Eye and ear preferences

This chapter covers methods of measuring preferences to use one eye or ear to attend to a stimulus, which reflects lateralized processing of sensory information. It begins with monocular occlusion as a way of measuring differences in strength or nature of response elicited by particular visual stimuli. Depending on the type of stimulus presented a preference for responding using the left or right eye can be found ( e.g. 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 on the sides of their head, this reflects differences in processing by the left and right sides of the brain. 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 rhe animal being tested 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 cause the test animal to turn its head 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 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.