Population Response Propagation to Extrastriate Areas Evoked by Intracortical Electrical Stimulation in V1

Abstract

The mouse visual system has multiple extrastriate areas surrounding V1 each with a distinct representation of the visual field and unique functional and connectivity profiles, which are believed to form two parallel processing streams, similar to the ventral and dorsal streams in primates. At the same time, mouse visual areas have a high degree of interconnectivity, in particular V1 sends input to all higher visual areas. The study of these direct connections can further our understanding of the cortical processing of visual signals in the early mammalian cortex. Several studies have been published about the anatomy of these connections, but an in vivo electrophysiological characterization and comparison of the transmission to multiple extrastriate areas has not yet been reported. We used intracortical electrical stimulation combined with RH1691 VSD imaging in adult C57BL/6 mice in urethane anesthesia to analyze interareal transmission from V1 to extrastriate areas in superficial cortical layers. We found seven extrastriate response sites (five lateral, two medial) in a spatial pattern similar to area maps of the mouse visual cortex and, by shifting the location of V1 stimulation, demonstrated that the evoked responses in LM and AL were in accordance with the visuotopic mappings of these areas known from anatomy and in vivo studies. These two sites, considered to be gateways to their processing streams, had shorter latencies and faster transmission speeds than other extrastriate response sites. Short latency differences between response sites, and that TTX injection into LM reduced but did not eliminate other extrastriate responses indicated that the evoked cortical activity was, at least partially, transmitted directly from V1 to extrastriate areas. This study reports on analysis of interareal transmission from V1 to multiple extrastriate areas in mouse using intracortical electrical stimulation in vivo.

Keywords: in vivo; interareal transmission; intracortical electrical stimulation; mouse; neural circuits; visual cortex; voltage-sensitive dye.

Figure 1

Cortical activity following V1 stimulation. Data from one trial, average of 12 repeated stimulations. (A) VSD signals following a 50 μA single-pulse current stimulus in V1 show statistically significant (above 3 × SD of pre-stimulus baseline) evoked activity around the stimulation site and separate secondary extrastriate spots. Frames taken at indicated delays after stimulation. (B) Time courses of the VSD fluorescence signal at labeled response locations. (C) Latency map at 25% of peak maximum at each pixel in the ROI demarcated with a red rectangle on (A). (A,C) Asterisk, stimulation site; ESL, ESM, lateral and medial extrastriate region, respectively; solid arrow, independent responses (labels A–G except D); hollow arrow, response suggested by contour protrusion, not independent here (label D); Ant, anterior; Post, posterior; Med, medial; Lat, lateral. False-color coding as shown on the color bars.

Figure 2

Shifts of B/LM and C/AL responses following changes in stimulation site location. Shifts in (B,C) locations were in accordance with known visuotopic maps of LM (inverted relative to V1 map along V1-LM border) and AL (inverted along V1-AL and LM-AL borders relative to V1 map). (A) VSD signal images taken at shown delays after stimulation, and (B) respective 50%-latency maps in the ROI (indicated by inset) in four trials in the same animal, each with a different V1 stimulation site location (asterisk). Contours (A, dashed curves) and locations (B, red dots) of the responses in the leftmost frames are overlaid on all other frames. Data from a different mouse than in Figure 1. (C) Each sub-panel: response locations from multiple trials in the same animal; data from three mice. Same-color rings belong to the same trial. Dashed lateral V1 borders are visual guides only. See Supplementary Figure 1 for additional data. (D) 50%-latency maps showing B/LM and C/AL response shifts, limited to the ROI, in five mice. Top rows show separate responses, lower rows show how these merged as the stimulation site was moved (further) in anterior or anterolateral direction (as indicated on the schematic). The responses are expected to shift in opposite direction (to each other) as the visuotopic maps of LM and AL are inverted at their common border. False-color coding as shown on the included color bars. (E) Comparison of merged and not merged B/LM and C/AL response time courses. Shown merged and not merged data are from two different trials in the same mouse. The merged and not merged time courses are very similar; the amplitude differences can be explained with variation between trials. ESL, ESM, lateral and medial extrastriate region, respectively.

Figure 3

Topographic map of extrastriate responses. (A) 50%-latency maps of three trials in the same animal, each showing the response sites from all three, marked with circles. Same color circles are from the same trial. Possible but not independent sites are marked with dashed circles. Response cluster maps in (B) were created from marked locations like these. (B) Response site locations, determined on 50%-latency maps, in multiple trials in four mice. Response clusters were defined based on simultaneously present responses and proximity. Cluster members share the same color. Striped black/red spots indicate merged black (B/LM) and red responses (C/AL). (C) Approximate spatial pattern of observed extrastriate response clusters. Overlapping B and C circles indicate that merging of responses was only observed between these two sites. ESL, ESM, lateral and medial extrastriate region, respectively.

Figure 4

Response latencies and population response transmission speeds. (A,C) Average (square) ± SD (bars) latencies after stimulation (A) and population response transmission speeds (C) at V1 and extrastriate response sites at 15% of peak maximum in all 33 mice. Population response transmission speeds calculated as separation between stimulation site and response divided by latency. Same data as in Table 1. Frames: black squares in each line show statistically significant differences between the site marked by asterisk and other sites, e.g., in (A) site B/LM was significantly different from all other sites except C/AL. (B) 15%-latency vs. separation, data from all 106 trials. Colors indicate response area. (D,E) extrastriate response latencies relative to B/LM (D) and C/AL (E) in individual trials. Data points from the same trial are connected. Blue horizontal lines were added at 0 ms latency for visual guidance. In all panels, B/LM data were restricted to unmerged cases (C/AL also present, n = 44 trials), and only these trials are shown in panels (D,E).

Figure 5

TTX in B/LM reduces response amplitude in extrastriate areas. (A) VSD images of evoked cortical activity at indicated delays after stimulation; each row shows a different trial in the same animal. First row (control), V1 electrode stimulation before application of TTX; second row, TTX electrode stimulation before application of TTX; third row (TTX), V1 electrode stimulation after application of TTX; fourth row (TTX–control), difference of V1 electrode stimulations pre- and post-application of TTX. Note that the color scales of the rows and the delays of frames in columns are slightly different. (B) Time courses of the fluorescence signal in V1 and labeled extrastriate sites. Data in (A,B) are from the same mouse. (A,B) Location “+” is an additional site to show the origin of the band appearing anterior to the visual cortex. (C) Maps of the maximum difference between V1 control and TTX trials in all four mice, demonstrating the spatial distribution of the strength of the TTX effect. (A,C) Asterisk, V1 stimulation site; triangle, TTX electrode for injection and stimulation; circles, extrastriate response sites as observed after V1 stimulation; Ant, anterior; Post, posterior; Med, medial; Lat, lateral; ESL, ESM, lateral and medial extrastriate region, respectively. False-color coding as indicated on color bars.