Читать книгу Bird Senses - Graham R. Martin - Страница 32

When enough trials are accumulated a relatively stable relationship between error rate and stripe width will emerge. This relationship is called a psychophysical function, and the point at which a bird gets 75% of the trials correct is considered the threshold of sensory performance (Figure 2.8). So, after all the training and testing, it is this 75% correct stripe width that the investigator is trying to determine, not what the bird can see 100% of the time.

FIGURE 2.8 An example of a psychophysical function of a Harris’s Hawk. Performance over a large number of trials has been accumulated on a two-choice task using the kind of setup shown in Figure 2.7. The average percentage ‘correct’ performance for panels showing stripes of different widths is accumulated. The black dots on the graph show the bird’s actual performance for different stripe widths and the red curve is the line of best fit to those data points. With wide stripes the bird is correct on nearly every trial, while with very narrow stripes its choices are random (50% correct). It is in the region between these two extremes that the threshold of the bird’s acuity lies. The region where the birds is 75% correct is usually taken as the threshold performance. In this example 75% correct would occur with stripes that are about 0.7 minutes of arc wide. Similar data are gathered for a number of birds using the same technique, and these are averaged to give an acuity estimate for the species. In the case of Harris’s Hawk, the published average acuity is 1 minute of arc (29 cycles per degree).

The important point to note is that the threshold is defined statistically. It is the stripe width at which on average the birds get the choice right 75% of the time (Figure 2.8). This statistical definition reflects the nature of sensory thresholds. The limit of performance can vary from moment to moment and from day to day, depending on the physiological state of the observer and on their motivation. Keeping up a bird’s motivation is very important because these investigations can last many months and the bird needs to be fit, healthy, and equally motivated throughout that period.

Once a threshold for visual acuity has been determined for a group of birds, an average value can be calculated as representative of that species. Individuals will differ, and so in any sample of birds there will be individuals above and below the average determined for the species. But it is usually the average value that is used in any comparisons between species; the best performance is no more representative of the species than the worst.