Pharmacological approach to the visual cortex

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Pharmacological approach to the visual codex A. M. Sillito Much of the neuronal processing of visual information which takes place relies on inhibitory pathways. The principal neurotransmitter in this system is probably y-aminobutyric acid (GABA). The pharmacologist has been able to provide a specific blocker to this transmitter, in the form of bicucuUine, which has been used to elaborate a number of the modulatory and processing functions of the inhibitory pathways. The considerable attention paid by neurobiologists to the visual system reflects more than a desire to unravel the processes involved in visual perception. Many aspects of the function and development of the visual system are becoming well documented. The correlation of this with the rapidly increasing knowledge of its neuronal organization at an anatomical, physiological, and pharmacological level offers the promise of an insight into brain mechanisms that can be generalized into many other areas. The application of neuropharmacological and histochemical techniques to the investigation of the visual system introduces the possibility of identifying transmitters in specific components of the system, functionally 'dissecting out' the action of these components, and examining visual responses in their absence. The recent application of neuropharmacological techniques to the study of the visual cortex of the cat provides a case in point, and the present account will be centred on this with particular reference to the action of inhibitory processes. The real question here concerns the involvement of these inhibitory processes in the generation of the highly selective receptive field properties of visual cortical cells. To present this work it is necessary to review briefly the background ideas in the field. Orientation colnnms and visual cortical organization Following the work of Hubel and WieseP ,2, we know that individual visual cortical cells are best activated by elongated visual stimuli moving over their receptive field at a particular orientation. This sensitivity to the orientation of a visual stimulus, such as a bar of light, characterizes visual cortical cells and constitutes the basis for distinguishing a system of radially oriented columns of cells extending from white matter to the surface of the cortex. Cells in a particular column are all best © Elsewer/North-Holland Biomedical Press 1979

activated by a bar of light at a particular orientation and the columns are therefore referred to as 'orientation columns'. Cells in adjacent columns are sensitive to slightly different orientations and this variation in orientation selectivity continues from one column to the next so that a group of orientation columns covers a complete 'round the clock' range of orientations. Each location in visual space can be viewed as being represented by an area of the visual cortex containing approximately two complete sets of orientation columns, each set covering all orientations and one being dominated by the left eye and the other by the right eye (these in turn are called eye dominance or ocular dominance columns). Although cells in a given orientation column respond best to a visual stimulus at a particular orientation, they exhibit a number of additional properties that vary between cells in the column 1. These addi-tional properties include the type of response seen to flashing stimuli, selectivity to the direction of motion of the stimulus, and selectivity to the length of the stimulus. To appreciate fully the operation of the visual cortex, one has to compare the

highly selective receptive field properties of visual cortical cells with those of cells in the lateral geniculate body (LGB) that provide its afferent input. The latter cells have concentric receptive fields, consisting of a central zone well activated by a flashing spot of light, and a surrounding region which will antagonize the response of the central zone. They are also strongly excited by elongated visual stimuli such as bars of light moving over their receptive field, but, unlike cortical cells, they respond equally well to a bar of light at any orientation and direction of motion. Med~animns generating visual cortical cell stimulus selectivity Clearly, the special stimulus selectivity of visual cortical cells must be generated by synaptic interactions within the cortex, because the input cells in the LGB do not exhibit this selectivity. A very simple example is the selectivity that many visual cortical cells exhibit to one of the two directions of motion of an optimally oriented slit over their receptive field (in this context the visual stimulus is always moved at right angles to its orientation). Because a geniculate cell providing the input would respond equally well in both directions of motion, the response of the cortical cell to the geniculate input must be suppressed in one direction, but not the other. This is illustrated in Fig. 1. This scheme includes the presence of inhibitory interneurones within the cortex. We know, on electrophysiological and anatomical grounds, that input fibres from the lateral geniculate body to the cortex are excitatory in nature, and that inhibitory effects are mediated intracortically by interneurones, the non-spiny and sparsely

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1 sec Fig. 2. (a) The three principal cell types in the visual cortex and the types o f interconnections they make. The large triangle is a pyramidal cell: this makes excitatory connections with other cells. The small cell indicated by the dosed circle is a non-spiny stellate cell: this is the putative inhibitory interneurone in the visual cortex and is thought to be GABAergic. Its terminals synapse primarily in the region o f the cell body o f other cortical cells. The cell indicated by the open circle is a spiny stellate cell which makes excitatory connections with other cells. Input fibres from the lateral geniculate body are shown on the right. These are excitatory and contact spiny stellate cells, non.spiny stellares, and the dendritic process o f some pyramidal cells. Note that excitatory conneca'ons tend to be limited to dendritic spines and dendrites. To the right is a multibarr elled micropipette o f the sort used to record the activity o f the cells and apply the G A B A antagonist bicuculline to inhibitory synapses in the region o f the cell body. (b) These peristimulus histograms (32 trials, 20 ms bins) show the action o f the G A B A antagonist bicuculline (bic)on simple cell directional selectivity. The slimulus is a n optimally oriented bar of light moving forwards and then backwards over the receptive field. Responses to the two directions are shown respectively to the right and left of the dotted line.

spined stellate cells`~. Pharmacological and neurohistochemieal evidence suggests that G A B A is the transmitter used by these neuronesS~'L Their terminals end predominantly on the cell bodies, axon hillocks, and proximal dendrites, of the target cells`. This contrasts with the distribution of excitatory terminals deriving from the lateral geniculate fibres and excitatory interneurones (spiny stellates and pyramidal cells) which end almost exclusively on dendritic spines and dendrites'. This situation is summarized in Fig. 2a. The differential location of inhibitory terminals in the region of the cell body suggests that the inhibitory interneurones exert a very powerful control over the activity of the cortical cells. The action of bicucnlline Intracellular recording techniques can be used to investigate the inhibitory control of visual cortical cell responses; however, in practice the interpretation of the data has proved to be difficult. An alternative method of attacking the problem is to attempt to produce a local block of the action of the inhibitory transmitter at the target cell. In this situation the response of

the cell should reflect the nature of its excitatory input. The alkaloid bicuculline is an effective antagonist of the action of GABA in the central nervous system and can be applied locally by iontophoresis from multibarrelled micropipettes. Because of the concentration of GABAergic terminals in the region of the cell body, a multibarrelled pipette effectively positioned for recording the cell's activity would also be well located for applying bicuculline to the inhibitory synapses acting on the cell. An example of the use of this technique is shown in Fig. 2b with respect to the responses of a simple cell in the visual cortex. (This cell type is thought to receive a direct geniculate input.) As can be seen, the cell normally responds to only one of the two directions of motion; however, when bicuculline is applied to the cell its response increases, and it responds to both directions of motion, just as a geniculate cell would. This effect is reversible. Some types of visual cortical cells do not lose their directional selectivity when bicucu$line is applied g. These presumably receive a directionally specific excitatory input from other cortical cells, as is illustrated at the top of Fig. 1.

Complex cell orientation selectivity As described above, one of the most distinctive functional properties of the visual cortex is the orderly columnar representation of orientation selectivity. The neuronal processes which underly this phenomenon are obviously of great interest to the visual neurophysioiogist. Hubel and Wiesel postulated that the orientation selectivity of a given column of cells was initially set up in the simple cells by virtue of the fact that they received a convergent excitatory input from a group of geniculate cells with receptive fields extending in a line through visual space. In this situation, only a bar of the appropriate orientation would activate all of the input cells simultaneously. Then the simple cells were thought to relay, in turn, the excitatory input to other cells in the column, the complex cells, which exhibited orientation selectivity because their excitatory input was derived solely from simple cells in the same parent column. Thus, a tight vertical organization of connectivity within a column provided the basis for the common orientation selectivity of its constituent cells. Several factors militate against this 'hierarchical' model. First, we know that many complex cells receive a direct geniculate input. Secondly, the morphological correlate of the complex cells in the visual cortex is the pyramidal cell: the dendritic fields of pyramidal cells extend considerably beyond the confines of a functionally determined orientation column. Hence, complex cells will sample excitatory inputs from cells in many other orientation columns. Moreover the excitatory axon collaterals of pyramidal cells, which make synaptic contact with other pyramidal cells, extend over even greater distances. All this indicates that at the complex cell level, excitatory connections between orientation columns are extensive, and that the normal orientation selectivity of complex cells must depend on inhibitory processes limiting their responsiveness to a particular range of orientations. This can obviously be tested by using iontophoreticaUy applied bicuculline to produce a block of the inhibitory processes acting on complex cells, and ascertaining the effect of this on their orientation selectivity. In fact, in accord with expectations, the application of bicuculline produces a reduction in the orientation selectivity of all complex cells and in many cases a complete loss of selectivity, as is illustrated in Fig. 38,1°. This substantiates the view that inhibitory processes are a major factor in the generation of complex cell orientation

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Fig. 3. Action of the GABA antagonist bicuculline (bic)on complex cell orientation selectivity. Peristimulus histograms (25 trials, 50 ms bins) show responses to four different orientations as indicated above the records. Normally (upper records) this cell only responded to one (the optimal) o f the four orientations; during bicuculline application (lower records) it responded to all four.

selectivity. O n e way in which this could occur is for there to be lateral inhibitory interactions between columns selective to different optimal orientations as suggested in the diagram in Fig. 4. Lateral inhibition is known to play an important role in many other aspects of the function of sensory systems and this simply extends it to the orientation domain in the visual cortex. A n o t h e r feature of this diagram is a recurrent collateral feedback from complex cells onto the inhibitory interneurone mediating these interactions. There is anatomical evidence for this type of recurrent collateral and, in accord with this, when complex cell resting discharge levels

are substantially increased by iontophoretic application of an excitatory amino acid there is a paradoxical increase in the effectiveness of visually evoked inhibitory influences acting on them ~°. Intravenous and topical application o f

drugs Other workers have attempted to study inhibitory interactions in the cortex by using intravenous or topical application of G A B A antagonists: unfortunately, the results are very difficult to interpret because of the wide population of neurones that are simultaneously affected by these procedures. However, this type of drug

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Fig. 4. Model suggesting possible role o f inhibitory mechanisms in the generation of complex cell orientation selectivity. Pyramidal cells are represented as open triangles, inhibitory interneuroncs as closed circles. Dendritic fields are not represented. Preferred orientation o f each column of cells is shown by the bar in the circles above each column. Within the cortical representation o f each point in visual space (the area between an adjacent pair of interrupted lines) only three orientations are shown. Connections are shown with reference to the lower pyramidal cell in the central column only. The inhibitory interneuronc controlling complex cell responsiveness receives excitatory connections from columns sensitive to different orientations in cortical areas representing the same and adjacent regions o f visual space. In addition there are connections from a recurrent collateral o[the recipient complex cell and the afferent input to the column.

application procedure has provided particularly exciting results in studies of the influence of the noradrenergic input to the visual cortex 5. In this case the interest is in a system that apparently has a more widespread and less discrete influence on neuronal processes in the visual cortex and hence the intravenous and topical application procedures are more appropriate. For example, it seems that if the noradrenergic input to a region of the visual cortex of young kittens is effectively eliminated by topically applying 6-hydroxydopamine to deplete the noradrenergic terminals, the changes in connectivity of the two eye inputs, normally caused by monocular deprivation, do not take place. This, and similar experiments, suggest that in young animals the noradrenergic input is an essential permissive or even causative factor for the plasticity seen in early life, In slmmim'y Although in the limited space available it has only been possible to present a small n u m b e r of somewhat simplified examples, I hope that this account demonstrates that neuropharmacological techniques can be used to make a precise and incisive analysis of inhibitory function in the visual cortex. The effect of iontophoretic application of the G A B A antagonist bicuculline on the response properties of visual cortical cells arguably provides a more convincing demonstration of the contribution of inhibitory mechanisms to these properties than any other technique used to date. This work, together with that on the noradrenergic system, which attacks quite a different problem, emphasizes the considerable potential value of the neuropharmacological approach to the visual cortex. Reading list I Hubel. D. H. and Wie~l,T. N. (lt~62)J. Physiol. (London) 160, 106-164. 2. tlubcl. D. H. and Wiesel.T. N. (It~771Proc. R. .~)c. London, Ser. B 198, 1-59. 3. I~crscn, L. L., Mitchel, S. F. and Srinivasan, V ( 1971) J. Physiol. (London) 212, 519-534. 4. Le Vay, S. (1973)J. Comp. Neurol 150,53-86. 5. Pettigrew,J. D. and Kasamatsu,T (1978) Nature (London) 271, 761-763. 6 Ribak. C. E, (1~178)J. Neurocvtol. 7, 46,1-478 7 Sillito. A. M. (19751J. Physiol tLondon) 25(k 287-304. b;. Sillito, A. M. (19751J. Physiol. (London1 250. 3(15 329. ~. Sillito, A. M. (1977)J. Physiol tLondon) 271, 699-720, 1(1. Sillito. A. M. (1979)J. PhysioL tLondon! 2~,~. 33-53. A. M. Sillito is Jrom the Dept oJ Ph),~iology, University of Birmingham, The Medical School. Vincent Drive, Birmingham BI5 2TJ, U.K