THE VISUAL SYSTEM

In considering the visual system (as well as the other sensory systems), it is important to discriminate between the physiological response of the peripheral sense organs to the stimulus energy and the psychophysiological (cognitive) percept of the sensation derived from neural processing. To emphasize this point Figs. 1-3 show three well known visual patterns that give rise to ambiguities in their cognitive interpretations. Fig. 1 represents an example of fictitious visual shape completions, in that the perception of the white square is based on seeing non-existent contours between the black corners. Fig 2 is an example of the well known Muller-Lyer illusion: the lengths of the two vertical lines are perceived as different, while their objective lengths are in fact identical. The pattern in Fig. 3 demonstrates alternation of figure and background: The viewer may see either a black candlestick on a white background or two smiling facial profiles facing each other on a black background.  

Figures 1-3. Visual Illusions

The meaningful interpretation of visual information is heavily dependent on central integrative mechanisms, as well as memories and correlates of past experience (learning). Because of time constraints, the present lectures are confined to the peripheral sensory end organ- the eye. Higher visual functions will be discussed in future courses.  

Figure 4. Similarities between the Eye and a Camera

THE EAR

 The ear is acomposite sensory (mechanoreceptive organ) which is comprised of two main parts, the auditory and vestibular systems. Phylogenetically, the vestibular component of the ear is the oldest. It appears in elementary form in early species such as the cyclostomes (i.e. lamprey). The auditory component appears to have evolved subsequently from vestibular organs, and consequently there are strong similarities in both the anatomy and physiology of mechano-neural transduction in the two types of organs.

The specific stimuli which are adequate to activate each of the two systems are quite different. The auditory system responds to high frequency pressure waves of air, while the vestibular system responds primarily during low frequency inertial forces. The activation of either system can lead to a reflex motor response. For example, a sudden loud sound generates a "startle response" in which the auditory system generates turning movements of the eyes and head, whereas the sudden activation of the vestibular system leads to postural and ocular stabilizing reflexes.     

THE AUDITORY SYSTEM

The ear is a mechanoreceptor responding to the vibration of air molecules thereby creating the subjective sensation of sound. Sound normally manifests as a pressure wave travelling through a gas, however such waves can also travel through liquids and solids. The normal human ear is exquisitely sensitive to sound waves: it can detect amplitudes of oscillation as small as 10-10 cm (this is less than the diameter of a Na+ ion). Sound has several measurable qualities, three of which will be considered here:

(1) Pitch - related to the frequency of sound waves (Fig. 1). The entire range of frequencies audible to a normal young subject extends from about 30 - 18,000 Hz (this range is considerably reduced in old age).

(2) Loudness - related to the amplitude of sound waves (Fig. 1). The total range of loudness extends from a threshold values of about 0.0002 dynes/cm2 (maximum sensitivity) to a pain threshold of about 2000 dynes/cm2. This is a total range of 107 or 140 decibels.

(3) Direction - determined by the spatial relationship between the subject and the source of the sound.

Figure 1. Definitions of Frequency (Pitch) and Amplitude (Loudness)

Although the total range of audible frequencies is about 30 - 18,000 Hz for the average normal subject, the sensitivity of the ear is not the same at all frequencies within this range. This is illustrated in Fig. 2, where the amplitude of the sound wave (y axis), necessary to produce a just audible (threshold) sound perception at a given frequency (x axis) is plotted. This curve is referred to as the Normal Audibility Curve, and has been derived by averaging data from many subjects. As can be seen, the normal auditory system is maximally sensitive between about 500 - 5,000 Hz (i.e. the threshold is lowest: minimal power is required to produce a just audible sound). This observed frequency response can be explained by considering the anatomy and functional properties of the ear.

Figure 2. The Normal Audibility Curve