Neural assemblies coordinated by cortical waves are associated with waking and hallucinatory brain states

Abstract

The relationship between sensory stimuli and perceptions is brain-state dependent: in wakefulness, suprathreshold stimuli evoke perceptions; under anesthesia, perceptions are abolished; and during dreaming and in dissociated states, percepts are internally generated. Here, we exploit this state dependence to identify brain activity associated with internally generated or stimulus-evoked perceptions. In awake mice, visual stimuli phase reset spontaneous cortical waves to elicit 3-6 Hz feedback traveling waves. These stimulus-evoked waves traverse the cortex and entrain visual and parietal neurons. Under anesthesia as well as during ketamine-induced dissociation, visual stimuli do not disrupt spontaneous waves. Uniquely, in the dissociated state, spontaneous waves traverse the cortex caudally and entrain visual and parietal neurons, akin to stimulus-evoked waves in wakefulness. Thus, coordinated neuronal assemblies orchestrated by traveling cortical waves emerge in states in which perception can manifest. The awake state is privileged in that this coordination is reliably elicited by external visual stimuli.

Keywords: CP: Neuroscience; VEP; anesthesia; feedback; hallucination; isoflurane; ketamine; state specific; traveling waves; travelling cortical waves; visual evoked potential.

Figure 1.. Spontaneous cortical waves exist during wakefulness, under isoflurane, and under ketamine, but are exclusively phase reset by visual stimuli in the awake brain

(A) Experimental design: 64-channel electrocorticography (ECoG) grid used to record local field potentials (LFPs) from the cortical surface of the left hemisphere (n = 32 mice). Thirty-two-channel laminar probes were placed in the primary visual cortex (V1) and the posterior parietal area (PPA) in n = 14 mice. Stimuli consisted of 100 ms of screen flashes (44% brightness, 33 cd/m²) delivered by a CRT monitor placed in front of the right eye (STAR Methods). (B) Five seconds of spontaneous V1 LFPs in the same mouse in the awake state (top), under isoflurane (middle), or under ketamine (bottom). (C) Forty single trials (gray) and average (red) visual evoked potentials (VEPs) from a representative mouse during wakefulness (left), under isoflurane (middle), and under ketamine (right). The vertical green line denotes stimulus onset. (D) Stimulus-evoked changes in spectral power (relative to pre-stimulus baseline) in V1 averaged over animals during wakefulness (left), under isoflurane (middle), or under ketamine (right). The vertical gray line denotes stimulus onset. (E) Intertrial phase coherence in V1 averaged over animals during wakefulness (left), under isoflurane (middle), or under ketamine (right). The vertical gray line denotes stimulus onset. (F) Single trial of LFPs filtered at 3–6 Hz recorded in a column of electrodes (each row is a single channel) at −2.0 mm lateral from bregma from the same representative mouse as in (A). (G) Intertrial average VEP filtered at 3–6 Hz from the same representative mouse as in (A) and (F).

Figure 2.. Spontaneous waves under ketamine and visual evoked waves in the awake brain both travel in the feedback direction

(A) The phase (relative to V1 shown by red diamond) averaged across trials and animals (n = 14) of the largest spontaneous 3–6 Hz spatiotemporal mode (STAR Methods) is shown by color (gray shows locations that did not meet statistical significance for phase coherence). PPA is shown by the blue diamond. The arrows show the spatial-phase gradient averaged across trials and animals. The length of the arrows encodes phase coherence (STAR Methods). (B) Same analysis as in (A) performed on the most visually responsive 3–6 Hz spatial mode (STAR Methods).

Figure 3.. V1 and PPA neurons are entrained to the 3–6 Hz visual evoked wave during wakefulness and to the spontaneous 3–6 Hz under ketamine

(A) Fraction of single units entrained to the 3–6 Hz oscillation in V1 averaged across animals (n = 11) in the awake state, as well as under isoflurane or ketamine anesthesia. Hashed bars show pre-stimulus, solid bars show post-stimulus. Error bars show standard error computed across animals. In the awake state, the fraction of entrained neurons is increased relative to the pre-stimulus baseline (χ²_V1 = 185.9201, p_V1 < 10⁻¹⁰, χ²; p_AwakeV1 < 10 ⁻¹⁰, Tukey’s post hoc). Under ketamine (p_KetV1 = 0.435, Tukey’s post hoc) or isoflurane (p_IsoV1 = 0.987, Tukey’s post hoc), no statistical differences in the fraction of entrained neurons are detected. Under ketamine, but not isoflurane anesthesia, the fraction of entrained neurons is comparable to that observed after the stimulus in the awake state (p_AwakeKetV1 = 0.999, Tukey’s post hoc). (B) Similar to (A), but for cells in the PPA. The fraction of PPA neurons entrained to the 3–6 Hz wave increases after the stimulus presentation in awake mice (χ²_PPA = 25.9103, p_PPA = 9.2883 × 10 ⁻⁶, χ²; p_AwakePPA = 0.034, Tukey’s post hoc), but not under isoflurane (p_IsoPPA = 0.998, Tukey’s post hoc) or ketamine (p_KetPPA = 1, Tukey’s post hoc). The fraction of PPA neurons entrained by the spontaneous wave under ketamine is similar to that in the post-stimulus period in the awake state (p_AwakePPA = 0.034, Tukey’s post hoc). (C) Example raster plots (top row) showing 100 trials (y axis) from a representative V1 neuron firing in relation to the visual stimulus (time t = 0 ms) in mice that are awake (left column), under isoflurane (middle column), or under ketamine (right column). Spike histogram is shown below the raster (bars show probability). The second row shows raster plots of the same neuron, after time warping to match the phases of the 3–6 Hz oscillations in the pre-stimulus period (STAR Methods). The third row shows a similar time-warped raster for the post-stimulus period 3–6 Hz.

Figure 4.. V1 and PPA cells form a transient oscillatory assembly at 3–6 Hz after stimulus presentation in awake mice and spontaneously under ketamine

(A) Single-trial (thin lines) and average (thick lines) waveforms from representative V1 (left) and PPA (right) single units in an awake mouse. (B) Average coherence of spike times from the V1 and PPA units in (A), averaged over trials (black vertical line shows the stimulus). The solid black outline represents pre-stimulus time from −600 to −100 ms before the flash onset. The dashed black outline represents post-stimulus time from 100 to 600 ms after the flash onset. The white dashed line represents the cone of influence. (C) Average coherence of all V1-PPA unit pairs over trials and animals in the −600 to −100 ms pre-stimulus (solid) and 100 to 600 ms post-stimulus (dashed) time frames in awake mice. (D) Average 3–6 Hz coherence of V1-PPA unit pairs over trials in each condition (p = 1.1959 × 10⁻¹⁷², Kruskal-Wallis; p_{Awakepre-post} < 10 ⁻¹⁰; p_Isopre-post < 10⁻¹⁰, p_Ketpre-post = 0.9999, p_{KetPreAwakePost} = 0.0668, p_{KetPreAwakePost} = 0.3514, p_{KetPreAwakePost} = 0.0668, p_{KetPreAwakePost} = 0.3514, Dunn-Sidak post hoc).

Figure 5.. Visual evoked 3–6 Hz waves have high signal-to-noise ratio, have consistent phase, and are reliably elicited only in the awake mice

(A) Distribution of ranks of the most visually responsive modes in each behavioral state. Visually responsive modes were more likely to be of lower rank in awake mice than in mice under isoflurane or ketamine (p = 1.5655 × 10⁻¹³⁰, Kruskal Wallis; p_Awake-Iso < 10⁻¹⁰, p_Awake-Ket < 10⁻¹⁰ , p_Iso-Ket = 0.8531, Dunn-Sidak post hoc). (B) Real part of the temporal component of the most visually responsive mode in a representative animal in the awake (left), isoflurane (middle), or ketamine (right) state. Each trace shows a single trial. The temporal components across trials align transiently after the stimulus in the awake animal, but not under ketamine or isoflurane. (C) Deviations of phases of the most visually responsive mode from uniform circular distribution averaged across trials and animals expressed as Z score (shading represents 95% confidence intervals). (D) Distribution of mean earth mover’s distance (EMD) between spatial amplitudes of the most visually responsive modes across trials and animals in the awake state (red), under isoflurane (purple), or under ketamine (green). Shading represents 95% confidence intervals. (E) Probability that a visually responsive mode was detected in all three states shown as a violin plot (each point is a probability estimated across all trials in a single animal) (p = 3.966 × 10⁻⁷, Kruskal-Wallis; p_Awake-Iso = 1.6435 × 10⁻⁶; p_Awake-Ket = 2.4853 × 10⁻⁵, p_Iso-Ket = 0.9999, Dunn-Sidak post hoc).

Figure 6.. Waves evoked by strong stimuli under isoflurane and ketamine resemble waves evoked by subthreshold stimuli in awake mice

(A) Fraction of electrodes in which phase of the slow waves is coherent with V1 (y axis) across trials within each mouse (individual points), when animals are shown 100 ms screen flashes of varying intensities or an LED flash, during wakefulness (left), under isoflurane (middle), or under ketamine (right). Under both ketamine and isoflurane, the number of coherent electrodes is consistently low for stimuli of all intensities. (B) At each stereotaxic location, the average phase offset of the most visually evoked mode from V1 (the red diamond) is plotted in color for lowest-intensity (1.5 cd/mm²) stimulus recorded in awake mice. The arrows depict the spatial gradient. Magnitude of the arrows show consistency of the phase angles over trials and animals. Gray locations were not statistically different from uniform circular distribution (Raleigh test) across trials and animals. The blue diamond denotes the PPA. (C) Similar phase plots for maximum-intensity stimuli (640 cd/mm²) recorded in mice under isoflurane and ketamine. Awake data (in B) are adapted from data presented in Figure S8B of Aggarwal et al. and are shown here for comparison to responses observed under ketamine and isoflurane.