(A) Schematic drawing of imaging field (blue hexagon) overlying the map of the visual areas (left hemisphere, the map is shown as a mirror image of the cortex due to conversion in the macroscope). Four optical detectors, 1- 4, were selected (total of 464 detectors) and their signal traces are shown on the right (B). V1B, V1M: binocular and monocular areas of V1; V2MM, V2ML: mediomedial and mediolateral areas of V2; RSD: retrosplenial dysgranular. The map showed was made according to the stereotaxic map of Paxinos and Watson (2005).
(B) Optical signals of visually evoked activity from four detectors (1- 4). A grating (0.05 cycle/degree, 50w × 38h degrees of viewing angle) was constantly presented to the contralateral eye. Drifting of the grating (3 cycles/sec) was used as visual stimulus, with onset time marked by the vertical line (St). The peak of the activity occurred sequentially from detector 1 to 4, indicating a forward propagating wave (primary wave) from V1 to V2 (left broken line). A reflected wave can be seen starting from detector 3 and propagating backward to detector 1 (right broken line). The two waves can be clearly seen in the bottom images.
(C) The pseudo-color images (0.6 ms snap shots) of the initial section of the evoked response. Twelve images (time marked by the doted line under the traces) are shown from a total of 8192 frames in a 5 second recording trial. On each detector, the amplitude of the signal was converted to pseudo-color according to a linear color scale (peak red; baseline blue). The first image was taken when the evoked primary wave first appeared in the V1 monocular area, approximately 104 ms after the grating started to drift.

(A) The visually evoked wave started in V1 (2^nd image) and compressed into a thin band (4^th image). The number below each image indicates the post-stimulus time for that frame.
(B) Images from the same field of view, activity evoked by an electrical shock to V1M of the contralateral cortex. The activity first appeared at the location of a bundle of afferent callosal fibers. The intensity of the electrical stimulation was small so that the activity in the imaging side was localized without propagation; increasing the stimulus intensity could cause the activity to expand and blur the initiation site.
(C and D) The location of the compression band (center of the band marked by a black line) is shown adjacent to the activity of callosal bundle (D). The two images are from the left images marked by *. The black line in (C) is redrawn on (D).

(A) Selected images (0.6 ms snap shots) of an evoked wave. The number below each image indicates the post-stimulus time for that frame. The wave initiated at 109 ms post-stimulus time (PST, first image) and the compression sustained for ∼35 ms (136 - 171 ms).
(B) Enlarged frame at 141 ms PST (* in A) overlaid with an anatomy map of V1/V2 border. A row of detectors, starting from the initiation site of the evoked wave and perpendicular to the V1M/V2 border, is selected (boxes) for making X-T maps in C.
(C) X-T map made from signals on the row of detectors (boxes in B) showing the space-time of the activity across the V1/V2 border (white dashed line). The thin stripe at the V1M/V2 border indicates that compression sustained for a long period. Supplemental Movie 1 shows the propagation pattern of this data set.

Images of two evoked (A) and 6 spontaneous waves (B). All images were taken from the same field of view in the same animal. The evoked response showed a clear compression at V1/V2 boarder in the middle of the imaging field (*). Spontaneous events initiated from different locations and propagated in various directions; none had wave compression. Note that images in (A) and (B) are presented with different inter-frame intervals for clarity, because spontaneous events propagated faster than evoked events.

(A) Nine recording trials from one animal. In each trial four images are chosen from four stages of the evoked wave: a, initiation of the primary wave; b, full expansion of the primary wave in V1 and starting of the compression; c, full compression (band at narrowest); and d, after the compression the waves move forward into V2 and reflect backward into V1. The evoked wave patterns were stable over a period of ∼2 hours.
(B) Evoked waves from seven animals (1-7) showing similar compression patterns. The animal in (A) is shown as animal 6 in (B).

(A) Sequential snap shots (bottom row follows the top row; inter-frame interval 15 ms) during an evoked wave. The wave was initiated in the V1 area (a) and compressed into a thin band at the V1M/V2 border (b). After the compression the wave continued to propagate into the V2 area (bottom row) and compressed again at the V2/RSD border (c).
(B and C) Enlarged images of b and c from the left showing the locations of compression bands.

X-T maps from the same field of view. (A) Under control conditions. (B) 5 μM bicuculline was added to the epidural surface. Note: Subthreshold concentrations (3 - 5 μM) were used to block the GABA_A receptors. Bicuculline will cause spontaneous interictal-like spikes with a threshold concentration of 5 - 10 μM (applied epi-durally). If spontaneous interictal-like spikes occur, the animal is excluded from the data set.

(A) Initiation sites of spontaneous and evoked events from one animal. The hexagons mark the imaging field. Left: The initiation sites of 20 evoked events; all were in area V1M. Right: The initiation sites of 123 spontaneous events are distributed in various locations. (When a spontaneous event was initiated outside of the field of view, its initiation site was marked as the center of the wavefront when entering the field of view.)
(B) Propagating velocity of the waves. Left: The peak time (t, the time of peak response at that detector) was measured for all detectors (When multiple wave peaks occurred, only the first peak was counted). Bold trace: initiation site, t = 0. Standard deviation (SD, bottom equations) of the peak time is used to quantify the distribution of propagation velocity over the imaging field. Right: Distribution of SDs in evoked and spontaneous events (443 events in 5 animals). The spontaneous events (white bars) have smaller SDs on average (80% between 0 -20ms), while the evoked events (black bars) have greater SDs on average (80%, 20∼40ms).