A neural circuit for spatial summation in visual cortex

Abstract

The response of cortical neurons to a sensory stimulus is modulated by the context. In the visual cortex, for example, stimulation of a pyramidal cell's receptive-field surround can attenuate the cell's response to a stimulus in the centre of its receptive field, a phenomenon called surround suppression. Whether cortical circuits contribute to surround suppression or whether the phenomenon is entirely relayed from earlier stages of visual processing is debated. Here we show that, in contrast to pyramidal cells, the response of somatostatin-expressing inhibitory neurons (SOMs) in the superficial layers of the mouse visual cortex increases with stimulation of the receptive-field surround. This difference results from the preferential excitation of SOMs by horizontal cortical axons. By perturbing the activity of SOMs, we show that these neurons contribute to pyramidal cells' surround suppression. These results establish a cortical circuit for surround suppression and attribute a particular function to a genetically defined type of inhibitory neuron.

Fig. 1. Different size tuning of three types of neurons in the visual cortex

a) Top: Schematic of the experimental setup. Center: Response of example unit to visual stimuli of three different sizes (top row: Raster plot; bottom row: PSTH). Shaded area represents period of stimulus presentation. Bottom: Average size tuning curve of 53 peak-aligned and normalized single units (6 animals, 11 recording sessions). Gray triangles and a dashed line: baseline firing rate. Inset: Average of the normalized but not peak-aligned 53 size tuning curves. b) Top left: Schematic of the experimental setup. Top right: td-Tomato expressing PV (red) with attached Alexafluor 488 filled recording pipette (green). Center: Response of PV to visual stimuli of three different sizes (top row: Raster plot; bottom row: PSTH). Bottom: Average size tuning curve (n = 11 peak-aligned and normalized size tuning curves; 3 animals). Inset: Average of the normalized but not peak-aligned 11 size tuning curves. c) As in (b) but for SOMs (n=7; 4 animals). d)– f) Distribution of SIs (left panels) and of preferred stimulus sizes (right panels) for single units (d), PVs (e) and SOMs (f). The SOM and PV data are superimposed onto the single unit data (gray, from (e)) for comparisons. All error bars are s.e.m.

Fig. 2. SOMs are selectively excited by horizontal cortical projections

a)Left: Schematic of the experimental setup. ChR2 is expressed selectively in layer 4 excitatory neurons. Recording electrodes in layer 2/3 target a SOM (orange) and a pyramidal cell (PC, black). Right: Excitatory currents simultaneously recorded in a SOM and a PC in response to photo-stimulation of layer 4 with a ramp of blue light (horizontal blue line). b)Left: Schematic of the experimental setup. As in (a) but whole cell recording electrodes in layer 2/3 target a PV (green) and a PC (black). Right: Excitatory currents simultaneously recorded in a PV and a PC in response to photo-stimulation of layer 4 as in (a). c)Summary statistics of the excitatory charge (as a fraction of that simultaneously recorded in the PC) recorded in SOMs (n=8) and PVs (n=8); p<0.05. d)Left: Schematic of the experimental setup. As in (a) except ChR2 is expressed selectively in PCs of layer 2/3. Right, excitatory (red, top traces) and inhibitory (bottom, blue traces) currents simultaneously recorded in a SOM and a PC in response to photo-stimulation of layer 2/3 with a ramp of blue light. e)Left: Summary statistics of excitatory charge (as a fraction of that simultaneously recorded in the PC) recorded in SOMs as compared to layer 2/3 PCs (n=7, p<0.05). Right: Ratio of excitation to inhibition (expressed as E/E+I) recorded in SOMs and PCs (n=10, p<0.05). All error bars are s.e.m.

Fig. 3. Suppression of PCs by SOMs as a function of the activated layer 2/3 area

a) Left: Schematic of the experimental setup Inset: Anatomical reconstruction of a biocytin-filled layer 2/3 SOM; dendrites: orange; axon: gray; dotted lines: top: border with layer 1; bottom: border with later 4. Right: Action potentials (black traces, top) recorded in the cell-attached mode in a SOM in response to light spot sizes of increasing diameter. Bottom: Excitatory currents (red traces) recorded subsequently in the whole cell voltage clamp configuration in the same SOM neuron in response to the same photo-stimuli. b) Summary graph for the spiking (black; n =14) and excitatory charge (red; n=6) of SOMs in response to light spots of five different diameters. Orange: summary statistics of the cumulative fraction of SOM dendritic arbor length within a sampled horizontal interval centered on the SOM cell body (n = 6). Inset: dendrites of the SOM illustrated in (a) but scaled to x-axis. c) Left: Schematic of the experimental setup. As in (a) but recording from PC. Right, top: Spiking of PC recorded in current clamp mode (black traces) in response to depolarizing current steps while layer 2/3 is photo-stimulated with increasingly large blue light spots. Bottom: Inhibitory currents recorded in a PC to the same light stimuli. d) Summary graph of the suppression of firing of PCs (black, n=7) and intracellularly recorded inhibitory charge (blue, n=6) to light spots of five different diameters. Photo-hyperpolarizing SOMs (blue arrow; see (e) below) reduces inhibitory charge in PCs. e) Schematic of the experimental setup. ChR2 is expressed in a fraction of layer 2/3 PCs and halorhodopsin is conditionally expressed in SOMs. Recording electrodes target a SOM (orange) and a PC (black). Full field blue light activates layer 2/3 PCs, while red light suppress SOMs. Traces: Spikes (black traces, top) recorded in the cell-attached mode in a SOM and inhibitory currents (blue traces, bottom) simultaneously recorded in a voltage clamped PC in response to blue light photo-stimulation (blue bar) of layer 2/3. Simultaneous illumination with red light (red bar, right panel) to photo-hyperpolarize SOMs abolishes SOM firing (left) and reduces inhibitory currents in the PC (right; see also blue arrow in (d)). f) Summary graph for halorhodopsin-mediated reduction of SOM firing (n=6) and concomitant reduction in inhibitory charge (IPSC) in layer 2/3 PCs (n=8).

Fig. 4. SOMs contribute to size tuning of layer 2/3 PCs

a) Schematic of the experimental setup. b) Section of the visual cortex of a SOM-CRE;Rosa-LSL-tdTomato mouse injected with AAV-flexed-Arch-GFP. All SOM-CRE cells express tdTomato (red) and infected neurons also express Arch-GFP (green). c) Size tuning of an isolated unit in control (black) and during photo-hyperpolarization of SOMs (orange). Dashed line is baseline firing rate; Inset: response ratio for this example unit. All error bars are s.e.m. d) Average peak-aligned and scaled size tuning curves for 28 isolated single units (3 animals; 7 recording sessions). Black: Control conditions; orange: SOM hyperpolarization. Inset: Average peak-aligned and control normalized size tuning curve where the SOM-hyperpolarization condition (orange curve). Note the lack of facilitation at the preferred or smaller size. All error bars are s.e.m. e) Scatter plot: SI under control conditions (x-axis) plotted against SI under SOM hyperpolarization (y-axis) for each of the 28 units. Blue data points are units that are size tuned at baseline (n=24, SI > 0; p<0.05). Gray data points are units not sized tuned at baseline. Stars are units that showeda significant reduction in SI (n=10; p<0.05). Histograms beside×and y-axes show SI distribution under control and SOM hyperpolarization, respectively. Oblique histogram illustrates the distribution of changes in SI with SOM hyperpolarization. f) The ratio of the average response during SOM photo-hyperpolarization to the average response under control conditions plotted against stimulus size relative to peak. Same units as (d) Inset: the same ratio without subtracting the baseline firing rate. g) Schematic illustration of the cortical circuit in layer 2/3 contributing to surround suppression. As a visual stimulus expands (shades of gray), recruitment of adjacent PCs increases SOM excitation through horizontal axons (horizontal arrows).