Two ways of looking
In half the rounds the target differs from everything else by one feature — it is the only red thing on the field. That kind of target does not have to be searched for: it appears by itself, and your time barely changes whether there are twelve objects or eighty. Psychologists call it pop-out.
In the other half the field also holds red squares and blue circles. Now neither colour nor shape identifies the target on its own — only the two together. Attention has to visit objects one at a time, and the time starts growing with the number of objects. The slope of that line, in milliseconds per object, is roughly how long one visit costs you.
What the chart shows
- A flat green line means the visual system handled the whole field in parallel — this is one of the few measurable things a person does that a straightforward program does not do at all.
- A rising blue line is a serial scan. Typical human numbers are somewhere around 20 to 40 ms per object, and about twice that when the target is absent.
- The difference between the two lines is the whole point of the test: same eyes, same field, same target — and two completely different mechanisms behind them.
And where the machine is
For a program both fields are the same work: run through the list of objects and check colour and shape. There is no pop-out, no attention and no difference between the two kinds of round — only a loop that takes a few microseconds either way.
Which is the honest shape of this comparison. The machine wins on time by a factor of about ten thousand, and still does not have the thing the flat green line demonstrates: a whole scene delivered at once, before anything has been examined.