Percent change in orientation tuning width vs. percent change in tuned response amplitude.

<p>In these scatter plots, the <i>x</i>-axis represents percent change in tuned response amplitude and the <i>y</i>-axis represents percent change in HWHH. A. Medium vs. high contrast, mixed contrasts condition. B. Low vs. high contrast, mixed contrasts condition. C. Low vs. medium contrast, mixed contrasts condition. In these three scatter plots, most data points are located in the quadrant delimited by 0 and 100% on both x and y axis, indicating that most cells showed both reduced response amplitude and reduced HWHH when contrast was decreased. However, the two variables were not significantly correlated. D. Medium vs. high contrast, constant contrast condition. E. Low vs. high contrast, constant contrast condition. F. Low vs. medium contrast, constant contrast condition. In the scatter plots in E and F, most data points can be found in the quadrant delimited by 0 and 100% on both x and y axes, indicating that most cells showed both reduced response amplitude and reduced HWHH when contrast was decreased. This is not the case for the scatter plot in D, reflecting the fact that orientation tuning width was not different, on average, between medium and high contrast after adaptation. There is, however, a significant inverse relationship between the two variables in this case. The line corresponds to the linear relationship between the two variables.</p

Comparison of tuned response amplitude with the same contrast in mixed and in constant contrast blocks.

<p>The distribution histograms show response amplitude obtained, for a given contrast, in constant contrast blocks as a percentage of that obtained in the mixed contrasts block. Upper histogram shows distribution for high contrast, middle histogram for medium contrast, and lower histogram for low contrast. Bar filling refers to significance of changes at the single cell level (t-test): black indicates significant decrease of response amplitude after matched adaptation compared to unmatched adaptation, gray indicates lack of significant changes (p>0.05), and hatched indicates significant increase. 100% on x-axis corresponds to no change in response amplitude. Relative to 100%, the distribution for high contrast is shifted to the left, indicating that response amplitude was larger for high contrast when neurons were adapted to a mixture of contrasts, compared to when neurons were adapted to the high contrast. On the contrary, the distribution of percent change for low contrast stimuli appears shifted to the right: response amplitude was lower on average for low contrast stimuli when neurons were adapted to mixed contrasts compared to when neurons were adapted to the low contrast. The distribution for medium contrast is more centered, although responses to medium contrast were slightly stronger, on average, after adaptation to medium contrast compared to adaptation to mixed contrasts.</p

Changes in orientation tuning width with different contrasts in mixed contrasts blocks.

<p>A. Cumulative distribution of HWHH for the three different contrasts. Horizontal dashed line corresponds to the median. B. Distribution of percent change in HWHH with different contrasts. HWHH for medium contrast is expressed as a percentage of the HWHH at high contrast in the upper histogram. HWHH for low contrast is expressed as a percentage of the HWHH at high contrast in the middle histogram, and as a percentage of the HWHH at medium contrast in the lower histogram. 100% on <i>x</i>-axis corresponds to no change in HWHH. Relative to 100%, all distributions are shifted to the left, indicating decreased HWHH with decreased contrast. Black bars in histograms correspond to significant decrease in HWHH when contrast decreases, and hatched bars to significant increase in HWHH, tested at the single cell level (p<0.05, t test).</p

Changes in orientation tuning width with different contrasts for constant contrast blocks.

<p>A. Cumulative distribution of HWHH for the three different contrasts. B. Distribution of percent change in HWHH with different contrasts. Same conventions as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004781#pone-0004781-g008" target="_blank">Figure 8</a>. Both cumulative distributions and percent change distributions show reduced effect of contrast on tuning width in the constant contrast conditions compared to the mixed contrasts condition (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004781#pone-0004781-g008" target="_blank">Fig. 8</a>).</p

Distribution of changes in tuned response amplitude with different contrasts.

<p>A. Mixed contrasts. B. Constant contrasts. Tuned response amplitude for medium contrast is expressed as a percentage of the tuned response amplitude at high contrast in the upper histograms. Tuned response amplitude for low contrast is expressed as a percentage of the tuned response amplitude at high contrast in the middle histograms, and as a percentage of the tuned response amplitude at medium contrast in the lower histograms. 100% on <i>x</i>-axis corresponds to no change in response amplitude. Relative to 100%, all distributions are shifted to the left, indicating decreased response strength with decreased contrast in nearly all cases. Black bars correspond to significant decreases, and hatched bars to significant increases in response amplitude, tested at the single cell level (p<0.05, t test). In the vast majority of cells, the response amplitude was significantly lower when contrast was decreased.</p

Protocol and orientation tuning with different contrasts, with and without matched adaptation, example.

<p>A. PSTH (bin width 1 sec) of the spiking response obtained with the four blocks of stimuli repeated 12 times in a marmoset V1 cell. Some of the grating stimuli, varying in contrast and orientation, are sketched below the PSTH. Sixteen orientations (from 0 to 168.75 deg, 11.25 deg steps) and 3 contrasts (16, 32 and 64% for this neuron) were randomly presented during the “mixed contrasts” block. Each grating presentation was 0.2 sec long, which is too short to allow for contrast adaptation. Since contrast varies at high rate, adaptation can only occur for a contrast level which is the mean of the different contrasts presented: there is a <i>mismatch</i> between the stimulus contrast presented at a particular time, and the contrast to which the cell is adapted. For the second, third and fourth stimulation blocks, the 16 orientations were still randomly presented, but only one contrast at a time was used: either low, medium or high. The duration of each block (>30 sec in this example) was long enough to allow for adaptation to each of the contrasts. The stimulus contrast presented at a particular time then <i>matched</i> the contrast to which the cell was adapted. Black lines on medium and high contrast responses correspond to the exponential decay fitted to the data. The time constant of adaptation was 0.54 sec with the medium contrast and 1.42 sec with the high contrast. There was no significant adaptation with the low contrast. B. Orientation tuning for data obtained during the mixed contrasts block. Symbols correspond to the mean firing rate for each orientation and contrast, and the lines correspond to the von Mises equation fitted to the orientation-response data. Inset shows fitted lines normalized to the same preferred orientation and to the same height, to facilitate comparison of tuning width. HWHH were 19.0, 16.8 and 12.2 deg for the tuning curves obtained with high, medium and low contrast stimuli, respectively. C. As in B, but for responses obtained after adaptation to either low, medium or high contrast. Spikes outside steady state adaptation, considered to begin at a time corresponding to 3 times the adaptation time constant, were not included in the calculation. HWHH were 19.2, 17.9 and 15.4 deg for the tuning curves obtained with the high, medium and low contrast stimuli, respectively. In this cell, adaptation led to a compression of the range of tuning widths and response amplitudes obtained with the different contrasts. The discrepancy between the spike rates in the PSTH in A and the orientation tuning curves in B and C is due to the fact that interstimulus intervals and responses to non-preferred orientations are included in the average for the long time-scale PSTH.</p

Changes in relative untuned response amplitude with different contrasts.

<p>A. Cumulative distribution for the three contrasts in the mixed contrasts condition. The RURA expresses the proportion of response amplitude that lacks orientation selectivity, relative to the total response amplitude. Values close to zero indicate null response to the orientation orthogonal to the preferred one. Values less than zero indicate firing rates lower than spontaneous activity, suggesting cross-orientation suppression. Values larger than zero indicate responses to orthogonal stimuli. B. Distribution of differences in RURA with different contrasts, in the mixed contrasts condition. Upper histogram: RURA obtained with high contrast minus RURA obtained with medium contrast. Middle histogram: RURA obtained with high contrast minus RURA obtained with low contrast. Lower histogram: RURA obtained with medium contrast minus RURA obtained with low contrast. At the population level, a significant difference was observed between high and medium contrast only, with larger RURA, on average, at high contrast. C. Cumulative distributions for each of the three contrasts, for the constant contrast blocks. D. Distribution of differences in RURA with different contrasts, for the constant contrast blocks. RURA values obtained with high contrast were significantly larger than those obtained with either medium (upper histogram) or low contrast (middle histogram). RURA did not differ between medium and low contrasts (lower histogram). We did not test differences in RURA at the single cell level as RURA calculation combines two parameters, each with its own associated standard error.</p

The effects of contrast and contrast adaptation on HWHH.

<p>A. Cumulative distribution of HWHH. Data in red were obtained with the mixed contrasts block and data in green obtained with constant contrast blocks. The effect of matched contrast adaptation is a narrowing of the distributions, with the largest shift observed for the low contrast. B. Distributions of percent change in HWHH for one contrast in two stimulation regimes. HWHH obtained in the constant contrast block for a given contrast (matched adaptation) is expressed as a percentage of the one obtained for the same contrast in the mixed contrasts block (mismatched adaptation). Cells showing significant (t-test, p<0.05) decrease in HWHH with adaptation to constant contrast are indicated in black, and cells showing significant increase in HWHH by hachure. Upper histogram: adaptation to high contrast resulted in a small but significant reduction of HWHH compared to adaptation to mixed contrasts (median: 96.3%). Middle histogram: adaptation to medium contrast resulted in a small but significant increase in HWHH compared to adapting to mixed contrasts (median: 105.0%). Lower histogram: adaptation to low contrast resulted in a significant and larger increase in HWHH compared to adapting to mixed contrasts (median: 120.7%).</p

Distribution of differences in preferred orientation with different contrasts.

<p>A. Mixed contrasts. B. Constant contrasts. Upper histograms show the differences for medium vs. high contrast. Middle histograms show the differences for low vs. high contrast. Lower histograms show the differences for low vs. medium contrast. Black bars correspond to cases for which the difference was found to be significant at the single cell level (p<0.05, t test).</p

Distribution histograms of contrast adaptation time constants.

<p>Adaptation time constants were determined from single exponential curves fitted to PSTHs obtained with high (upper histogram), medium (middle histogram) and low (lower histogram) contrasts in constant contrast blocks, as exemplified in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004781#pone-0004781-g001" target="_blank">Fig. 1A</a>.</p