Recent years have seen increased exploration of the sensory development of the foetus . All five senses, auditory, visual, cutaneous, olfactory and gustatory and been demonstrated to begin functioning in the prenatal period. It is the aim of this chapter to review the evidence of haptic perception in the human foetus. The development and function of other senses is briefly presented. However before reviewing this evidence it is necessary to pause to consider some general issues which must be borne in mind when evaluating the evidence presented by these studies.
Sensory and perceptual development in the foetus
First, it is important to distinguish between sensation and perception . Often researchers reporting studies in this area use the two terms interchangeably but this is an error as the two are different. Sensation refers to the transduction of the physical signal by the sensory receptor and turning this physical signal into neural impulses. Thus a sound stimulates the hair cells of the inner ear and neural impulses are generated by the hair cell and transmitted to the cochlear nerve for onward neural transmission to the brain. Sensation refers simply to the act of transducing the physical stimulus into neural stimuli within the nervous system. Perception is the process which adds meaning to these neural impulses as they interact with the various centres and pathways in the brain. Thus if the sound stimulus were the voice of the infant’s mother, the sensation experienced would be the various parameters of the sound, for example its frequency, duration, intensity. The perception experienced would include, for example, recognition of the voice as mother, the meaning of the words spoken by the mother, and potentially the urgency or otherwise of the information conveyed. Caution must thus be exercised in interpreting the results of studies examining foetal sensory and perceptual development. Although the foetus may respond to the sound of its mother’s voice, whether the foetus perceives this stimulus as its mother’s voice or is responding to a sensation due to the presence of a sound, needs careful experimentation and consideration.
A second issue concerns the onset of the development of sensory abilities. To determine whether the foetus detects a particular stimulus we have to rely on observation of a change in its behaviour, e.g., a sudden behavioural jump to the onset of a loud sound. Observation of the foetus’s movement may be achieved through ultrasound [2, 3] or recording its heart rate [4, 5].