Influence on cortical responses to patterned flashes of contrast-depth and of the ratio between the combined light and dark areas

Summary

The influence of contrast-depth and of the ratio between the combined light and dark areas was studied in 5 subjects. The subjects were dark-adapted and their pupils were dilated. Flash duration was 4 μsec; flashes were presented at a frequency of 2/sec; flash luminance was 3 mL unless otherwise noted. Unipolar leads were used (active electrode 21/2 cm above the inion, reference electrode at one earlobe, ground electrode over vertex).

Responses are averaged over 100 flashes, with the aid of a Nuclear Chicago data retrieval computer.

Results can be summarized as follows:

• 1. Changes in contrast-depth at constant flash intensity as well as at constant overall flux result in a sigmoid shaped relation between the latency of the main negative component (invertedT-wave) and the contrast-depths; the amplitude decreases in a monotonical way.

• 2. An increase in total flux results in a steeper, sigmoid, relation.

• 3. The latency vs. contrast-depth function is independent of the spatial frequency of the pattern, for raster patterns as well as for checkerboards.

• 4. An increase of the dark to light ratio up to 50/50 results in a linear decrease of the latency of the negative component. Beyond this value the latency does not change any more.

• 5. The shape of the latency vs. contrast-depth function is dependent on the dark to light ratio, up to a ratio of 50/50, and independent at higher ratios.