Article Figures & Data
Figures
- Figure 1.
Random noise pattern used as visual stimulus. The pattern has a mean intensity of 0.5 (1 being white and 0 being black). Each pattern is multiplied by a temporal sinusoid ½cisin(2πfit), where ci and fi are the contrast and frequency, respectively, and added to a uniform background of mean intensity of 0.5. Two patterns are superimposed by displaying interleaved video lines.
- Figure 2.
Amplitude spectrum of the SSVEPs recorded at Oz. The labels denote the frequency of the response components in terms of multiples of the input frequencies. A, B, Mask (f1) and test (f2) stimulus presented separately at 10% contrast. The stimulus-driven components are clearly greater than the background EEG and are seen at integer multiples of the input frequencies f1 (blue) and f2 (red). In these panels, the resolution of the spectrum is 0.103 Hz, 10 times better than the resolution available in the main experiment, because the analysis window is 10 times as long. Note the precise isolation of the SSVEPs to specific frequencies in the spectrum. C, Concurrent presentation of mask and test of equal contrast (10%). Responses corresponding to harmonics of the stimulus frequencies are present, in addition to intermodulation terms (green), some of which are labeled. D, Concurrent presentation of the mask and test, with the mask contrast at 40% and the test contrast at 10%. The pattern of the spectrum closely resembles the responses elicited by the mask stimulus alone (cf. A). The frequencies corresponding to the test stimulus are notably absent even though the test stimulus is shown at the same contrast in both B and D.
- Figure 3.
Group mean (n = 10) of cortical current amplitude in V1. Error bars denote SEM. Colors denote mask contrast: red, 0%; black, 5%; blue, 10%; green, 20%. A, Response to the test stimulus (measured at the frequency f2). B, Response to the mask stimulus (measured at the frequency f1). C, Amplitude of the second-order sum IM term (measured at the frequency f1 + f2). Arrows indicate the point of equality between test and mask contrasts. D, Amplitude of the second-order difference term (measured at the frequency f1 − f2).
- Figure 4.
A–C, Fitting of three variants of the normalization model (A, memory-less; B, long-memory; C, short-memory) to the contrast response data from Figure 3. Colors correspond to different mask contrast: red, no mask; black, 5%; blue, 10%; green, 20%. All three models show similar goodness-of-fit in the mask (first column) and test (second column) terms. However, the short-memory model best captures the characteristics of the intermodulation terms, including the peak of the sum response function when the test and mask contrasts are equal (third column) and the diminished signal/noise at the difference frequency relative to the sum (fourth column).
- Figure 5.
Data from Figure 3 replotted as a function of the ratio of the test to mask contrast. Left and middle, The contrast response functions essentially overlap for different mask contrasts. Right, Peak of the IM response occurs at a ratio of one.
Tables
Rmax σmask_1 (%) σmask_2 σmask_3 σmask_4 p q τ (ms) Memory-less 0.05 (0.02, 0.07) 3.5 (1.7, 5.3) 7.1 (4.4, 9.7) 9.2 (5.9, 12.4) 17 (12, 21) 1.49 (1.2, 1.8) 1.52 (1.2, 1.8) n/a Long-memory 0.08 (0.03, 0.12) 3.9 (2.3, 5.4) 8.2 (6.1, 10) 11 (7.9, 13) 15 (11, 19) 1.6 (1.4, 1.8) 1.9 (1.7, 2.1) n/a Short-memory 23 (12, 34) 37 (8, 66) 66 (23, 111) 81 (30, 133) 128 (51, 204) 2.2 (1.7, 2.7) 2.4 (1.9, 2.9) 26 (16, 36) Values in parentheses are 95% confidence intervals.
