| Popis: |
Incremental thresholds (ΔI) were obtained behaviourally for six turtles (Chrysemys picta) at different levels of background illumination (I). The results did not conform to Weber's Law, but were well fitted by the relationshipΔI = K(I+X), whereK andX are constants. When the value ofI was low (5.1 × 10−4ft lamberts) the threshold was affected by temperature, the animals being about 0.5 log units less sensitive at 30°C than at 20°C. Temperature did not affect the thresholds when the value ofI was high (1.6 ft lamberts). These results were interpreted as showing that the thresholds were limited by visual noise when the value ofI was low, and that this noise depends on temperature. Possible sources of the noise were discussed. Psychometric functions (frequency of seeing curves) were obtained for the different experimental conditions. These were steeper at high values ofI than at low, and at 30°C than at 20°C. |
