Activity recall in a visual cortical ensemble

Abstract

Cue-triggered recall of learned temporal sequences is an important cognitive function that has been attributed to higher brain areas. Here recordings in both anesthetized and awake rats demonstrate that after repeated stimulation with a moving spot that evoked sequential firing of an ensemble of primary visual cortex (V1) neurons, just a brief flash at the starting point of the motion path was sufficient to evoke a sequential firing pattern that reproduced the activation order evoked by the moving spot. The speed of recalled spike sequences may reflect the internal dynamics of the network rather than the motion speed. In awake rats, such recall was observed during a synchronized ('quiet wakeful') brain state having large-amplitude, low-frequency local field potential (LFP) but not in a desynchronized ('active') state having low-amplitude, high-frequency LFP. Such conditioning-enhanced, cue-evoked sequential spiking of a V1 ensemble may contribute to experience-based perceptual inference in a brain state-dependent manner.

Figure 1

Sequential spiking of neuronal ensemble in rat V1 evoked by moving spot. (a) Schematic of experimental setup. Visual stimuli were presented to left eye, multielectrode array (red dots) was inserted into right V1. Diagram of rat cortex was adopted from Paxinos and Watson, shown on 1 × 1 mm grid. A, anterior; P, posterior; L, lateral. V1, primary visual cortex. V1M, monocular region of V1; V1B, binocular region of V1; V2M, medial secondary visual cortex; V2L, lateral secondary visual cortex. λ, skull landmark lambda. (b) Multiunit RFs recorded simultaneously by 16 electrodes in an awake rat. Red ellipse, contour of Gaussian fit at one standard deviation. The two repeats of RF measurement were separated by 30 min. “*”, units excluded from analyses because of low firing rate or extraneous RF position (see Online Methods). (c) Superposition of RFs and visual stimuli. Colored ellipses, Gaussian fits of RFs. White dashed circles, “Starting point” (S) and “End point” (E) of conditioning spot. (d) PSTHs of the 14 units shown in (c) during conditioning, ordered by distance between RF center and S. Red curves, PSTHs smoothed with Bayesian adaptive regression splines.

Figure 2

Conditioning-induced increase of sequential spiking in anesthetized and awake rats. (a,b) Example spike trains of simultaneously recorded units in anesthetized (a, 13 units) and awake (b, 12 units) rats evoked by S → E conditioning (“cond”; middle) and by test cue at S before (top) and after (bottom) 100 trials of conditioning at a speed of 180° s⁻¹. Each row shows responses in 7 consecutive trials, 0 – 800 ms (a, middle row of b) or 0 – 250 ms (b, top and bottom rows) after stimulus onset. Units are ordered by distance between RF center and S. (c,d) Top three rows, pairwise cross-correlation averaged from anesthetized (c, n = 19 experiments) and awake (d, n = 18) rats with 100 trials of conditioning at 180° s⁻¹. Bottom row, difference between cross-correlation functions before and after conditioning.

Figure 3

Specificity of cue-triggered recall of spike sequence. (a,b) Cumulative histograms of Spearman CCs between test cue-evoked spike sequence and RF position, before (dotted line) and the first 2 min after (solid line) 100 trials of conditioning at 180° s⁻¹ for anesthetized (a, n = 19 experiments) and awake (b, n = 18) rats. Note that the CC distribution before conditioning (dotted line) shows a rightward shift from CC = 0, due to symmetric spread of activity evoked by the stimulus at S. (c,d) Cumulative histograms of CCs for spike sequences evoked by test cues at mid-point between S and E (c, n = 19) and at E (d, n = 18) before (dotted line) and the first 2 min after (solid line) conditioning in awake rats. (e,f) Cumulative histograms of CCs for test stimuli at S (e) and E (f) before (dotted line) and the first 2 min after (solid line) 100 trials of flashed bar conditioning (n = 18). Since there is no directionality in the bar stimulus, the starting and end points were chosen randomly for each experiment as either of the two ends of the bar, referred to as pseudo-S (e) and pseudo-E (f), respectively. Diagram in upper left region of each plot illustrates conditioning and test stimuli used in that experiment.

Figure 4

Persistence of conditioning-induced increase in sequential spiking in awake rats. (a) Difference in percentage of sequence matches before and after conditioning vs. CC threshold for test cues at S (black) and E (gray). The difference was significant at all CC thresholds below 0.75 for S (“*”, P < 0.05; “**”, P < 0.01; “***”, P < 0.001; Wilcoxon signed rank test), but not at any threshold for E. Error bar, s.e.m. (b) Time course for decay of conditioning-induced increase in percentage of matches (at CC threshold of 0.6). For S, the increase was significant at 2 min (P = 8.6 × 10⁻⁴; Wilcoxon signed rank test) and 4 min (P = 0.022), but not at 6 min (P = 0.57). For E, the effect was not significant at any time. Error bar, s.e.m. (c) Cumulative histograms of Spearman CCs at different periods after conditioning (solid lines). Dotted line, histogram before conditioning. The difference was significant at 2 and 4 min (P = 1.5 × 10⁻⁴; P = 9.8 × 10⁻³; Kolmogorov-Smirnov test), but not at 6 min (P = 0.1).

Figure 5

Dependence of sequence learning on conditioning parameters and NMDA receptor activation, measured in anesthetized rats. (a) Dependence of conditioning-induced increase in sequential firing on the number of conditioning trials. Each line represents decay time course of the increase in percentage of matches (at CC threshold of 0.6) induced by a given number of conditioning trials at a speed of 180° s⁻¹. Red: 200 trials, P = 0.002, 0.01, 0.002 at 2, 4, 6 min after conditioning, n = 15; black: 100 trials, P = 3.3 × 10⁻³, 0.044, 0.027, n = 19; magenta: 50 trials, P = 0.03, 0.30, 0.42, n = 15; cyan: 10 trials, P = 0.90, 0.39, 0.87, n = 14; Wilcoxon signed rank test. (b) Dependence on conditioning speed. Each line represents decay time course following 100 trials of conditioning at a given conditioning speed. Dash: 360° s⁻¹, P = 2.8 × 10⁻⁴, 0.01, 0.23, n = 18; Solid: 180° s⁻¹ (same as black line in a); Dotted: 60° s⁻¹, P = 0.64, 0.99, 0.41, n = 21. (c) Effect of local application of 75 μM APV. Black/gray line shows decay time course following 100 trials of conditioning at a speed of 180° s⁻¹ with/without APV. Gray line, same as black solid line in (a) and (b). Black line, P = 0.34, 0.59, 0.18, n = 18. Error bar, s.e.m.

Figure 6

Dependence of sequence recall on brain state in awake rats. (a) Brain state switch indicated by changes in LFP power spectrum. Red and blue lines, periods of synchronized (“Synch”) and desynchronized (“Desynch”) states, corresponding to red and blue clusters in (b). Insets above, sample LFP traces during periods marked by red and blue blocks. (b) Clusters in 2-D state space (Ratio 1, ratio of LFP powers between 1 – 10 Hz and 1 – 25 Hz; Ratio 2, between 15 – 30 Hz and 15 – 60 Hz). (c,d) Pairwise cross-correlations (similar to Fig. 2c, d) in synchronized (c) and desynchronized (d) states. (e,f) Cumulative histograms of Spearman CCs for test cue at S before (dotted line) and the first 5 min after (solid line) conditioning during synchronized (e) and desynchronized (f) states for the 7 experiments with frequent switching between the two brain states.