When there is point-to-point correspondence between the stimulus and response, verbal behaviour may be classified as echoic. A further requirement is that the verbal stimulus and the echoic response must be in the same mode (auditory, visual, etc.) and have exact physical resemblance (e.g., same sound pattern). An echoic is a class of verbal operants regulated by a verbal stimulus in which there is correspondence and topographic similarity between the stimulus and response. Saying "This is a dog" to the spoken stimulus "This is a dog" is an example of an echoic response in human speech.