7.1  The electrical responses to light stimuli in single optic nerve fibres

investigations of h. k. hartline and s. w. kuffler 73

flash of light after an appreciable latency period caused a discharge of impulses in the fibre which lasted for several seconds. The total number of impulses elicited and the discharge frequency were found to increase with the intensity of the flash. These findings were in accord with what might be expected. Surprisingly, however, Hartline found that the magnitude of each impulse discharge was the same when intensity or duration of the light stimulus was varied. Also, he found that different test wavelengths could be made to evoke the same frequency of impulses from a given receptor cell by adjusting the incident light energy (Graham & Hartline, 1935). On this evidence the authors concluded that single receptors were unable to discriminate­ between wavelength and intensity, a characteristic of receptors later termed ‘the Principle of Univariance’. This principle implies that at least two different types of photoreceptor must be present in the retina in order to distinguish between different colour qualities of different wavelengths.

With exposure of short duration, Hartline found, in accord with the reciprocity law of photochemistry (the Bunsen-Roscoe law), that the response pattern of the nerve fibre did not vary as long as the total amount of light energy (light intensity multiplied with duration) remained constant, strongly suggesting that the phototransduction­

process was photochemical in nature. Based on this suggestion he could determine the absorption spectrum of the photosensitive substance of single receptors in Limulus (responsible for excitation of a single optic nerve fibre) simply by measuring the reciprocal of the relative light energy of different wavelengths needed to produce a specific burst of impulses (Graham & Hartline, 1935).

The impulse pattern obtained from single optic nerve fibres ofLimulus was also found to be affected by light and dark adaptation. Thus, during light adaptation the frequency of the discharge of impulses from single nerve fibres of a light stimulus declined to a steady level, while it showed a progressive recovery during dark adaptation.

Hartline(1940)ascribedthedeclineandrecoveryofthefrequency of impulses to depletion and regeneration of the photosensitive­

74development of the duplicity theory from 1930–1966

substance of the receptors. Furthermore, he explained the steady discharge of impulses recorded after light adaptation had stabilized on the assumption that the rate of decomposition and regeneration of the photosensitive substance had reached a state of equilibrium.

7.3  The electrical responses in single optic nerve fibres of the frog

Hartline also made use of cold-blooded vertebrate frog retina in his investigation. In accord with his results on Limulus, the frequency of discharge in different fibres was found to be greater the higher the intensity of the retinal illumination, and it could be reduced and increased by light and dark adaptation. However, while the response pattern in nerve fibres of Limulus was found to be essentially similar in character, Hartline found three quite different response patterns in the frog retina:

1.  In some fibres (so-called ‘on’ fibres) the discharge was similar to that of Limulus; the impulses were discharged regularly as long as the light was on.

2.  Other fibres (‘on-off’ fibres) discharged impulses briefly when the light was turned on and again when it was turned off with no activity in between.

3.  A third category of fibres (‘off’ fibres) discharged impulses only when the light was turned off.

This diversity of response was ascribed to the complex nervous structure interposed between the receptor elements and the ganglion cells of the frog retina.

An interesting feature of the impulses in the ‘off’ fibres was their strong suppression by re-illumination. This phenomenon was interpreted to mean that excitation of nerve cells may inhibit nerve activity. Hartline also found the ‘on-off’ fibres of the frog retina to be extremely sensitive to movements of retinal images: the more rapid and more extensive the movement, the greater the number of impulses at ‘on’ and ‘off’.

Moreover, exploring the retina of the frog using a small spot of light, Hartline found that the receptive field of a ganglion cell

investigations of h. k. hartline and s. w. kuffler 75

(defined as the area of the retina which when illuminated may cause a discharge in the particular ganglion cell, i.e. the retinal region occupied by photoreceptors whose connections converge upon the ganglion cell) was maximally sensitive in the centre of the field, and that the sensitivity gradually fell off with increasing distance from this centre. Also, when the area of the receptive field illuminated was increased, the response of the ganglion cell increased, demonstrating spatial summation in the retina of the frog. Clearly, several retinal pathways had to converge upon the ganglion cell producing a combined effect.

7.4  Receptive field organization of rods and cones: Kuffler’s investigation

The investigation of Hartline was followed up by Kuffler (1953). He made an extensive study of the configuration and functional ­organization of receptive fields in the retina of cats. Discharge patterns of single ganglion cells of the cat retina were recorded with a microelectrode while light stimuli were applied to different areas of its receptive field. Two small independently controlled light beams and also a background light field that determined the adaptation level of the retina were employed.

With this technique, Kuffler (1953) obtained several very interesting results. He showed that the retina contained receptive fields with central and peripheral areas that had opposite discharge patterns. Fields with ‘on’-centre regions had ‘off’ surrounds and vice versa. In intermediary regions between the central and peripheral areas ‘on-off’ response patterns could be recorded. Both transient and maintained discharge patterns were found.

The boundaries and extent of the receptive fields were found to vary with light and dark adaptation. With high background ­illumination the receptive fields tended to shrink, while they tended to expand during dark adaptation. He also found that different regions within a single receptive field tended to interact: ‘off’-areas tended to suppress the discharges from ‘on’-areas and vice versa. In fact, all

76development of the duplicity theory from 1930–1966

degrees of mutual modifications were obtained when different zones within the receptive field were activated simultaneously by the two test lights.

Furthermore, Kuffler found that the ratio of regions dominated by ‘on’ or ‘off’ differed in different receptive fields.

To explain his results, Kuffler suggested that specific small areas within a receptive field of a ganglion cell gave rise to ­predominantly inhibitory or excitatory impulses and that the discharge pattern of larger areas of the field depended on the summed effect of the pathways converging on the given ganglion cell.

The important question of whether the receptive field organization of a ganglion cell was the same for the rod and cone receptor systems was pursued by Barlow, Fitzhugh and Kuffler (1957). In the lightadapted state, they found, in accord with Kuffler (1953), the receptive field of ganglion cells of the cat retina to be arranged so that the central and peripheral regions of the field were mutually antagonistic, evoking electrical potentials of opposite sign regardless of whether the ganglion cell explored was an ‘on’- or ‘off’-centre unit. In the dark-adapted state, on the other hand, they did not find this antagonistic interaction, suggesting that the receptive field organization of the rod and cone systems, in fact, was based on quite different principles.

Further investigation, however, revealed that the mutually antagonistic interaction between the central and peripheral parts of the receptive field could be obtained under conditions where only cones and only rods were test stimulated. Hence, it was concluded that dark adaptation, not a change in the receptor system, reduced the antagonistic interactions of the receptive field.

To explain the lateral inhibition found, the authors pointed to several possibilities: horizontal, amacrine or bipolar cells could all be responsible.

They also suggested that lateral inhibition could play a role in the simultaneous contrast effects observed psychophysically and also explain the well-known psychophysical fact stressed by Lythgoe (1940, p. 40) that summation of the light stimulus was more marked

investigations of h. k. hartline and s. w. kuffler 77

in the darkthan the light-adapted state. In opposition to Lythgoe’s explanation, however, they suggested, based on their own results, that less interaction existed in the dark-adapted state, and that the increase of the summation effect obtained when the eye became dark adapted was due to a reduction of lateral inhibition, not an increase in lateral summation.