International Journal of Comparative Psychology

A model will be presented in which mirror image confusion is employed as an example of perception of shape orientation, occurring as a result of evolutionary change in vision and movement. In the most primitive condition, vertical and horizontal coordinates are absent and shapes are equivalent in terms of orientation. In this condition directionality in external space is not objectified and movement is reflexively toward or away from the visual target. In the second condition, only the horizontal axis is present. Changes in orientation from upward to downward are perceptually salient. Quadrupedal movement patterns and locomotion across land, dominated by the horizon, are associated with the evolution of a mammalian eye with enhanced acuity across the vertical axis. Vertical mirror image confusion ceases to exist. In the third condition, the vertical axis appears. Factors in primate evolution associated with the appearance of enhanced acuity along these visual axes are related to perception of lateral rotations. In the fourth condition, upright posture and development of lateral bias in eye movement are related to the human proclivity to differentiate right and left orientation of shapes while exhibiting increased difficulty in tasks that involve changes in the vertical orientation. Thus structural changes in evolution associated with posture and movement are demonstrated to account for differences in perceptual responses to orientation of shapes.

Lateral asymmetries are not confined to humans. Palaeozoic trilobites and calcichordates are now known to have been asymmetrical; song control in passerines is vested in the left cerebral hemisphere; learning which is lateralized to the left forebrain of chicks includes imprinting, visual discrimination learning and auditory habituation, while responses to novelty, attack and copulation are activated by the right; in rats the right hemisphere is involved in emotional behavior and spatial discriminations, and there are numerous other behavioral, anatomical and pharmacological asymmetries; the left hemisphere of the female mouse is superior at processing its pups' calls, and there are reports of behavioral asymmetries in impala, cats and dogs. Anatomical asymmetries in the primate brain, from monkeys upwards, are matched by increasing evidence of behavioral asymmetries in visual pattern discrimination, discrimination of species-specific calls, and handedness. We discuss the interaction of preexisting behavioral and brain asymmetries with the evolution in hominids of an upright bipedal posture and tool use, and the origins of language, and conclude that there may be a continuity with earlier species of our two most obvious asymmetries, language lateralization and hand preferences. There may be an ancient left-brain specialization for sensory and motor discrimination learning, which is complemented by a relegation to the right of primitive spatial and emotional functions.

The capacity of the two cerebral hemispheres for temporal processing was investigated in two experiments concerned with sensory and motor processing, respectively. The temporal processing of sensory information was examined in a task requiring simultaneity judgement of pairs of tactile stimuli delivered unimanually or bimanually. Unimanual stimulation permitted presentation of both events to the same hemisphere while bimanual stimulation involved both hemispheres and necessarily required interhemispheric communication to compare stimulus onset asynchrony (SOA). The order of presentation of asynchronous pairs determined which cerebral hemisphere was activated first. Pairs of stimuli were judged as simultaneous at longer SOAs in the bimanual than the unimanual conditions whilst unimanual left and right simultaneity thresholds did not differ. These results suggest that the two hemispheres are equally capable of temporally resolving a pair of simple tactile stimuli. A structural model proposing that temporal comparisons are carried out in the hemisphere receiving the second stimulus provides the best account of the results.

The temporal processing of motor information was examined in a task requiring the planning and execution of sequences of finger movements. A predetermined number of double-tap responses with the index and middle fingers of a given hand were required in response to a visual cue in the ipsilateral visual field. The restriction of the performance cue in each trial to the hemisphere controlling the response permitted assessment of the contribution of each cerebral hemisphere to differences in hand skill. Movement time increased linearly for both hands with increasing length of tap sequence and did not differentiate hand performance. Response preparation time, however, increased linearly with increasing task load for the preferred hand but varied quadratically for the non-preferred hand. These results indicate that differences in hand skill may be determined by the mode of response preparation within the contralateral hemisphere. They also suggest that studies of hand differences involving fixed levels of motor demand would not properly differentiate hand performance.

Together, these studies indicate that both cerebral hemispheres are capable of the temporal processing of sensory and motor information but that the hemisphere primarily involved is determined by side of stimulus or response, respectively.