Eye and ear preferences

This chapter covers methods of measuring eye and ear preferences to attend to sensory information and to hemispheric differences in processing sensory information. It begins with monocular occlusion as a way of measuring differences in type and strength of response elicited by visual stimuli. Depending on the type of stimulus presented a preference for responding using the left or right eye can be found (e.g., domestic chicks show a right-eye preference when searching for food grains and a left-eye preference when attacking a conspecific or responding to a predator). Especially in species with their eyes positioned laterally and hence with small binocular visual fields, differences between responses elicited after applying a patch to the left or right eye reflect differences in processing by the left and right sides of the brain respectively. In these species it is also possible to test responses to stimuli presented in the left versus right monocular visual fields without having to apply eye patches. A method of determining the extents of the monocular and binocular visual fields is explained. Then a modification of the monocular testing method involving rotation of the stimulus around the animal is discussed: as shown in frogs and toads, response to prey moved in this manner differs between clockwise and anticlockwise rotation. Eye preferences can also be determined using binocular presentation of stimuli that elicit head turning to permit monocular fixation of the stimulus before a specific response is made (e.g., before attacking a conspecific, as scored in chicks and horses). Angles adopted by fish when viewing their image in a mirror have been used to measure lateralization of attending to a 'conspecific'. Another approach is simultaneous introduction of identical stimuli into the monocular field of each eye and assessment of side biases in responding (as in toads striking at insect prey). Visual pathways and strategies of monocular viewing are discussed briefly to help explain how eye preferences reveal brain lateralization. Next, several methods of measuring lateralization of processing and responding to auditory stimuli are covered and finally some points are made about future directions of research along these lines. The suitability of these methods for testing different species is considered in all sections of the chapter.