eleven trials) reflects the trajectory of averaged membrane potential. R1n, 105 MQ. Neurone no. PlCl.C3/89.

was injected (dotted trace, ten trials, superimposed on depolarized trace for comparison) we observed a smaller peak hyperpolarization of 4-5 mV. This is what would be anticipated if the hyperpolarization were due to a postsynaptic inhibitory potential with a reversal potential negative to the baseline potential. The opposite would have occurred if the hyperpolarization was due to removal of excitation. These experiments (Figs 2-4) show clearly that the S-type fields can be excited by bars of one contrast and inhibited by bars of the opposite contrast. This characteristic is a defining characteristic of simple cell subfields (Hubel & Wiesel, 1959, 1962). The experiments demonstrate that the inhibition was a postsynaptic process, but was not associated with the large increases in conductance that would be required for shunting inhibition (Koch et al. 1990). A second example of the response to flash stimuli is shown in Figs 5 and 6 for a layer 2 pyramidal neurone. This neurone had an SI-type Off receptive field. The neurone was excited by a flashed dark bar and inhibited by a flashed light bar. The

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response to the dark bar was biphasic, with a strong initial excitation followed by a hyperpolarization of the membrane and inhibition of the discharge (Fig. 5, top and middle). The latency of the depolarization leading to impulses was 35 ms, with the first impulses occurring 38 ms after stimulus onset. The time required to reach peak

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0.6 0.6 0 -0.2 Time (s) Time (s) Fig. 6. Responses of the layer 2 pyramidal cell shown in Fig. 5 to a light bar flashed over the Off subfield (left column) and electrical stimulation from OR2 (right column). Top left, response of membrane potential to flash (single sweep). Middle left, dotted trace, averaged response of membrane potential to flash (six trials; mean potential of control period, -50 mV). Note action potentials (averaged down) occur during hyperpolarizing response. Continuous line, averaged response to flash, with current pulses (-0-2 nA; seven trials). Bottom left, percentage change in conductance measured from trials above, normalized against control values (±S.E.M., n = 7). Top right, response to electrical stimulation showing long duration hyperpolarizing IPSP (single trial). Middle right, dotted trace, averaged response to electrical stimulation (six trials). Continuous trace; averaged response to electrical stimulation, with current pulses (-0-2 nA; seven trials). Bottom right, mean percentage change (n = 7) in conductance during response above. Neurone no. PtCt.C3/89.

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spike frequency was considerably longer, occurring only 150 ms after stimulus onset (Fig. 5, bottom). This mirrors the trend seen for the layer 5 pyramidal cell illustrated in Figs 2-4. Reversing the polarity of the bar produced a sustained hyperpolarization of the neurone (Fig. 6, top left). By comparison, the amplitude of the electrically evoked IPSP was larger and showed a distinct excitatory rebound abut 400 ms after the pulse (Fig. 6, top right). The constant-current pulses used to measure the input 23-2

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0 2 Time (s) Fig. 7. S2-type receptive field of layer 6 pyramidal neurone. The latency of response to OR2 stimulation was 12 ms, indicating monosynaptic activation by lateral geniculate nucleus (LGN) afferents. In this and all subsequent figures the drawing alongside each window indicates receptive field subfield composition and type, and direction, orientation and contrast of the bar stimulus. Each window consists of a 2 s control period, followed by a 2 s test period during which the stimulus appeared and moved across the receptive field. Top two windows, action potential discharge evoked by light and dark bars moving across the S2-type receptive field in most preferred direction and orientation. These histograms (20 ms bins, five trials) were derived from intracellular records of the kind shown in window three. Note the antagonistic effects of bars of opposite contrast on the On and Off subfields. Third window, membrane potential response (single sweep) to light bar stimulus reveals the fluctuations that underlie extracellular response shown above. Rapid discharge is followed by a hyperpolarization (open arrow). This is the characteristic antagonistic response to a light bar entering the Off subfield. Filled arrow points to region

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conductance (Fig. 6, middle left and right) produced similar trends in both cases (compare Fig. 6 bottom left with right). The relative change in conductance was modest compared to that seen in Fig. 3, at best about 10-20% above the control value. This is far less than the 100-200% expected from the theoretical estimate (Koch et al. 1990). Response to moving stimuli The inhibition that was demonstrated with flashed stimuli (above) is responsible for the characteristic subfield antagonism of S-type or simple cells. The data presented here indicate that the inhibition is not associated with a large synaptic conductance that typifies shunting inhibition. The same conclusion can be drawn from experiments involving moving stimuli. The response of a layer 6 neurone with a simple or S2-type receptive field is shown in Fig. 7. The response to light and dark bars moving in the same direction showed that the subfields were excited by a bar of one contrast and inhibited by a bar of the opposite contrast. Note in particular the subfield excitation in the dark bar response indicated by the open arrow (second window). This occurred as the dark bar left the On subfield. The light bar response recorded intracellularly (third window) showed an antagonistic inhibitory response at this position; the membrane hyperpolarized. In addition, a second region of inhibition was also evident in this neurone. In response to a bar of either contrast, a hyperpolarizing inhibitory period occurred (filled arrow) towards the end of the test period. This was also seen in the response to a dark bar (not shown). Unlike the subfield inhibition described above (open arrow), this inhibition (filled arrow) was independent of the stimulus contrast: both light and dark bars or edges had the same effect. The inhibition was of relatively long duration (up to several hundred milliseconds) and was found in both simple and complex cells. It may have been due in part to non-synaptic mechanisms like the medium and long after-hyperpolarizations that are mediated by intrinsic potassium currents induced by discharges (Schwindt, Spain, Foehring, Stafstrom, Chubb & Crill, 1988). However, similar inhibition (following a discharge) has been reported in which the membrane potential simply returns to baseline and does not hyperpolarize (Creutzfeldt & Ito, 1968). This cannot be simply explained by an intrinsic post-train where contrast-independent hyperpolarizing inhibition was seen in the intracellular traces below. Fourth window, membrane potential response during injection of current pulses (-0-1 nA, 30 ms, 10 Hz) and light bar stimulus. There is no marked change in the amplitude of the voltage deflections produced by constant current pulses used to measure input resistance, even in the region of hyperpolarization. Inset shows expanded time base example of voltage deflection from control period (for this and subsequent insets: calibration bar, 5 mV, 5 ms; plot resolution, 0-22 ms). Fifth window, averaged membrane potential (three trials; average membrane potential of control period -50 mV, dashed line) clearly showing a long-lasting hyperpolarization (arrows) that follows the depolarizing response to a light bar. The later hyperpolarization (filled arrow) was also seen at this latency in the averaged response to a dark bar (not shown). Bottom window, input conductances derived from three trials similar to window 4 above. The conductances have been normalized against their mean value during the control period to facilitate comparison. There is no large and sustained change in the conductance of the neurone anywhere in the response. The current-voltage plot of Rin is shown in Fig. 1. (Bar width 0 3 deg, length 13-1 deg, velocity 3-3 deg s-'). Neurone no. P1C4.E38.

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after-hyperpolarization. In the neurone illustrated in Fig. 7 we were able to compare the conductance changes associated with the stimulus polarity-dependent and the stimulus polarity-independent hyperpolarizations. To measure the input resistance of the neurone, a train of hyperpolarizing current pulses (see expanded time base example shown in inset, Fig. 7) was injected into the neurone during the trials. The top trace shows a single trial with the current pulses. The amplitude of the voltage deflections remained approximately constant across the control and trial periods, indicating that the input resistance did not change markedly during the period of inhibition, even when the light bar was moving into an Off subfield or leaving the On subfield. The normalized measurements (window 6) for several such trials show that the percentage change in conductance through the entire trial was not larger than about 20 % of control. This result was typical for all the S-type receptive fields: in none did we detect the large sustained conductance increases that would be anticipated if the inhibition were due to a shunting inhibitory mechanism. We thus failed to find large changes in input resistance during optimal stimulation with moving stimuli. The next step was to look at the effects of non-optimal stimuli.

Direction selectivity The principal mechanism suggested for direction selectivity in the mammalian visual system is that of Barlow & Levick (1965) devised for the retina and extended to the cortex (Barlow, 1981). In their model the excitation arriving at the neurone has to be inhibited in the non-preferred direction. In the preferred direction the same inhibition is present, but occurs after the excitation, and is therefore ineffective. Evidence for inhibitory mechanisms in directionality is considerable (see Palmer & Davis, 1981 a, b; Ganz & Felder, 1984; Orban, 1984; Emerson, Citron, Felleman & Kaas, 1985; Eysel, Muche & Worgotter, 1988). Several authors have suggested that a 'shunting' mechanism is responsible for the directionality (Torre & Poggio, 1978; Emerson et al. 1985; Koch & Poggio, 1985; Emerson, Citron, Vaughn & Klein, 1987). For the excitatory load to be inhibited in the null direction the inhibition has to be effective for the duration that the stimulus moves over the receptive field. Given the size of receptive fields recorded here and the velocity of the moving stimulus, the inhibition would have to be active for several hundred milliseconds or more. If the inhibition was due to a large conductance change the conductance would have to last as long as the excitatory synapses were active (Koch & Poggio, 1985). The receptive field sizes and stimulus velocities used here ensured that any large inhibitory conductances would be active sufficiently long to be detected by the methods used here. We measured the input resistance of nine directionally sensitive neurones for the response to stimuli moving in the preferred and the non-preferred directions. An example of the directional response of a neurone with an S1-type receptive field is shown in Fig. 8. In the preferred direction (top, single trial) the neurone gave a train of action potentials (arrowed) as the stimulus moved across the receptive field. In the reverse direction (bottom, single trial) no action potentials were evoked by the stimulus and there was no deflection in the membrane potential. In both the preferred and non-preferred directions the size of the voltage deflections produced by

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the constant-current pulses were comparable. The normalized plots show that there were no large conductance changes in the non-preferred direction. Even neurones with relatively wide S-type fields can sustain their directionality, as Fig. 9 shows. The directionality of this neurone was previously shown to be Hl1Tn-9n Pirt?9 FIRMA F-I I V.4V r I kot-COVIOU

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independent of the polarity of the stimulus (Fig. 7 in Douglas et al. 1991). Also the directionality of this neurone was not associated with any clear hyperpolarization in the non-preferred direction. Simply, the number of spikes was significantly reduced (compare top single trial with bottom single trial). This neurone was thus a good candidate to test for the presence of 'silent' or shunting inhibition. Again, if shunting inhibition is involved in this directionality then it would have to act for the duration

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the stimulus moved over the receptive field, in this case for well over 1 s. The normalized measurements in the non-preferred direction (bottom trace) shows that there was no clear change in conductance from control to test. Very few neurones with complex or C-type receptive fields responded to movement only in one direction. Figure 10 shows a C-type receptive field with a directional

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Fig. 9. S3-type receptive field of a layer 5 pyramidal neurone. The latency to stimulation at OR1 was 3 0 ms and at OR2 was 2-2 ms, indicating polysynaptic activation by LGN afferents. The neurone had a strong directional preference that was contrast independent. Top set, response to preferred direction. Bottom set, response to non-preferred direction. The amplitude of the voltage deflections due to constant current pulses does not change markedly for the non-preferred directions of motion, and normalized conductance for non-preferred direction is similar to that of controls and preferred direction. Inset shows voltage deflection example. Mean membrane potential of control period, -60 mV. Rin, 123 MQl. Bar width 0 3 deg, length 12-0 deg, velocity 3-0 deg s-5. Neurone no. P4C4.E36.

preference, i.e. the response in the reverse direction was weaker than that of the optimal direction. Obviously in this reverse direction the membrane continues to depolarize. However, the diminished response to the non-optimal direction of motion might be due to a shunt that was not fully effective in preventing all the synaptic current from being delivered to the site of action potential generation. We tested this

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possibility by injecting hyperpolarizing pulses into the neurones. The input resistance of the neurone in the two directions of motion remained approximately equal (compare top single trial with bottom single trial). Note that the receptive field was large and the response endured for almost the full 2 s test period. In the non-optimal P3Cl.E32/86 H2T3.38

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direction the neurone responded for about half as long and the normalized trace shows that the conductance did not change from control to test. None of the directionally tuned neurones showed an obvious increase in the conductance during the test period when the stimulus moved in the non-preferred direction. Therefore we conclude that both simple and complex neurones show no

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evidence that their selectivity for the direction of stimulus motion is under control of the shunting type of inhibitory mechanism. We can also effectively rule out the Barlow & Levick (1965) mechanism because we found no evidence for the temporally delayed inhibition that is predicted by their model for motion in the optimal direction.

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Fig. I1. S3-type receptive field of layer 5 pyramid whose response to optimal orientations is shown in Fig. 9. Here the receptive field is stimulated with a bar at 90 deg to the optimal orientation in both directions of motion. The amplitude of the voltage deflections due to constant current pulses does not differ between the control and test period for either direction of motion (upper and lower single traces), indicating small changes in input resistance during the response to visual stimulation. The upper and lower normalized plots show small fluctuations, indicating that large sustained changes in conductances are absent or cross-orientation response. Mean membrane potential and stimulus parameters as for Fig. 9. Neurone no. P4C4.E36.

Orientation selectivity Recordings from both simple and complex cells (Douglas et al. 1991) show that, in general, distinct hyperpolarizing potentials are not evoked by the stimuli moving at non-optimal orientations. However 'hidden' or 'silent' inhibition may still be

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present but will remain undetected at resting potentials. Thus, in this section we use the method of measuring input resistance to examine the possibility that a shunting

inhibitory mechanism underlies orientation selectivity in some neurones. Figure 11 shows the response of a neurone with an S3-type receptive field when presented with a stimulus at 90 deg to the optimal orientation. The response to the P1C4.E86 H2T1.18

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optimal orientation is shown in Fig. 9. For both directions of motion at the crossorientation the amplitude of the voltage deflections produced by the current pulses did not appear to change noticeably between the control and the test period. Figure 12 shows an example of an S2-type receptive field where the bar was moved at 90 deg to the optimal orientation. The response to a bar moved at the optimal orientation is shown in Fig. 7 (a similar strong response to a dark bar moving in the optimal orientation is shown in the extracellular histograms of Fig. 1 of Douglas et al. 1991). At the cross-orientation there was no response, but the membrane did not hyperpolarize. The amplitude of the voltage deflections arising from the current pulses did not appear to change between control and the test period. The normalized conductance measurements show that there was no large sustained increase in conductance during the test period. The same tests were made for neurones with C-type or complex receptive fields. Figure 13 shows the results obtained for a CST receptive field that was also highly directional (see Fig. 10 for optimal response). In this instance depolarizing current pulses were used to measure the input resistance. The voltage deflections produced by the constant current pulses had a similar amplitude in the test as in the control

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period, indicating that the input resistance was not altering markedly. The normalized conductances in both cases (Fig. 13) also confirm that the orientation response is not associated with a large change in the conductance.

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2 0 Time (s) Fig. 13. CsT receptive field of a layer 3 neurone recorded with a potassium citrate-filled pipette. Response to optimal orientation shown in Fig. 10. Here the receptive field is stimulated with a bar at 90 deg to optimal orientation in both directions of motion. The amplitude of the depolarizing voltage deflections due to constant current pulses does not differ between control and test periods (single traces). Normalized plots show that large sustained changes in conductance are absent during cross-orientation response. Mean membrane potential and stimulus parameters as for Fig. 10. Neurone no. P3C1.E32.

DISCUSSION

Mechanisms of inhibition Simulations of synaptic inputs onto anatomically characterized cortical pyramidal cells have shown that shunting inhibition is accompanied by significant increases in somatic input conductance (Koch et al. 1990). Minimal inhibition, able to attenuate modest excitation to 0 7 of its control amplitude, produced more than a 30 % increase in input conductance. More effective attenuation to 0-2 produced an increase in

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conductance of about 200 %. However, the experiments of the present study have shown that in cortical neurones the inhibition evoked by visual stimulation is not accompanied by a large increase in input conductance. This suggests that shunting inhibition does not play a major role in the selective responses of cortical neurones. The lack of strong inhibitory conductances is surprising and raises questions about the adequacy of the technique, the nature of intracortical inhibitory processes, and even whether we are studying intracortical inhibition at all. As we have discussed in the previous paper (Douglas et al. 1991), a wealth of evidence from 30 years of extracellular studies allows us to answer the last question in the affirmative for at least three conditions: that of subfield inhibition, directionality, and orientation selectivity in neurones with simple or S-type receptive fields. In that paper we presented data from intracellular recordings which showed that cortical inhibition is not associated with large hyperpolarizing potentials. This was puzzling because electrical stimulation showed IPSPs of large amplitude. Some difference between electrically evoked and visually evoked IPSPs might be expected since electrical stimulation evokes a synchronous volley of synaptic activity, whereas during visual stimulation the synapses are activated in a more temporally dispersed manner. Even when a flashed stimulus was used, we were unable to evoke inhibitory potential changes of the magnitude seen with electrical stimulation. The conductance change associated with the electrical stimulation was no more than a 20-40% increase over the control. Smaller conductance changes would be expected with the smaller potential change that visual stimulation evoked. Thus intracortical inhibition does not appear to act by shunting the excitatory currents. Because this conclusion has far-reaching implications for the nature of cortical processing, we will spell out the potential pitfalls of the technique and the counter-examples from other work. We have suggested previously (Douglas et al. 1988) that some of our results might be explained if we were recording a withdrawal of excitation due to inhibition at an earlier stage of the circuit. This explanation would not apply to the subfield antagonism, which we demonstrated here to be a postsynaptic phenomenon (Fig. 4). We also found that classical IPSPs (Phillips, 1959) could be evoked by electrical stimulation in all the neurones we tested (e.g. Figs 3 and 6). Similarly, the observation that the GABAergic (presumed inhibitory) neurones project mainly to spiny neurones within the same vertical column, suggests that they will be activated by the same stimulus that drives their target cells (see Martin, 1988; Douglas & Martin, 1990). Thus, even in cases where we could detect little evidence of inhibitory conductances, it would be surprising if the inhibitory synapses remained completely inactive under all the stimulus conditions we used. A more reasonable explanation is that the inhibitory conductance changes were small and thus beneath the sensitivity of our sampling method (i.e. less than 15 % change in conductance). This explanation is consistent with the results of experiments where the membrane was depolarized to increase the size of the IPSP (Innocenti & Fiore, 1974; Ferster, 1986, 1987, 1988). Even with depolarizations to the EPSP reversal potential, the IPSP amplitudes increased by only a few millivolts. The method we used was limited both temporally and spatially. Large conductances that occur for only a few milliseconds will not be detected by this method, because the current pulses endure for 15-50 ms. However, theory shows that the conductance

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must endure for as long as the excitation if the neurone is to be inhibited effectively (Koch, Poggio & Torre, 1983; Koch & Poggio, 1985). Thus, for the properties of direction and orientation selectivity, the increase in inhibitory conductance must endure for several hundred milliseconds, which is certainly long enough to be detected by our methods. The method of measuring the input resistance is also limited primarily by the passive cable properties of the neurone. In particular, changes in conductance produced by proximal synapses will have a greater effect than distally placed synapses on the total conductance measured at the soma. Therefore we cannot exclude the possibility that large inhibitory conductances occurring on the distal dendrites or spine heads (Koch & Poggio, 1983) would be masked from an electrode in the soma by the impedance properties of the intervening dendritic tree. Against this interpretation, however, is the evidence that the axonal boutons of the smooth neurones (see White, 1989) are preferentially located on the soma and proximal dendritic shafts (Somogyi, Kisvarday, Martin & Whitteridge, 1983; Kisvairday, Martin, Friedlander & Somogyi, 1987). These smooth neurones form type 2 synapses and have GABA-like immunoreactivity (Krnjevic & Schwartz, 1967; Somogyi & Soltesz, 1986; Somogyi, 1989) and are generally supposed to be inhibitory in function. It is only in layer 1 that the apical tufts of pyramidal cell dendrites appear to receive a significant GABAergic input (Martin, Friedlander & Alones, 1989). The layer 1 input would be ineffectual in preventing excitation from reaching the soma from the basal dendritic tree and much of the apical dendrite, so can be excluded from present considerations. Counterexamples of large conductance changes have been detected under two conditions in cortical neurones. The first is that reported by Dreifuss et al. (1969) from in vivo recordings in cat cortex, the other is from in vitro recordings from cortical slices (e.g. Connors et al. 1982). In both experimental conditions a hyperpolarizing IPSP, produced by electrical stimulation, was found to be associated with a large and sustained increase in conductance, in some cases as high as 200 % of the initial conductance. The high conductances found in vitro have been confirmed by us in rat and cat cortical slices (Berman et al. 1989; N. J. Berman, R. J. Douglas & K. A. C. Martin, unpublished observations), using identical apparatus and methods to those used in this study. The reason for the difference between our in vivo recordings and the large conductances reported by others remains unclear. One explanation might be that the large sustained conductances are produced if the metabolic processes clearing extrasynaptic GABA are somehow compromised, as they would be in vitro. Dreifuss et al. (1969), using direct stimulation of the pericruciate cortical surface in vivo, reported a very large variance in the conductance changes (mean percentage increase 54%, S.D. 77 %). They appear to have used substantially stronger stimulus strengths than we did and they found a strong positive correlation between the input conductance of the neurone and the conductance change during the IPSP. They did not test their neurones with natural stimuli. An alternative explanation might be that there exists at all times a balance between the conductance changes produced by inhibitory synapses and those produced by excitatory synapses (Douglas, Martin & Whitteridge, 1989). The

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conductance increase due to active inhibition would then be balanced by the decreased conductance due to inactivation of the excitatory synapses. Such a balance would relegate postsynaptic inhibition to a tidying-up role, since it would not operate to inhibit full-blown excitation, but would only operate when the excitation was diminishing. Such a role for inhibition has been suggested to account for orientation tuning (Bishop et al. 1973), but seems insufficient to account for other selective properties like subfield antagonism, directionality, or end inhibition. In the event this notion of balance does not account for the lack of change in the input resistance between the control period, where most synaptic inputs are relatively quiescent, to the test period when the synapses would be at their most active. The conclusion drawn from this study is that large inhibitory conductances are not a characteristic of cortical inhibition during visual 'stimulation. This makes it difficult to explain how cortical inhibition can suppress the strong excitatory inputs that are thought to arrive from the lateral geniculate afferents, or other cortical neurones. However, there is one important issue that we cannot entirely rule out from the above experiments, and that is the possibility that the synaptic input to spines may play a larger role than we imagine. Although the percentage of spines that receive a dual excitatory-inhibitory input is small (7 %) (Beaulieu & Colonnier, 1985), it may be a particular subset of spines that receive the inhibitory input. The most obvious set would be the spines receiving an excitatory synapse from the lateral geniculate afferents. Spine-based inhibition is particularly attractive because it offers the most selective means of postsynaptic inhibition in cortical neurones (Jack et al. 1975) and is the closest the cortex could come to producing a functional equivalent of presynaptic inhibition, since there is no structural evidence for a presynaptic cortical inhibition. An inhibitory veto of excitation a la Barlow & Levick (1965) at the level of the geniculocortical input to spine heads would provide a convenient explanation for many of the experimental results reported by Douglas et al. (1991), for such an action might be relatively invisible to a somatic recording (Koch et al. 1990). Thus, it seemed important if we were to sustain the above argument about the absence of shunting inhibition, that we establish whether or not the geniculocortical input is subject to an inhibitory veto. The following paper by Dehay, Douglas, Martin & Nelson (1991) analyses this aspect. We thank John Anderson and Dr S. J. Thomas for excellent technical assistance. We are indebted to the Wellcome Trust for support. R. J. D. and N.J. B. were supported by the Medical Research Council (South Africa). The Guarantors of Brain provided additional support to R. J. D. N. J. B. was a Von Karajan Neuroscience Trust Scholar and Blaschko Scholar for part of the period of this research. K. A.C.M. is the Henry Head Research Fellow of the Royal Society.

REFERENCES

BARLOW, H. B. (1981). The Ferrier Lecture. Critical limiting factors in the design of the eye and visual cortex. Proceedings of the Royal Society B 212, 1-34. BARLOW, H. B. & LEVICK, W. R. (1965). The mechanism of directionally selective units in rabbit's retina. Journal of Physiology 178, 477-504.

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BEAULIEU, C. & COLONNIER, M. (1985). A laminar analysis of the number of round-asymmetrical and flat-symmetrical synapses on spines, dendritic trunks, and cell bodies in area 17 of the cat. Journal of Comparative Neurology 231, 180-189. BERMAN, N. J., DOUGLAS, R. J. & MARTIN, K. A. C. (1989). The conductances associated with inhibitory postsynaptic potentials are larger in visual cortical neurones in vitro than in similar neurones in intact, anaesthetized rats. Journal of Physiology 418, 107P. BISHOP, P. O., COOMBS, J. S. & HENRY, G. H. (1973). Receptive fields of simple cells in the cat striate cortex. Journal of Physiology 231, 31-60. BLOMFIELD, S. (1974). Arithmetical operations performed by nerve cells. Brain Research 69, 115-124. BONDS, A. B. (1989). Role of inhibition in the specification of orientation selectivity of cells in the cat striate cortex. Visual Neuroscience 2, 21-55. CONNORS, B. W., GUTNICK, M. J. & PRINCE, D. A. (1982). Electrophysiological properties of neocortical neurons in vitro. Journal of Neurophysiology 48, 1302-1320. CONNORS, B. W., MALENKA, R. C. & SILVA, L. R. (1988). Two inhibitory postsynaptic potentials, and GABAA and GABAB receptor-mediated responses in neocortex of rat and cat. Journal of Physiology 406, 443-468. COOMBS, J. S., ECCLES, J. C. & FATT, P. (1955). The specific ion conductances and the ionic movements across the motoneuronal membrane that produce the inhibitory post-synaptic potential. Journal of Physiology 130, 326-373. CREUTZFELDT, 0. D. & ITO, M. (1968). Functional organization of primary visual cortex neurones in the cat. Experimental Brain Research 6, 324-352. CRILL, W. E. & SCHWINDT, P. C. (1983). Active currents in mammalian central neurones. Trends in Neuroscience 6, 236-240. DEAN, A. F., HESS, R. F. & TOLHURST, D. J. (1980). Divisive inhibition involved in direction selectivity. Journal of Physiology 308, 84-85P. DEHAY, C., DOUGLAS, R. J., MARTIN, K. A. C. & NELSON, C. (1991). Excitation by geniculocortical synapses is not 'vetoed' at the level of dendritic spines in cat visual cortex. Journal of Physiology 440, 723-734. DEISZ, R. A. & PRINCE, D. A. (1989). Frequency-dependent depression of inhibition in guinea-pig neocortex in vitro by GABAB receptor feed-back on GABA release. Journal of Physiology 412, 513-541. DOUGLAS, R. J. & MARTIN, K. A. C. (1990). Neocortex. In Synaptic Organisation of the Brain, ed. SHEPPARD, G., pp. 220-248. Oxford University Press, New York. DOUGLAS, R. J. & MARTIN, K. A. C. (1991). A functional microcircuit for cat visual cortex. Journal of Physiology 440, 735-768. DOUGLAS, R. J., MARTIN, K. A. C. & WHITTERIDGE, D. (1988). Selective responses of visual cortical cells do not depend on shunting inhibition. Nature 332, 642-644. DOUGLAS, R. J., MARTIN, K. A. C. & WHITTERIDGE, D. (1989). A canonical microcircuit for neocortex. Neural Computation 1, 480-488. DOUGLAS, R. J., MARTIN, K. A. C. & WHITTERIDGE, D. (1991). An intracellular analysis of the visual responses of neurones in cat visual cortex. Journal of Physiology 440, 659-696. DREIFUSS, J. J., KELLY, J. S. & KRNJEVI6, K. (1969). Cortical inhibition and gamma-aminobutyric acid. Experimental Brain Research 9, 137-154. ECCLES, J. C. (1961). The nature of central inhibition. Proceedings of the Royal Society B 153, 445-476. EMERSON, R. C., CITRON, M. C., FELLEMAN, D. J. & KAAS, J. H. (1985). A proposed mechanism and site for cortical direction selectivity. In Models of the Visual Cortex, ed. ROSE, D. R. & DOBSON, V. G., pp. 420-431. John Wiley & Sons, Chichester, New York. EMERSON, R. C., CITRON, M. C., VAUGHN, W. J. & KLEIN, S. A. (1987). Non-linear directionally selective subunits in complex cells of cat striate cortex. Journal of Neurophysiology 58. 33-65. EYSEL, U. TH., MUCHE, T. & WORG6TTER, F. (1988). Lateral interactions at direction-selective striate neurones in the cat demonstrated by local cortical inactivation. Journal of Physiology 399, 657-675. FERSTER, D. (1986). Orientation selectivity of synaptic potentials in neurons of cat primary visual cortex. Journal of NVeuroscience 6, 1284-1301.

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Mechanisms of inhibition in cat visual cortex.

1. Neurones from layers 2-6 of the cat primary visual cortex were studied using extracellular and intracellular recordings made in vivo. The aim was t...
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