Electrophysiological measurements inside the V2 LPZ.
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A The left panel shows a parasagittal slice of an anatomical MR scan of macaque L02. The red arrows show the borders of the V1 lesion in this plane. The red-dashed line represents the position of the image plane shown on the right. In the right panel, the recording electrode can be seen to penetrate the lesioned portion of area V1 and lie with its tip (brown arrow) located in V2 near the fundus of the lunate sulcus (LS), where the horizontal meridian representation separates areas V2 and V3. The red arrow tips mark the boundary of the V1 lesion in this plane (from ∼20 eccentricity to near the midline, see Figure 2). The white dotted line outlines the cortical gray matter located in the lunate sulcus, whose posterior bank corresponds to area V2. The yellow dotted line outlines the dural boundary. Note that the yellow and the white dotted line come together near the lateral red arrow, marking the end of V1 gray matter and the beginning of the lesion. During the penetration shown, multi-unit activity (MUA) was recorded from 4 different electrode positions (positions 1 to 4 shown in panel b). Recordings were first performed in position 1 (brown arrow in the lunate fundus) and then the electrode was stepwise withdrawn 500 µm at a time, until it exited gray matter (total distance travelled ∼2 mm). Positions 2, 3, and 4 traverse the posterior bank of the lunate, positioned solidly inside the V2 LPZ. Retracting the electrode beyond position 4 led it to exit gray matter, a further confirmation that the corresponding portion of V1 had been completely lesioned (this has also been visualized anatomically with a high resolution MR image, and confirmed functionally –see Figure 1C, 2B, 4C). (*) marks the foveal representation in V1. LS = Lunate Sulcus, STS = Superior Temporal Sulcus, A = Anterior, P = Posterior, D = Dorsal, V = Ventral. B Receptive field (RF) maps of multi-unit activity (MUA) recorded during the penetrations illustrated in panel A, (positions 1–4 in panel B), and during two separate, more lateral penetrations inside the V2 LPZ (receptive fields 5, 6). RFs were mapped manually using small oriented bars (width×length = 0.50×10); see Materials and Methods). Note that electrode location #1 yields a receptive field map close to the horizontal meridian (as expected from the electrode position in the lunate fundus near the border between V2, V3). Locations 2,3,4 of the same tract were situated squarely in area V2, on the posterior bank of the lunate sulcus and the corresponding multi-unit receptive fields lie closer to the vertical meridian. Note that receptive fields overlap with the LPZ of fMRI maps (Figures 4 & 6), but are not necessarily confined to it. Importantly, responses could be elicited even when visual stimulation was entirely restricted within the scotoma confirming that they arise via a subcortical pathway that bypasses area V1. C Multi-unit RF-plot obtained from a linear array of 8 electrodes during a separate experiment from the same V1-lesioned animal as in panel B. Electrode number 3 broke on entry and recorded no multi-unit activity. Multi unit receptive fields from the interior of the LPZ (sites 4–8) are represented by solid lines, whereas receptive fields obtained outside the LPZ, in non-deafferented V2, by dashed lines (control sites 1,2). Two multi-unit receptive fields illustrated in the right inferior quadrant were obtained from a monkey without a V1 lesion (control exp #3). Note that multi unit receptive fields obtained inside the V2 LPZ are much larger than those obtained at similar eccentricities in non-deafferented V2 cortex, and on one occasion (electrode #7) bipartite. Receptive fields obtained at a second cortical depth (500 µm away) for each electrode penetration were found to be commensurate. Here only one set is illustrated. D Multi unit activity peri-stimulus-time-histogram (PSTH) centered at the onset of full-field visual stimulation (Materials and Methods) recorded from electrode position 5 (panel B). The signal was high-pass filtered at 312 Hz and thresholded at 3 standard deviations beyond the mean. Each bar height corresponds to the spike rate calculated within 100 ms bins, and averaged over 10 stimulation cycles. The multi-unit firing rate at this position increases by more than 50% when the stimulus is on. E Summary of multi unit activity elicited from all recording locations shown in panels B and C inside the V2 LPZ. To elicit visual responses under the same conditions as in the fMRI experiments (Figure 2 B), the same full field rotating checkerboard stimulus was alternated with periods of background illumination. On average, MUA increased by ∼30% during visual stimulation (p = 0.01, one-tailed paired-samples t-test) compared to baseline.