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An Emergent Model of Orientation Selectivity in Cat Visual Cortical Simple Cells

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An Emergent Model of Orientation Selectivity in Cat Visual Cortical Simple Cells The Journal of Neuroscience, August 1995, 75(8): 5448-5465 An Emergent Model of Orientation Selectivity in Cat Visual Cortical Simple Cells David C. Sometql Sacha B. Nelson,2 and Mriganka Surl ‘Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 and *Department of Biology and Center for Complex Systems, Brandeis University, Waltham, Massachusetts 02254 It is well known that visual cortical neurons respond vigor- thalamocortical inputs (feedforward excitation) or a combination ously to a limited range of stimulus orientations, while their of intracortical inhibition and thalamocortical convergence. primary afferent inputs, neurons in the lateral geniculate nu- However, “feedforward” and “inhibitory” models (see Fig. 1) cleus (LGN), respond well to all orientations. Mechanisms are inconsistent with key pieces of physiological data and ne- based on intracortical inhibition and/or converging thala- glect the effects of connectionsfrom intracortical excitatory neu- mocottical afferents have previously been suggested to un- rons, which provide the majority of synapsesonto cells in all derlie the generation of cortical orientation selectivity; how- cortical layers (LeVay and Gilbert, 1976; Peters and Payne, ever, these models conflict with experimental data. Here, a 1993; Ahmed et al., 1994). I:4 scale model of a 1700 pm by 200 pm region of layer IV Feedforward models suggestthat cortical neuronsobtain ori- of cat primary visual cortex (area 17) is presented to dem- entation selectivity from elongatedpatterns of converging LGN onstrate that local intracortical excitation may provide the inputs (see Fig. la) (Hubel and Wiesel, 1962; Ferster, 1987). Ex- dominant source of orientation-selective input. In agreement perimentsqualitatively supportthis idea; on average,cortical sim- with experiment, model cortical cells exhibit sharp orienta- ple cell subfieldsare elongatedalong an axis parallel to the pre- tion selectivity despite receiving strong iso-orientation in- ferred responseorientation (Jonesand Palmer, 1987; Chapmanet hibition, weak cross-orientation inhibition, no shunting in- al., 1991). But for many simplecells subfieldlength-to-width ra- hibition, and weakly tuned thalamocortical excitation. Sharp tios (aspectratios) are quantitatively insufficient to accountfor the tuning is provided by recurrent cortical excitation. As this sharp orientation selectivity exhibited (Watkins and Berkley, tuning signal arises from the same pool of neurons that it 1974; Jonesand Palmer,1987). Reportsof low meanaspect ratios excites, orientation selectivity in the model is shown to be (Chapmanet al., 1991; Pei et al., 1994) are alsoinconsistent with an emergent property of the cortical feedback circuitry. In feedforward modelsof orientationselectivity. Weakly biasedfeed- the model, as in experiment, sharpness of orientation tuning forward inputs can be sharpenedby using high firing thresholds is independent of stimulus contrast and persists with si- (the “iceberg” effect; Creutzfeldt et al., 1974b), but this mecha- lencing of ON-type subfields. The model also provides a uni- nism incorrectly predicts broadening of orientation tuning with fied account of intracellular and extracellular inhibitory increasingstimulus contrast (Sclar and Freeman, 1982; Wehmeier blockade experiments that had previously appeared to con- et al., 1989). Pure feedforward models also cannot account for flict over the role of inhibition. It is suggested that intra- the loss of orientation selectivity observedunder iontophoresisof cortical inhibition acts nonspecifically and indirectly to bicuculline, a GABA, antagonist,which reducesinhibition over maintain the selectivity of individual neurons by balancing a localized population of cortical neurons(Sillito, 1975; Tsumoto strong intracortical excitation at the columnar level. et al., 1979; Sillito et al., 1980). [Key words: visual cortex, orientation selectivity, com- Mechanismsthat utilize shunting (“divisive”) inhibition (e.g., putational neuroscience, recurrent excitation, intracortical Koch and Poggio, 1985; Carandini and Heeger, 1994), hyper- inhibition, cortical circuits] polarizing (“subtractive”) inhibition at nonpreferredorientations (see Fig. lb) (e.g., Wehmeier et al., 1989; Wiirgotter and Koch, Visual cortical orientation selectivity is one of the most thor- 1991), and/or inhibition between spatially overlapping cells with oughly investigatedreceptive field propertiesin all of neocortex; opposite contrast polarity (e.g., ON-OFF) (Heggelund, 1981) however, its underlying neural mechanismshave yet to be fully can sharpentuning in cells that have mildly oriented thalamo- illuminated (see Ferster and Koch, 1987; Martin, 1988). Cur- cortical inputs, can produce contrast-invariant orientation tuning, rently favored models rely on either oriented convergence of and can account for bicuculline-induced tuning loss. However, inhibitory modelsare inconsistentwith other experimental data. Received Jan. 9, 1995; revised Mar. 3, 1995; accepted Mar. 8, 1995. Shunting inhibition plays little role in orientation selectivity This work supported by the McDonnell-Pew Center for Cognitive Neuro- (Douglas et al., 1988; Berman et al., 1991; Dehay et al., 1991; science at MIT, NIH Grants MH-10671, EY-06363, EY-07023; and a grant of High Performance Computing time from the Project Scout at MIT-LCS, spon- Ferster and Jagadeesh,1992), and silencing ON-type subfields sored under ARPA Contract MDA 972-92-J-1032. does not disrupt sharp orientation selectivity (Schiller, 1982; Correspondence should be addressed to David C. Somers, Department of Sherk and Horton, 1984). Inhibitory postsynaptic potentials Brain and Cognitive Sciences, MIT, E25-618, 45 Carleton Street, Cambridge, MA 02139. (IPSPs) are evoked strongly by stimuli presentedat the preferred Copyright 0 1995 Society for Neuroscience 0270.6474/95/155448-18$05.00/O orientation, while cross-orientation stimuli evoke weak IPSPs The Journal of Neuroscience, August 1995, 7~178) 5449 Feedforward Inhibitory Recurrent Figure 1. Models of visual cortical orientation selectivity. a, In feedforward models all “first-order” cortical neurons (triangle, excitatory; hexagon, inhibitory) receive converging input (gray arrow) from a population of LGN neurons that cover a strongly oriented region of visual space. The bandwidth or sharpness of a cortical cell’s orientation tuning is determined by the aspect ratio of its LGN projection. b, Many inhibitory models employ a mild feedforward bias to establish the initial orientation preference of cortical neurons and utilize inhibitory inputs (white arrows), from cortical neurons preferring different orientations, to suppress nonpreferred responses. Here, we present a model, c, in which recurrent cortical excitation (hluck arrows)among cells preferring similar orientations, combined with iso-orientation inhibition from a broader range of orientations, integrates and amplifies a weak thalamic orientation bias, which is distributed across the cortical columnar population. (Ferster, 1986; Douglas et al., 1991a; for a differing view, see layer IV circuitry under a 1700 pm by 200 p,m patch of Pei et al., 1994). Such inhibitory tuning conflicts with cross- cortical surface and was composed of more than 3000 spik- orientation inhibitory models (Bishop et al., 1973; Morrone et ing neurons with over 180,000 synapses.Cortical excitatory al., 1982), and strong iso-orientation suppression poses problems and inhibitory neurons were modeled separately using intra- even for models that use other forms of hyperpolarizing inhi- cellular parameters from regular-spiking and fast-spiking bition to sharpen thalamocortical input (e.g., Wiirgotter and Koch, 1991). Furthermore, recent results from our laboratory neurons (Connors et al., 1982; McCormick et al., 1985). Ret- conflict with all orientation models that rely heavily on direct inal and geniculate cell populations spanning a 4” by 4” mo- inhibitory input. Whole-cell, intracellular blockade of inhibition nocular patch of the central visual field were also represent- had negligible effect on sharpnessof orientation tuning of ed. The model was organized as three sequential layers: ON blocked cells (Nelson et al., 1994). These results also appear to and OFF retinal ganglion cells, ON and OFF lateral genic- conflict with reports that orientation tuning can be abolishedby ulate nucleus neurons, and simple cells in layer IV of cortical bicuculline-inducedextracellular inhibitory blockade (Sillito et area 17. Oriented flashed bar stimuli were presented to the al., 1980; Nelson, 1991). retinal cells and the responseproperties of cortical cells were Here, we demonstratethat this apparent paradox can be re- solved by considering the effects the two inhibitory blockades studied. have on the tuning of cortical excitatory inputs. Our computer Retinal ganglion cells. Four hundred forty-one ON and simulationsalso demonstratethat local, recurrent cortical exci- 441 OFF retinal ganglion cells (RGCs) were configured as tation can generate sharp, contrast-invariant orientation tuning two 21 by 21 arrays of neurons with center-surround recep- in circuits that have strong iso-orientationinhibition and weakly tive field antagonism. Center fields were 30’ wide and the oriented thalamocorticalexcitation (seeFig. 1~). This modelpri- center-to-center spacing between neighboring receptive marily addressesthe circuitry within a single cortical “hyper- fields was 12’ of visual angle.
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