Article Figures & Data
Figures
- Figure 1.
Hypothetical orientation and contrast response functions mediating fine discrimination. A, Orientation tuning curves with preferred orientations of 0 (vertical) and 20°, shown in black and gray, respectively. The differential response to a small orientation change around vertical is much greater in the population tuned 20° away than in the population tuned to vertical, making the tilted population much more sensitive for orientation discrimination. B, Contrast response functions for the same two populations. The contrast response function is monotonic with the largest response in the population that matches the vertical target orientation and a scaled-down response for other orientations. The differential response to a change in contrast of a vertical target appears to be greater in the population tuned to 0°, than in the population tuned to 20°. But as the higher response level is likely associated with a larger variance, the tuning for contrast discrimination may have a broad peak centered on populations tuned for the target orientation.
- Figure 2.
Stimuli and methods. A, Psychophysics. Observers adapted to a full field grating (for 1 min on the first trial and for 10 s on subsequent trials) whose orientation was fixed at 0, 10, 20, 30, or 40 ° from the vertical test. Orientation and contrast increment thresholds were measured in separate blocks. The adaptor had a contrast of 90% and flickered on and off at 5 Hz. After the adaptation period, two near-vertical targets appeared 5° to the left and right of fixation. The targets were 2° in diameter and had a base contrast of 50% and a spatial frequency of 2 c/deg. They were presented for 200 ms after the offset of the adaptor. B, Steady-state EEG. Observes fixated a marker at the center of the screen. A cue indicating the task and the location of the increment was presented at fixation (here a contrast modulation task is indicated). Stimuli appeared in the lower visual field, 1.5° below and 4.5° to the left and right of fixation. The static target grating was surrounded by a “reporter” annulus with the same spatial frequency. The contrast of the reporter grating modulated on and off in a square wave at 15 and 20 Hz on the left and right, respectively, and generated unique frequency-tagged responses in the SSVEP. The faint blue circle (not part of stimulus) depicts the observer's attentional window over the target and shows attention spilling over onto the flickering annulus on the cued side. The contrasts of the reporter gratings were set at 75 and 83%, respectively, so they appeared perceptually matched in contrast and generated EEG responses of approximately equal amplitude. Each trial lasted 2 s and started with the appearance of the cue at the fixation point indicating the task (contrast or orientation discrimination) and the location (left or right) of the increment. The target and grating stimuli came on 600 ms before the start of the trial to eliminate onset transients. C, Pipeline to convert SSVEP to cortical current density data.
- Figure 3.
Average proportion correct data in the orientation and contrast discrimination tasks in the SSVEP experiment, with error bars showing SE across observers. The orientation and contrast increments were approximately equally visible for the two tasks, across different annulus orientations.
- Figure 4.
Sample cortical source waveforms from one observer and the corresponding Fourier spectra in the contrast detection condition when the observer attended to the 15 Hz stimulus on the left or the 20 Hz stimulus on the right. A, Estimated cortical source waveform in V1 in the right hemisphere. The waveforms for attend-left and attend-right conditions are shown in blue and green, respectively. B, The corresponding Fourier spectra for the attend-left (blue) and attend-right conditions (green). Strong evoked responses can be seen at the two stimulus frequencies, 15 and 20 Hz. C, Comparison of the 15 Hz response in the attend-left and attend-right condition. The central bar in the triplet is the 15 Hz response, and the flanking bars show the amplitude of the responses at 15 ± 0.5 Hz.
- Figure 5.
Cortical current density in area V1 for the attend-contrast conditions with a vertical annulus, averaged across 13 observers. The upper and lower rows show frequency-tagged responses at 15 and 20 Hz, which are the first harmonics of the annuli on the left and right, respectively. Blue and green represent the attend-left and attend-right conditions. The central bar in the triplet shows response at the stimulus frequency while the flanking bars show responses at frequencies ± 0.5 Hz on either side of this frequency and serve as a measure of noise. The response is clearly contralateral and shows modulation due to attention. The data in each panel are similar to that in Figure 4C, except that they show data averaged across all observers with error bars indicating SE.
- Figure 6.
Cross-talk matrix for the 13 observers in our study. The columns show activation in the receiving area (j) when a particular seed area (i) is activated. Activation in the seed areas (the diagonal terms) is much stronger than in other areas (off-diagonal terms), indicating that the cross talk is modest. The matrix within the dashed lines shows the cross talk for the three foveal clusters (V1, hV4, and V3A) reported in the study. The outer rows and columns show the cross talk for areas V2 and V3 that are part of the V1 cluster.
- Figure 7.
Psychophysical threshold elevation for orientation and contrast discrimination following adaption to gratings of different orientations. These data indicate that orientation discrimination is mediated by off-channel populations tuned ∼20° from the target orientation while contrast discrimination is mediated by neurons broadly tuned to the target orientation.
- Figure 8.
Scalp topography of the SSVEP response. A–C, Plot of the topography for annulus orientations of 0, 20, and 45°, respectively. The on–off 15 Hz stimulation in the left visual field generated a strong response at the first harmonic in contralateral scalp regions. Within each of A–C, the rows show responses in the orientation and contrast tasks, while the first, second, and third columns show data for the attended and unattended conditions, and the difference between them. The attended and unattended conditions for each surround orientation share the same color bar, but the scale of the color bar changes with surround orientation. The difference response has its own color bar, which has the same scale for all surround orientations.
- Figure 9.
Source-imaged measurements of EEG in functionally defined ROIs. The plots show the differential response due to attention in the orientation and contrast tasks, in areas V1, V3A, and hV4, respectively. The gray, black, and white bars plot responses for surround orientations of 0, 20, and 45°, respectively (*p < 0.05, **p < 0.01).
- Figure 10.
Source-imaged EEG measurements of the attentional modulation index. A, The modulation index was calculated as the increase in response of the attended over the unattended condition, relative to the sum of the attended and unattended condition. Modulation in V1 and area hV4 show a similar trend to the pattern obtained from psychophysics for orientation discrimination: populations tilted 20° away from the target orientation are selectively modulated in the orientation task, while area V3a does not show such a trend (*p < 0.05, **p < 0.01). For contrast discrimination, there is no discernable pattern across orientation in any of the ROIs. B, The data are replotted to isolate the effect of feature attention in each cortical area. Feature attention is the difference between the attend-orientation and attend-contrast conditions. Only area V1 shows clear peak at 20° in the orientation tuning function.
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