Visual stimuli induce waves of electrical activity in turtle cortex

Abstract

The computations involved in the processing of a visual scene invariably involve the interactions among neurons throughout all of visual cortex. One hypothesis is that the timing of neuronal activity, as well as the amplitude of activity, provides a means to encode features of objects. The experimental data from studies on cat [Gray, C. M., Konig, P., Engel, A. K. & Singer, W. (1989) Nature (London) 338, 334-337] support a view in which only synchronous (no phase lags) activity carries information about the visual scene. In contrast, theoretical studies suggest, on the one hand, the utility of multiple phases within a population of neurons as a means to encode independent visual features and, on the other hand, the likely existence of timing differences solely on the basis of network dynamics. Here we use widefield imaging in conjunction with voltage-sensitive dyes to record electrical activity from the virtually intact, unanesthetized turtle brain. Our data consist of single-trial measurements. We analyze our data in the frequency domain to isolate coherent events that lie in different frequency bands. Low frequency oscillations (<5 Hz) are seen in both ongoing activity and activity induced by visual stimuli. These oscillations propagate parallel to the afferent input. Higher frequency activity, with spectral peaks near 10 and 20 Hz, is seen solely in response to stimulation. This activity consists of plane waves and spiral-like waves, as well as more complex patterns. The plane waves have an average phase gradient of approximately pi/2 radians/mm and propagate orthogonally to the low frequency waves. Our results show that large-scale differences in neuronal timing are present and persistent during visual processing.

Figure 1

Experimental procedure (a and b) and the response from single cortical locations (c–f). (a) Schematic of the set-up. A 3.5-mm-diameter region of stained cortex that encompasses parts of lateral (D2) and medial (D1) dorsal cortex was imaged onto an array of photodiodes (135 μm/pixel). The focal plane was typically ≈200 μm below the pial surface. A metal electrode was used to measure the LFP. (b) Fluorescent micrograph of a thin section of stained turtle cortex, prepared as in ref. , along with the normalized emission from the section as a function of depth below the pial surface. (c) The optical signal after low-pass filtering at 40 Hz. The large depolarization appears ≈400 ms after stimulus onset of the loom (t = 0). Insert shows the approximate location of the pixel relative to the reconstructed D1/D2 border (Methods). (d) Overlay of the optical signal at one pixel and the adjacent LFP, as indicated in Insert in c. The large depolarization in the optical signal was removed with a median filter (400 ms wide), and the relative amplitudes of the two signals were adjusted to give the maximal overlap during the stimulus interval. (e) Spectral power (logarithmic scale) in the LFP during the 1- to 2-s epoch prior to stimulation and the 1- to 2-s epoch after the onset of stimulation (2WT = 5.0; K = 3). Note the emergence of a broad peak at 18 Hz (∗) during stimulation. (f) Spectral power in the optical signal during the 1- to 2-s epoch prior to stimulation and the 1- to 2-s epoch after the onset.

Figure 2

The spatiotemporal optical signal. (a) The average of the power spectra for 72 pixels over the most active part of cortex, computed over sliding 1-s intervals (2WT = 4.0; K = 2). The logarithm of the data is false colored, with red corresponding to the maximum and purple corresponding to minimum values. The transition near 0.3 s reflects the onset of stimulus-induced activity. The star indicates the stimulus-induced band at 18 Hz. (b) The spectrum for a particular 1-s interval (bar in a). (c) The broad-band filtered optical data. We plotted every eighth frame (126 Hz) of the single-trial response. The data were spatially denoised and low-pass filtered at 60 Hz (Methods), and the large stimulus-induced depolarization was removed with a median filter (400 ms width). The color scale for each frame was separately normalized by the magnitude of the largest optical signal within that frame. Note that the vertical stripes apparent in some images are a motion artifact whose frequency content lies below 1 Hz and thus does not effect subsequent analysis of the data. Orientation and size of the optical field are as in Fig. 1a.

Figure 3

The magnitude and phase of the spatial coherence of the optical signal from a space–frequency SVD analysis (Methods). (a) The coherence averaged over a T = 3 s interval both prior to and subsequent to the onset of stimulation. In the latter case the interval encompassed the entire epoch of significant spectral power in the band at 18 Hz (Fig. 2a). The coherence was estimated at successive frequency bins (2WT = 3.0; K = 7); a value of C(f) ≅ 0.14 indicates the lack of significant coherence. (b) Phase (contour lines) and amplitude (false color) of the coherence at f = 3 Hz prior to stimulation. The image formed by the magnitude of Ṽ₁(x, y, f = 3) defines the spatial distribution of coherence, and the phase of Ṽ₁(x, y, 3) defines the temporal delays between different regions. The relative magnitude is false colored with red for maximum and blue/green for zero, and the phase is overlaid as a contour plot with π/12 radians per contour. The arrow indicates the dominant direction of the gradient. (c–f) Phase and amplitude at f = 3, 8, 18, and 22 Hz, respectively, during stimulation by a loom. (g) Phase and amplitude at 18 Hz for the next trial with the same animal. Orientation of the optical field is as in Fig. 1a.

Figure 4

Temporal evolution of the band-limited electrical activity from a demodulation procedure. (a) The magnitude and phase of the optical response centered at 18 Hz as a function of time, beginning with the onset of the loom. We plotted the demodulate, Ṽ_f(x, y, t), as a sequence of false-colored images with the magnitude of the demodulate coded by the saturation level and the phase of the demodulate coded by the hue; a single normalization is used for the entire sequence. Note the presence of approximately linear (∗ and ∗∗∗) and circular (∗∗) phase shifts. Each frame corresponds to an independent sample (9 Hz); a phase shift of 2π radians corresponds to a cycling through the chromatic scale (e.g., red → green → blue → red). The final row includes contour maps of the phase only at times t = 0.68 s (∗), t = 1.45 s (∗∗), and t = 2.22 s (∗∗∗); each contour corresponds to π/12 radians. Orientation of the optical field is as in Fig. 1a. (b) The magnitude and phase of the demodulate centered at f = 8, 10, 13, 18, and 23 Hz, filtered at W = f/4 (2WT = 2; K = 1), and averaged over a period of four cycles. The center time of the interval was t = 1.51 s (∗∗ in a).